Insights on the Synthesis, Crystal and Electronic Structures, and Optical and Thermoelectric Properties of Sr1- xSbxHfSe3 Orthorhombic Perovskite

Article abstract of DOI:10.1021/acs.inorgchem.8b01038

Single-phase polycrystalline powders of Sr_1-xSb_xHfSe₃ (x = 0, 0.005, 0.01), a new member of the chalcogenide perovskites, were synthesized using a combination of high temperature solid-state reaction and mechanical alloying approaches. Structural analysis using single-crystal as well as powder X-ray diffraction revealed that the synthesized materials are isostructural with SrZrSe₃, crystallizing in the orthorhombic space group Pnma (#62) with lattice parameters a = 8.901(2) ? b = 3.943(1) ? c = 14.480(3) ? and Z = 4 for the x = 0 composition. Thermal conductivity data of SrHfSe₃ revealed low values ranging from 0.9 to 1.3 W m^-1 K^-1 from 300 to 700 K, which is further lowered to 0.77 W m^-1 K^-1 by doping with 1 mol % Sb for Sr. Electronic property measurements indicate that the compound is quite insulating with an electrical conductivity of 2.9 S/cm at 873 K, which was improved to 6.7 S/cm by 0.5 mol % Sb doping. Thermopower data revealed that SrHfSe₃ is a p-type semiconductor with thermopower values reaching a maximum of 287 μV/K at 873 K for the 1.0 mol % Sb sample. The optical band gap of Sr_1-xSb_xHfSe₃ samples, as determined by density functional theory calculations and the diffuse reflectance method, is ～1.00 eV and increases with Sb concentration to 1.15 eV. Careful analysis of the partial densities of states (PDOS) indicates that the band gap in SrHfSe₃ is essentially determined by the Se-4p and Hf-5d orbitals with little to no contribution from Sr atoms. Typically, band edges of p- and d-character are a good indication of potentially strong absorption coefficient due to the high density of states of the localized p and d orbitals. This points to potential application of SrHfSe₃ as absorbing layer in photovoltaic devices.

Full text of DOI:10.1021/acs.inorgchem.8b01038

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Insights on the Synthesis, Crystal and Electronic Structures, and
Optical and Thermoelectric Properties of Sr Sb HfSe
1
−x
x
3
Orthorhombic Perovskite
†
†
†
†
†
Nicholas A. Moroz, Christopher Bauer, Logan Williams, Alan Olvera, Joseph Casamento,
‡
‡
§
§
†
Alexander A. Page, Trevor P. Bailey, Ashley Weiland, Stanislav S. Stoyko, Emmanouil Kioupakis,
‡
§
,†
Ctirad Uher, Jennifer A. Aitken, and Pierre F. P. Poudeu*
†
Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States
Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, United States
‡
§
Department of Chemistry and Biochemistry, Duquesne University, Pittsburgh, Pennsylvania 15282, United States
ABSTRACT: Single-phase polycrystalline powders of
Sr1−xSb HfSe (x = 0, 0.005, 0.01), a new member of the
x
3
chalcogenide perovskites, were synthesized using a combina-
tion of high temperature solid-state reaction and mechanical
alloying approaches. Structural analysis using single-crystal as
well as powder X-ray diffraction revealed that the synthesized
materials are isostructural with SrZrSe , crystallizing in the
3
orthorhombic space group Pnma (#62) with lattice parameters
a = 8.901(2) Å; b = 3.943(1) Å; c = 14.480(3) Å; and Z = 4 for
the x = 0 composition. Thermal conductivity data of SrHfSe
3
−1
−1
revealed low values ranging from 0.9 to 1.3 W m
K
from
3
00 to 700 K, which is further lowered to 0.77 W m−1 K−1 by doping with 1 mol % Sb for Sr. Electronic property measurements
indicate that the compound is quite insulating with an electrical conductivity of 2.9 S/cm at 873 K, which was improved to 6.7 S/
cm by 0.5 mol % Sb doping. Thermopower data revealed that SrHfSe is a p-type semiconductor with thermopower values
3
reaching a maximum of 287 μV/K at 873 K for the 1.0 mol % Sb sample. The optical band gap of Sr1−xSb HfSe samples, as
x
3
determined by density functional theory calculations and the diffuse reflectance method, is ∼1.00 eV and increases with Sb
concentration to 1.15 eV. Careful analysis of the partial densities of states (PDOS) indicates that the band gap in SrHfSe is
3
essentially determined by the Se-4p and Hf-5d orbitals with little to no contribution from Sr atoms. Typically, band edges of p-
and d-character are a good indication of potentially strong absorption coefficient due to the high density of states of the localized
p and d orbitals. This points to potential application of SrHfSe as absorbing layer in photovoltaic devices.
3
INTRODUCTION
based on the prototype crystal of CaTiO , formula
3
■
XII 2+VI 4+ 2−
A
B X 3, forming in the orthorhombic Pnma symmetry.
Ternary transition metal chalcogenides have garnered a great
deal of attention within the materials research community as
they have the potential to offer advanced and unique
In the idealized cubic system, the A cation is stationed at
Wyckoff position 4a (0, 0, 0), while B cations populate the
body-center 4a site (1/2, 1/2, 1/2). Many variations of this
structure have been investigated, most commonly the distorted
orthorhombic, tetragonal, and hexagonal perovskite ver-
1
−5
6−11
12−26
thermal,
electronic,
and magnetic properties,
as
27−33
well as interesting optical responses.
