Ba2Ln1-xMn2Te5(Ln = Pr, Gd, and Yb; X = Ln vacancy): Syntheses, crystal structures, optical, resistivity, and electronic structure

Article abstract of DOI:10.1039/d1dt00057h

Three new isostructural quaternary tellurides, Ba2Ln1-xMn2Te5 (Ln = Pr, Gd, and Yb), have been synthesized by the molten-flux method at 1273 K. The single-crystal X-ray diffraction studies at 298(2) K showed that Ba2Ln1-xMn2Te5 crystallize in the space group -C2/m of the monoclinic crystal system. There are six unique crystallographic sites in this structure's asymmetric unit: one Ba site, one Ln site, one Mn site, and three Te sites. The Ln site in the Ba2Ln1-xMn2Te5 structure is partially filled, which leaves about one-third of the Ln sites vacant (□) for Pr and Gd compounds. These structures do not contain any homoatomic or metallic bonding and can be charge-balanced as (Ba2+)2(Gd/Pr3+)2/3(Mn2+)2(Te2-)5. The refined composition for the Yb compound is Ba2Yb0.74(1)Mn2Te5 and can be charge-balanced with a mixed valence state of Yb2+/Yb3+. The crystal structures of Ba2Ln1-xMn2Te5 consist of complex layers of 2∞[Ln1-xMn2Te5]4- stacked along the [100] direction, with Ba2+ cations separating these layers. The Ln atoms are bound to six Te atoms that form a distorted octahedral geometry around the central Ln atom. Each Mn atom in this structure is coordinated to four Te atoms in a distorted tetrahedral fashion. These LnTe6 and MnTe4 units are the main building blocks of the Ba2Ln1-xMn2Te5 structure. The optical absorption study performed on a polycrystalline Ba2Gd2/3Mn2Te5 sample reveals a direct bandgap of 1.06(2) eV consistent with the DFT study. A semiconducting behavior was also observed for polycrystalline Ba2Gd2/3Mn2Te5 from the resistivity study. The temperature-dependent magnetic studies on a polycrystalline sample of Ba2Gd2/3Mn2Te5 did not reveal any long-range magnetic order down to 5 K. The effective magnetic moment (μeff) of 10.37μB calculated using the Curie-Weiss law is in good agreement with the theoretical value (μcal) of 10.58μB.

Full text of DOI:10.1039/d1dt00057h

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Ba2Ln1−xMn2Te5 (Ln = Pr, Gd, and Yb; x = Ln
vacancy): syntheses, crystal structures, optical,
Cite this: DOI: 10.1039/d1dt00057h
resistivity, and electronic structure†
a
Gopabandhu Panigrahi, a Subhendu Jana, a Mohd Ishtiyak,
S. Narayanswamy,b Pinaki P. Bhattacharjee,b K. V. Ramanujachary,c
Manish K. Niranjan d and Jai Prakash
*
a
Three new isostructural quaternary tellurides, Ba2Ln1−xMn2Te5 (Ln = Pr, Gd, and Yb), have been syn-
thesized by the molten-flux method at 1273 K. The single-crystal X-ray diffraction studies at 298(2) K
showed that Ba2Ln1−xMn2Te5 crystallize in the space group C32h –C2/m of the monoclinic crystal system.
There are six unique crystallographic sites in this structure’s asymmetric unit: one Ba site, one Ln site, one
Mn site, and three Te sites. The Ln site in the Ba2Ln1−xMn2Te5 structure is partially filled, which leaves
□
about one-third of the Ln sites vacant ( ) for Pr and Gd compounds. These structures do not contain any
homoatomic or metallic bonding and can be charge-balanced as (Ba2+)2(Gd/Pr3+ 2/3(Mn2+)2(Te2−)5. The
)
refined composition for the Yb compound is Ba2Yb0.74(1)Mn2Te5 and can be charge-balanced with a
mixed valence state of Yb2+/Yb3+. The crystal structures of Ba2Ln1−xMn2Te5 consist of complex layers of
21[Ln1−xMn2Te5]4− stacked along the [100] direction, with Ba2+ cations separating these layers. The Ln
atoms are bound to six Te atoms that form a distorted octahedral geometry around the central Ln atom.
Each Mn atom in this structure is coordinated to four Te atoms in a distorted tetrahedral fashion. These
LnTe6 and MnTe4 units are the main building blocks of the Ba2Ln1−xMn2Te5 structure. The optical absorp-
tion study performed on a polycrystalline Ba2Gd2/3Mn2Te5 sample reveals a direct bandgap of 1.06(2) eV
consistent with the DFT study. A semiconducting behavior was also observed for polycrystalline
Ba2Gd2/3Mn2Te5 from the resistivity study. The temperature-dependent magnetic studies on a polycrystal-
line sample of Ba2Gd2/3Mn2Te5 did not reveal any long-range magnetic order down to 5 K. The effective
magnetic moment (µeff) of 10.37µB calculated using the Curie–Weiss law is in good agreement with the
theoretical value (µcal) of 10.58µB.
Received 7th January 2021,
Accepted 7th April 2021
DOI: 10.1039/d1dt00057h
rsc.li/dalton
sis,4 magnetism,5,6 and superconductivity.7 Similarly, the
ternary chalcogenides of lanthanides (Ln) with alkali or alka-
1. Introduction
Transition-metal (M) based chalcogenides containing alkali line earth metals are also well-known for their rich crystal-
(A) or alkaline earth (AE) metals have long been a subject of chemistry and a variety of physical properties.8 The main
interest due to their applications in various fields, such as framework of most of these chalcogenide crystal structures is
thermoelectric materials,1,2 transparent conductivity,3 cataly- formed by the sharing of transition metal and chalcogen-
based octahedra and/or tetrahedra.9 The addition of lantha-
nides that prefer octahedral10 and trigonal prism-like geome-
tries11 to transition-metal based chalcogenides results in the
stabilization of various new complex low-dimensional
materials that display a variety of one-dimensional, layered,
and tunnel structures.12,13 Among these, layered chalcogenides
have additional advantages due to the possibility of fine-
tuning their physical properties using host–guest intercalation
as well as controlling their electronic properties by manipulat-
ing the chemical composition of the spacing layer.14 For
example, the superconducting transition temperature (Tc) of
Fe-based chalcogenides could be enhanced from 8 K to
30–46 K with various intercalated guest species.15,16
aDepartment of Chemistry, Indian Institute of Technology Hyderabad, Kandi,
Sangareddy, Telangana 502285, India. E-mail: jaiprakash@chy.iith.ac.in
bDepartment of Materials Science & Metallurgical Engineering, Indian Institute of
Technology Hyderabad, Kandi, Sangareddy, Telangana 502285, India
cDepartment of Chemistry and Biochemistry, Rowan University, Glassboro, New
Jersey 08028, USA
dDepartment of Physics, Indian Institute of Technology Hyderabad, Kandi,
Sangareddy, Telangana 502285, India
†Electronic supplementary information (ESI) available: Experimental details,
PXRD patterns, and details of the crystallographic parameters for
□
Ba2Ln2/3 1/3Mn2Te5 (Ln
= Pr, Gd, and Yb). CCDC 2049695, 1961083, and
2049699 for Pr, Gd, and Yb-based structures, respectively. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/d1dt00057h
This journal is © The Royal Society of Chemistry 2021
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Furthermore, the quaternary compounds of transition and mium (Alfa Aesar 99.9%), gadolinium (Alfa Aesar 99.9%), ytter-
lanthanide metal-based chalcogenides are expected to show bium lumps (Alfa Aesar 99.9%), manganese pieces (Alfa Aesar
complex magnetic behavior due to the possible exchange–cor- 99.9%), sodium chloride powder (Alfa Aesar 99.9%), and tellur-
relation between the d-electrons of M atoms and 4f-electrons ium ingot (Sigma-Aldrich 99.999%). Prior to use, these chemi-
of Ln atoms.17 Therefore, the crystal-chemistry and the associ- cals were stored inside an argon-filled dry glove box due to
ated physical properties of the quaternary chalcogenide com- their air and moisture sensitivity. All chemical manipulations
pounds that contain both transition metals and lanthanides were performed inside the argon-filled glove box. The surfaces
are indeed very intriguing. For example, compounds like of praseodymium, gadolinium, and ytterbium lumps were first
BaLn2ZnS5 (Ln = Ce, Pr, and Nd),18 BaNd2MnS5,18 BaLn2MnS5 shaved to remove the oxidized layer, and then small pieces of
(Ln = La, Ce, and Pr),19 BaGdCuSe3,20 BaGdCuS3,20 and these metals were cut for the reactions.
