A3V5O14 (A = K+, Rb +, or Tl+), New polar oxides with a tetragonal tungsten bronze related structural topology: Synthesis, structure, and functional properties

Article abstract of DOI:10.1021/ic1008297

Three polar noncentrosymmetric (NCS) oxide materials, A₃V ₅O₁₄ (A = K⁺, Rb⁺, or Tl ⁺), have been synthesized by hydrothermal and conventional solid state techniques. Their crystal structures and functional properties (second-harmonic generation, piezoelectricity, and polarization) have been determined. The iso-structural materials exhibit a layered structural topology that consists of corner-sharing VO₄ tetrahedra and VO₅ square pyramids. The layers stack parallel to the c-axis direction and are separated by the K⁺, Rb⁺, or Tl⁺ cations. Powder second-harmonic generation (SHG) measurements using 1064 nm radiation indicate the materials exhibit moderate SHG efficiencies of ～100× α-SiO₂. Additional SHG measurements, that is, particle size versus SHG efficiency, indicate the materials are type-I phase-matchable. Converse piezoelectric measurements for K₃V₅O ₁₄, Rb₃V₅O₁₄, and Tl ₃V₅O₁₄ revealed d₃₃ values of 28, 22, and 26 pm/V, respectively. Pyroelectric measurements, that is, temperature-dependent polarization measurements, resulted in pyroelectric coefficients of -2.2, -2.9, and -2.8 μC/m²K at 65 °C, for K₃V₅O₁₄, Rb₃V₅O ₁₄, and Tl₃V₅O₁₄ respectively. Frequency-dependent polarization measurements confirmed that all of the materials are nonferroelectric, consistent with our first principle density functional theory (DFT) electronic structure calculations. Infrared, UV-vis, thermogravimetric, and differential scanning calorimetry measurements were also performed. Crystal data: K₃V₅O₁₄, trigonal, space group P31m (No. 157), a = 8.6970(16) A, c = 4.9434(19) A, V = 323.81(15), and Z = 1; Rb₃V₅O₁₄, trigonal, space group P31m (No. 157), a = 8.7092(5) A, c = 5.2772(7) A, V = 346.65(5), and Z = 1; Tl₃V₅O₁₄, trigonal, space group P31m (No. 157), a = 8.7397(8) A, c = 5.0846(10) A, V = 336.34(8), and Z = 1.

Full text of DOI:10.1021/ic1008297

6986 Inorg. Chem. 2010, 49, 6986–6993
DOI: 10.1021/ic1008297
A3V5O14 (A = Kþ, Rbþ, or Tlþ), New Polar Oxides with a Tetragonal Tungsten
Bronze Related Structural Topology: Synthesis, Structure, and
Functional Properties
Jeongho Yeon, Sang-Hwan Kim, and P. Shiv Halasyamani*
Department of Chemistry, University of Houston, 136 Fleming Building, Houston, Texas 77204-5003
Received April 27, 2010
Three polar noncentrosymmetric (NCS) oxide materials, A3V5O14 (A=Kþ,Rbþ,orTlþ), have been synthesized by hydrother-
mal and conventional solid state techniques. Their crystal structures and functional properties (second-harmonic generation,
piezoelectricity, and polarization) have been determined. The iso-structural materials exhibit a layered structural topology that
consists of corner-sharing VO4 tetrahedra and VO5 square pyramids. The layers stack parallel to the c-axis direction and are
separated by the Kþ, Rbþ, or Tlþ cations. Powder second-harmonic generation (SHG) measurements using 1064 nm
radiation indicate the materials exhibit moderate SHG efficiencies of ∼100 ꢀ R-SiO2. Additional SHG measurements, that is,
particle size versus SHG efficiency, indicate the materials are type-I phase-matchable. Converse piezoelectric measurements
for K3V5O14, Rb3V5O14, and Tl3V5O14 revealed d33 values of 28, 22, and 26 pm/V, respectively. Pyroelectric measurements,
that is, temperature-dependent polarization measurements, resulted in pyroelectric coefficients of -2.2, -2.9, and -2.8 μC/
m2 K at 65 ꢀC, for K3V5O14, Rb3V5O14, and Tl3V5O14 respectively. Frequency-dependent polarization measurements
3
confirmed that all of the materials are nonferroelectric, consistent with our first principle density functional theory (DFT)
electronic structure calculations. Infrared, UV-vis, thermogravimetric, and differential scanning calorimetry measurements
were also performed. Crystal data: K3V5O14, trigonal, space group P31m (No. 157), a = 8.6970(16) A, c = 4.9434(19) A, V =
˚
˚
˚
˚
323.81(15), and Z = 1; Rb3V5O14, trigonal, space group P31m (No. 157), a = 8.7092(5) A, c = 5.2772(7) A, V = 346.65(5),
˚
˚
and Z = 1; Tl3V5O14, trigonal, space group P31m (No. 157), a = 8.7397(8) A, c = 5.0846(10) A, V = 336.34(8), and Z = 1.
Introduction
varies as a function of temperature.1,6 Thus all ferroelectrics are
pyroelectrics, but the converse is not true. With respect to syn-
thesizing new polar inorganic materials, as well as noncentrom-
metric materials as a whole, a variety of strategies have been
proposed.7-36 We have focused on using cations susceptible to
Polar materials, that is, those that exhibit a macroscopic
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lize in one of 10 polar crystal classes, (1, 2, 3, 4, 6, m, mm2, 3 m,
4 mm, or 6 mm).4 The two most common functional properties
associated with polarity are ferroelectricity and pyroelectricity.