Their wide range of
crystal structure types and chemical composition allows for
creative systematic structural engineering that has produced
some of the most well-known photovoltaic and thermoelectric
4
2−46
sions.
These relatively dense structures offer various
47,48
interesting physical properties such as superconductivity,
colossal magnetoresistance,
ity, large dielectric constant,
coefficients.
4
9,50
3
4−36
37−41
ultralow thermal conductiv-
and high optical absorption
materials, such as CuInSe2
and CuAgSe.
The
51
52,53
flexibility and ease with which these materials have been
synthesized spurs the need to investigate new chalcogenide-
based crystal structures, potentially offering novel properties
and transport mechanisms and adding to the general
understanding of how to engineer these materials to power
our future.
Ternary transition metal chalcogenides with perovskite
structure have been notably absent from recent literature
concerning this important material family. The perovskite
structure, named for Russian mineralogist Lev Perovski, is
54−57
These unique properties have led to the
application of inorganic perovskite materials in photovoltaic
5
8,59
solar cells, thermoelectric devices,
organic memory devices,
and even inorganic−
60,61
often taking advantage of the
specific electronic transport properties the high symmetry
structure exhibits, as well as high temperature stability.
Received: April 16, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b01038
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Typically, the perovskite structures require significantly high
temperatures and pressures for stable formation. Consequently,
this has led to a focus on the production of oxides, which has
deterred the creation and elucidation of chalcogenide-based
perovskites. To date, of the chalcogenide-based perovskites,
only sulfide-based compounds have been studied, specifically
mm × 55 mm), which was placed inside a silica tube. The 8-in. tube
−4
was flame-sealed under residual pressure of 10 Torr. The tube was
then heated to 1073 K in 24 h, held for 120 h, and then cooled to
room temperature in 24 h. This process was successful in creating
>
98% phase pure SrHfSe and Sb-doped SrHfSe samples, based on X-
3
3
ray diffraction (XRD) data. However, attempts to synthesize Ba and
Ca compounds were not successful. Further processing of Ba and Ca
compounds by cold-pressing the products of the first reaction at ∼300
MPa into 13 mm diameter discs and reheating the pellets at 1073 and
1173 K for an additional 120 h did not complete the reaction. Binary
compounds such as CaSe and BaSe constituted a majority of the
products.
6
2
62
63
64,65
63
EuZrS3,
EuHfS3,
CaZrS3,
BaZrS3,
and BaHfS3, but
the electronic and thermal properties of these compounds have
not been extensively studied. Recently, Sun et al. published the
promising results of a theoretical study on a series of ABX3
compounds with A = Ca, Sr, and Ba, B = Ti, Zr, and Hf, and X
Single Crystal Growth. Single crystals of SrHfSe suitable for X-ray
=
S and Se, predicting the photovoltaic behavior of the
3
6
6
structural analysis were successfully grown by annealing approximately
distorted perovskite, needlelike, and hexagonal phases.
0
.25 g of the presynthesized SrHfSe polycrystalline powder. The
3
Results of this research predicted that the distorted perovskite
powder was flame-sealed into a 7 mm diameter × 12-in.-long silica
phase of ABSe , where A = Ca, Sr, and Ba and B = Hf and Zr,
−4
3
tube under residual pressure of 10 Torr. The tube was heated to
exhibits band gaps between 1.3 and 1.7 eV, suggesting excellent
potential for high efficiency photovoltaic devices. Furthermore,
needle-like (orthorhombic) phases were predicted to possess
band gaps of 0.75−1.0 eV, which can be quite useful for
thermoelectric applications. More recently, Meng et al.
1
373 K in 24 h and held for 240 h and then cooled to room
temperature in 24 h. Lustrous black crystals were harvested from the
products. Attempts to produce single crystals of BaHfSe and other
3
compounds failed but did consistently create crystals, which were too
small for characterization by single crystal X-ray diffraction methods.
Densification. Polycrystalline single-phase powder of SrHfSe and
calculated the electrical and optical properties of BaZrS from
3
3
Sb-doped SrHfSe samples were consolidated into pellets using the
3
density functional theory (DFT) calculations and were able to
determine that BaZr Ti S could exhibit a band gap of 1.46
hot-press method. Approximately 1 g of powder sample was added to a
0
.9 0.1 3
1
0 mm diameter graphite die and pressed between graphite anvils.
eV, ideal for single- or multijunction solar cells, and potentially
Densification of pellets was conducted under flowing Ar gas at 40 mL/
min, a pressing temperature of 1123 K for 2 h, and an applied pressure
of 100 MPa with a ramp up and down rate of 225 K/h. The pellets
were then removed from the graphite dies and polished to mirror
finishes with SiC polishing paper, up to 1200 grit. The densities of the
pellets were determined by both the geometric method (measured
mass/measured volume) and the “true” density measurement using a
67
obtain >20% photovoltaic conversion efficiency. Though the
synthesis was challenging, they were also able to create BaZrS3
with distorted-perovskite structure and confirm that the band
gap of 1.85 eV matches that of the calculations.