10
BaNdAgTe3 show interesting magnetic properties. Examples
2.1.1. Syntheses of single crystals of Ba2Ln1−xMn2Te5 (Ln =
of quaternary chalcogenides that show optical properties are Pr, Gd, and Yb). Black crystals of the new quaternary phases,
CsLnCdTe3 (Ln = La, Pr, Nd, Sm, Gd, Tm, and Lu),21 Ba2Ln1−xMn2Te5, were first discovered by exploratory solid-
CsLnMnSe3 (Ln = Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Y),22 state syntheses carried out in the Ba–Ln–Mn–Te system. The
BaLaCuS3,20 and BaLaCuSe3.20 Recent theoretical studies on following synthesis methodology was used: the reactants were
these 1113-type compounds show that these compounds have weighed and then transferred into a carbon-coated fused silica
the potential to exhibit excellent thermoelectric properties.23
tube inside an Ar-filled dry glove box. These fused silica tubes
The phase-diagrams of ternary metal chalcogenides are were then sealed under vacuum (ca. 10−4 Torr) with a flame
relatively well studied as compared to quaternary metal chalco- torch. The heat treatment of the tubes containing the reaction
genides. As a result, the number of phases known in the qua- mixture was carried out in a programmable muffle furnace.
ternary AE–Ln–M–Q system are fewer than those known in the Many different heating profiles were explored to synthesize
corresponding ternary AE/Ln/Q or AE/M/Q systems. So far, a Ba2Ln1−xMn2Te5 crystals in good yield. The optimized best
total of about fifty-three compounds are known in the AE–Ln– heating profile is as follows: the temperature of the furnace
M–Q quaternary system.24 These compounds can be broadly was increased from room temperature to 1273 K in 48 h, where
divided into four families based on their compositions: the reaction mixtures dwelled for 48 h and then slowly cooled
AELnMQ3 (1-1-1-3 type), Ba4Ln2CdQ10 (4-2-1-10 type), down to 1123 K at a cooling rate of 3 K h−1 followed by anneal-
BaLn2MS5 (1-2-1-5 type), and Ba2LaAg5S6 (2-1-5-6 type). The ing for 48 h at 1123 K. Subsequently, the furnace temperature
compounds with the general formula Ba4Ln2CdQ10, was cooled to 773 K at a rate of 4 K h−1. Finally, the furnace
BaLn2MS5, and Ba2LaAg5S6 crystallize in the orthorhombic was switched off and radiatively cooled to room temperature.
(space group: Cmc21), tetragonal (I4/mcm), and monoclinic The reaction tubes loaded with La and Ce were found broken
(C2/m) structures, respectively. The members of the 1-1-1-3 after the heat-treatment.
family adopt seven structure types depending on the nature
and identity of metals and chalcogens.25
Synthesis of Ba2Pr1−xMn2Te5. Black irregular shaped crystals
of Ba2Pr1−xMn2Te5 (Ba : Pr : Mn : Te in 6 : 2 : 6 : 15 ratio) were
The crystal-chemistry of quaternary AE–Ln–M–Te tellurides obtained from the reaction mixtures of Ba (24.6 mg,
is least explored, with only the 1-1-1-3 type of compounds 0.179 mmol), Pr (8.4 mg, 0.060 mmol), Mn (9.8 mg,
known for the tellurides. Thus, we have initiated a study to 0.179 mmol), Te (57.1 mg, 0.448 mmol), and 100 mg of NaCl
explore the AE–Ln–M–Te quaternary system to discover new flux.
phases in an effort to study their structure–property relation-
Synthesis of Ba2Gd1−xMn2Te5. Black irregular shaped crystals
ships. In this article, we present the syntheses and crystal of Ba2Gd1−xMn2Te5 (Ba : Gd : Mn : Te in 9.5 : 19 : 10 : 48 ratio)
structures of three new members of this family, were obtained from the reaction mixtures of Ba (11.9 mg,
Ba2Ln1−xMn2Te5 (Ln = Pr, Gd, and Yb; x = Ln vacancy), that 0.087 mmol), Gd (27.2 mg, 0.173 mmol), Mn (5.0 mg,
show partial occupancy at the lanthanide sites. These layered 0.091 mmol), and Te (55.8 mg, 0.438 mmol) in 100 mg of NaCl
quaternary tellurides are the first examples of compounds in that acted as a molten flux.
the AE–Ln–M–Te system, which do not adopt the 1-1-1-3 type
Synthesis
of
Ba2Yb1−xMn2Te5.
Black
crystals
of
structure. The physical properties, including variable tempera- Ba2Yb1−xMn2Te5 (Ba : Yb : Mn : Te in 6 : 2 : 6 : 15 ratio) were
ture resistivity and magnetization, of the Ba2Gd2/3Mn2Te5 com- obtained from the reaction mixtures of Ba (24.1 mg,
pound have also been studied in detail. We also present the 0.176 mmol), Yb (10.1 mg, 0.058 mmol), Mn (9.6 mg,
electronic structure of this compound using density functional 0.176 mmol), and Te (56.1 mg, 0.439 mmol) in 100 mg of NaCl
(DFT) calculations.
used as a flux.
The energy dispersive X-ray spectroscopic (EDS) method
was employed to analyze the elemental compositions of the
reaction products using the octane elite energy dispersive X-ray
spectrometer (EDAX Inc., USA) attached to the field-emission
scanning electron microscope (FE-SEM) (make: JEOL, Japan;
2. Experimental section
2.1. Syntheses
The following reactants were used as received for the syntheses model: JSM 7800F). The EDS data were collected with an accel-
of Ba2Ln1−xMn2Te5: barium rod (Alfa Aesar 99+%), praseody- erating voltage of 15–20 kV. The sizes of the crystals obtained
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Fig. 1 (a) The EDS mapping and point analysis of Ba2.03(2)Gd0.66(5)Mn2.07(4)Te5.11(6) crystals and (b) the PXRD pattern of the polycrystalline
Ba2Gd2/3Mn2Te5 compound. The asterisk indicates the reflection from the secondary BaTe phase.
from the reactions were in the range of ∼20 μm–200 μm. A few shaped crystals subjected to the EDS study were first placed on
crystals were selected from the reaction products and placed a glass slide under Paratone-N oil. The unit cell determi-
on carbon tape affixed to a stub. The EDS data were collected nations of few crystals were performed to ascertain the quality
on these crystals. Some of the black crystals showed four of these crystals. A suitable sized single crystal was then
elements, Ba, Ln, Mn, and Te, using a semiquantitative EDS mounted on a KM4/Xcalibur Goniometer. The crystal-to-detec-
analysis. The composition of these black crystals was close to tor distance was fixed at 45 mm and an exposure time of 20 s
Ba2Ln2/3Mn2Te5 composition. The EDS mapping of a typical per frame was used for data collection in a series of 0.6° in ω
crystal of Gd-compound (shown in Fig. 1(a)) shows the homo- scans. The CrysAlispro program26 was used to collect intensity
geneous distribution of all four elements consistent with data, cell refinement, and data reduction. The absorption cor-
Ba2.03(2)Gd0.66(5)Mn2.07(4)Te5.11(6) composition. Our EDS study rections of the measured intensities of the reflections were per-
did not detect any trace of sodium/chlorine/silicon in the formed by the multi-scan method using the SCALE3
black crystals of Ba2Ln1−xMn2Te5. These crystals were carefully ABSPACK.27
removed from the carbon tape after the EDS studies and used
Room temperature (298(2) K) single-crystal X-ray diffraction
for single crystal diffraction studies.
