Engineering Polar Oxynitrides: Hexagonal Perovskite BaWON2

Article abstract of DOI:10.1002/anie.202006519

Non-centrosymmetric polar compounds have important technological properties. Reported perovskite oxynitrides show centrosymmetric structures, and for some of them high permittivities have been observed and ascribed to local dipoles induced by partial order of nitride and oxide. Reported here is the first hexagonal perovskite oxynitride BaWON₂, which shows a polar 6H polytype. Synchrotron X-ray and neutron powder diffraction, and annular bright-field in scanning transmission electron microscopy indicate that it crystalizes in the non-centrosymmetric space group P6₃mc, with a total order of nitride and oxide at two distinct coordination environments in cubic and hexagonal packed BaX₃ layers. A synergetic second-order Jahn–Teller effect, supported by first principle calculations, anion order, and electrostatic repulsions between W⁶⁺ cations, induce large distortions at two inequivalent face-sharing octahedra that lead to long-range ordered dipoles and spontaneous polarization along the c axis. The new oxynitride is a semiconductor with a band gap of 1.1 eV and a large permittivity.

Full text of DOI:10.1002/anie.202006519

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
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Accepted Article
Title: Engineering Polar Oxynitrides: Hexagonal Perovskite BaWON2
Authors: Judith Oró-Solé, Ignasi Fina, Carlos Frontera, Jaume
Gàzquez, Clemens Ritter, Marina Cunquero, Pablo Loza-
Alvarez, Sergio Conejeros, Pere Alemany, Enric Canadell,
Josep Fontcuberta, and Amparo Fuertes
This manuscript has been accepted after peer review and appears as an
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202006519
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Engineering Polar Oxynitrides: Hexagonal Perovskite BaWON2
Judith Oró-Solé,[a] Ignasi Fina,[a] Carlos Frontera,[a] Jaume Gàzquez,[a] Clemens Ritter,[b] Marina
Cunquero,[c] Pablo Loza-Alvarez,[c] Sergio Conejeros,[d] Pere Alemany,[e] Enric Canadell,[a] Josep
Fontcuberta,*[a] and Amparo Fuertes*[a]
Abstract: Non-centrosymmetric polar compounds have important
technological properties. Reported perovskite oxynitrides show
centrosymmetric structures and for some of them high permittivities
have been observed, ascribed to local dipoles induced by partial order
of nitride and oxide. Here we report the first hexagonal perovskite
oxynitride BaWON2, that shows a polar 6H polytype. Synchrotron X-
ray and neutron powder diffraction, and annular bright-field in
scanning transmission electron microscopy indicate that it crystalizes
in the non-centrosymmetric space group P63mc, with total order of
nitride and oxide at two distinct coordination environments in cubic
and hexagonal packed BaX3 layers. Synergetic second order Jahn-
Teller effect supported by first principle calculations, anion order and
electrostatic repulsion between W6+ cations induce large distortions at
two inequivalent face sharing octahedra that lead to long-range
ordered dipoles and spontaneous polarization along the c axis. The
new oxynitride is a semiconductor with a band gap of 1.1 eV and a
large permittivity.
ferroelectricity among other relevant properties.[5,6] Large room-
temperature permittivities (εr) have been reported for BaTaO2N,
SrTaO2N and LaTiO2N, of 320-620,[7] 450[8] and 750
respectively.[9] Piezoelectric measurements showed fingerprints
of ferroelectricity in epitaxial films of SrTaO2N[10] and BaTaO2N
crystals.[11]
All previously reported perovskite oxynitrides show
centrosymmetric structures derived from the Pm-3m cubic
aristotype, with symmetry lowering for Goldschmidt tolerance
factors (t) below 1.[5] Reported tungsten perovskite oxynitrides are
RWO3-xNx (R=rare earth)[12,13] and SrWO2N[14], with t values
between 0.927 and 0.989. In pseudocubic perovskites all anion
sites show similar coordination environments with cations,
favouring total disorder of nitrogen and oxygen or partial order
with local cis configuration of B-N bonds.[15] The origin of
ferroelectricity in tantalum and titanium perovskite oxynitrides is
still under discussion, and large dielectric polarizability is ascribed
to the presence of randomly oriented local dipoles induced by the
cis order of nitrides in the BO4N2 octahedra.[9, 10, 16]
In the quest for polar oxynitrides, a suitable strategy is to
look for structures combining SOJT, octahedral d0 transition
metals and anion positions with well differentiated environments
that could promote total order of N3- and O2-. Hexagonal
perovskites ABX3 formed by sequences of cubic and hexagonal
close packing of AX3 layers show two different environments for
anions; those positioned in hexagonal packed layers are in
sharing faces of the octahedra, while anions in cubic packed
layers are in sharing corners. Face sharing BX6 octahedra often
show distortions because of B-B repulsions. These perovskites
are stabilized for t>1, and the majority of reported compounds are
formed by the large alkali or alkaline earth cations such as Ba2+
at the A site and transition metals at B sites.