To date, the only previously known structures of the series of
perovskite materials of the type ABSe where A = Ca, Sr, and
3
He gas Quantachrome Micro Ultrapyc 1200e pycnometer, and they
Ba and B = Zr and Hf are SrZrSe , which was successfully
3
3
68
were compared to the theoretical density of SrHfSe , 6.58 g/cm ,
3
synthesized as phase pure by Tranchitella et al. in 1997, and
69,70
obtained from the crystal structure refinement. All Sr1−xSb HfSe
x 3
BaZrSe synthesized in 1964 by Aslanov.
However, it was
found that both compounds do not adopt the anticipated
distorted perovskite structure. SrZrSe crystallizes in a new
3,
pellets yielded both a geometric and true density that were greater
than 97% of the theoretical density. The pellets were then dissected
into prismatic bars roughly 3 mm × 2 mm × 8 mm using a wire saw
and a cutting solution consisting of a mixture of water, glycerin, and
silicon carbide 300 mesh powder. The prismatic bars were again
polished with metallographic paper to 1200 grit in order to perform
electronic transport measurements. The excellent match of the peak
intensities on the XRD patterns of the samples prior to and after
densification indicates little to no preferred orientation, suggesting
isotropic response of the transport properties of the material.
3
68
orthorhombic needlelike structure, whereas BaZrSe forms in
3
the hexagonal CsNiCl structure (space group P63/mmc
3
6
9,70
#
194).
Tranchitella’s approach to the creation of
selenide-based perovskites with the GdFeO structure type is
3
notable in that relatively low-pressure synthesis techniques
were employed (<1 GPa). This work follows a similar approach
while incorporating mechanical alloying (high energy ball-
milling) to produce a novel series of ABSe3 materials,
maintaining an approach that requires less instrumental
sophistication than typical perovskite synthesis at high
temperature (>1273 K) and pressure (>1 GPa). Hence, we
report in this study the synthesis and crystal structure of
SrHfSe , which adopts the needle-like (orthorhombic) SrZrSe -
Characterization. Single Crystal X-ray Diffraction. A needle-
shape single crystal of SrHfSe was harvested from an annealed sample
3
and examined by X-ray diffraction. Intensity data was recorded at 300
K on a STOE IPDS-2T diffractometer using graphite-monochromated
Mo Kα radiation (λ = 0.71073 Å) at 50 kV and 40 mA. The diffraction
data were indexed in the orthorhombic crystal system with lattice
parameters a = 8.901(2) Å; b = 3.943(1) Å; c = 14.480(3) Å; and Z =
3
3
4
. The structure solution was obtained by direct methods in the space
type structure, and their thermoelectric and optical properties.
group Pnma (#62) and refined by full-matrix least-squares techniques
Substitutional doping of Sr by Sb in SrHfSe is also investigated
3
71
using the SHELTXL package. The asymmetric unit cell contains two
in order to probe the effects on the electronic and thermal
transport properties as well as the optical band gap.
crystallographically independent metal positions: the M1 (4c) site,
which is located in an octahedral environment of six Se atoms, and the
M2 (4c) site, which is surrounded by nine Se atoms in a tricapped
trigonal prismatic geometry; and three Se-atom positions (Se1−Se3),
which are all located at Wyckoff site 4c. In the initial refinement model,
the smaller Hf atom was located at the M1 site, whereas the Sr atom
EXPERIMENTAL DETAILS
Synthesis and Processing. Synthesis. Stoichiometric amounts of
Sr granules (Alfa-Aesar, 99%, 19 mm long granules), Hf powder
■
(
American Elements, 99.5%), Sb powder (Alfa Aesar, 99%), and Se
was located at the M2 site. The refinement of this model yielded R =
1
powder (Sigma-Aldrich, 99.5%) were weighed in a glovebox under Ar
atmosphere and added to hardened steel jars with hardened steel ball
bearings and sealed. 2.5 g of total reactants were added to each jar with
7% with reasonable atomic displacement parameters for all atoms
except the Sr atom, which showed thermal displacement parameters
twice that of Hf and all Se atoms. Nevertheless, bond valence sum
(BVS) calculation results also fully supported the above-mentioned
occupation of each metal site (Table 1). The final refinement of this
model, while including a secondary extinction correction and
anisotropic displacement parameter for all atoms, resulted in the
7
2
2
5 g of ball bearings. The jars were mounted on a SPEX 8000D mill
and processed for 30 min in order to ensure complete mixing of the
reactants. The jars were then transferred to the glovebox, and the
contents were transferred to a graphite crucible with a lid (diameter 7
B
DOI: 10.1021/acs.inorgchem.8b01038
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 1. Bond Valence Sum (BVS) Calculations for SrHfSe3
at 300 K
Table 4. Selected Interatomic Distances (Å) in SrHfSe at
300 K
3
a
Atom position
BVS
Oxidation state
Bond type
Distance [Å]
Bond type
Distance [Å]
v
vi
Sr1
Hf1
Se1
Se2
Se3
2.11
4.03
2.16
2.27
2.14
2+
4+
2−
2−
2−
Hf1Se3
2.559(2)
2.676(9)
2.698(8)
2.720(9)
Sr1Se(3 ,3 )
3.174(2)
3.198(2)
3.212(2)
3.229(2)
3.395(2)
3.844(2)
i
vii
Hf1Se(1,1 )
Sr1Se2
ii iii
viii ix
Hf1Se(2 ,2 )
Sr1Se3 ,3 )
iv
Hf1Se2
Sr1Se(1,1 )
v
Sr1Se1
iv
Sr1Se2
final value, R ∼ 3%. The final charge balanced composition of the
a
1
Operators for generating equivalent atoms: (i) x, 1 + y, z; (ii) 1 − x, 1
2
+
4+
2−
crystal obtained from the refinement was (Sr )(Hf )(Se ) . A
3
− y, 1 − z; (iii) 1 − x, −y, 1 − z; (iv) x, y − 1, z; (v) 1 − x, −1 − y, 1
summary of the crystallographic data is given in Table 2. The atomic
−
−
z; (vi) 1 − x, −y, 1 − z; (vii) −1/2 + x, −1 + y, 3/2 − z; (viii) 3/2
x, −y, 1/2 + z; (ix) 3/2 − x, −1 − y, 1/2 + z; (x) 1/2 + x, 1 + y, 3/2
Table 2. Selected Crystallographic Data for SrHfSe at 300 K
− z; (xi) 3/2 − x, 1 − y, 1/2 + z; (xii) 3/2 − x, −y, −1/2 + z; (xiii) 3/
3
2
− x, −1 − y, −1/2 + z; (xiv) 3/2 − x, 1 − y, −1/2 + z.