data of Ba2Ln1−xMn2Te5 (Ln = Pr and Yb) crystals were
2.1.2. Syntheses of polycrystalline Ba2Ln2/3Mn2Te5. After recorded using a Bruker D8 Venture instrument equipped with
establishing the composition and crystal structure of a Photon III mixed mode detector. A graphite monochroma-
Ba2Ln2/3Mn2Te5, polycrystalline samples of these quaternary tized Mo-Kα radiation with a wavelength of 0.71073 Å was used
phases were synthesized using the sealed tube solid-state to collect the data. The EDS analyzed crystals were picked
method. A stoichiometric amount of Ba, Ln, Mn, and Te was under paratone oil for unit cell determination. A suitable
loaded inside a carbon-coated fused silica tube having 10 mm crystal was chosen on the goniometer head to collect the data.
inner diameter and 12 mm outer diameter inside an argon The working voltage and operating currents were 50 kV and
filled glove box. The total mass of the reactants was about 1.4 mA, respectively, throughout the experiment. The detector
400 mg in each reaction. The fused silica tube containing the to crystal distance was fixed at 50 mm, and an exposure time
reaction mixture was first evacuated (ca. 10−4 Torr) and sealed of 2 s per frame was used for data collection. The data were col-
using a flame torch. The sealed ampoule was then placed lected as a series of 0.5° frame width ω and ϕ scans. The data
inside a programmable muffle furnace, and the temperature collection, cell refinement, and data reduction were carried
was ramped up to 1273 K with a heating rate of ∼78 K h−1. The out with the help of APEX3 software.28 The absorption correc-
reaction mixture was annealed at 1273 K for 120 h before the tions were done by the multi-scan method using SADABS.29
furnace was switched off and allowed to cool to room tempera-
The crystal structures of Ba2Ln1−xMn2Te5 (Ln = Pr, Gd, and
ture. The reaction tube was opened under ambient conditions Yb) were solved using the SHELX-14 suite programs.30 The
to reveal a homogeneous looking black product that was XPREP program31 suggested monoclinic symmetry with the
further ground into fine powder inside the argon-filled glove C-centering of the cell and indicated three possible space
box. The powder X-ray diffraction data of this black polycrystal- groups (C2, C2/m, and Cm) consistent with the observed reflec-
line product were collected for phase identification.
tions’ extinction condition. The statistical intensity (|E2 − 1|)
value indicated a centrosymmetric structure. Hence, a centro-
symmetric monoclinic C2/m (no. 12) space group was used for
2.2. Single-crystal X-ray diffraction
Single-crystal X-ray diffraction data of Ba2Gd1−xMn2Te5 were the structure solution. The crystal structure’s initial model was
collected at 298(2) K using a Rigaku Supernova X-Calibur obtained by the direct methods using the SHELXS program32
equipped with an Eos CCD detector with a graphite monochro- that identified six crystallographic independent atomic posi-
matic Mo-Kα radiation (λ = 0.71073 Å). The black irregular- tions. These sites were modeled with one manganese, one
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lanthanide, one barium, and three tellurium positions as per to 100% and ∼74%, respectively. We found that the refined
their coordination environments and interatomic distances. occupancy values of Ba and Yb sites were very close when we
The isotropic thermal parameters of the Ln atoms in all three varied the occupancies of Ba and Yb together or when Yb was
structures were relatively larger. The Ln site’s occupancy refine- fixed and Ba was free and vice versa. Ytterbium is well known
ment reveals that this site is partially vacant, with a site occu- for exhibiting a mixed valence state (+2/+3) in solid-state com-
pancy factor close to 65–68% for Pr–Gd structures and about pounds, and hence we have not fixed the occupancy of the Yb
74% for the Yb-structure. Further occupancy refinements of site during the final cycles of the structure refinements. The
2
the Ln sites lead to a reduction in the R(F)/Rw(Fo ) values. The final refined formula for the Yb compound is
occupancy refinements of Ba sites in these three structures did Ba2Yb0.74(1)Mn2Te5.
not show any vacancy when refined freely while keeping all the
To show the atomic vacancy at the Pr/Gd site, the general
other atomic sites fixed to unity. However, for Pr and Gd struc- chemical formula of Pr and Gd-based quaternary compounds
□
tures, simultaneous refinements of site occupancy factors (sof) will now be referred to as Ba2Ln2/3 1/3Mn2Te5 (Ln = Pr and Gd;
□
of Ba and Ln sites show vacancy at the Ba sites as well (about
= vacancy at Ln sites) in this article. The precession images
90–92%). The occupancy refinement results were dependent of the single crystal-data did not show the presence of any
on whether one varied the occupancies of Ba and Ln sites superstructure/satellite reflections. Finally, the parameters like
while fixing the occupancies of Mn, and three Te sites as one scale factors, atomic positions with anisotropic displacement
(i.e., 100%) or whether one varied the occupancies of Ba, Mn, parameters, and extinction parameters were refined. The final
Te1, Te2, and, Te3 while keeping the occupancy of Ln fixed. structure was run through the ADDSYM program33 of
The agreement factors [R(F)/Rw(Fo )] were found to be a PLATON33 that shows no missing symmetry. The atomic posi-
2
minimum when occupancies of all sites were freely refined. tions were finally standardized using the program
However, the resulting compositions did not charge balance. It STRUCTURE TIDY.34 Further details are given in Tables 1–3.
should be noted that the vacancy at the Ba site along with the
2.3. Powder X-ray diffraction (PXRD) study
vacant Pr/Gd sites is not expected as it will result in an overall
negative charge on these compounds. It is well known that in
many cases, the refinement of X-ray occupancies is not reliable
and can have large errors associated with it, especially in the
crystals with high X-ray mass absorption coefficients. In such
cases, the refinements can be stable and converge to a false
minimum. Furthermore, charge balancing in Pr and Gd com-
pounds requires one-third (∼33%) of the Ln sites to be unoccu-
pied if all the other sites are completely filled. The refined
occupancies of the Pr/Gd sites were indeed close to 65–68%.
Hence, instead of reporting the refined compositions as
Ba2−yLn1−xMn2Te5, we have fixed the Ln sites’ occupancies to
2/3 (i.e., 0.6667) in the final cycles of least square refinements
to achieve charge neutrality, which leads to Ba2Ln2/3Mn2Te5
(Ln = Pr and Gd) compositions with two formula units (Z) per
cell of the structure. For the Ba2Yb1−xMn2Te5 structure, the
refined occupancies of Ba and Yb sites were found to be close
The phase purity of the polycrystalline reaction products was
evaluated by PXRD studies. The PXRD data of the finely
ground samples were collected at room temperature using a
Rigaku Ultima IV diffractometer with a Cu-Kα radiation (λ =
1.5406 Å) source. The PXRD data were recorded over a 2θ range
of 5° to 70° using a θ–2θ geometry. The experimental PXRD
patterns were compared with the calculated PXRD patterns
simulated from the single-crystal structural data. The pres-
ence/absence of a secondary phase(s) was established by the
phase analysis of the PXRD patterns using the Match3!
Software.35
2.4. UV-visible-near infrared (UV-vis-NIR) absorption study
□
The optical bandgap study of Ba2Gd2/3 1/3Mn2Te5 was carried
out at room temperature (298(2) K).
