PbMn(IV)TeO6: A New Noncentrosymmetric Layered Honeycomb Magnetic Oxide

Article abstract of DOI:10.1021/acs.inorgchem.5b02677

PbMnTeO₆, a new noncentrosymmetric layered magnetic oxide was synthesized and characterized. The crystal structure is hexagonal, with space group P62m (No. 189), and consists of edge-sharing (Mn⁴⁺/Te⁶⁺)O₆ trigon

Full text of DOI:10.1021/acs.inorgchem.5b02677

Article
pubs.acs.org/IC
PbMn(IV)TeO6: A New Noncentrosymmetric Layered Honeycomb
Magnetic Oxide
Sun Woo Kim,† Zheng Deng,† Man-Rong Li,† Arnab Sen Gupta,‡ Hirofumi Akamatsu,‡
Venkatraman Gopalan,‡ and Martha Greenblatt*,†
†Department of Chemistry and Chemical Biology, Rutgers, the State University of New Jersey, 610 Taylor Road, Piscataway, New
Jersey 08854, United States
‡Department of Materials Science and Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, United States
S
* Supporting Information
ABSTRACT: PbMnTeO6, a new noncentrosymmetric lay-
ered magnetic oxide was synthesized and characterized. The
crystal structure is hexagonal, with space group P62m (No.
̅
189), and consists of edge-sharing (Mn4+/Te6+)O6 trigonal
prisms that form honeycomb-like two-dimensional layers with
Pb2+ ions between the layers. The structural difference
between PbMnTeO6, with disordered/trigonal prisms of
Mn4+/Te6+, versus the similar chiral SrGeTeO6 (space group
P312), with long-range order of Ge4+ and Te6+ in octahedral
coordination, is attributed to a difference in the electronic
effects of Ge4+ and Mn4+. Temperature-dependent second harmonic generation by PbMnTeO6 confirmed the
noncentrosymmetric character between 12 and 873 K. Magnetic measurements indicated antiferromagnetic order at TN ≈ 20
K and a frustration parameter (|θ|/TN) of ∼2.16.
crystal structure of BaGeTeO6 (P312) was also reported, but it
is not isostructural with SrGeTeO6.20 If the substitution of
magnetic cations (3d−5d transition metals) into the chiral
PbSb2O6 structure is possible, such new phases should be good
candidates for the design of new NCS magnetic oxides.
Interestingly, the crystal structure of SrMnTeO6 was
reported with a similar layered structure (NCS space group
INTRODUCTION
■
Noncentrosymmetric (NCS) oxide materials have been studied
for decades because of their interesting physical properties,
including ferroelectricity, piezoelectricity, pyroelectricity, multi-
ferroicity, magnetoelectricity, and second harmonic generation
(SHG).1−5 Recently, much experimental research focused on
finding new multiferroic and magnetoelectric transition metal
oxides along with theoretical calculations to understand the
origin of multiferroic/magnetoelectric behavior and to predict
new structural chemical properties for the design of new
materials with optimal characteristics.6−11
P62m, No. 189) as that of SrGeTeO , but it consists of edge-
̅
6
sharing disordered (Mn4+/Te6+)O6 trigonal prisms, which also
form honeycomb-like layers separated by Sr2+ cations (SrO6
trigonal prisms).21,22 This result suggests that two similar
layered structures may be observed in A(II)B(IV)Te(VI)O6:
The crystal structure of PbSb2O6-type materials (general
formula ABB′O6) basically exhibit a layered structure
the chiral space group P312 or the NCS space group P62m
̅
depending on the B-site cation arrangement.