Crystal system
Orthorhombic
Pnma (#62)
502.99
Space group
powder X-ray diffraction (XRD) techniques. XRD data were collected
using a graphite monochromated Cu Kα (λ = 1.54056 Å) on a Rigaku
Miniflex 600 operating at 40 kV and 15 mA and a Rigaku Ultima with
rotating arm anode operating at 40 kV and 100 mA. Rietveld
refinement pattern fitting was performed on the diffraction patterns of
the pellet samples of SrHfSe using the FullProf software package in
order to determine the lattice constants and unit cell volume using the
single crystal structure data of SrHfSe as a starting model.
Wavelength Dispersive X-ray Fluorescence (WDXRF). Powder
samples of SrHfSe and Sb-doped SrHfSe were loaded into plastic
boats with Prolene film bottoms. Samples were then characterized with
a Rigaku Supermini 200 emitting radiation from a Pd source at 50 kV
and 4 mA to determine the concentration of each constituent element
and the overall chemical composition.
Differential Scanning Calorimetry (DSC). DSC was performed
using a Netzsch DSC404 F1, by placing approximately 25 mg of pellet
material in a platinum crucible with lid. An empty platinum crucible
was used as a reference. The measurement was performed under
flowing nitrogen gas, 20 mL/min, from room temperature to 1300 K
in order to determine phase purity and to identify thermal events
associated with phase changes, such as melting and crystallization of
the compound.
Formula weight (g/mol)
3
Calculated density (g/cm )
6.576
Lattice parameters (Å)
a =
b =
c =
8.901(2)
3.943(1)
3
14.480(3)
74
3
3
V (Å ); Z
508.07(2); 4
λ (Mo Kα) = 0.71073
522
Radiation (Å)
μ (cm−1
3
3
)
2
θ range
7° ≤ 2θ ≤ 67°
−13 ≤ h ≤ 13
index range
−
−
5 ≤ k ≤ 5
21 ≤ l ≤ 21
Transmission factors
Diff. elec. density [eÅ−3]
0.029−0.198
+3.32 to −2.77
0.029
a
R (F > 4σ(F ))
1
o
o
b
wR (all)
0.065
2
GOF
1.148
a
R = ∑||F | − |F ||/∑|F |. wR = [∑w(F − F ) /∑w(F ) ]1/2.
b
2
o
2
2
2 2
1
o
c
o
2
c
o
X-ray Photoelectron Spectroscopy (XPS). XPS was used to assess
the oxidation state of various metal atoms in the samples. Powder
samples of each composition were hand pressed into 3 mm diameter
pellets under ambient conditions using a Pike Technologies press. XPS
spectra were collected for each pellet using a Kratos Axis Ultra XPS
equipped with a monochromatic Al source (15 kV, 10 mA). Specimens
were loaded into the transfer chamber and allowed to evacuate for a 12
coordinates and isotropic displacement parameters of all atoms are
listed in Table 3. Selected interatomic distances are gathered in Table
Table 3. Wyckoff Positions (WP), Atomic Coordinates, and
−
4
Equivalent Isotropic Displacement Parameters (U /10
eq
2
Å ) for All Atoms in the Asymmetric Unit Cell of SrHfSe at
3
−7
h period reaching less than 5 × 10 Torr. The analysis chamber was
3
00 K
−9
kept at approximately 10 Torr during measurement. Wide scans
a
were acquired using an analyzer pass energy of 160 at 1 eV steps.
Selected regions were scanned using a pass energy of 20 at 0.02 eV
steps. Postprocessing analysis was performed using Casa XPS.
Thermal Transport Measurement. Thermal diffusivity and heat
Atom
WP
x
y
z
Ueq
Hf1
Sr1
Se1
Se2
Se3
4c
4c
4c
4c
4c
0.8308(6)
0.5628(2)
0.6611(2)
0.9792(2)
0.7093(2)
1/4
3/4
1/4
1/4
1/4
0.4431(3)
0.6779(8)
0.5120(8)
0.6073(6)
0.2829(7)
80(2)
112(3)
73(2)
59(2)
78(3)
capacity (C ) data for each pelletized sample was measured using the
p
laser flash method on a Linseis LFA 1000 in order to calculate the
thermal conductivity. Pellets were cleaned with acetone and lightly
coated with graphite spray; then thermal diffusivity was measured from
a
Ueq is defined as one-third of the trace of the orthogonalized Uij
3
00 to 723 K under flowing N gas (>30 mL/min). The data collected
tensor
2
was compared with a reference Pyroceram 9606 sample, and the
accuracy of the data was determined to be within 2%, with machine
precision of ±3%.