Non-centrosymmetric compounds are pivotal in advanced
applications such as actuators and sensors, data storage and
computing,
non-linear
and
non-reciprocal
optics
or
photovoltaics.[1] In the search of non-centrosymmetric structures,
transition metals with d0 configurations in octahedral coordination
have attracted attention because they may suffer distortions
caused by electronic effects -e.g. second order Jahn-Teller effect
(SOJT)-, often showing asymmetric environments and off-center
displacements that lead to large permittivities and polar
materials.[2] Anisotropic bond network, electrostatic repulsions
and lattice stress reinforce the structural distortions,[3] and the
existence of different anions in an ordered arrangement provide
further sources of bond asymmetries and the possibility to create
permanent dipoles and acentric structures.[4]
The high charge of anion nitride stabilizes high oxidation
states of the transition metal, and the combination of Ba2+ with
highly charged small cations such as W5+ or W6+ predicts large
tolerance factors, i.e. 1.052 and 1.058 for BaWO2N or BaWON2
respectively, compatible with a hexagonal perovskite. Here we
report the new compound BaWON2 that is the first example of a
hexagonal perovskite oxynitride. It is non-centrosymmetric and
shows the 6H polytype, formed by cubic (c) packed BaN3 layers
and hexagonal (h) packed BaO3 layers stacked in the sequence
cchcch (Figure 1a). The new perovskite is a semiconductor, with
band gap of about 1.1 eV, and shows a high permittivity ascribed
to the presence of inequivalent octahedral B sites with distortions
induced by SOJT effect and electrostatic repulsions between W6+
cations.
Black colored powder samples of BaWON2 were prepared
starting either from mixtures of stoichiometric amounts of BaCO3
and WO3 or from BaWO4, by treatment under flowing NH3 at 750-
800 oC. Experimental details are reported in the Supporting
Information (SI). EDX analyses gave Ba:W ratios between 0.97
and 1.04, and N contents determined by combustion analysis
Perovskite oxynitrides ABO2N or ABON2 (A=rare earth or
alkaline earth metal; B=transition metal) are important
heteroanionic materials that show high permittivities and
[a]
Dr. J.Oró-Solé, Dr. I.Fina, Dr. C.Frontera, Dr. J.Gàzquez, Prof.
E.Canadell, Prof. J.Fontcuberta, Prof. A.Fuertes
Institut de Ciència de Materials de Barcelona(ICMAB-CSIC)
Campus UAB, 08193 Bellaterra(Spain)
E-mail: fontcuberta@icmab.cat
E-mail: amparo.fuertes@icmab.es
[b]
[c]
Dr. C.Ritter
Institut Laue-Langevin, 71 Av. de Martyrs, Grenoble 38000(France)
M.Cunquero, Dr. P.Loza-Alvarez
ICFO-Institut de Ciències Fotòniques, The Barcelona Institute of
Science and Technology, Castelldefels (Spain)
Dr. S.Conejeros
Departamento de Química, Universidad Católica del Norte, Av.
Angamos 0610, Antofagasta 124000(Chile)
Prof. P.Alemany
[d]
[e]
Departament de Ciència de Materials i Química Física and Institut
de Química Teòrica i Computacional (IQTCUB), Universitat de
Barcelona, Martí i Franquès 1, Barcelona 08028(Spain)
Supporting information for this article is given via a link at the end of
the document.
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Figure 2. Rietveld fits to room temperature a) SXRD and b) ND patterns in
P63mc space group for BaWON2 and BaWO1.11N1.89 respectively. The inset in
(a) shows the high Q region enlarged.