(centrosymmetric space group P31m, No. 162) consisting of
̅
On the basis of these observations, the exploratory synthesis
of A(II)M(IV)TeO6 (A(II) = Sr, Ba, Pb; M(IV) = V, Mn, Ru,
Re, Os, Ir) phases was undertaken to find new multifunctional
materials. Here, we report the successful synthesis and
characterization of the new NCS compound PbMnTeO6.
edge-shared B/B′O6 octahedra that form honeycomb-like layers
separated by A cations (AO6 octahedra).12 PbSb2O6-type
compounds containing 3d or 4d transition metals show
interesting magnetic behavior. For example, divalent 3d
transition metal arsenates AAs2O6 (A = Mn2+, Co2+, Ni2+)
exhibit long-range antiferromagnetic ordering at around 30
K,13,14 and the 4d transition metal arsenate Pd(II)As2O6 shows
EXPERIMENTAL SECTION
Reagents. PbO (Alfa Aesar, 99.99%), MnO2 (Strem Chemical,
99.995%), and H2TeO4·2H2O (Alfa Aesar, 99+%) were used without
any further purification.
■
a relatively high Nee
́
l temperature (TN) of 140 K.15,16 More
interestingly, SrRu2O6 (Ru5+, S = /2) shows an extraordinarily
high antiferromagnetic transition temperature (TN) of 565
K.17,18
3
Synthesis. Polycrystalline PbMnTeO6 was prepared by a conven-
tional solid-state reaction. Stoichiometric amounts of PbO (0.3348 g,
If the B and B′ cations order crystallographically, the crystal
structure adopts the lower-symmetry chiral space group P312
(No. 149).12 To date, only one compound, SrGeTeO6, has
been reported with the chiral PbSb2O6-type structure.19,20 The
Received: November 22, 2015
© XXXX American Chemical Society
A
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1.5 mmol), MnO2 (0.1304 g, 1.5 mmol), and TeO3 (0.2634 g, 1.5
mmol; amorphous TeO3 was prepared by heating H2TeO4·2H2O at
400 °C for 12 h in air23) were thoroughly ground and pressed into a
pellet. The pellet wrapped in Pt foil was placed in a quartz tube that
was evacuated and flame-sealed. The sealed ampule was heated to 700
°C for 12 h and then cooled to room temperature (the heating and
cooling rates were 200 °C/h). After that, the resulting product was
washed with dilute HNO3 solution several times to remove
Pb2MnTeO6 impurities24 (see Figure S1). The final product obtained
was dark-brown-black polycrystalline PbMnTeO6. Energy-dispersive
X-ray spectroscopy (EDX) analysis of the final product PbMnTeO6
indicated a chemical composition of Pb0.99(8)Mn1.04(7)Te0.98(1)Ox; thus,
the compound is stoichiometric within the standard deviation range of
error (see Figure S2).
infra). Finally, Rietveld refinement of PbMnTeO6 with the
SrMnTeO structural model (space group P62m) yielded R =
̅
6
p
5.18%, Rwp = 6.92%, and χ2 = 1.46 with lattice parameters of a =
b = 5.10143(5) Å, c = 5.39643(6) Å, V = 121.62(4) Å3, and Z =
1. The Pb atoms are located at the 1a (0, 0, 0) positions, Mn/
Te atoms at the 2d (2/3, 1/3, 1/2) positions, and O atoms at the
6i (x, 0, z) positions in space group P62m. The Rietveld
̅
refinement plot of the PXRD data is shown in Figure 1; atomic
coordinates and atomic displacement parameters are summar-
ized in Table 1.
Powder X-ray Diffraction. The final product was characterized by
powder X-ray diffraction (PXRD) (Bruker AXS D8 Advance
diffractometer, Cu Kα radiation (λ = 1.5406 Å), 40 kV, 30 mA, step
scan 10−120°/0.02°/10.5s) for purity and phase identification.
Diffraction data analysis and Rietveld refinement were performed
with the TOPAS25 and GSAS-EXPGUI26 software packages.
Second Harmonic Generation. The temperature dependence of
the optical SHG by PbMnTeO6 was measured for a pellet between 12
and 873 K in normal-incidence reflection geometry using an 800 20
nm fundamental input generated by a Ti:sapphire laser (Coherent
Libra, 2 mW, 80 fs, 2 kHz). The SHG signal was detected with a
photomultiplier tube (Hamamatsu H7926). The sample was cooled
and heated at a rate of 6.0 °C/min between 12 and 320 K in a cryostat
and 7.0 °C/min between 300 and 873 K on a heating stage.
