Masayuki Fukuda

Ikuya Yamada

Hidenobu Murata

Hajime Hojo

Olivier J Hernandez

Clemens Ritter

Katsuhisa Tanaka

Koji Fujita

Perovskite-Type CuNbO 3 Exhibiting Unusual Noncollinear Ferrielectric to Collinear Ferroelectric Dipole Order Transition

We report a perovskite oxide with a new type of polar structure, CuNbO 3 , thanks to the use of high-pressure and high-temperature synthesis. The perovskite-type CuNbO 3 was previously obtained under high-pressure and hightemperature conditions, but its crystal structure has not been solved yet. Our structural analysis reveals that CuNbO 3 perovskite crystallizes in a polar monoclinic space group (Pc) with a a pc  a pc  2a pc unit cell (a pc being the pseudo-cubic 2 2 lattice parameter), which is one maximal non-isomorphic subgroup of the polar rhombohedral space group R3c. This compound exhibits a remarkable "noncollinear ferrielectric" structure, due to parallel displacements of Cu + and antiparallel displacements of Nb 5+ along different axes, representing a new type of polar phase in the perovskite structure. We also observe that the noncollinear ferrielectric Pc structure transforms around 470 K into the collinear ferroelectric R3c structure that features parallel displacements of Cu + and Nb 5+ in the same direction. The present work extends the accessible composition range of the perovskite niobate series and demonstrates the role of A-O covalency in determining their crystal structure.

INTRODUCTION

Perovskite niobates, ANb 5+ O 3 (A = Li, Na, K, and Ag), constitute an important class of oxide materials, exhibiting a wide range of properties including piezoelectricity, ferroelectricity, antiferroelectricity, and electro-optical and nonlinear optical effects. [START_REF] Saito | Lead-free piezoceramics[END_REF][START_REF] Zuo | Sintering and electrical properties of lead-free Na 0.5 K 0.5 NbO 3 piezoelectric ceramics[END_REF][START_REF] Wu | Potassium-Sodium Niobate Lead-Free Piezoelectric Materials: Past, Present, and Future of Phase Boundaries[END_REF][START_REF] Cohen | High-Q microphotonic electro-optic modulator[END_REF] Recently, they also have attracted much attention due to their excellent photocatalytic and photovoltaic properties. [START_REF] Liu | Hydrothermal synthesis and photocatalytic properties of ATaO 3 and ANbO 3 (A = Na and K)[END_REF][START_REF] Shi | Photophysical and photocatalytic properties of ANbO 3 (A=Na, K) photocatalysts[END_REF][START_REF] Huan | Enhanced ferro-photocatalytic performance for ANbO 3 (A = Na, K) nanoparticles[END_REF] The lattice instabilities and structural distortions of perovskite oxides are often discussed in terms of the tolerance factor, t = R A-O / (R B-O ), where R A-O and R B-O are 2 the ideal A-O and B-O bond lengths for a particular ABO 3 material, respectively, calculated using the bond valence model [START_REF] Brese | Bond-Valence Parameters for Solids[END_REF] . For example, KNbO 3 is ferroelectric with t > 1 (t = 1.06), displaying so-called B-site driven ferroelectricity. [START_REF] Shirane | Phase Transitions in Ferroelectric KNbO 3[END_REF][START_REF] Postnikov | Ferroelectric structure of KNbO 3 and KTaO 3 from first-principles calculations[END_REF][START_REF] Kawamura | Electronic Polarization in KNbO 3 Visualized by Synchrotron Radiation Powder Diffraction[END_REF] The B-site cation (i.e., Nb 5+ ) is nominally too small for its octahedral site and can off center. In the room-temperature phase of KNbO 3 , second-order Jahn-Teller distortions [START_REF] Cohen | Origin of ferroelectricity in perovskite oxides[END_REF][START_REF] Kunz | Out-of-Center Distortions around Octahedrally Coordinated d 0 Transition Metals[END_REF][START_REF] Ok | Distortions in Octahedrally Coordinated d 0 Transition Metal Oxides: A Continuous Symmetry Measures Approach[END_REF] for Nb 5+ (4d 0 ) ions yield their off-center displacements along the pseudo-cubic <110> pc axis, leading to an orthorhombic ferroelectric structure (space group Amm2). Conversely, LiNbO 3 is ferroelectric with t < 1 (t = 0.85), exhibiting A-site driven ferroelectricity. [START_REF] Abrahams | Ferroelectric Lithium Niobate. 3. Single Crystal X-Ray Diffraction study at 24°C[END_REF][START_REF] Weis | Lithium Niobate: Summary of Physical Properties and Crystal Structure[END_REF] It can be described by a grossly distorted perovskite-type structure. Generally, perovskites with t < 1 are not ferroelectric at all, and instead find favorable bond lengths through tilts of BO 6 octahedra about various axes, which preserve the inversion symmetry. The notable exceptions, including LiNbO 3 , are perovskites with very small t values, where a combination of the out-of-phase tilts of BO 6 octahedra around the [111] pc axis (a -a -a -tilt system in Glazer notation [START_REF] Glazer | The Classification of Tilted Octahedra in Perovskites[END_REF] ) and polar displacements of Asite cations along the same axis yield a rhombohedral ferroelectric structure (space group R3c).