With the former, the observed polarization may be switched in
the presence of an external electrict field. It is this “switchabli-
lity” that is the basis for the technological relevance of ferroelec-
tric materials.5 With the latter, the polarization is not switch-
able; however, the magnitude of the spontaneous polarization
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Published on Web 07/07/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 15, 2010 6987
second-order Jahn-Teller (SOJT) distortions,37-43 octahed-
rally coordinated d0 transition metal cations and cations with a
lone-pair. In doing so, we have synthesized and characterized a
host of polar materials.44-55 When the individual polar poly-
hedra, that is, d0 transition metal octahedra and lone-pair
polyhedra, are aligned, the material exhibits a macroscopic
polarization. Thus, the origin and magnitude of the aforemen-
tioned functional properties are critically dependent on the
individual asymmetric units as well as their alignment in the
crystal structure. It remains a challenge, however, to control the
alignment and to reliably predict the strength of the functional
properties.
A structural topology that is amenable to polarity is the
tetragonal tungsten bronze (TTB) structure.56,57 TTB oxides
have been studied extensively attributable to their fascinating
structural chemistry and technologically relevant physical pro-
perties.56-61 The TTB structure consists of a three-dimensional
corner-sharing framework of MO6 octahedra (M=Ti, Nb, Ta,
W, Fe, Co, etc.) that contain interstitial sites where a variety of
metal cations (alkali, alkaline earth, and lanthanide) may
reside. In this paper we report on the synthesis, structure,
and functional properties of three polar oxides, A3V5O14 (A =
Kþ, Rbþ, or Tlþ), that exhibit a TTB layer-like topology.
K3V5O14 was first reported in 1959 by Bystrom,62 with a more
thorough crystal structure published in 1994.63 With the latter,
however, no physical properties were reported. In a more
recent publication on K3V5O14, powder second-harmonic
generation (SHG) measurements were reported that suggested
a very large SHG efficiency, ∼ 20 ꢀ KDP or ∼800 ꢀ R-SiO2.64
This efficiency, nearly double that of BaTiO3, seemed incon-
sistent with the structural topology and cation coordination
environments. To confirm the SHG efficiency, and investigate
structure-property relationships, we resynthesized K3V5O14
as well as the new materials Rb3V5O14 and Tl3V5O14. We were
unable to synthesize Cs3V5O14 attributable to the stability of
CsVO365 and Cs2V4O14.66
In this paper, we report on the synthesis, structure, and
functional properties of K3V5O14, Rb3V5O14, and Tl3V5O14.
We show that the previously reported SHG efficiency for
K3V5O14 is erroneously high through a careful examination
of its crystal structure. In addition, although all three mate-
rials are polar, they do not exhibit ferroelectric behavior, that
is, their polarization is not switchable. Finally, we discuss
structure-property relationships, as well as present theore-
tical calculations that provide insight into the functional
properties of the reported materials.
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Experimental Section
Reagents. KNO3 (Alfa Aesar, 99%), RbNO3 (Alfa Aesar,
99%), Tl2CO3 (Alfa Aesar, 99.9þ%), and V2O5 (Aldrich,
99þ%) were used as received.
Synthesis. Single crystals of K3V5O14 and Rb3V5O14 were
grown from reactions where 0.120 g (0.247 g) (1.19 mmol (1.67
mmol)) of KNO3 (RbNO3) and 0.180 g (0.253 g) (1.00 mmol (1.40
mmol)) of V2O5 were placed in a platinum crucible. The crucible
was gradually heated to 600 ꢀC in air, held for 24 h, and then cooled
slowly to room temperature (RT) at a rate of 6 ꢀC h-1. For
K3V5O14, red block-shaped crystals were found as a pure phase,
whereas with Rb3V5O14 red block-shaped crystals were recovered in
∼11% yield based upon V2O5. Tl3V5O14 was hydrothermally
synthesized by combining 0.422 g (0.900 mmol) of Tl2CO3, 0.327 g
(1.80 mmol) of V2O5, and 0.251 g (4.05 mmol) of H3BO3 with
4 mL of H2O in a 23 mL Teflon-lined autoclave. The autoclave was
closed, gradually heated to 230 ꢀC, held for 48 h, and cooled slowly
to room temperature at a rate of 6 ꢀC h-1. The mother liquor was
decanted, and the products were recovered by filtration and washed
with excess distilled water and acetone. Red multifaceted crystals,
subsequently determined to be Tl3V5O14, were recovered in ∼20%
yield based on V2O5.
Bulk samples of K3V5O14, Rb3V5O14 and Tl3V5O14 were pre-
pared by conventional solid-state methods. Separate stoichiomet-
ric amounts of KNO3 (0.400 g, 3.96 mmol), RbNO3 (0.584 g, 3.96
mmol), Tl2CO3 (0.928 g, 1.98 mmol), and V2O5 (0.600 g, 3.30
mmol) were thoroughly ground and pressed into pellets. The pellets
were placed in alumina crucibles and were heated to 380 ꢀC in air,
held for 4 days, and then cooled to RT. The pellets were reground
three to five times, heated and cooled as before. The materials were
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134, 52.

6988 Inorganic Chemistry, Vol. 49, No. 15, 2010
Yeon et al.