Magnetic Measurements. The magnetic measurements for
PbMnTeO6 were performed with a commercial Quantum Design
SQUID magnetometer. The dc magnetic susceptibility data were
collected at 2 K ≤ T ≤ 300 K under applied magnetic fields of 1000
and 10 000 Oe. Isothermal magnetization curves were obtained for
magnetic fields −5 T ≤ H ≤ 5 T at T = 2 and 300 K.
Figure 1. Rietveld refinement plot of the PXRD data for PbMnTeO6.
PbMnTeO6 exhibits a two-dimensional crystal structure
consisting of edge-sharing Mn(1)/Te(1)O6 trigonal prisms
that form honeycomb-like layers in the ab plane with Pb2+
cations (PbO6 trigonal prisms) located between the layers (see
Figure 2). The crystal structures of SrGeTeO6 (chiral space
group P312) and PbMnTeO6 (as well as SrMnTeO6, NCS
RESULTS AND DISCUSSION
■
Synthesis. Previously, Woodward et al.20 reported the
synthesis and crystal structures of AGeTeO6 (A = Sr2+, Ba2+),
which were prepared at ∼900 °C in air. Because the ionic radii
of Mn4+ and Ge4+ in a sixfold coordination environment are
identical (0.53 Å),27 the AMn(IV)TeO6 (A = Sr2+, Ba2+) phase
space group P62m) are similar (see Figure 3), but two major
̅
differences come from the order/disorder character of the
cations and the arrangement of oxygens: ordered/octahedra in
SrGeTeO6 versus disordered/trigonal prisms in PbMnTeO6
(and SrMnTeO6). The origin of these differences is likely due
to electronic differences between Mn4+ and Ge4+. Interestingly,
these differences affect the long-range ordering of M4+ cations
(M = Mn, Ge) in the layer (Ge4+−O−Te6+−O−Ge4+ vs
(Mn4+/Te6+)−O−(Mn4+/Te6+)).20
should also form. Interestingly, Wulff and Muller-Buschbaum22
̈
reported the crystal structure of SrMnTeO6 from a single-
crystal sample obtained from the reaction of Sr(OH)2·8H2O,
MnCO3·xH2O, and TeO2 at 800 °C for 8 days in air, which also
produced crystals of Sr2MnTeO6 as a byproduct.22 However,
our attempts to prepare phase-pure AMnTeO6 (A = Sr2+, Ba2+)
with synthetic methods similar to those used for PbMnTeO6
have not been successful to date (see Figures S3 and S4).
Structure. Initially, the structural refinements were
performed on PXRD data based on the two possible structural
models (space group P312, No. 149, for SrGeTeO6 and space
The Mn(1)/Te(1)−O(1) bond distances in PbMnTeO6
range between 1.9091(2) and 1.9095(1) Å, and the Mn(1)/
Te(1)−O(1)−Mn(1)/Te(1) bond angle is 100.94(4)°. It is
noteworthy that the Pb(1)−O(1) bonds in the PbO6 trigonal
prisms are equivalent with lengths of 2.5572(1) Å, which
indicates that the 6s2 lone pair on the Pb2+ cation is non-
stereoactive, as also observed in other Pb2+-containing layered
oxides.12,28 Selected bond distances and bond angle for
PbMnTeO6 are summarized in Table 2. The local coordination
environments are shown in Figure S5. Bond valence sum
(BVS) calculations29,30 resulted in values of 1.80, 3.93, and 6.13
for Pb2+, Mn4+, and Te6+, respectively (see Table 2).
Second Harmonic Generation. The SHG intensity for
PbMnTeO6 as a function of temperature is shown in Figure 4.
Finite SHG signals were observed over the whole temperature
measurement range, confirming that PbMnTeO6 is NCS.