A polycrystalline
□
Ba2Gd2/3 1/3Mn2Te5 sample was homogenized into a fine
Table 1 Crystallographic data and structural refinement details of Ba2Ln1−xMn2Te5 structuresa
□
□
Chemical formula
Ba2Pr2/3 1/3Mn2Te5
Ba2Gd2/3 1/3Mn2Te5
Ba2Yb0.74(1)Mn2Te5
Space group
C2/m
15.246(3)
4.5479(9)
10.688(2)
117.20(3)
659.1(3)
2
5.681
21.85
0.0296
0.0742
a (Å)
b (Å)
c (Å)
β°
15.268(3)
4.5817(9)
10.737(2)
117.23(3)
667.8(3)
15.266(3)
4.5784(9)
10.664(2)
116.92(3)
664.6(3)
V (Å3)
Z
ρ (g cm−3
)
5.551
20.68
0.032
0.081
5.754
23.58
0.028
0.067
μ (mm−1
)
R(F)b
Rw(Fo
2 c
)
a λ = 0.71073 Å, T = 298(2) K. b R(F) = ∑||Fo| − |Fc||/∑|Fo| for Fo2 > 2σ(Fo2). c Rw(Fo2) = {∑[w(Fo2 − Fc2)2]/∑wFo
}
4 1/2. For Fo2 < 0, w = 1/[σ2(Fo2) + (rP)2
2
+ mP] where P = (Fo + 2Fc2)/3, (r = 0.0199 and m = 22.5849 for Ba2Pr2/3 1/3Mn2Te5, r = 0.0158 and m = 30.3751 for Ba2Gd2/3 1/3Mn2Te5, and r =
□
□
0.0107 and m = 13.0832 for Ba2Yb0.74(1)Mn2Te5).
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Table 2 Fractional atomic coordinates and Uiso/Ueq values of Ba2Ln1−xMn2Te5 structures
a
Atoms
Wyckoff position
Site symmetry
S.O.F.
x
y
z
Uiso/Ueq
□
Ba2Pr2/3 1/3Mn2Te5
Ba1
Pr1
Mn1
Te1
Te2
Te3
4i
2a
4i
4i
4i
2d
m
2/m
m
m
m
2/m
1
2/3
1
1
1
0.17486(4)
0.000000
0.56260(11)
0.16438(5)
0.38396(4)
0.000000
0.000000
0.000000
0.000000
0.000000
0.000000
0.500000
0.72576(7)
0.000000
0.30161(15)
0.31604(7)
0.06589(6)
0.500000
0.02645(18)
0.0204(2)
0.0214(3)
0.02503(18)
0.01964(16)
0.01895(19)
1
□
Ba2Gd2/3 1/3Mn2Te5
Ba1
Gd1
Mn1
Te1
Te2
Te3
4i
2a
4i
4i
4i
2d
m
2/m
m
m
m
2/m
1
2/3
1
1
1
0.17437(5)
0.00000
0.56300(12)
0.16410(5)
0.38556(5)
0.00000
0.00000
0.00000
0.00000
0.00000
0.00000
0.50000
0.72809(8)
0.00000
0.30167(18)
0.31200(8)
0.06390(7)
0.50000
0.02680(2)
0.01980(2)
0.02050(3)
0.02320(2)
0.01848(18)
0.01820(2)
1
Ba2Yb0.74(1)Mn2Te5
Ba1
Yb1
Mn1
Te1
Te2
Te3
4i
2a
4i
4i
4i
2d
m
2/m
m
m
m
2/m
1
0.17538(3)
0.000000
0.56160(8)
0.16387(4)
0.38440(3)
0.000000
0.000000
0.000000
0.000000
0.000000
0.000000
0.500000
0.72820(5)
0.000000
0.29775(12)
0.31528(6)
0.06434(5)
0.500000
0.02135(12)
0.01950(19)
0.0198(3)
0.02327(13)
0.02029(13)
0.01898(15)
0.745(3)
1
1
1
1
a Uiso/Ueq is the one-third value of the trace of orthogonalized Uij tensor.
Table 3 Selected interatomic distances (Å) of Ba2Ln1−xMn2Te5 structures
□
□
Atomic pair
Ba2Pr2/3 1/3Mn2Te5
Ba2Gd2/3 1/3Mn2Te5
Ba2Yb0.74(1)Mn2Te5
Ln1–Te1
Ln1–Te2
Mn1–Te1
Mn1–Te2
Mn1–Te3
Ba1–Te1
Ba1–Te2
Ba1–Te2
Ba1–Te3
Ba1⋯Ba1
Ba1⋯Ln1
Ba1⋯Mn1
Ln1⋯Mn1
Ln1⋯Ln1
Mn1⋯Mn1
Te1⋯Te1
Te1⋯Te2
Te1⋯Te3
Te2⋯Te2
Te2⋯Te3
Te3⋯Te3
3.1654(15) × 2
3.1676(6) × 4
2.7325(9) × 2
2.744(2)
2.7043(15)
3.5329(8) × 2
3.5861(18)
3.5913(8) × 2
3.5114(11) × 2
4.582(1)
4.792(1)
4.151(1)
3.717(1)
4.582(1)
4.582(1)
4.270(1)
4.454(1)
4.470(1)
4.323(1)
4.160(1)
4.582(1)
3.1229(16) × 2
3.1258(6) × 4
2.7209(11) × 2
2.7371(22)
2.6975(17)
3.5190(9) × 2
3.5616(19)
3.5784(9) × 2
3.5010(11) × 2
4.548(1)
4.751(1)
4.159(1)
3.696(1)
4.548(1)
4.548(1)
4.308(1)
4.371(1)
4.479(1)
4.289(1)
4.163(1)
4.548(1)
3.1539(14) × 2
3.1498(6) × 4
2.7292(8) × 2
2.7233(17)
2.7153(12)
3.5366(7) × 2
3.5602(17)
3.5772(7) × 2
3.5258(10) × 2
4.578(1)
4.753(1)
4.148(1)
3.674(1)
4.578(1)
4.578(1)
4.277(1)
4.413(1)
4.454(1)
4.327(1)
4.156(1)
4.578(1)
powder inside an Ar-filled glove box. The absorption study was 2.5. Temperature-dependent resistivity study
then performed against dried BaSO4 powder, which served as
a standard reference for the UV-vis-NIR absorption study. A The temperature-dependent four-probe resistivity data were
JASCO V-770 UV/vis/NIR spectrometer was used to collect the collected for a sintered polycrystalline rectangular pellet of
□
data in diffuse reflectance mode over the wavelength range of Ba2Gd2/3 1/3Mn2Te5 over a temperature range of 95–300 K. A
□
2000 nm (0.62 eV) to 250 nm (4.96 eV). The Kubelka–Munk polycrystalline sample of Ba2Gd2/3 1/3Mn2Te5 was compressed
equation, α/S = (1 − R)2/2R, was used to transform the reflec- into a cylindrical disk with a diameter of 8 mm and a thick-
tance data into absorption data. Here α, S, and R are the ness of ≈3 mm under a pressure of ≈10 MPa. The pellet was
absorption coefficient, scattering coefficient, and reflectance, placed inside a carbon-coated fused silica tube and vacuum-
respectively.36 The absorption vs. energy plot was used to esti- sealed using a flame torch. The sealed pellet was sintered at
mate the bandgap by absorption edge study.
673 K for 24 hours. Finally, the cylindrical pellet was cut into a
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rectangular pellet for resistivity measurement. The procedure Ba2Yb0.74(1)Mn2Te5 crystals were about 20% to 40% based on
and the equipment details are the same as we described the manganese. The use of NaCl flux is essential for the
previously.37
growth of the crystals since the corresponding flux-free reac-
tions only produced mixtures of polycrystalline products.