73
4
. The software Diamond was utilized to create the graphic
representation of the crystal structure with ellipsoid representations
98% probability level) for all atoms. Detailed crystallographic data can
Electronic Transport Measurements. Prismatic bars of each
composition were cut from the pellets, polished to a mirror finish,
and mounted into a ULVAC-RIKO ZEM-3. The thermopower and
electrical resistivity of each sample were measured simultaneously from
room temperature to 723 K under a low pressure of He. The
instrument precision on the electrical resistivity and Seebeck
coefficient data is ±4%.
(
be obtained from Fachinformationszentrum Karlsruhe, 76344
Eggenstein-Leopoldshafen, Germany (fax +49-7247-808-666; e-mail
crysdata@fiz.karlsruhe.de), on quoting the depository number CSD-
4
34309.
Powder X-ray Diffraction and Rietveld Refinement. Phase analysis
and the crystal structure identification of various Sr1−xSb HfSe3
Hall-Effect Measurements. Prismatic bars of each composition with
approximate size 3 × 3 × 8 mm were polished to a mirror finish, and
x
compositions in both powder and pellet form were performed by
C
DOI: 10.1021/acs.inorgchem.8b01038
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
room temperature Hall coefficients, R , were measured using the AC
H
4
-probe setup in a Quantum Design Physical Properties Measurement
System (PPMS) under a magnetic field of ±1 T and using an
excitation current of 0.1 mA. The carrier concentration was then
−1
calculated as n = (R *e) , where e is the electronic charge.
H
H
Optical Band Gap Measurements. A Varian Cary 5000
spectrometer, equipped with a Harrick Praying Mantis diffuse
reflectance accessory, was used to collect the optical diffuse reflectance
spectra of the compounds over the ultraviolet, visible, and near-
infrared spectral regions (UV/vis/NIR). Each sample was ground and
placed in a sample holder to a depth of 3 mm. Barium sulfate (Fisher,
9
9.92%) was used as a 100% reflectance standard. The measurement
was conducted at a scan rate of 600 nm/min. The optical band gaps of
the samples were estimated from the UV/vis/NIR spectra by
extrapolation of the absorption edge to the baseline. The absorption
spectra were calculated from the reflectance spectra via the Kubelka−
75
Munk equation. The direct allowed transmission model was used to
fit each set of data in order to produce Tauc plots to estimate the
optical band gap.
First-Principles Calculations. DFT calculations were performed
7
6−79
using the Vienna Ab Initio Simulation Package (VASP),
projector augmented wave pseudopotentials,
6
with
including 10, 10, and
valence electrons for Sr, Hf, and Se, respectively. The band structure
without spin−orbit coupling effects was calculated utilizing the hybrid
80,81
Figure 1. (a) X-ray diffraction patterns of polycrystalline samples of
8
2,83
Sr1−xSb HfSe phases synthesized using solid-state reactions of the
functional of Heyd, Scuseria, and Ernzerhof (HSE06)
in
x
3
elements compared to the theoretical pattern calculated using single
crystal structure data. (b) Differential scanning calorimetry (DSC)
conjunction with interpolation based on maximally localized Wannier
84
functions as implemented in the wannier90 code. The functional of
8
5
curves for SrHfSe showing a single endothermic peak of melting at
Perdew−Burke−Ernzerhof (PBE) was used to evaluate the
magnitude of spin−orbit coupling. The plane wave basis set cutoff
was set at 400 eV. Gamma-centered grids of 4 × 8 × 3 were used to
sample the first Brillouin zone for all calculations.
3
1
273 K and a single peak of crystallization at similar temperature. The
small peak indicated with (*) on the XRD patterns of Sb- doped
samples can be associated with HfSe binary impurity phase.
2
RESULTS AND DISCUSSION
3m; #164). The phase purity and thermal stability of the
synthesized Sr1−xSb HfSe phases were confirmed by differ-
■
x
3
Synthesis and Crystal Structure. Single-phase fine black
ential scanning calorimetry (DSC) measurements (Figure 1B),
which showed a single peak of melting and a single
crystallization peak at 1273 K. This indicates that the
compound melts congruently and crystallizes without disorder
or decomposition. No additional peaks, which would suggest
the presence of minor impurity phases, could be detected from
the DSC curves.