N/O ratio of this sample, and also showed better agreement
factors in the P63mc space group (RBragg=2.89%, Rwp=2.11%)
than in P63/mmc (RBragg=3.36%, Rwp=2.22%). N/O occupancies in
P63mc model were refined independently for the three 6c anion
sites, resulting in an ordered distribution with full 1.0 oxygen
occupancy at the site in hexagonal packed layers (X1), and N/O
occupancies of 0.96(3)/0.04 and 0.94(3)/0.06 at the two sites of
cubic layers X2 and X3 respectively. The refined N stoichiometry
was 1.90, in excellent agreement with the chemical analysis of the
sample.
Figure 1. Crystal structure of BaWON2: a) Projection along a-axis showing the
sequence of hexagonal (h) and cubic (c) close packed BaO3 and BaN3 layers
respectively in the 6H polytype. b) Coordination environment of W atoms.
were within 1.89-2.13 atoms per formula unit. N excess from the
stoichiometric content in some samples can be ascribed to
residual nitrogen-rich impurities not detected by X-ray powder
diffraction. Rietveld refinement to synchrotron X-ray powder
diffraction (SXRD) data was performed for the BaWON2
composition using the structural models of the 6H hexagonal
perovskite in the non-centrosymmetric P63mc (No 186) and in
centrosymmetric P63/mmc (No 194) space groups, that are
compatible with the reflection conditions determined by electron
diffraction(See SI). The refinement in P63mc (Figure 2a, Table S1)
lead to cell parameters a=5.85073(2) and c=14.53220(9) Å with
agreement factors RBragg=3.90% and Rwp=6.19%, whereas
refinement using the space group P63/mmc lead to poorer
residuals, RBragg= 4.41% and Rwp= 6.47%. Refinement to neutron
diffraction data (ND) of a ≈300 mg sample with 1.89 nitrogens per
formula (Figure 2b, Table S3) was performed to investigate the
anion distribution. This lead to smaller cell parameters (a=
5.84618(11), c= 14.3845(6) Å), which is consistent with the lower
The
P63mc
structure
of
BaWON2
has
three
crystallographically independent positions for W in octahedral
environments with distortions along the C3 axis, located between
hexagonal BaO3 and/or cubic BaN3 packed layers (Figure 1). W2
and W3 are placed at the face-sharing octahedra WO3N3 with fac
configuration of nitrides and oxides (i.e. 3 N and 3 O occupying
respectively two faces of the octahedron), and W1 is placed at the
corner sharing octahedra WN6. The WO3N3 octahedra show
larger distortions (Δd as defined in [2] is 1.47 for W2 and 0.82 for
W3) than that of the WN6 octahedron (Δd=0.18), as
a
consequence of the repulsion between highly charged W6+
cations along the shared face, separated 3.161(8) Å. The
distortion in the WO3N3 octahedra is enhanced by SOJT effect -
also affecting the WN6 octahedra- and by the higher covalency of
the W-N bond compared to W-O bond. The preferred occupation
of the more electronegative O atom at face-shared positions may
be understood considering the weaker bonding for these sites,
2
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Figure 3. a) Atomic resolution contrast-inverted ABF image of BaWON2 viewed along a-axis. The superimposed structure corresponds to the P63mc model refined
from SXRD. b) Intensity profiles drawn in a) along W2-O1 and W3-O1 directions. Vertical dashed lines mark the maximum of the intensity profile of O1 in each
direction.
with W-X-W angles ≈90°, compared with corner-shared sites, with
W-X-W angles≈180°.[17] The spontaneous polarization, calculated
from the coordinates and formal electrical charges of all atoms, is
≈17.7 µC/cm2, parallel to the c axis. This value is lower than that
estimated for MnTaO2N (55 µC/cm2) with LiNbO3-type
structure.[18]
High-resolution aberration corrected annular bright field
(ABF) images in scanning transmission electron microscopy
allowed to map the position of all atoms including O and N
columns in real space. The contrast-inverted ABF image of
BaWON2 along a-axis (Figure 3a) nicely matches the
superimposed P63mc structure determined from SXRD data.