Attempts towards synthesizing Ba2Ln1−xMn2Te5 (Ln = La, Ce,
2.6. Temperature-dependent magnetic measurements
43
Nd, and Sm) crystals failed, and only crystals of Ba2MnTe3
Magnetization (M) measurements of polycrystalline samples of
and respective binary lanthanum telluride phases were
observed in their final reaction products. A series of stoichio-
metric flux free solid-state reactions of Ba, Ln, Mn, and Te
elements were carried out with varying temperature profiles to
optimize the reaction conditions and to obtain monophasic
polycrystalline samples of Ba2Ln1−xMn2Te5. Most of these reac-
tions produced the target phase as a minor phase along with
known ternary and binary phases [see the ESI†]. Only for the
□
Ba2Gd2/3 1/3Mn2Te5 were obtained using a Quantum Design
Physical Properties Measurement System (PPMS). The temp-
erature-dependent magnetization data were collected in the
zerofield cooling (ZFC) mode (5 K–300 K) and field cooling
(FC) mode (300 K–5 K) under an applied magnetic field (H) of
1000 Oe.
2.7. Electronic structure calculations
□
Gd compound, i.e., Ba2Gd2/3 1/3Mn2Te5, the target phase was
The ab initio quantum mechanical calculations are performed
using the plane-wave basis set formalism within the frame-
work of density functional theory (DFT)38 as implemented in
the VASP Package.39 The interactions between the core and
valence electrons are approximated using the projected aug-
mented wave (PAW) potentials.40 In the case of Mn atoms, the
3p semicore states are treated as valence states. The GGA+U
approach41 is used to include the on-site correlations for the
Mn-3d orbitals. For Gd atoms, the localized 4f states and semi-
core 3p states are treated as the core states and valence states,
respectively. It may be noted that f-electrons are generally not
handled well within the DFT framework due to self-interaction
errors. However, Gd is described reasonably well as seven elec-
trons occupy the majority f-shell. In the case of Ba atoms,
semicore 5s and 5p states are treated as valence states. The
exchange–correlation effects are treated using the Perdew–
Burke–Ernzerhoff (PBE)42 form of generalized gradient
approximation (GGA). A kinetic energy cutoff of a minimum of
300 eV is used to expand the Kohn–Sham orbitals in the plane-
wave basis set. The integrations in the k-space are performed
by sampling the Brillouin zone using a 6 × 6 × 8 Monkhorst–
Pack k-point mesh. The lattice parameters and the fractional
coordinates of the atoms in the unit cell are relaxed until the
Hellmann–Feynman forces become less than 30 meV Å−1. Self-
consistency in calculations is achieved with total energies con-
verging up to 10−5 eV per cell. The unit cell of
obtained as the main phase along with small amounts of BaTe
as a secondary phase (Fig. 1b). This polycrystalline sample of
Ba2Gd2/3 1/3Mn2Te5 was obtained by the stoichiometric reac-
□
tion of the corresponding elements at 1273 K. The reactants
were heated at 1273 K for five days to produce a black lump
that was homogenized and used for further characterization.
3.1.1. Crystal structures of Ba2Ln1−xMn2Te5 (Ln = Pr, Gd,
and Yb). Single-crystal X-ray diffraction studies at 298(2) K
were used to establish the crystal structures of these three qua-
ternary compounds. These compounds crystallize in the
monoclinic C23h –C2/m space group with two formula units per
□
cell. The refined cell constants for Ba2Pr2/3 1/3Mn2Te5 are a =
15.268(3) Å, b = 4.5817(9) Å, c = 10.737(2) Å, and β = 117.23(3)°.
The lattice parameters of Ba2Ln2/3 1/3Mn2Te5 were observed to
□
decrease with the substitution of smaller Gd atoms (Table 1).
The observed reduction in the lattice parameters is consistent
with lanthanide contraction. However, the refined a and
b-lattice constants and cell volume for the Yb compound are
slightly larger than those observed for the Gd compound. This
could be due to the mixed valency of Yb (+2/+3) species. These
structures can be viewed as a defect variant of the Ba2UCu2S5
structure type (space group: C2/m).44 The asymmetric unit of
the Ba2Ln1−xMn2Te5 (Ln = Pr, Gd, and Yb) structure comprises
six independent crystallographic sites: one Ba1, one Ln1, one
Mn1, and three Te sites (Te1, Te2, and Te3). All the atomic
sites have a site symmetry of .m. except for Ln1 and Te3 that
occupy the sites with .2/m. symmetry. All these crystallographic
sites are fully occupied except for the Ln1 site. Only two-thirds
of the Ln site is filled for Pr and Gd-compounds, leaving one-
third of the Pr1/Gd1 sites unoccupied. For Ba2Yb0.74(1)Mn2Te5,
the Yb site occupancy is about 74%, slightly larger than the Pr/
Gd compounds. The site symmetries and Wyckoff positions of
these atoms are listed in Table 2. The overall crystal structure
of these three compounds can be described as pseudo-two
dimensional, as shown in Fig. 2. Each Ln atom in this struc-
ture is located at the center of a distorted octahedron formed
by two Te1 and four Te2 atoms. In contrast, Mn atoms are
□
Ba2Gd2/3 1/3Mn2Te5 contains two formula units (Z = 2), which
□
correspond to Ba4Gd4/3 2/3Mn4Te10. The fractional occupancy
of a Gd atom in the (1 × 1 × 1) one-unit cell with the
□
Ba4Gd4/3 2/3Mn4Te10 composition is modeled using a larger (1
× 3 × 1) unit cell with 2 Gd atoms and one vacancy per unit
□
cell, i.e., the Ba6Gd2 Mn6Te15 composition.
3. Results and discussion
3.1. Syntheses and crystal structures of Ba2Ln1−xMn2Te5 (Ln
= Pr, Gd, and Yb)
The black-colored crystals of these new quaternary compounds bound to four Te atoms (2 × Te1, 1 × Te2, and 1 × Te3) in a
were first synthesized by the solid-state reactions of elements tetrahedral fashion.
□
at 1273 K using an excess of NaCl molten flux. The approxi-
The Pr–Te bond distances in Ba2Pr2/3 1/3Mn2Te5 range
□
mate yields of Ba2Ln2/3 1/3Mn2Te5 (Ln = Pr and Gd) and from 3.165(2) Å to 3.168(1) Å which are in good agreement
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Å.50 The coordination geometry around the Yb atom in
YbGa6Te10 is severely distorted octahedral, and the oxidation
state of ytterbium is reported as +2 based on the crystal struc-
ture refinement results. The Yb2+–Te distances in YbGa6Te10
are also slightly longer than the Yb–Te distances observed in
our quaternary compound. Thus, based on the comparison of
the Yb–Te distances in Ba2Yb0.74(1)Mn2Te5 with the related
structures, a mixed valency of Yb (+2/+3) is expected in
Ba2Yb0.74(1)Mn2Te5. However, detailed X-ray photoelectron
spectroscopy and magnetic studies are needed to support this
conjecture. Regrettably, we could not synthesize either bigger
crystals or a single-phase polycrystalline Ba2Yb0.74(1)Mn2Te5 for
such measurements. We have obtained a mixture of polycrys-
talline YbTe,49 Ba2MnTe3,43 and the target phase in the reac-
tion aimed for the synthesis of bulk Ba2Yb1−xMn2Te5 (see the
ESI†).
The Mn–Te bond distances of 2.697(2) Å to 2.744(2) Å in the
tetrahedral units of MnTe4 of Ba2Ln1−xMn2Te5 are in fair agree-
ment with the Mn2+–Te distances reported in related structures
with Mn in tetrahedral coordination. Examples include
CsMnInTe3 (Mn2+–Te distances of 2.707(2) to 2.771(2) Å),51
Ba2MnTe3 (2.640(1) to 2.733(1) Å),43 and Cs2Mn3Te4 (2.7379(7)
Fig. 2 A unit cell view of the Ba2Ln1−xMn2Te5 crystal structures along
the [010] direction.