Since the t values of NaNbO 3 (t = 0.97) and AgNbO 3 (t = 0.97) are slightly less than 1, the coexistence of off-center Nb displacements and NbO 6 octahedral tilts provides complex crystal structures and phase diagrams. [START_REF] Megaw | The seven phases of sodium niobite[END_REF][START_REF] Mishra | Competing antiferroelectric interaction in NaNbO 3 : Neutron diffraction and theoretical studies[END_REF][START_REF] Koruza | Phase transitions of sodium niobate powder and ceramics, prepared by solid state synthesis[END_REF][START_REF] Johnston | The Polar Phase of NaNbO 3 : A Combined Study by Powder Diffraction, Solid-State NMR, and First-Principles Calculations[END_REF][START_REF] Fábry | Silver niobium trioxide, AgNbO 3[END_REF][START_REF] Sciau | Structural investigation of AgNbO 3 phases using x-ray and neutron diffraction[END_REF][START_REF] Fu | AgNbO 3 : A lead-free material with large polarization and electromechanical response[END_REF][START_REF] Yashihma | Structure of Ferroelectric Silver Niobate AgNbO 3[END_REF] The room-temperature phase of NaNbO 3 is characterized by a sequence of two octahedral tilts system (a -a -c + /a -a -c -), and adopts an orthorhombic antiferroelectric structure (space group Pbcm), where the displacements of Nb 5+ ions as well as of Na + ions are ordered in an antiferroelectric way and thereby the respective dipole moments are cancelled within the unit cell. [START_REF] Sakowski-Cowley | The Structure of Sodium Niobate at Room Temperature, and the Problem of Reliability in Pseudosymmetric Structures[END_REF][START_REF] Johnston | The Polar Phase of NaNbO 3 : A Combined Study by Powder Diffraction, Solid-State NMR, and First-Principles Calculations[END_REF] Despite the similar ionic radii (twelvecoordinated) of Ag + (1.37 Å) and Na + (1.39 Å), AgNbO 3 crystallizes in a different structure from NaNbO 3 . Recent A c c e p t e d M a n u s c r i p t studies have revealed that AgNbO 3 is ferroelectric at room temperature, forming an orthorhombic ferroelectric structure (space group Pmc2 1 ). [START_REF] Yashihma | Structure of Ferroelectric Silver Niobate AgNbO 3[END_REF] The Pmc2 1 AgNbO 3 exhibits the same octahedral tilts system as the Pbcm NaNbO 3 , but their cation displacements are ordered in a different way. In the case of AgNbO 3 , both displacements of Ag + and Nb 5+ ions are ordered in a ferrielectric way, and thus the respective dipole moments are not completely cancelled within the unit cell. The uncompensated ionic displacements lead to a net polarization and weak ferroelectricity. [START_REF] Fu | AgNbO 3 : A lead-free material with large polarization and electromechanical response[END_REF][START_REF] Yashihma | Structure of Ferroelectric Silver Niobate AgNbO 3[END_REF] Here, it should be noted that the ordered dipole moments from the displacements of Ag + and Nb 5+ ions are collinear to the <100> pc axis. Namely, AgNbO 3 adopts a collinear ferrielectric structure. Yashima and Matsuyama pointed out that the formation of ferrielectric structure is ascribed to covalency of Ag-O bonds as well as of Nb-O bonds; the covalent character arises from the Ag 4d-O 2p and Nb 4d-O 2p orbital overlaps. [START_REF] Yashima | Origin of the Ferrielectricty and Visible-Light Photocatalytic Activity of Silver Niobate AgNbO 3[END_REF] The Ag-O and Nb-O covalencies are also responsible for visible photocatalytic properties of AgNbO 3 .