Table 1. Crystallographic Data for A3V5O14 (A = Rbþ, and Tlþ)
Table 2. Atomic Coordinates and Equivalent Isotropic Displacement Parameters
2
(A ) for A3V5O14 (A= Rbþ, and Tlþ).a
˚
formula
fw
T(K)
crystal system
space group
˚
a (A)
˚
c (A)
Rb3V5O14
735.11
296.0(2)
trigonal
P31m (No. 157)
8.7092(5)
5.2772(7)
90
Tl3V5O14
1091.81
296.0(2)
trigonal
P31m (No. 157)
8.7397(8)
5.0846(10)
90
b
x
y
z
Ueq
Rb3V5O14
Rb(1)
V(1)
V(2)
O(1)
O(2)
O(3)
O(4)
0.6122(1)
0.3333
0.2305(1)
0.3333
0.22526(5)
0.8326(4)
0.1698(3)
0
0.6667
0
0.6667
0.2144(1)
0.6912(2)
0.7067(2)
0
0.0083(7)
0.6470(8)
0.5797(5)
0.022(1)
0.012(1)
0.014(1)
0.021(1)
0.026(1)
0.023(1)
0.017(1)
R (deg)
γ (deg)
120
120
3
V (A )
˚
346.65(5)
1
3.521
13.776
57.98
0.0420
1.109
0.0174
0.0435
0.000(8)
336.34(8)
1
5.390
39.173
57.44
0.0267
1.096
0.0156
0.0309
0
Z
0
0.4610(3)
F
calcd (g/cm3)
μ(mm-1
)
2 θmax (deg)
R (int)
Tl3V5O14
GOF (F2)
Tl(1)
V(1)
V(2)
O(1)
O(2)
O(3)
O(4)
0.6085(1)
0.3333
0.2285(1)
0.3333
0.2464(7)
0.8324(6)
0.1692(4)
0
0.6667
0
0.6667
0.2089(3)
0.6785(3)
0.6869(3)
0
0.024(1)
0.011(1)
0.013(1)
0.024(2)
0.025(1)
0.021(1)
0.017(1)
R(F)a
Rw(Fo
2 b
)
Flack parameter
-0.004(8)
0
0
P
P
P
aR(F) =
||Fo| - |Fc||/ |Fo|. bRw(Fo2) = [ w(Fo - Fc2)2/
2
0
0.4616(5)
0.6188(11)
0.5616(7)
P
w(Fo2)2]1/2
.
a Equivalent atomic coordinates are presented. b Ueq is defined as one-
third of the trace of the orthogonalized Uij tensor.
determined to be pure by powder X-ray diffraction (see Supporting
Information, Figure S1 and Powder Diffraction Section). We also
attempted to synthesize the Liþ, Naþ, and Csþ analogues, but were
unsuccessful. With Liþ and Naþ, it is clear that the 10-coordinate
environment observed in the Kþ, Rbþ, and Tlþ phases (see Crystal
Structure Description) is too large for these cations, whereas for
Csþ a mixture of CsVO365 and Cs2V4O1166 is always found.
Single-Crystal X-ray Diffraction. A red block-shaped crystal
(0.13 ꢀ 0.10 ꢀ 0.07 mm3) of K3V5O14, a red block-shaped crystal
(0.04 ꢀ 0.04 ꢀ 0.04 mm3) of Rb3V5O14, and a red multifaceted
crystal (0.01 ꢀ 0.01 ꢀ 0.01 mm3) of Tl3V5O14 were used for
single-crystal data collection. Data were collected using a
Siemens SMART diffractometer equipped with a 1K CCD area
detector using graphite-monochromated Mo KR radiation. A
hemisphere of data was collected using a narrow-frame method
with scan widths of 0.30ꢀ in ω and an exposure time of 10 s, 50 s,
and 40 s per frame for K3V5O14, Rb3V5O14, and Tl3V5O14,
respectively. The data were integrated using the Siemens SAINT
program,67 with the intensities corrected for Lorentz polariza-
tion, air absorption, and absorption attributable to the variation
in the path length through the detector faceplate. Multiscans for
K3V5O14 and ψ-scans for Rb3V5O14 and Tl3V5O14 were used for
the absorption correction on the hemisphere of data. All of the
data were solved by direct methods using SHELXS-97 and
refined using SHELXL-97.68,69 All of the atoms were refined
with anisotropic thermal parameters and converged for I >
2σ(I ) (Tables 1, 2). All calculations were performed using the
WinGX-98 crystallographic software package.70
UV-vis-NIR spectrophotometer over the spectral range
200-1500 nm at RT. Poly-(tetrafluoroethylene) was used as a
reference material. Reflectance spectra were converted to ab-
sorbance with the Kubelka-Munk function71,72 (see Suppor-
ting Information, Figure S3).
Thermogravimetric Analysis (TGA). Thermogravimetric ana-
lyses were carried out on a TGA 951 thermogravimetric analy-
zer (TA Instruments). The sample was placed into a platinum
crucible and heated under air at a rate of 10 ꢀC min-1 to 900 ꢀC
(see Supporting Information, Figure S4).
Differential Scanning Calorimetry Analysis (DSC). Differen-
tial scanning calorimetry (DSC) analysis was performed on an
EXSTAR 6000 Differential Scanning Calorimeter (SII Nano
Technology Inc.). Approximately 10 mg of the samples were
placed into an aluminum pan covered by an aluminum lid,
heated to 500 ꢀC, and then cooled at a rate of 10 ꢀC min-1 to RT
under flowing air. Alumina was used as the reference during the
measurements (see Supporting Information, Figure S4).
Second Harmonic Generation. Powder SHG measurements
were performed on a modified Kurtz-NLO system73 using a pulsed
Nd:YAG laser with a wavelength of 1064 nm. A detailed descri-
ption of the equipment and methodology has been published
elsewhere.2 As the powder SHG efficiency has been shown to
depend strongly on particle size,73 the reported materials were
ground and sieved into distinct particle size ranges (<20, 20-45,
45-63, 63-75, 75-90, >90 μm). Relevant comparisons with
known SHG materials were made by grinding and sieving crystal-
line R-SiO2 and LiNbO3 intothesameparticlesizeranges. Noindex
matching fluid was used in any of the experiments.