Overall, the SHG intensity gradually decreases in the
temperature range up to 873 K. A noncentrosymmetric-to-
group P62m, No. 189, for SrMnTeO ). The two models gave
̅
6
comparable results by the Le Bail fit (Rp = 5.07%, Rwp = 6.84%,
and χ2 = 1.43 for SrGeTeO6 and Rp = 5.09%, Rwp = 6.86%, and
χ2 = 1.43 for SrMnTeO6). From Rietveld refinement with the
SrGeTeO6 structure model (space group P312), when the
Mn4+ and Te6+ ions were fixed as ordered, the thermal
parameters of both atoms were negative, and thus, the
refinement was unreliable. When the Mn4+ and Te6+ ions
were randomly distributed, the refinement was stable with Rp =
5.39%, Rwp = 7.15%, and χ2 = 1.56, but because of the
symmetry constraints, this structure had to adopt the higher-
symmetry centrosymmetric space group P31m rather than the
̅
chiral space group P312.12 Moreover, the SHG measurements
clearly established the NCS character of PbMnTeO6 (vide
B
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a
Table 1. Atomic Coordinates and Atomic Displacement Parameters for PbMnTeO6
b
atom
Pb(1)
Wyckoff position
x
y
z
Ueq (Å2)
SOF
1a
2d
6i
0
0
0
0.0063(6)
0.0064(8)
0.0260(2)
1
2
1
1
Mn(1)/Te(1)
/
/
/
2
0.5/0.5
3
3
O(1)
0.6139(1)
0
0.3022(1)
1
a
From PXRD Rietveld refinement using space group P62m (No. 189): R = 5.18%, R = 6.92%, and χ2 = 1.46. Unit cell parameters: a = b =
̅
p
wp
b
5.10143(5) Å, c = 5.39643(6) Å, V = 121.6240(30) Å3, Z = 1. Ueq is defined as one-third of the trace of the orthogonal Uij tensor.
Table 2. Selected Bond Distances, Bond Angle, and BVS for
PbMnTeO6
cation
Pb(1)
Mn(1)/Te(1)
anion
bond length (Å)
BVS
1.80
3.93/6.13
O(1)
O(1)
O(1)
O(1)
2.5572(1) × 6
1.9091(1) × 2
1.9092(5) × 2
1.9095(1) × 2
bond angle (deg)
Mn(1)/Te(1)−O(1)−Mn(1)/Te(1)
100.94(4)
A pseudosymmetry analysis was performed in order to gain
insight into the origin of the noncentrosymmetry in
PbMnTeO6. The optical SHG intensity gradually decreases
with increasing temperature, although it does not reach zero up
to 873 K, indicating a second-order phase transition from a
noncentrosymmetric phase to a centrosymmetric phase.
Assuming that the phase transition is second order, the
centrosymmetric phase should belong to supergroups of the
Figure 2. Ball-and-stick diagram of the PbMnTeO6 structure in the (a)
bc and (b) ab planes.
noncentrosymmetric P62m (No. 189) structure shown in
̅
Figure 5a. The possible supergroup structures were searched
within atomic displacements of 2 Å using the PSEUDO
program.31 We found one possible structure with P6/mmm
symmetry. Figure 5b shows the P6/mmm (No. 191) structure
along with arrows representing a distortion that leads to the
centrosymmetric phase transition was observed to be above 873
K, likely ∼900 K. The sample was not heated above 873 K in
the SHG experiment because a thermogravimetric analysis
(TGA) measurement for PbMnTeO6 at 1023 K in air indicated
the decomposition of the compound (see Figures S6 and S7).
P62m structure. The distortion corresponds to rotations of
̅
oxygen prisms enclosing the Mn/Te sites, which break the
Figure 3. Comparison of ball-and-stick diagrams for the three similar crystal structures: (a) PbSb2O6 in the bc plane,12 (b) SrGeTeO6 in the ab
plane,20 and (c) PbMnTeO6 in the ab plane (this work).
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Figure 4. Temperature dependence of the optical SHG intensity for a
PbMnTeO6 pellet between 12 and 873 K. The inset shows the
schematic of the SHG experiment, where the sample pellet is irradiated
with an 800 nm laser and the 400 nm SHG is scattered off the sample.
Figure 7. Inverse magnetic susceptibility of PbMnTeO6 with a Curie−
Weiss fit (solid line).