powders of Sr1−xSb HfSe phases were synthesized by
x
3
mechanical alloying of a stoichiometric mixture of high purity
elements followed by solid state reaction at moderate
temperatures. A comparison of the X-ray diffraction patterns
of the synthesized compounds with that of GdFeO reveals
sharp differences, meaning that the compounds do not adopt a
3
distorted Perovskite structure. However, striking similarity was
observed when comparing the XRD data of the Sr1−xSb HfSe3
To confirm the crystal structure of SrHfSe , suitable single
3
x
crystals were successfully grown by annealing polycrystalline
powder samples at 1373 K for 10 days, as described in the
experimental procedure. Long black needle-like crystals were
harvested from the resulting annealed ingots, and analyzed
using single crystal X-ray diffraction techniques. Intensity data
were indexed in the orthorhombic space group Pnma (#62),
with a = 8.9001(18) Å, b = 3.9424(8) Å, and c = 14.480(3) Å°
phases to that of SrZrSe , suggesting that they crystallize in the
3
orthorhombic structure with space group Pnma (#12) (Figure
A). The observed deviation from the perovskite structure type
could be anticipated from the rather low Goldschmidt tolerance
factor of t = 0.82, calculated using the formula
1
t = (r + r )/ 2 (r + r ), where rA is the ionic radius of
A
0
B
0
the A-cation (Sr), r is the ionic radius of the B-cation (Hf),
(Table 2). The result suggests that SrHfSe is isostructural with
B
3
8
6
and r is the ionic radius of the anion (Se). Composition
SrZrSe (prototype structure NH CdCl ) first developed by
0
3
4
3
68
analysis of the synthesized powder of SrHfSe3 using the
wavelength dispersive X-ray florescence (WDXRF) technique
showed a slightly Sr-rich composition, Sr1.16Hf0.90Se . This
structure remained consistent through hot-pressing, signaling
that the synthesized compound is single phase and is thermally
stable. A similar synthesis approach was also used to synthesize
Tranchitella et al. Polyhedral representations of the crystal
structures of SrHfSe projected along the b-axis are shown in
3
3
Figure 2, along with the connectivity between similar
polyhedra. The structure can be divided into two substructures
based on the packing arrangement of [HfSe ] or [SrSe ]
6
9
polyhedra. All metal atoms reside at Wyckoff position 4c, with
Hf atoms located in a distorted octahedral geometry [1 + 2 + 2
+ 1] and Sr atoms located in a distorted tricapped trigonal
prismatic geometry [2 + 1 + 2 + 2 + 1 + 1]. The Hf−Se bond
distances range from 2.560(2) Å to 2.720(8) Å, whereas the
Sr−Se bond distances range from 3.173(2) Å to 3.395(2) Å
with a longer bond of 3.844(2) Å (Table 4). The observed
high purity Sb-doped samples of SrHfSe . Attempts to
3
synthesize Sr1−xSb HfSe phases exclusively by mechanical
x
3
alloying were not successful in creating pure needle-like or
distorted perovskite structure after 100 h of high-energy ball-
milling. XRD patterns of the resulting products revealed a
mixture of binary impurity compounds, such as HfSe3,
Hf1.35Se , and HfSe . For example, the small peak at 15 deg
bond distances in SrHfSe are comparable to the reported
2
2
3
on the XRD patterns of Sb-doped samples (Figure 1A) can be
bonding in SrZrSe , which is to be expected due to the
3
associated with the binary phase, HfSe (P-3m space group P-
similarity in the oxidation state (+4) and ionic radius of Zr (230
2
D
DOI: 10.1021/acs.inorgchem.8b01038
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Inorganic Chemistry
Article
Figure 2. Projection of the crystal structure of SrHfSe along the b-axis
3
highlighting: (a) the octahedral coordination around Hf atoms and (b)
the tricapped trigonal prismatic coordination around Sr atoms; (c)
double chains of edge-sharing HfSe octahedra running parallel to the
6
b-axis; and (d) adjacent SrSe tricapped trigonal prisms sharing edges
9
in all directions to form a 3D network separating adjacent double
chains of edge-sharing HfSe octahedra.
6
pm) and Hf (225 pm). The cell volume of SrHfSe , 508.1(2)
3
3
3
Å , is also within 1% of the cell volume of SrZrSe , 513.0(4) Å .
3
Adjacent [HfSe ] octahedra within the crystal structure of
6
SrHfSe share edges to form isolated double-chains running
parallel to the b-axis (Figure 2C). The double-chains separated
3
Figure 3. (a) XPS spectrum and curve-fitting analysis of Sr2+ in
SrHfSe . Two sets of doublet peaks indicate possible Sr-based impurity
by [SrSe ] polyhedra are arranged in layers with alternating
3
9
4
+
in small amounts. (b) XPS spectrum and curve-fitting analysis of Hf
in SrHfSe . (c) XPS spectrum and curve-fitting analysis of Se in
orientations along the c-axis to build the three-dimensional
structure (Figure 2A).
Alternatively, the crystal structure of SrHfSe3 can be
described as consisting of tricapped trigonal prismatic [SrSe9]
polyhedra that share edges (Figure 2D) in all directions to form
a three-dimensional framework with channels that are filled by
2
−
3
SrHfSe3.
doublet (Figure 3B) with binding energies 15.3 eV/16.9 eV,
4
+
similar to the values (16.1 eV/17.8 eV) reported for Hf in
88,89
HfS2.
No indication of any Hf-based impurity phase could
double chains of edge sharing [HfSe ] octahedra.