Powder X-ray Diffraction. Powder X-ray diffraction (PXRD)
data of the materials were collected using a PANalytical X’Pert
PRO diffractometer operating with Cu-KR radiation. The data
were taken in the 2θ range of 5ꢀ-90ꢀ with a step size of 0.008ꢀ
and a fixed time of 0.3 s. No impurities were observed, and the
calculated and experimental PXRD patterns are in excellent
agreement (see Supporting Information, Figure S1).
Piezoelectric Measurements. Converse piezoelectric measure-
ments were performed using a Radiant Technologies RT66A piezo-
electric test system with a TREK (model 609E-6) high voltage
amplifier, Precision Materials Analyzer, Precision High Voltage
Interface, and MTI 2000 Fotonic Sensor. The samples were pressed
into pellets (∼1.3 cm diameter, ∼1 mm thick) and sintered at 360 ꢀC
for 7 days. Silver paste was applied to both sides of the pellets, and
the pellets were cured at 200 ꢀC for 12 h. The same pellets were also
used in polarization measurements (see below).
Infrared (IR) Spectroscopy. Infrared spectra were recorded on
a Matteson FT-IR 5000 spectrometer in the 400-4000 cm-1
range (see Supporting Information, Figure S2).
UV-vis Diffuse Reflectance Spectroscopy. UV-vis diffuse
reflectance spectra were collected on a Varian Cary 500 scan
Polarization Measurements. The polarization was measured
on a Radiant Technologies RT66A ferroelectric test system with
a TREK high-voltage amplifier between RT and 165 ꢀC in a
(67) SAINT, Program for Area Detector Absorption Correction, 4.05;
Siemens Analytical X-ray Systems: Madison, WI, 1995.
(68) Sheldrick, G. M. SHELXS-97, A program for automatic solution of
crystal structures; University of Goettingen: Goettingen, Germany, 1997.
(69) Sheldrick, G. M. SHELXS-97, A program for Crystal Structure
Refinement; University of Goettingen: Goettingen, Germany, 1997.
(70) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.
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Article
Inorganic Chemistry, Vol. 49, No. 15, 2010 6989
Delta 9023 environmental test chamber. The unclamped pyro-
electric coefficient, defined as dP/dT (change in polarization
with respect to the change in temperature), was determined by
measuring the polarization as a function of temperature. A
detailed description of the methodology used has been published
elsewhere.2 To measure any possible ferroelectric behavior, the
polarization was measured at RT under a static electric field
from 10-14 kV/cm between 1000-4000 Hz. For the pyro-
electric measurements, the polarization was measured statically
from RT to 165 ꢀC in 20 ꢀC increments, with an electric field of
7-12 kV/cm. The temperature was allowed to stabilize before
the polarization was measured.
Electronic Structure Calculations. First principles density
functional theory (DFT) electronic band structure calculations
were performed using the plane-wave pseudopotential and tight-
binding linear muffin-tin orbital (TB-LMTO) methods. Norm-
conserving Martins-Troullier (MT) pseudopotentials generated
from FHI-98 code were utilized with a generalized gradient approx-
imation (GGA) for the exchange-correlation corrections, as im-
plemented in the Quantum ESPRESSO (4.0.1 version) package.74
A plane wave energy cutoff of 37 Ry was fixed. A Gaussian
smearing of 0.01 Ry for Brillouin zone integrations was adopted
with a 4 ꢀ 4 ꢀ 6 k-point mesh in the unit cell. TB-LMTO
calculations were also performed within the atomic sphere approx-
imation (ASA). The von Barth-Hedin local exchange-correlation
potential was used for the local density approximation (LDA). The
radial scalar relativistic Dirac equation was solved for obtaining the
partial waves. A total of 9 and 10 empty spheres were necessary to
achieve space filling for K3V5O14 and Rb3V5O14, and for Tl3V5O14
respectively. A total of 40 irreducible k-points from 6 ꢀ 6 ꢀ 4 grid
was used for Brillouin zone integrations by tetrahedron method.
The basis set consisted of K-4s/[4p], Rb-5s/[5p]/[4d]/[4f], Tl-6s/
6p/[5d]/[5f], V-3d/4s/4p/ and O-2s/2p orbitals where the orbital in
bracket was treated with the downfolding technique. Total energy
changes of less than 10-5 Ry indicated self-consistency. For all of
the structural figures and electronic structure calculation results, the
program VESTA was used.75
Figure 1. Polyhedral diagram of A3V5O14 in the (a) ab- and (b) bc-planes.
Note that in (b) all of the polyhedra are pointing in the same direction.
environments between the A cation indicate that the lone-
pair electrons on the Tlþ cation are inert, rather than stereo-
chemically active.76,77 This “inertness” of the lone-pair is in
agreement with our DFT and electron localization function
(ELF) calculations (see Electronic structure). Bond valence
calculations78,79 resulted in values between 5.03-5.22 for
vanadium and 1.11-1.17 for the alkali metals and Tlþ cation
(see Table 4). One of the most interesting features of the
A3V5O14 materials is its topological structural relationship
with respect to a layer in the tetragonal tungsten bronze
(TTB) topology. Polyhedral representations of a TTB and
A3V5O14 layer are shown in Figure 2. The “A” cations in both
structures are shown as green spheres, whereas the d0 cation
polyhedra are in blue, with oxygen atoms in red. With both
structures, the “A” cations are surrounded by several poly-
hedra. In the TTB structure, the “A” cation is surrounded by
four or five MO6 octahedra, whereas in the A3V5O14 struc-
ture the analogous cations are surrounded by five polyhedra,
three VO5 square pyramids and two VO4 tetrahedra. As a
result, the TTB structure has triangular, square, and penta-
gonal “channels”, whereas in A3V5O14 only triangular and
pentagonal “channels” are observed (see Figure 2). The lack
of square channels in A3V5O14 is attributable to the occur-
rence of the VO4 tetrahedra. It should be emphasized that the
structural relationships between the TTB and A3V5O14
structures is strictly topological and with respect to the layers
only. The TTB structures are three-dimensional, whereas
A3V5O14 exhibits a two-dimensional crystal structure.