́
Neel transition temperature (TN) at ∼20 K, consistent with
low-dimensional magnetic behavior.32,33 No significant diver-
gence between the zero-field-cooled (ZFC) and field-cooled
(FC) magnetization curves was observed. From the plot of 1/χ
versus T shown in Figure 7, the susceptibility data were fit to
the Curie−Weiss (CW) law, χ = C/(T − θ) for T > 150 K,
where C is the Curie constant and θ is the Weiss constant; the
values C = 1.81 emu K mol−1 and θ = −43.22 K were extracted
from the CW fit of the data. On the basis of the CW fit, the
effective magnetic moment, μeff = 3.81μB per Mn, is in good
agreement with the theoretical spin-only value of 3.87μB for
Mn4+ (S = 3/2). The negative Weiss constant indicates
antiferromagnetic interactions, which could arise from super-
exchange interactions of nearest-neighbor Mn4+−O2−−
Mn4+.34−36 It is noteworthy that the frustration parameter
(|θ|/TN)37 of ∼2.16 in PbMnTeO6 indicates that some degree
of magnetic frustration is present in this quasi-two-dimensional
honeycomb lattice. The origin of magnetic frustration in
PbMnTeO6 is likely due to the competition between nearest-
neighbor and next-nearest-neighbor antiferromagnetic inter-
actions, but the degree of magnetic frustration is relatively
lower because magnetic interactions are diluted by non-
magnetic Te6+ cations compared with an only Mn4+-containing
honeycomb lattice antiferromagnet, Bi3Mn4O12(NO3).33,38
As shown in Figure 8, the isothermal magnetization of
PbMnTeO6 measured at 2 and 300 K as a function of the
applied field H is linear, which indicates that no ferromagnetic
interactions are involved. The magnetic interactions of Mn4+
and Mn4+ between layers are negligible because the layers are
well-separated by the Pb2+ ions with distances longer than 5.0
Å.
Figure 5. (a) Noncentrosymmetric P62m (No. 189) structure of
PbMnTeO6 and (b) possible parent centrosymmetric structure with
space group P6/mmm (No. 191). The black arrows in (b) represent a
̅
distortion that leads to the P62m structure.
̅
inversion symmetry. This is likely the origin of the non-
centrosymmetry in PbMnTeO6.
Magnetic Behavior. The dc magnetic susceptibility of
PbMnTeO6 was measured at 1000 and 10 000 Oe over the
temperature range 2−300 K, and the results are shown as plots
of χ and 1/χ versus T in Figures 6 and 7, respectively.
PbMnTeO6 exhibits antiferromagnetic behavior with a broad
CONCLUSION
■
A new noncentrosymmetric layered honeycomb magnetic
oxide, PbMnTeO6, was successfully synthesized by a conven-
tional solid-state reaction. The crystal structure of PbMnTeO6
exhibits a two-dimensional structure (space group P62m)
̅
consisting of edge-sharing (disordered Mn4+/Te6+)O6 trigonal
prisms, which form honeycomb-like layers. In contrast, the
similar layered SrGeTeO6 presents long-rage order of Ge4+ and
Te6+ ions in octahedral coordination, forming a chiral PbSb2O6-
type structure (space group P312). The origin of these
differences is likely due to electronic differences between
Figure 6. Temperature dependence of the magnetic susceptibility of
PbMnTeO6 measured at 1000 and 10 000 Oe. The inset shows a
close-up of the low-temperature region revealing the Nee
́
l temperature
of ∼20 K.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: martha@chem.rutgers.edu.
Author Contributions
The manuscript was written through contributions of all
authors.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
S.W.K., Z.D., M.R.L., and M.G. gratefully acknowledge support
from the ARO-434603 grant (DOD-VV911NF-12-1-0172).
A.S.G., H.A., and V.G. gratefully acknowledge support from the
National Science Foundation MRSEC Center for Nanoscale
Science at Penn State through Grant DMR-1420620. S.W.K.
thanks Prof. Mark Croft for fruitful discussions and Graeme
Gardner for EDX measurements.
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