Oxygen atoms (O1) at h layers are shifted along c above Ba1
planes (horizontal dashed line), evidencing the acentric nature of
the structure. Figure 3b, depicting the projected distances along
W-O directions, clearly shows that W2-O1 bond (red profile) is
longer than W3-O1 (blue profile), as determined from powder
diffraction data (Figure 1b, Tables S1-S3).
micrometers) regions of the particles. Propagation through thicker
sections would result in absorption of both fundamental and
frequency doubled light. To collect the recorded images, the
required laser power was relatively large (~100 mW), which
sometimes seemed to compromise the integrity of the sample.
The structure of P63mc and P63/mmc models of BaWON2
was optimized using DFT calculations with the HSE06 functional
[20] (details are reported in the SI). The P63mc structure was found
to be 3.4 kcal/mol more stable than P63/mmc. Whereas the
centrosymmetric structure is semimetallic (Figure 5b), a band gap
(indirect) of 0.64 eV opens in the polar structure (Figure 5a)
because the two bands labeled with an asterisk (and degenerate
at Γ) are pushed up when the mirror plane relating the face-
sharing W octahedra (labelled W2 in the P63/mmc model) is lost.
These two bands are made of an out-of-phase combination of the
pair of e-type d orbitals originating from the t2g set of the distorted
W2 octahedra (see in the inset in Figure 5b the illustration of one
of these two equivalent bands at Γ). The in-plane O px and py
orbitals can only mix into these bands in the polar structure, while
the mirror plane forbids mixing in the centrosymmetric structure.
Since these bands are W-O antibonding and the W-O bonds are
already long in the centrosymmetric structure, the stabilization
The capability of BaWON2 to generate a second harmonic
generation (SHG) signal was assessed using
a tunable
Ti:sapphire ultrashort pulse laser as excitation source (details are
reported at the SI) coupled to a nonlinear microscope. BaWON2
powder was mixed with starch (wheat) that has been reported as
an ideal SHG signal indicator. [19] Signals from the sample are
filtered in the wavelength range corresponding to the SHG signal
generated with the laser, using a filter centered at 405 nm with a
bandwidth of 10 nm. Under these conditions, we were able to
obtain a signal of BaWON2 that nicely mimics that generated by
the reference sample (Figure 4a), suggesting that it arises from
SHG conversion process. Recorded images were obtained from
selected regions of the observed particles (Figures 4b and 4c).
The fact that only regions of the particles could be imaged may
result from their polycrystalline nature, implying that not all
crystallites have the polar axis aligned with the polarization
direction of the incoming light. In addition, this may also imply that
even if the polar axis is properly aligned, the sample itself, being
black, highly absorbs the excitation or the generated wavelengths
at different regions. Therefore, the SHG signal may only come
from thin (smaller than one coherent length, i.e. less than a few
Figure 4. a) SHG signals obtained from BaWON2 (blue) and starch granules
(red) by scanning the excitation wavelength along the spectra of the Ti:sapphire
laser. b) Transmission image of the starch granules (white arrow-heads) mixed
with BaWON2 powder. The regions analysed are marked in red and blue. c)
Image obtained by using a filter centred at 405 nm.
3
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Figure 6. Temperature dependence in BaWON2 of: (a) electrical resistivity ρ(T)
(left axis) and ρ(T-1/4) (right axis); (b) permittivity εr(T) and losses (tan δ)
recorded at various frequencies. Optical absorption Tauc plots for (c) indirect
and (d) direct transitions.
consistent with a narrow band gap semiconducting material. The
εr value (≈ 200–230 in the 10-100 K temperature range), is in line,
although somewhat smaller, with values reported for SrTaO2N. [21]
The semiconducting behavior indicated by data in figures 6a and
6b is compatible with the observed black color of the samples.
Optical absorption spectroscopic measurements were conducted
on BaWON2 powders diluted in KBr. Tauc plots for indirect and
direct band absorption (Figures 6c and 6d) show that indirect
transitions rule optical absorption, and the optical band gap is
about 1.1 eV. Both observations agree reasonably with DFT
predictions.