Results and Discussion
Structures. The crystal structure of A3V5O14 (A = Kþ,
Rbþ, and Tlþ) is shown in Figures 1a and 1b. All three
materials exhibit two-dimensional (2D) cationic layers of
(V5O14)3þ that are composed of corner shared VO4 tetrahe-
dra and VO5 square pyramids. The 2D layers stack parallel to
the c-direction, with Aþ cations residing between the layers.
There are two crystallographically unique vanadium sites,
hereafter referred to as V(1) and V(2) that have tetrahedral
and square pyramidal coordination environments respec-
tively. Three corner-shared V(2)O5 square pyramids form a
V(2)3O12 unit that is connected to V(1)O4 tetrahedra that
form the 2D layer. In connectivity terms, the structure may
1- 3-
be written as {2[V(1)O1/1O3/2]0 3[V(2)O1/1O4/2
]
}
with
charge balance maintained by three Aþ (Kþ, Rbþ, or Tlþ)
cations. The local coordination environments of V5þ and Aþ
cations are shown in Supporting Information, Figure S5. The
V(1)-O (V(2)-O) bond distances range between 1.617-
˚
˚
(6)-1.739(3) A (1.600(6)-1.931(4)A). As seen in Supporting
Information, Figure S5, all three Aþ cations have similar
coordination environments. Each Aþ cation is surrounded
by 10 oxygen atoms with the A-O bond distances between
IR Spectroscopy. The IR spectra of K3V5O14, Rb3V5O14,
and Tl3V5O14 reveal V-O vibrations between 400 and
˚
2.813(5)-3.338(6) A (Table 3). The similar coordination
(76) Stoltzfus, M. W.; Woodward, P.; Seshadri, R.; Park, J.-H.; Bursten,
B. Inorg. Chem. 2007, 46, 3839.
(77) Yeon, J.; Kim, S.-H.; Halasyamani, P. S. J. Solid State Chem. 2009,
182, 3269.
(78) Brown, I. D.; Altermatt, D. Acta Crystallogr., Sect. B 1985, B41, 244.
(79) Brese, N. E.; O’Keeffe, M. Acta Crystallogr., Sect. B 1991, B47, 192.
(74) Baroni, S.; Dal Corso, A.; de Gironcoli, S.; Giannozzi, P.; Cavazzoni,
C.; Ballabio, G.; Scandolo, S.; Chiarotti, G.; Focher, P.; Pasquarello, A.;
Laasonen, K.; Trave, A.; Car, R.; Marzari, N.; Kokalj, A. http://www.
quantum-espresso.org/.
(75) Momma, K.; Izumi, F. J. Appl. Crystallogr. 2008, 41, 653.

6990 Inorganic Chemistry, Vol. 49, No. 15, 2010
Yeon et al.
Table 3. Selected Bond Distances (A) for A3V5O14 (A= Kþ, Rbþ, and Tlþ)
˚
K3V5O14
Rb3V5O14
Tl3V5O14
K(1)-O(1) ꢀ 2
K(1)-O(2) ꢀ 2
K(1)-O(2)
2.864(2)
3.156(2)
3.266(5)
2.813(5)
2.873(3)
2.925(3)
1.617(6)
1.739(3)
1.606(5)
1.824(5)
1.917(3)
Rb(1)-O(1) ꢀ 2
2.925(2)
3.162(4)
3.315(4)
2.983(4)
2.990(2)
3.096(2)
1.630(4)
1.741(2)
1.602(4)
1.824(4)
1.924(2)
Tl(1)-O(1) ꢀ 2
2.897(3)
3.179(2)
3.338(6)
2.859(5)
2.925(3)
2.992(4)
1.635(7)
1.747(3)
1.600(6)
1.825(2)
1.931(3)
Rb(1)-O(2) ꢀ 2
Tl(1)-O(2) ꢀ 2
Rb(1)-O(2)
Rb(1)-O(3)
Tl(1)-O(2)
Tl(1)-O(3)
K(1)-O(3)
K(1)-O(4) ꢀ 2
K(1)-O(4) ꢀ 2
V(1)-O(1)
Rb(1)-O(4) ꢀ 2
Rb(1)-O(4) ꢀ 2
V(1)-O(1)
Tl(1)-O(4) ꢀ 2
Tl(1)-O(4) ꢀ 2
V(1)-O(1)
V(1)-O(4) ꢀ 3
V(1)-O(4) ꢀ 3
V(1)-O(4) ꢀ 3
V(2)-O(2)
V(2)-O(2)
V(2)-O(2)
V(2)-O(3) ꢀ 2
V(2)-O(4) ꢀ 2
V(2)-O(3) ꢀ 2
V(2)-O(4) ꢀ 2
V(2)-O(3) ꢀ 2
V(2)-O(4) ꢀ 2
Table 4. BVS, BSI, and GII Values, Dipole Moments, SHG Efficiencies (Relative to R-SiO2), Piezoelectric Responses, and Pyroelectric Coefficients for A3V5O14 (A = Kþ,
Rbþ, and Tlþ)
BVS
dipole moments
V(1)5þ V(2)5þ
piezo.
d33 (pm/V)
pyro. coeff.