with the Pr3+–Te distances in CsPrCdTe3 (3.192(2) Å to 3.195(2) to 2.7515(8) Å).52
Å),21 CsPrZnTe3 (3.1394(4) Å to 3.1616(4) Å),45 PrCu0.45(1)Te2
Based on the coordination geometries of the Ln and Mn
(3.150(1) Å to 3.3175(7) Å),46 and CsPr2Ag3Te5 (3.151(2) Å to atoms, the building blocks of the Ba2Ln1−x Mn2Te5 structures
3.223(2) Å).47 The Gd–Te bond distances of 3.123(2) Å to 3.126 (Ln = Pr, Gd, and Yb) could be assigned to the LnTe6 octahedra
□
(1) Å in the Ba2Gd2/3 1/3Mn2Te5 structure are somewhat and MnTe4 tetrahedra. Each LnTe6 octahedron is connected to
shorter than the Pr-analogue due to the smaller size of Gd3+ two neighboring LnTe6 units by sharing two opposite edges
relative to the Pr3+ ion.48 The Gd3+–Te distances are compar- along the b-axis to form infinite chains of [Ln1−xTe4]6−. These
able to those observed in related structures such as CsZnGdTe3 [Ln1−xTe4]6− chains are missing one-third and about 26% of
(3.0859(3) Å to 3.1025(3) Å)45 and CsCdGdTe3 (3.130(1) Å to Pr/Gd and Yb atoms, respectively, on average. Three membered
3.144(1) Å).21
linear units of Mn1–Te3–Mn1 link two neighboring chains of
The Yb–Te bond distances of 3.150(1) Å to 3.154(2) Å in [Ln1−xTe4]6− to form infinite layers of 2 [Ln1−xMn2Te5]4− as
1
1
Ba2Yb0.74(1)Mn2Te5 are slightly longer than the corresponding shown in Fig. 3(a). The infinite layers of 2 [Ln1−xMn2Te5]4− are
distances in the Gd compound. The chemistry of ytterbium- separated by the Ba2+ cations. Each MnTe4 tetrahedron in this
based tellurides is relatively less explored, resulting in very few structure is connected to three neighboring MnTe4 tetrahedra
known compounds.24 To the best of our knowledge, there are via the sharing of two vertices (2 × Te1) along the b-axis and
only two known examples of Yb containing quaternary tellur- one vertex (Te3) roughly along the c-axis. Finally, the Ln-based
ides: RbYbZnTe3 and CsYbZnTe3.22 These two tellurides octahedra and Mn-based tetrahedra are joined via edge-
contain ytterbium in the +3 oxidation state, and the coordi- sharing in the infinite layers of 2 [Ln1−xMn2Te5]4−. The LnTe6
1
nation environments around the Yb atoms are also octahedral. and
MnTe4
based
polyhedral
networks
of
the
The ionic size of Yb2+ (1.02 Å) with a six coordination number 2 [Ln1−xMn2Te5]4− layers are connected along the [001] direc-
1
is larger than the Yb3+ (0.868 Å).48 Thus, it is expected that the tion in the…..oct tet tet oct tet tet….. fashion.
Yb2+–Te distances will be longer than the Yb3+–Te in similar
The Ba atoms in these structures are coordinated with
coordination environments. The Yb3+–Te distances found in seven Te atoms (2 × Te1, 3 × Te2, and 2 × Te3) that form a dis-
RbYbZnTe3 (3.0264(4) Å to 3.0501(4) Å)22 and CsYbZnTe3 torted monocapped trigonal prism-like geometry (Fig. 3(b)).
(3.0328(6) Å to 3.0417(5) Å)22 are indeed slightly shorter than The interatomic distances of Ba–Te in the monocapped trigo-
the Yb–Te bond distances in Ba2Yb0.74(1)Mn2Te5.
nal prism BaTe7 vary from 3.501(1) Å to 3.5912(8) Å. These dis-
One of the most stable tellurides of ytterbium is cubic YbTe tances are consistent with the previously reported distances of
49
3.451(1) Å to 3.882(1) Å in Ba2GaGdTe5,53 3.393(1) Å to 3.714(1)
ˉ
(space group: Fm3m), where divalent ytterbium is present.
The coordination environment around the Yb atom in YbTe is Å in Ba2MnTe3,43 and 3.4007(2) Å to 3.6408(3) Å in Ba2ZnTe3.54
octahedral with six equal Yb2+–Te bond distances of 3.185(1)
As discussed earlier, the Ba2Ln1−xMn2Te5 structure can be
Å. These distances are slightly longer than those observed in described as a defect variant of the Ba2UCu2S5 structure.44 It is
Ba2Yb0.74(1)Mn2Te5. The Yb–Te distance in ternary phase derived from the Ba2UCu2S5 structure by the aliovalent substi-
YbGa6Te10 (Yb is ∼65% occupied), which adopts the filled tution of Cu+ and U4+ sites with Mn2+ and Ln3+ ions, respect-
beta-manganese structure, varies from 3.180(1) Å to 3.260(1) ively, and the isovalent substitution of S2− sites with
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Fig. 3 (a) A segment of the 21[Ln1−xMn2Te5]4− anionic layers and (b) the Ba coordination environment in Ba2Ln1−xMn2Te5.
Te2−species. Such a complete substitution of corresponding atoms. It is interesting to note that removing any Ln3+ species
elements in Ba2UCu2S5 should give
Ba2LnMn2Te5 composition. It is apparent that there is a the occupancy of Te atoms since these are all bonded to neigh-
charge-imbalance of +1 in this ideal Ba2LnMn2Te5 compo- boring Ln and Mn atoms.
a
hypothetical from the Ln sites in the 2 [Ln1−xMn2Te5]4− layer will not affect
1
sition when all crystallographic sites are fully occupied. To
3.1.3. Bond valence sum calculation. As discussed earlier,
compensate for this extra +1 charge, the Pr3+/Gd3+ sites are the Ba2Ln2/3 1/3Mn2Te5 (Ln = Pr and Gd) compounds do not
only two-thirds occupied, which leads to the stabilization of show any homoatomic bonding and hence can be easily
□
□
□
the Ba2Ln2/3 1/3Mn2Te5 structure where
represents the charge-balanced based on the Zintl concept with 2 × Ba2+, 2/3
× Ln3+, 2 × Mn2+, and 5 × Te2−. Furthermore, bond valency
vacancies at the Ln (= Pr and Gd) sites.
3.1.2. Assignment of formal oxidation states. The shortest sum (BVS) calculations57,58 for the Pr and Gd-based structures
Te⋯Te distance of 4.156(1) Å (Table 3) in the Ba2Ln1−xMn2Te5 also suggest the same formal charge assignments as discussed
structure implies that there is no bonding interaction between above. The complete skeletal structure does not show any dis-
the Te atoms. In addition, this structure type does not show tortion even after removing Ln from the structure, which can
any metal–metal homoatomic bonding. Thus, assignments of explain the slightly lower or higher BVS values of Ln and Mn
formal oxidation states of +2 for Ba and −2 for Te atoms in due to their under bonding and over bonding. Although the
□
Ba2Ln2/3 1/3Mn2Te5 structures are straightforward. The most BVS calculation method is strictly an empirical method, the
stable oxidation state of Pr and Gd is +3. The oxidation of Mn decrease in the BVS value of lanthanide atoms going from Pr
atoms in the Ba2Ln1−xMn2Te5 structures could be assigned to to Yb could be due to the mixed valency of Yb atoms and
+2, the most stable oxidation state known for manganese in increased atomic number in a common coordination environ-
the tellurides. Thus, the charge balance can be achieved as ment.59 The more deviation of BVS values in the case of Te3
(Ba2+)2(Ln3+)2/3(Mn2+)2(Te2−
) for the Pr and Gd compounds. compared to Te1 and Te2 may be due to its shortest bond
5
The
charge-balancing
in
the
Yb
compound, length with the metal in the compound’s distorted geometry.
Ba2Yb0.74(1)Mn2Te5, is arbitrary due to the possibility of a The BVS details of all the atoms of Ba2Ln1−xMn2Te5 com-
mixed-valence state of Yb (+2/+3).
pounds are given in Table 4.