To summarize, we have shown that compositional and
steric design allows obtaining the new polar oxynitride BaWON2,
where oxide and nitride are ordered in two distinct coordination
environments of the 6H hexagonal perovskite structure. Anion
order, together with electrostatic repulsion between W6+ cations,
leads to anisotropic M-X bonds and high distortions of the face-
sharing octahedra. Electronic effects are found to promote a
second order Jahn-Teller distortion that lowers the energy of the
BaWON2 lattice, favoring a non-centrosymmetric structure by
suppressing a mirror plane. As a result, a gap opens in the
electronic band structure and a semiconducting ground state
emerges, as confirmed by optical and electron transport data. The
observed remarkably large permittivity is consistent with
anisotropic, multi-anionic bond network in the structure, which in
turn shall promote anharmonicity of the bond dynamics and
subsequently the observed non-linear optical response. This
achievement contrasts with earlier results on ABO2N perovskites
with pseudocubic centrosymmetric structures that lead to large
dielectric permittivity and piezoresponse under electric field bias,
ascribed to field-response of nanopolar regions, but without long
range polar order. The relatively smaller permittivity of polar
BaWON2 compared to that of BaTaO2N is also consistent with this
view. These findings show the potentiality of hexagonal
Figure 5. DFT band structure for optimized P63mc (a) and P63/mmc (b) crystal
structures of BaWON2. The inset in b) shows the nature of one of the two
equivalent (at Γ) crystal orbitals in the P63/mmc model, noted with an asterisk.
afforded by the lengthening of one of these bonds is more than
compensated by the destabilization due to the strengthening of
the other bond. This is why the two bands are destabilized in the
polar structure leading to a gap opening. These bands are empty
and thus do not contribute to the total energy of the system, but
exactly for the same reasons their two bonding counterparts
(occurring within the sea of the occupied orbitals) are stabilized
when the symmetry plane disappears and the O px and py orbitals
mix into, thus increasing the W-O bonding. This second-order
Jahn-Teller effect is thus responsible for the preference for the
polar structure.
The temperature-dependent electrical resistivity ρ(T) of a
sintered pellet of BaWON2 (Figure 6a) indicates a semiconductor-
like behavior, with ρ(300 K) ≈ 40 Ωcm and resistivity rapidly
increasing upon lowering the temperature (ρ(90 K) ≈ 170 Ωcm).
ρ(T) is better described by variable range hopping model (VRH)
than by an Arrhenius law, suggesting that the motion of thermally
activated carriers is limited by defects, prompting some
localization and subsequent thermally stimulated hopping (VRH).
The dielectric properties have been determined by impedance
spectroscopy. Frequency-dependent permittivity (Figure 6b)
shows that εr decreases when increasing frequency, signaling the
contribution of a leakage-related resistive channel in the RC
(resistance-capacitance) equivalent circuit of the measured
sample. The strong frequency-dependent permittivity at high
temperature is fully consistent with the corresponding ρ(T) data.
The reduction of losses (tan δ) observed upon lowering
temperature or increasing frequency (Figure 6b) is also fully
perovskite oxynitrides as
a new group of heteroanionic
compounds to explore properties by considering different cation
compositions, in particular for the search of polar materials, but
also for important applications of oxynitrides such as visible light-
active photocatalytic activity.
4
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Acknowledgements
This work was supported by the Ministerio de Ciencia e
Innovación, Spain (MAT2017-86616-R, MAT2017-85232-R,
SEV-2015-0496, MDM-2017-0767, PGC2018-096955-B-C44
and PGC2018-093863-B-C22) and from Generalitat de Catalunya
(2017SGR1377,
2017SGR581,
2017SGR1506
and
2017SGR1289). We thank Alba and ILL (experiments
2018082958 and 5-23-721, DOI: 10.5291/ILL-DATA.5-23-721) for
beam time provision, and Dr. François Fauth for assistance in
data collection at Alba. I.F and J.G. acknowledge the Ramon y
Cajal contracts RYC-2017-22531 and RYC-2012-11709. S.C.
was supported by FONDECYT, Chile (project 111071063). ICTS-
CNME at UCM is acknowledged for offering access to STEM and
expertise.
Keywords: oxynitrides • perovskite phases • polar materials•
transition metals • hexagonal perovskites
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5
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Entry for the Table of Contents
N
Ba
O
W
The new 6H hexagonal perovskite oxynitride BaWON2 is a non-centrosymmetric polar compound that shows total order of nitrogen
and oxygen in two distinct coordination environments. The combination of anion order, electrostatic repulsion between W6+ cations
and second order Jahn-Teller effect induce different distortions at three inequivalent tungsten octahedra leading to long range
ordered dipoles and spontaneous polarization along the c axis.
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