(65 ꢀC, μC/m2 K)
compounds
Aþ
V(1)5þ
V(2)5þ
BSI
GII
SHG
3
K3V5O14
Rb3V5O14
Tl3V5O14
1.11
1.13
1.17
5.22
5.14
5.06
5.06
5.05
5.03
0.058
0.058
0.059
0.103
0.085
0.065
1.05
0.90
1.03
1.65
1.98
1.87
130
80
100
28
22
26
-2.2
-2.9
-2.8
1000 cm-1. The short terminal V-O (VdO) vibrations are
observed in the range of 900-980 cm-1, and the bands
occurring at 650-750 cm-1 and 400-450 cm-1 can be
assigned to V-O and V-O-V vibrations, respectively.
These assignments are consistent with those previously
reported.48,80-84 The IR spectra and assignments have been
deposited in the Supporting Information, Figure S2.
UV-vis Diffuse Reflectance Spectroscopy. All the re-
ported compounds, K3V5O14, Rb3V5O14, and Tl3V5O14
are red. The UV-vis spectra reveal that the absorption
energy for all three compounds is approximately 2 eV.
Absorption (K/S) data were calculated from the Kubelka-
Munk function:71,72
2
ð1 - RÞ
2R
K
S
FðRÞ ¼
¼
with R representing the reflectance, K the absorption, and S
the scattering. In a K/S versus E (eV) plot, extrapolating the
linear part of the rising curve to zero provides the onset of
absorption at 2.11 eV, 2.08 eV, and 1.96 eV for K3V5O14,
Rb3V5O14, and Tl3V5O14, respectively. This observation is
consistent with the results from our electronic structure
calculations. For all three materials, the top of the valence
band mainly consists of O-2sp orbitals, whereas the bottom
of the conduction band is composed of contributions from
V(2)-3d orbitals (major) and V(1)-3d and O-2sp orbitals
(both minor). Thus, the optical band gap may be attributed
to ligand-to-metal, that is O-2sp to V(2)-3d, charge trans-
fer. The UV-vis diffuse reflectance spectra for the reported
compounds have been deposited in the Supporting Infor-
mation, Figure S3.
Figure 2. Polyhedral diagrams of the tetragonal tungsten bronze
structure (top) and A3V5O14 (bottom). Note that the A3V5O14 struc-
ture does not have any square “channels” attributable to the VO4
polyhedra.
Thermal Analyses. The thermal behavior of K3V5-
O14, Rb3V5O14, and Tl3V5O14 was investigated using
(80) Devi, R. N.; Vidyasagar, K. J. Chem. Soc., Dalton Trans. 1998, 3013.
(81) Sykora, R. E.; Ok, K. M.; Halasyamani, P. S.; Wells, D. M.;
Albrecht-Schmitt, T. E. Chem. Mater. 2002, 14, 2741.
(82) Frost, R. L.; Erickson, K. L.; Weier, M. L. Spectrochim. Acta, Part A
2004, 60A, 2419.
(83) Frost, R. L.; Erickson, K. L.; Weier, M. L.; Carmody, O. Spectro-
chim. Acta, Part A 2005, 61A, 829.
(84) Sullens, T. A.; Albrecht-Schmitt, T. E. Inorg. Chem. 2005, 44, 2282.
thermogravimetric analysis (TGA) and differential scan-
ning calorimetry (DSC). No weight loss was observed up
to 900 ꢀC for all the materials. DSC data were collected
from RT to 500 ꢀC. The DSC data indicates that K3V5O14
melts congruently. Transitions are observed at ∼418 ꢀC
(∼360 ꢀC) in the heating (cooling) cycle consistent with

Article
Inorganic Chemistry, Vol. 49, No. 15, 2010 6991
melting (recrystallization). With Rb3V5O14, DSC measure-
ments revealed an endothermic peak at ∼402 ꢀC during
heating, indicating decomposition of Rb3V5O14 to RbVO3,
RbV3O8, and Rb3V5O14. During the cooling cycle, a peak
observed at ∼353 ꢀC indicates recrystallization of the mate-
rials after decomposition. The final residues of RbVO3,
RbV3O8, and Rb3V5O14 were confirmed by powder XRD
measurements. For Tl3V5O14, a peak was observed at
∼392 ꢀC during heating that indicated decomposition of
Tl3V5O14 to TlVO3 and TlV3O8. The additional peak at
∼420 ꢀC is thought to be attributable to the melting point of
TlVO3.85 An additional peak at ∼346 ꢀC during cooling
suggested recrystallization of the materials after decomposi-
tion. The final residues of TlVO3 and TlV3O8 were confirmed
by powder XRD measurements. Powder XRD patterns of
the compounds quenched from 450 ꢀC have been deposited
to the Supporting Information (see Supporting Information,
Figure S6).
hysteresis loops, the materials are not ferroelectric.86 In
other words, the macroscopic polarization cannot be
switched under an external electric field. The observed
polarization “loops” are thought to be attributable to
dielectric loss. To understand this irreversibility, the local
polarizations in the materials need to be examined. The
macroscopic polarity in these compounds is attributable
to the local polarity observed in the VO4 and VO5 poly-
hedra. The VO4 polyhedra, however, exhibit much less
polarity, attributable to its tetrahedral coordination envi-
ronment, compared with the VO5 polyhedra. For the mate-
rials to exhibit ferroelectric behavior, the macroscopic pola-
rization must be “switchable”, reversed in the presence of an
external electric field. For d0 transition metals in octahedral
oxide coordination environments, it is possible for the cation
to be switched from one corner, edge, or face to the opposite,
for example, BaTiO3 (corner) and LiNbO3 (face),87,88
in the presence of an external electric field resulting in
ferroelectric behavior. The potentially “switchable” V5þ
cation is in the VO5 square pyramids, as the V5þ cation in
the VO4 tetrahedra is not in a polar coordination environ-
ment. In the VO5 square pyramid, the dipole moment may
be considered switched when the vanadium atom moves
toward to the basal plane. This movement is suggested to be
energetically unfavorable, as the absence of an axial oxide
ligand on the other side of the square pyramid prevents
stabilization of the shifted vanadium atom.