Given the versatility and functionality of perovskite niobates, it is interesting to extend their compositional space to other systems. In this study, Cu + -based perovskite niobate, CuNbO 3 , is targeted because of the expectation that as in the case of AgNbO 3 , the covalent A-O and B-O bonding due to Cu 3d-O 2p and Nb 4d-O 2p orbital overlaps would result in an unusual electric dipole order. However, although ANbO 3 (A = Li, Na, K, and Ag) can be synthesized under ambient conditions, CuNbO 3 crystallizes in a nonperovskite structure in such conditions; the ambientpressure phase of CuNbO 3 (AP-CuNbO 3 ) adopts a RbTaO 3type structure with a nonpolar monoclinic C2/m symmetry. [START_REF] Joshi | Photoelectrochemical Investigation and Electronic Structure of a p-Type CuNbO 3 Photocathode[END_REF][START_REF] Harb | Screened coulomb hybrid DFT investigation of band gap and optical absorption predictions of CuVO 3 , CuNbO 3 and Cu 5 Ta 11 O 30 materials[END_REF] AP-CuNbO 3 is characterized by a corner-and edge-sharing NbO 6 lamellar structure, in contrast to the perovskite structure with corner-sharing NbO 6 octahedra. Utilizing an ion exchange in AP-CuNbO 3 , a new form of LiNbO 3 with a lamellar structure was synthesized for the electrochemical applications. [START_REF] Pralong | A New Form of LiNbO 3 with a Lamellar Structure Showing Reversible Lithium Intercalation[END_REF] The perovskite instability in CuNbO 3 is partly ascribed to the small Cu + ions on the Asites, leading to a small t value (t = 0.89). We show here that the powerful high-pressure synthesis allows us to isolate a polar CuNbO 3 perovskite. The application of high pressure stabilizes phases that are inaccessible metastable products under ambient pressures. In particular, new perovskites and related oxides have emerged through exploration of the high-pressure and high-temperature phases of ABO 3 ternary systems. [START_REF] Rodgers | High-pressure / High-temperature Synthesis of Transition Metal Oxide Perovskites[END_REF][START_REF] Belik | High-pressure synthesis, crystal chemistry and physics of perovskites with small cations at the A site[END_REF] In 1970, Sleight et al. reported the synthesis of a monoclinic perovskite-type CuNbO 3 at 6.5 GPa and 1273-1473 K, but no space group was unambiguously identified. [START_REF] Sleight | Preparation of CuNbO 3 and CuTaO 3 at high pressure[END_REF] Clearly, the crystal structure of high-pressure-synthesized CuNbO 3 (HP-CuNbO 3 ) has remained elusive so far.

Herein, we present a comprehensive structural characterization of the metastable perovskite phase of HP-CuNbO 3 using X-ray, neutron, and electron diffraction as well as optical second harmonic generation, combined with first-principles lattice dynamics calculations. The results show that HP-CuNbO 3 crystallizes in a monoclinic perovskite-type structure (space group Pc), featuring an unusual electric dipole order that represents a new type of polar phase in the perovskite structure. The perovskitetype Pc phase is a weakly polar material due to a "noncollinear ferrielectric" dipole order, i.e., a combination of parallel displacements of Cu + ions along [111] pc with respect to the closed-packed oxygen planes and antiparallel displacements of Nb 5+ ions along <100> pc with respect to the octahedral centroids. We also observe that, upon heating to 470 K in ambient pressure, the weakly polar Pc phase transforms into the strongly polar R3c phase with parallel displacements of Cu + and Nb 5+ ions. Our results highlight that the bonding nature of A-site cations, as well as their ionic size, has great impact on electric dipole order in small-tolerance-factor perovskites.

EXPERIMENTAL AND COMPUTATIONAL DETAILS

Polycrystalline samples of CuNbO 3 were prepared by the solid-state reaction at high pressure and high temperature. Stoichiometric amounts of the starting materials, Cu 2 O (99.9%, Rare Metallic) and Nb 2 O 5 (99.99%, Sigma-Aldrich) were ground and mixed carefully. The mixed powder was placed into a Pt capsule and put into a high-pressure cell. The solid-state reaction was performed at 12 GPa and 1273 K for 30 min with a Walker-type high-pressure apparatus, followed by a rapid temperature quench and then a gradual pressure release.