These structures will be electronically unstable if one
assumes full Ln site occupancy that corresponds to the
Ba2LnMn2Te5 composition. It is apparent that this leads to an
overall +1 charge on the Ba2LnMn2Te5 formula for Ln = Pr and
Gd. For the Yb compound, the hypothetical Ba2YbMn2Te5
composition will be electronically stable if Yb is only present
in the +2 oxidation state. However, the deficiency at the Yb site
in this compound indicates a mixed valence state of Yb
(+2/+3). Thus, Ln-deficiency stabilizes the Ba2Ln1−xMn2Te5
structures electronically. There are many examples known in
the literature, where partial occupancy leads to electronic
stabilization. For example, stabilization of Ba0.5CuZrSe3,55
which is isostructural to CsCuZrSe3,56 requires the replace-
ment of monovalent Cs atoms with half-filled divalent Ba
3.2. Structural relationships of Ba2Ln1−xMn2Te5 with known
related structures
We next discuss the structural commonalities of the
Ba2Ln1−xMn2Te5 structures with other known related struc-
tures found in the quaternary AE–Ln–M–Q system (AE = alka-
line-earth; Ln = lanthanides, M = 3d transition metals; Q =
chalcogens) and the Ba2UCu2S5 structure type.44 There are a
total of three unique structure types (including
□
Ba2Ln2/3 1/3Mn2Te5 structure) known in this quaternary
system. The general formulae of these quaternary compounds
are AELn2MQ5 (1215-family) or AELnMQ3 (1113-family). The
AELn2MQ5 type compounds crystallize in the Cs3CoCl5 struc-
ture type60 in the tetragonal space group (I4/mcm) and the
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Table 4 Bond valency sum (BVS) of atoms in Ba2Ln1−xMn2Te5
□
□
Ba2Pr2/3 1/3Mn2Te5
Ba2Gd2/3 1/3Mn2Te5
Ba2Yb0.74(1)Mn2Te5
Site
Ba1
Ln1
Mn1
Te1
Te2
Te3
BVS
1.97
2.92
2.47
2.30
2.32
2.56
BVS
2.04
2.78
2.53
2.34
2.32
2.62
BVS
1.98
2.08
2.50
2.16
2.12
2.48
AELnMQ3 type compounds crystallize in two different struc- atoms are coordinated by seven Te atoms in a monocapped tri-
ture types (structure type: Eu2CuS3,61 space group = Pnma for gonal prismatic arrangement. The main difference between
62
Ln = La–Nd; KCuZrS3 type with space group Cmcm for Ln = these two structure types is the sequence of the tetrahedral
Gd–Er) depending upon the size of the Ln3+ ion. All these com- and octahedral units, as evident in Fig. 4. The ALnMTe3 struc-
pounds have layered structures that are separated by the AE2+ ture has the ..oct tet oct tet oct.. ordering along the c-axis in
cations.
contrast to the …oct tet tet oct…ordering in Ba2Ln1−xMn2Te5
The AE atoms in these compounds mainly act as electron structures.
donors and are involved in predominantly non-directional
ionic bonding. Therefore, these AE2+ cations are also referred
to as space fillers in these structures. The main building
blocks of these structures are chalcogen-based polyhedra of
lanthanide and transition elements. The coordination geome-
tries of Ln atoms are either octahedral or bicapped trigonal
prismatic in these structure types. On the other hand, the tran-
sition metals prefer octahedral or tetrahedral coordination
geometries.
□
3.3. Optical bandgap study of Ba2Gd2/3 1/3Mn2Te5
□
The absorption spectrum of powdered Ba2Gd2/3 1/3Mn2Te5
(Fig. 5) was collected at room temperature (298 (2) K) over a
range of 2000 nm to 250 nm wavelength. The optical bandgap
of the compound can be deduced by the extrapolation method
and was found in the near IR region (∼0.86 eV).
To calculate the value of the direct bandgap, we have
plotted the Tauc plot that is shown in Fig. 5(b). The direct
□
The structure type, Ba2Ln1−xMn2Te5, shows some resem-
blance to the well-known layered ALnMTe3 (A = alkali or alka-
line–earth metal; M = transition element; Ln = lanthanides or
actinides) structure type (space group Cmcm).8 In both cases,
the lanthanide and transition metals form distorted octahedra
and distorted tetrahedra by coordinating with six and four Te
atoms, respectively. The 21[LnMTe3]n− (n = 1 for alkali and n = 2
for alkaline–earth metal) layers in the ALnMTe3 structures are
separated by electropositive alkali or alkaline earth metals.
These An+ (n = 1 for alkali and +2 for alkaline–earth) cations
are surrounded by eight Te atoms in a bicapped trigonal-pris-
matic arrangement. In contrast, in Ba2Ln1−xMn2Te5, the Ba
bandgap value of the compound Ba2Gd2/3 1/3Mn2Te5 was
found to be 1.06(2) eV. The indirect bandgap was also esti-
mated, and it was found to be 0.81(2) eV. Thus, it is safe to
□
conclude that Ba2Gd2/3 1/3Mn2Te5 is
semiconductor.
a narrow bandgap
3.4. Temperature-dependent resistivity study of
□
Ba2Gd2/3 1/3Mn2Te5
Fig. 6 shows the variation in the dc-electrical resistivity of a
polycrystalline Ba2Gd2/3 1/3Mn2Te5 sample with temperature.
The resistivity starts to increase exponentially upon cooling the
sample below room temperature. This behavior is character-
□
Fig. 4 The unit cell views of (a) BaLnCuTe310 and (b) Ba2Ln1−xMn2Te5 structures.
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□
Fig. 5 (a) The optical absorption spectrum, (b) the direct, and (c) indirect bandgap Tauc plots for polycrystalline Ba2Gd2/3 1/3Mn2Te5.
Fig. 6 The temperature-dependent resistivity plot for polycrystalline
Fig. 7 Temperature dependence of the molar magnetic susceptibility
□
Ba2Gd2/3 1/3Mn2Te5. The inset shows the corresponding Arrhenius plot
□
(χmol) of Ba2Gd2/3 1/3Mn2Te5 powder under an applied magnetic field of
□
for Ba2Gd2/3 1/3Mn2Te5.
1000 Oe. The inset of the figure shows a linear Curie–Weiss fitting of
the inverse magnetic susceptibility data in the temperature range of
200–300 K.
istic of a semiconducting sample. The room temperature value
of resistivity was found to be about 10 Ω cm. The semiconduct-
□
ing nature of the Ba2Gd2/3 1/3Mn2Te5 sample is also consistent
linearly based on the modified Curie–Weiss law equation: χ(T)
= C/(T − Θ) to calculate values for the Curie constant (C) and
the Weiss constant (Θ). The linear fit of inverse magnetic sus-
ceptibility against temperature (inset of Fig. 7) adheres to the
Curie–Weiss law at the high-temperature range (200 K to
300 K). The calculated value of C was found to be 13.43(2) emu
K mol−1 and a negative Weiss constant (Θ) of −92.38(2) K was
observed. The large negative Weiss constant indicates a sub-
stantial degree of local antiferromagnetic coupling of 4f and
3d electrons of Gd3+ and Mn2+.22 The effective magnetic
moment (µeff) of 10.37µB calculated from the Curie–Weiss law,
with the optical absorption bandgap study.
To estimate the activation energy, we have plotted a graph
(inset of Fig. 6) of ln(ρ) vs. 1/kT (in e V−1), which is known as
the Arrhenius plot. The symbols ρ and k represent electrical
resistivity and the Boltzmann constant, respectively. The
Arrhenius plot showed a nearly straight line over a temperature
region of 105–300 K. The activation energy was estimated to be
about 0.03(2) eV from the slope of this line.
□
3.5. Magnetic properties of Ba2Gd2/3 1/3Mn2Te5
The temperature-dependent magnetization studies of a poly- is in good agreement with the theoretical value (µcal) of
□
crystalline sample of Ba2Gd2/3 1/3Mn2Te5 were carried out over 10.58µB. The value of µcal was calculated using the formula µcal
3+ 2
the temperature region of 5 K–300 K. The temperature-depen- = {2/3 × (µGd
) . The effective magnetic
+ 2 × (µMn2+)2}1/2 22
□
□
dent magnetic susceptibility plot for Ba2Gd2/3 1/3Mn2Te5 is moment of Ba2Gd2/3 1/3Mn2Te5 is due to the spin only contri-
shown in Fig. 7. The magnetic susceptibility (χ) was found to bution from the Gd3+ (8S7/2) and Mn2+ (6S5/2) ions as their
increase on cooling the sample till 5 K, with no evidence of orbital magnetic moment value is zero.
long-range magnetic ordering. The inverse of magnetic suscep-
A nonlinear fitting of the inverse of magnetic susceptibility
tibility was plotted with temperature, and the data were fitted data with temperature was also performed based on the
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equation χ(T) = χD + C/(T − Θ) to estimate the effect of diamag-
netic susceptibility contributions (χD) to the C and Θ values.