Second-Harmonic Generation. Since K3V5O14, Rb3-
V5O14, and Tl3V5O14 crystallize in the noncentrosymmetric
space group, P31m, SHG measurements were performed.
Powder SHG measurements using 1064 nm radiation
revealed efficiencies of approximately 130, 80, and 100 ꢀ
R-SiO2 for K3V5O14, Rb3V5O14, and Tl3V5O14, respectively,
in the 45-63 μm particle size range. Additional SHG
measurements, SHG efficiency versus particle size (20-125
μm), revealed that all three compounds are type 1 phase-
matchable, and fall into the class C category of SHG
materials.73 On the basis of the SHG efficiency and phase-
matching behavior, the average NLO susceptibility Ædeffæexp
may be estimated. The values are 13.1, 10.3, and 11.5 pm/V
for K3V5O14, Rb3V5O14, and Tl3V5O14, respectively. These
moderate SHG efficiencies may be attributable to alignment
of VO4 and VO5 polyhedra. As seen in Figure 2, the VO4 and
VO5 polyhedra are aligned in the same direction. We suggest
that this alignment is responsible for the observed SHG
efficiencies. It should be noted that although all three
reported materials contain V5þ, the cations do not undergo
second-order Jahn-Teller (SOJT) distortions. SOJT distor-
tions in oxides containing d0 transition metals are observed
when the cation is octahedrally coordinated.43 In K3V5O14,
Rb3V5O14, and Tl3V5O14, the V5þ cations are in tetrahedral
and square pyramidal coordination environments. The ab-
sence of an out-of-center displacement, that is, SOJT distor-
tion, is likely the reason for the moderate, as opposed to
strong, SHG efficiency. We suggest that the earlier reported
Although the reported materials are not ferroelectric, they
do exhibit macroscopic polarization indicating the materials
are pyroelectric. To determine the pyroelectric coefficient, p,
the change in polarization as a function of temperature, was
measured. The pyroelectric coefficients are -2.2, -2.9, and
-2.8 μC/m2 K for K3V5O14, Rb3V5O14, and Tl3V5O14,
3
respectively, at 65 ꢀC. These values are consistent with
other nonferroelectric pyroelectric materials such as ZnO
(-9.4 μC/m2 K) and tourmaline (-4.0 μC/m2 K).1
3
3
Electronic Structure Analysis. We performed electronic
structure calculations on K3V5O14, Rb3V5O14, and Tl3V5O14
using pseudopotential methods. Electronic structure calcu-
lations have been reported for K3V5O14.89 In the published
report, the density of states for a wide range of energies, -30
to þ5 eV, were examined, whereas we focused on energies
above and below the Fermi level, -5 to þ5 eV. With our
calculations, the total and projected density of states, TDOS
and PDOS, respectively, are shown in Figure 3. An energy
gap at the Fermi level, indicative of their semiconducting
behavior, is observed in the TDOS for each material. The
calculated energy gaps are approximately ∼2.3 and 2.2 eV
and 1.8 eV for K3V5O14, Rb3V5O14, and Tl3V5O14 respec-
tively, consistent with the UV-vis diffuse reflectance data.
Overall, their electronic structures are similar. In the PDOSs
(see Figure 3), the top of the valence band is mainly
composed of O-2sp orbitals, whereas the bottom of the
conduction band is composed of V(2)-3d orbitals (major)
and V(1)-3d and O-2sp orbitals (both minor). The larger
contribution of V(2)-3d relative to V(1)-3d is attributable
to the shorter V(1)-O bond compared with V(2)-O.
64
SHG data for K3V5O14 are erroneously high, and are
inconsistent with the structural features of the materials.
Piezoelectric Measurements. Converse piezoelectric
measurements were performed on K3V5O14, Rb3V5O14,
and Tl3V5O14. With each sample, a maximum voltage of
500 V was applied and 20 measurements were done and
averaged. The d33 piezoelectric charge constants of 28, 22,
and 26 pm/V for K3V5O14, Rb3V5O14, and Tl3V5O14 res-
pectively, were estimated. The piezoelectric data have been
deposited in the Supporting Information, Figure S10.
Polarization Measurements. All of the materials are not
only noncentrosymmetric but also polar, that is, they
possess a permanent dipole moment indicating the pos-
sibility of ferroelectric behavior. Although polarization
loops were observed that appear similar to ferroelectric
(86) Scott, J. F. J. Phys.: Condens. Matter 2008, 20, 021001.
(87) Shirane, G.; Danner, H.; Pepinsky, R. Phys. Rev. 1957, 105, 846.
(88) Abrahams, S. C.; Hamilton, W. C.; Reddy, J. M. J. Phys. Chem.
Solids 1966, 27, 1013.
(89) Huang, S. P.; Wu, D. S.; Shen, J.; Cheng, W. D.; Lan, Y. Z.; Li, F. F.;
Zhang, H.; Gong, Y. J. J. Phys.: Condens. Matter 2006, 18, 5535.
(85) Gaplovska, K.; Zurkova, L. J. Therm. Anal. 1981, 20, 463.

6992 Inorganic Chemistry, Vol. 49, No. 15, 2010
Yeon et al.
Figure 4. COHP curves for Tl-O, V(1)-O, and V(2)-O in Tl3V5O14
calculated using the TB-LMTO-ASA method.
Figure 5. Electron localization function (ELF) isosurface with η = 0.9
isshown. Thesphere-likeisosurfacesclearly indicate thatthe lone-pairs on
the Tlþ cations are inert, as opposed to stereoactive.