6
be detected. One can conclude based on XPS data that the
The accuracy of the structure solution and completeness of
oxidation states of Sr, Hf, and Se atoms in SrHfSe are 2+, 4+,
3
the structure refinement were validated by bond valence sum
7
2
and 2−, respectively.
calculations, X-ray photoelectron spectroscopy (XPS), and
comparing the theoretical X-ray powder diffraction pattern
created from the single crystal structure solution to
Optical Properties. The optical band gap of each material
was determined by collecting the diffuse reflectance of each
sample over the near-IR and UV−visible light spectra. Tauc
experimental XRD patterns from the synthesized Sr1−xSb HfSe3
x
plots of the response of Sr1 Sb HfSe samples are shown in
−x
x
3
phases (Figure 1). Bond valence sum calculations (Table 1)
suggest ordering of Hf and Sr atoms within 4c sites with
oxidation 4+ and 2+, respectively. To further confirm the
assigned oxidation states and chemical environment of Hf and
Figure 4. The direct transmission model is applied to all
samples as direct band gaps were estimated from the density
functional theory analysis performed by Sun et al. associated
90
with SrHfSe and BaHfSe . For Sr1−xSb HfSe samples, the
3
3
x
3
Sr atoms in SrHfSe , XPS was performed on selected samples.
3
band gap is estimated at 1.02 eV for the undoped sample, and
increasing to 1.06 and 1.15 eV for the 0.5 and 1.0 mol %
The spectrum of Sr_3d shell electrons for the pristine SrHfSe3
sample is shown in Figure 3A along with fitting curves. Two
sets of the Sr_3d5/2/ Sr_3d3/2 doublet peaks with binding
energies 132.2 eV/134 and 136.2 eV/138 eV can be observed,
indicating the presence in the polycrystalline sample of Sr
atoms with two distinct chemical environments. However, the
difference in the binding energies for each set of Sr_3d5/2
/
Sr_3d3/2 peaks is 1.8 eV, which is consistent with the value
2+
87
reported for Sr in SrS (Sr_3d5/2: 132.7 eV). Since only one
crystallographically independent Sr site is observed in the
structure of SrHfSe , the second doublet may be associated
3
with the presence of a small fraction of Sr-containing impurity
phase within the sample. This analysis is consistent with the
presence of two sets of the Se_3d5/2/ Se_3d3/2 doublet peaks
with binding energies 52 eV/52.9 and 53.7 eV/54.6 eV in the
spectrum of Se_3d shell electrons (Figure 3C). The spectrum
of Hf_4f shell electrons shows a single Hf_4f7/2/ Hf_4f5/2
Figure 4. Optical band gaps of Sr1−xSb HfSe showing increasing band
gap with increasing Sb content.
x
3
E
DOI: 10.1021/acs.inorgchem.8b01038
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Inorganic Chemistry
Article
samples, respectively. The observed increase in band gap offers
evidence that Sb doping significantly changes the electronic
structure of the system in addition to raising the Fermi level.
This is an intriguing result as it identifies a possible avenue for
engineering the band gap of the material for photovoltaic
applications. The ideal direct band gap for photovoltaic
applications is expected to be between 1.1 and 1.6 eV,
corresponding to efficiencies eclipsing 28%. Thus, Sb-doped
anticipated for the hypothetical needle-like structure of
90
SrHfSe3, and is close to the experimental value of 1.02 eV
obtained from diffuse reflectance measurements (Figure 4).
Both experimental and theoretical values of the band gap are
consistent with the black color of the SrHfSe polycrystalline
3
powder sample.
Careful analysis of the partial densities of states (PDOS)
indicates that the highest occupied valence band (or HOMO)
primarily consists of Se-4p orbitals, whereas the lowest
unoccupied conduction band (or LUMO) is mainly composed
of Hf-5d orbitals (Figure 6). This implies that the optical
needle-like SrHfSe appears to provide a promising opportunity
3
91
for further development in photovoltaics.
Electronic Structure. DFT calculations using the HSE06
functional were performed to investigate the band structure
absorption of SrHfSe can be mainly ascribed to the promotion
3
(Figure 5) and the partial density of states (PDOS) (Figure 6)
of carriers from the Se-4p to the Hf-5d orbitals. The valence
bands between −5 and 0 eV (referenced to the HOMO)
mostly consist of Se-4p orbitals, whereas the conduction bands
from 0.88 to 5 eV are dominated by Hf-5d states. Therefore,
the band gap in SrHfSe is essentially determined by the Se-4p
3
and Hf-5d orbitals with little to no contribution from Sr atoms.
Typically, band edges of p- and d-characters are a good
indication of a potentially strong absorption coefficient due to
the high density of states of the localized p and d orbitals. This
points to a potential application of SrHfSe as an absorbing
3
layer in photovoltaic devices. However, significant d-character
of the conduction band could result in large carrier effective
masses and low carrier concentration, which can lead to low
intrinsic carrier mobility and low overall electrical conductivity.
Thermoelectric Properties. To probe the thermoelectric
behavior of SrHfSe , high-density pellets of pristine and Sb-
3
doped SrHfSe samples were fabricated and their thermal
3
conductivity, electrical conductivity, and thermopower in the
Figure 5. Band structure of SrHfSe3 calculated using the HSE06
functional. Energies are referenced to the valence band maximum. The
compound is a direct gap material with a calculated gap of 0.88 eV.
midtemperature range, 300−700 K, were measured. Thermal
conductivity results indicate that SrHfSe exhibits relatively low
3
values ranging from 1.3 W/mK at 300 K to 0.9 W/mK at 700
K, which is suitable for thermoelectric applications (Figure 7C).