The diamagnetic susceptibility (χD) value obtained from the
above non-linear equation is −3.24 × 10−4 emu mol−1, which is
very small and comparable with the theoretical value of −2.77
× 10−4 emu mol−1 63
and hence can be neglected since it does
,
not affect the values of C and Θ values obtained from the
linear fit.
□
3.6. Electronic structure of Ba2Gd2/3 1/3Mn2Te5
The spin-polarized and non-polarized calculations were per-
formed using the GGA+U method to explore the atomic and
□
electronic structure of Ba2Gd2/3 1/3Mn2Te5, which could also
□
be considered as Ba6Gd2 Mn6Te15 as discussed in section 2.7.
Table 5 shows the fractional coordinates of atoms in the
unit cell obtained from the spin-polarized calculation with U =
2 eV for Mn-3d orbitals. As can be seen, the theoretical esti-
mates are in good agreement (<3%) with the experiments. For
spin-polarized calculations, the antiferromagnetic configur-
ation is found to be lower in energy by ∼200 meV f.u−1 as com-
pared to the ferromagnetic configuration and thereby more
stable. The absolute value of the magnetic moment of each
Mn atom is found to be ∼4.3μB. In our calculations, only anti-
ferromagnetic (AF) ordering, where the (010) intra-plane coup-
ling is ferromagnetic while the (010) inter-plane coupling is
antiferromagnetic, is considered. This AF ordering is similar
to the A-type ordering generally considered in the perovskite
magnetic oxides. However, it may be noted that though AF
ordering is found to be more stable, the true ground state may
□
Fig. 8 The total density of states for Ba2Gd2/3 1/3Mn2Te5 in the (a) anti-
ferromagnetic, (b) ferromagnetic, and (c) non-magnetic configurations.
The densities of the spin-up and spin-down states are indicated by the
consist of a more complex spin structure or magnetic ordering. positive (up arrow) and negative values (down arrow).
Furthermore, nanoregions with a different type of AF ordering
and/or competing magnetic spin structures may also exist in
the system.64,65
ations. As can be seen, the bandgap is found to be ∼0.97 eV
Fig. 8 shows the total density of states (DOS) for the anti-
ferromagnetic, ferromagnetic, and non-magnetic configur-
for the antiferromagnetic and ∼0.54 eV for the ferromagnetic
configurations. However, the bandgap disappears for the non-
magnetic configuration. This suggests that the appearance of
□
Table 5 The computed fractional atomic coordinates of a Ba2Gd2/3
1/
□
the bandgap in Ba2Gd2/3 1/3Mn2Te5 is strongly related to the
3Mn2Te5 unit cell
underlying magnetic structure and thereby exchange inter-
actions. It may be noted that different values of the U (on-site
Coulomb correlations) parameter for Mn-3d orbitals, usually
in the range 2.0–5.0 eV, have been used in several earlier
reported works.66,67 Thus, in order to test the dependence of
the bandgap magnitude on the U parameter, we have also per-
formed electronic structure calculations with different values
of U, i.e., U = 2 eV, 3 eV, and 3.9 eV. The bandgap is found to
be increased by ∼0.14 eV as U is increased from 2 eV to 5 eV.
The bandgap is computed to be 1.06 eV and 1.09 eV for U = 3.0
eV and U = 4.0 eV, respectively, for the AF configuration.
Site
X
y
z
Ba-1
Ba-2
Ba-3
Ba-4
Gd-1
Gd-2
Mn-1
Mn-2
Mn-3
Mn-4
Te-1
Te-2
Te-3
Te-4
Te-5
Te-6
Te-7
Te-8
Te-9
Te-10
Cal. (exp.)
Cal. (exp.)
Cal. (exp.)
Cal. (exp.)
Cal. (exp.)
Cal. (exp.)
Cal. (exp.)
Cal. (exp.)
Cal. (exp.)
Cal. (exp.)
Cal. (exp.)
Cal. (exp.)
Cal. (exp.)
Cal. (exp.)
Cal. (exp.)
Cal. (exp.)
Cal. (exp.)
Cal. (exp.)
Cal. (exp.)
Cal. (exp.)
0.175 (0.1744)
0.675 (0.6744)
0.825 (0.8256)
0.325 (0.3256)
0.0 (0.0)
0.000 (0.0)
0.500 (0.5)
0.000 (0.0)
0.500 (0.5)
0.716 (0.7281)
0.716 (0.7281)
0.284 (0.2719)
0.284 (0.2719)
0.0 (0.0)
−0.026 (0.0)
0.526 (0.5)
0.000 (0.0)
0.499 (0.5)
0.001 (0.0)
0.499 (0.5)
0.001 (0.0)
0.518 (0.5)
0.000 (0.0)
0.516 (0.5)
0.000 (0.0)
0.504 (0.5)
0.000 (0.0)
0.504 (0.5)
0.498 (0.5)
0.0 (0.0)
0.500 (0.5)
0.001 (0.0)
0.562 (0.563)
0.061 (0.063)
0.438 (0.437)
0.938 (0.937)
0.181 (0.1641)
0.656 (0.6641)
0.819 (0.8359)
0.344 (0.3359)
0.384 (0.3855)
0.878 (0.8855)
0.616 (0.6144)
0.122 (0.1144)
0.0 (0.0)
0.290 (0.3016)
0.289 (0.3016)
0.710 (0.6983)
0.709 (0.6983)
0.357 (0.312)
0.318 (0.312)
0.642 (0.688)
0.682 (0.688)
0.059 (0.0639)
0.058 (0.0639)
0.941 (0.9361)
0.941 (0.9361)
0.5 (0.5)
Next, the spin-polarized total and projected densities of the
□
electronic states of Ba2Gd2/3 1/3Mn2Te5 (AF configuration)
shown in Fig. 9 are analyzed. The valence band (VB) width is
∼5 eV with VB extending from ∼−5.5 eV to valence band
maximum (VBM) at 0 eV. The states in the VB from 0 to −3 eV
are mainly comprised of Te-5p along with a lesser contribution
from the Mn-3d orbitals. The VB in the energy range of −3 eV
0.5 (0.5)
0.5 (0.5)
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based octahedra (LnTe6) and Mn-based tetrahedra (MnTe4).
These LnTe6 and MnTe4 units are condensed to form the
21[Ln1−xMn2Te5]4− layers that are separated by the Ba2+ cations.
The optical absorption study of the polycrystalline
□
Ba2Gd2/3 1/3Mn2Te5 compound shows the direct and indirect
bandgaps of 1.06(2) eV and 0.81(2) eV, respectively, which are in
good agreement with the theoretical study and consistent with
the black color of the compound. The temperature-dependent
resistivity study confirms the semiconducting behavior of poly-
□
crystalline Ba2Gd2/3 1/3Mn2Te5. The magnetic studies of polycrys-
□
talline Ba2Gd2/3 1/3Mn2Te5 did not show any long-range ordering
□
down to 5 K. The narrow bandgap of Ba2Gd2/3 1/3Mn2Te5 is
promising for future thermoelectric studies.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
JP thanks DST-SERB, Government of India for the financial
support under the early career research (ECR) award (grant
number: ECR/2017/000822) and IIT Hyderabad for seed grant
and research facilities. SJ and GP thank MHRD, IIT Hyderabad
for the research fellowships. MI thanks DST India for the
research fellowship. The authors also gratefully acknowledge
DST-FIST (SR/FST/ETI-421/2016) for the SEM-EDS facility and
DST-FIST (SR/FST/PSI-215/2016) for computational resources
used in this work. The authors thank Prof. James A. Ibers
(Northwestern University, USA) for helpful discussions on
crystal structure refinements.
□
Fig. 9 (a) The total density of states for Ba2Gd2/3 1/3Mn2Te5 with anti-
ferromagnetic ordering for Mn magnetic moments. (b) The partial
density of states (PDOS) projected onto the orbitals of Ba atom, (c) Gd
atom, (d) Mn atom, and (e) Te atom.
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