To examine the lone-pair character on the Tlþ cation,
electron localization function (ELF) calculations were
performed using the TB-LMTO-ASA method. The
ELF visualization with η = 0.9 is shown in Figure 5
where a sphere-like iso-surface is observed at the Tlþ site.
Thus, the lone-pair on Tlþ may be considered inert, as
opposed to stereoactive.76,77 This indicates that the po-
larization associated with the Tlþ is expected to be
negligible. Therefore, the polarization associated with
all the reported materials is wholly attributable to the
vanadium oxide polyhedra.
Local Dipole Moment Calculations. As shown in Sup-
porting Information, Figure S5, we classified the V-O
bonds into apex bonds (V-Oap) along the c-direction and
equatorial bonds (V-Oeq) in the ab-plane. In all three
materials, the V(1)O4 tetrahedron has three longer V(1)-
Oeq bonds and a shorter V(1)-Oap bond. Also, the V(2)O5
square pyramid has four longer V(2)-Oeq bonds and a
shorter V(2)-Oap bond. Thus, the geometric center of
oxygen atoms in each polyhedron is near the vanadium
atom position on each V-Oap bond axis parallel to the
c-axis. Thus, it is expected that the local dipole moments90,91
associated with the V(1)O4 and V(2)O5 polyhedra are small
(see Table 4). Therefore, we anticipate that the macroscopic
Figure 3. Total and projected density of states for (a) K3V5O14,
(b) Rb3V5O14,and(c) Tl3V5O14 calculated using the pseudopotential method.
For Tl3V5O14 in the PDOSs, attributable to the lone-pair, a
small contribution of the Tl-6s orbital is observed at the top
of the valence band, whereas the contribution of the Tl-6p
orbitals is negligible.
The crystal orbital Hamilton population (COHP) ana-
lysis was carried out using the TB-LMTO-ASA method.
Figure 4 shows the -COHP curves for the V(1)-O,
V(2)-O, and Tl-O bonds in Tl3V5O14. Bonding and
antibonding characteristics of the V-O bonds are ob-
served below and above the Fermi level, respectively.
Also, the -COHP curve for the Tl-O reveals the anti-
bonding character near the Fermi level that corresponds
to antibonding Tl(6s)-O interactions. Since the Tl-6p
orbital contribution into the Tl-O bonds is negligible
below the Fermi level, the lone-pair on the Tlþ cation is
anticipated to be highly symmetric, that is, there is
negligible mixing between the Tl-6s and -6p orbitals.
(90) Debye, P. Phys. Z. 1921, 22, 302.
(91) Debye, P. Polar Molecules; Chemical Catalog Company, Inc.: New
York, 1929.

Article
Inorganic Chemistry, Vol. 49, No. 15, 2010 6993
polarization is rather small, consistent with the SHG and
polarization measurements.
Conclusion
Three polar vanadates, K3V5O14, Rb3V5O14, and Tl3V5O14,
were synthesized and characterized. All of the materials
exhibit a layered crystal structure consisting of TTB-like
sheets of corner-shared VO4 and VO5 polyhedra. The alkali
metal or thallium cations are found between the layers. The
materials exhibit moderate SHG efficiencies of ∼100 ꢀ R-
SiO2. The magnitude of the SHG efficiency can be under-
stood by a close examination of the cation coordination
environments. In addition, although all three materials are
polar, none are ferroelectric as the polarity associated with
the V5þ cation, in the VO4 and VO5 polyhedra, cannot be
reversed. Finally, DSC measurements indicate that K3V5O14
melts congruently suggesting that large (>5 mm) single
crystals could be grown. We are currently in the process of
growing such crystals.
BSI and GII Indices. Bond strain index (BSI)92 and
global instability index (GII)93 were calculated (see
Table 4). Values of BSI and GII greater than 0.05 vu
(valence unit) are indicative of structural strain.94 The
BSI and GII values in Table 4 are substantially above
zero indicating the materials exhibit electronic and
lattice induced strains. Interestingly, for all materials,
GII values are bigger than BSI values, suggesting that
the lattice induced strain is stronger than the electronic
induced strain. This is not unexpected as none of the
cations undergo SOJT distortions. It is interesting to
note that although BSI values remain constant among
the materials, the GII values decrease with increasing A
cation size, that is, GII (K3V5O14) > GII (Rb3V5O14) >
GII (Tl3V5O14). This decrease may be attributable to the
differences in the V-O bond distances. The V-O bond
distance, on average, decreases with decreasing A cation
size producing a larger amount of lattice strain thereby
increasing the GII for the smallest A cation material,
K3V5O14.
Acknowledgment. We thank the Robert A. Welch
Foundation (Grant E-1457), the ACS PRF 47345-
AC10, and the NSF (DMR-0652150) for support. We
also thank the Youngstown State S&CIF X-ray Facility
funded by NSF Grant 0087210, Ohio Board of Regents
Grant CAP-491, for the single crystal data.
(92) Preiser, C.; Losel, J.; Brown, I. D.; Kunz, M.; Skowron, A. Acta
Crystallogr., Sect. B: Struct. Sci. 1999, B55, 698.
(93) Salinas-Sanchez, A.; Garcia-Munoz, J. L.; Rodriguez-Carvajal, J.;
Saez-Puche, R.; Martinez, J. L. J. Solid State Chem. 1992, 100, 201.
(94) Brown, I. D. The Chemical Bond in Inorganic Chemistry; Oxford
University Press: New York, 2002; Vol. 12.
Supporting Information Available: Additional information is
provided in Figures S1-S10 and Table S1, and crystallographic
data in CIF format. This material is available free of charge via
the Internet at http://pubs.acs.org.