The substitution of 0.5 mol % and 1 mol % Sb for Sr reduces
the thermal conductivity further to 0.77 W/mK at 700 K. At
temperatures below 673 K, 1 mol % Sb yields the lowest
thermal conductivity, which can be expected due to mass
Figure 6. Calculated partial density of states (PDOS) of SrHfSe . The
3
valence band has primarily Se-p character, and the conduction band
primarily Hf-d character.
of SrHfSe . The band structure calculated along high-symmetry
3
directions of the first Brillouin zone shows that both the
conduction band (CB) minimum and the valence band (VB)
maximum are located at the Γ point, indicating that SrHfSe is a
3
direct gap semiconductor with a band gap of 0.88 eV (Figure
5). This result does not include the effects of spin orbit
coupling. Calculations with the PBE functional showed that
spin−orbit coupling effects on the gap are minor, narrowing the
gap by approximately 50 meV. Interestingly, the band gap
obtained from our calculation is similar to the value previously
Figure 7. Thermoelectric properties of Sr1−xSb HfSe samples. (a)
electrical conductivity; (b) thermopower; (c) thermal conductivity;
and (d) figure of merit, ZT.
x
3
F
DOI: 10.1021/acs.inorgchem.8b01038
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
fluctuation phonon scattering since the atomic mass of Sb is
nearly 1.5 times greater than that of Sr. Above 673 K, there
appears to be little difference in the thermal conductivity of 0.5
and 1 mol % Sb samples.
rapidly with rising temperature, which is attributed to bipolar
conduction arising from thermal excitation of extrinsic electrons
to the conduction band.
Electronic transport data showed that the synthesized
ASSOCIATED CONTENT
■
pristine SrHfSe sample is a poor metal with very low values
3
Accession Codes
of the electrical conductivity (∼0.5 S/cm) and thermopower
CCDC 1837976 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
(
∼7 μV/K) at 300 K (Figures 7A and 7B). Hall effect
measurements indicate that SrHfSe is a high p-type semi-
3
1
9
−3
conductor with carrier concentration of 1.05 × 10 cm at
00 K. Upon increasing the temperature, both the electrical
3
conductivity and thermopower surprisingly rapidly increase
simultaneously, reaching maximum values (2.5 S/cm and ∼260
μV/K) at 700 K and remain constant with further increase in
temperature to 800 K. The trend suggests the presence of
impurity donor states within the band gap of the synthesized p-
1
EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: ppoudeup@umich.edu.
ORCID
type SrHfSe sample. Such states upon thermal excitation of
3
extrinsic carriers to the conduction band (CB) become donor
states leading to bipolar conduction where thermally activated
extrinsic carriers from impurity states gradually compensate
Pierre F. P. Poudeu: 0000-0002-2422-9550
intrinsic holes. Partial substitution of Sr atoms in SrHfSe by
Author Contributions
3
electron richer Sb atoms results in a large suppression of hole
concentration around 300 K, leading to a large increase in the
thermopower value at 300 K from ∼7 μV/K for the pristine
sample to 135 μV/K for the sample containing 0.5 mol % Sb.
Upon increasing the temperature, the thermopower and
electrical conductivity of the Sb-doped samples rapidly increase,
reaching maximum values of 6.7 S/cm and 284 μV/K at 873 K
for the sample containing 0.5 mol % Sb. Another alternative
explanation for this simultaneous increase in both electrical
NAM synthesized the samples and carried out structural
characterization, performed high temperature electronic and
thermal conductivity measurements, performed data analysis,
and cowrote the manuscript. CB helped in the synthesis and
carried out structural characterization. LW and EK performed
theoretical studies and cowrote the manuscript. AO, JC, AP,
TB, and CU performed high temperature electronic and
thermal conductivity measurements, performed data analysis,
and coedited the manuscript. AW, SS, and JA performed optical
properties measurements and coedited the manuscript. P.F.P.P.
conceived the experiment, performed data analysis, and
cowrote the manuscript.
conductivity and thermopower of SrHfSe upon Sb-doping is
3
that the added electrons resulting from the n-type doping move
the Fermi level toward the conduction band, resulting in
bipolar conduction where extrinsic electrons counter intrinsic
holes, increasing both the carriers effective mass and mobility.
While the overall thermoelectric performance of this material is
not impressive, Sb-doping did substantially improve both the
Notes
The authors declare no competing financial interest.
2
ACKNOWLEDGMENTS
maximum power factor, 0.522 μW/cm K for 0.5 mol % doped
■
2
at 798 K compared to 0.182 μW/cm K at 698 K for SrHfSe ,
This work was supported by the National Science Foundation
under award # DMR-1561008. C.U. and P.F.P.P. gratefully
acknowledge financial support for electrical conductivity, Hall
Effect, and thermopower measurements from the Department
of Energy, Office of Basic Energy Sciences under Award # DE-
SC-0008574. We also thank the University of Michigan M-
Cubed program as well as the generous gift from Kendall and
Susan Warren to the UM College of Engineering in support of
research in renewable energy generation. Computational
resources were provided by the DOE NERSC facility under
Contract No. DE-AC02- 05CH11231.
3
and the figure of merit, 0.046 for 0.5 mol % doped at 698 K
compared to 0.014 at 698 K for SrHfSe3.
CONCLUSION
In summary, we have synthesized the ternary chalcogenide
■
perovskite SrHfSe through solid-state reaction of the elements
3
and investigated its crystal structure as well as the optical and
thermoelectric properties. We found that the compound
crystallizes with the SrZrSe structure type rather than the
3
distorted orthorhombic perovskite structure reported for
SrHfS . Optical spectroscopy data indicate that SrHfSe is a
3
3
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■
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