Electron diffraction (ED) patterns were taken from single-crystal domains at room temperature using a JEM-ARM200F (JEOL) transmission electron microscope. Synchrotron X-ray diffraction (SXRD) data were recorded at room temperature (λ = 0.75003 Å) and in the temperature range of 300 to 680 K (λ = 0.77565 Å) on the BL02B2 beamline at SPring-8. The powder sample was loaded into a Lindemann glass capillary with an inner diameter of 0.1 mm, continuously rotated during the measurement to reduce the effect of preferential orientation. Neutron powder diffraction (NPD) data were collected at room temperature on the D2B instrument at the Institut Laue-Langevin, Grenoble (λ = 1.594 Å). A 475.7-mg sample was encased in a vanadium can. The crystal structure was refined using the Rietveld analysis programs Fullprof 34 and JANA2006. [START_REF] Petříček | Crystallographic Computing System JANA2006: General features[END_REF] In the former case, a Thompson-Cox-Hastings pseudo-Voigt profile function was used, as well the Berar & Baldinozzi model for asymmetry correction. Anisotropic broadening was taken into account through spherical harmonics coefficient (size effect) or generalized S HKL strain parameters. The width of few isolated reflections was additionally selectively relaxed. An absorption correction A c c e p t e d M a n u s c r i p t was applied for the SXRD refinement. DICVOL06 [START_REF] Louër | Powder pattern indexing and the dichotomy algorithm[END_REF] was used to index the diffraction pattern. The structure solution was carried out through the direct method with EXPO2013. [START_REF] Altomare | a kit of tools for phasing crystal structures from powder data[END_REF] The crystal structures were drawn using the program VESTA [START_REF] Izumi | VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data[END_REF] based on the structural refinement results. The cation ratio within the sample was determined with inductively coupled plasma optical emission spectroscopy (ICP-OES) on a SPECTROBLUE (SPECTRO).

To investigate the presence of noncentrosymmetry, a second-harmonic generation (SHG) response was measured at room temperature using a pulsed Nd:YAG laser (λ, 1064 nm; pulse duration, 25 ps; repetition frequency, 10 Hz) as the light source. The SHG light from the sample was detected with a photomultiplier tube through a 532-nm narrow band-pass filter. X-ray absorption near edge structure (XANES) spectroscopy at Cu and Nb K-edges were collected by the transparent method on the BL14B2 beamline at SPring-8. The magnetic susceptibility data were recorded using a SQUID magnetometer MPMS-XL (Quantum Design) between 5 and 300 K under an applied field of 100 Oe.

Electronic and crystal structure of CuNbO 3 and related compounds were calculated within the framework of density functional theory using the projector augmentedwave (PAW) method [START_REF] Blöchl | Projector augmented-wave method[END_REF] implemented in the Vienna Ab-initio Simulation Package (VASP) code. [START_REF] Kresse | Ab intio molecular dynamics for openshell transition metals[END_REF][START_REF] Kresse | Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set[END_REF][START_REF] Kresse | From ultrasoft pseudopotentials to the projector augmented wave method[END_REF] The cut-off energy of plane waves set to 600 eV. The k-points sampling was performed by Γ-centered Monkhorst-Pack 43 4×4×3, 4×4×4, and 6×6×6 meshes for Pc perovskite-type and R3c LiNbO 3type CuNbO 3 and Cu 2 O, respectively. Exchangecorrelational functional were evaluated with the PBEsol functional. [START_REF] Perdew | Restoring the Density-Gradient Expansion for Exchange in Solids and Surfaces[END_REF] The effect of Hubbard U was considered using Dudarev's approach. [START_REF] Dudarev | Electron-energy-loss spectra and the structural stability of nickel oxide: An LSDA+U study[END_REF] The effective on-site Coulomb term, U eff , was set at 3 eV. The initial electronic configurations for PAW potentials were 3s 2 3p 6 3d 10 4p 1 , 1s 2 2s 1 , 4s 2 4p 6 5s 2 4d 3 , and 2s 2 2p 6 for Cu, Li, Nb, and O, respectively. The crystal structures were fully optimized until residual forces and stresses were less than 1.0×10 -4 eV Å -3 and 1.0×10 -3 eV Å -3 , respectively. Convergence tests were performed by comparing the results with more severe conditions, e.g., 800 eV and two times denser k-meshes. We confirmed that total energies were converged within 3 meV atoms -1 .

Lattice dynamics calculations were performed using Parlinski-Li-Kawazoe method implemented by phonopy code. [START_REF] Togo | Transition pathway of CO 2 crystals under high pressures[END_REF][START_REF] Togo | First-principles calculations of the ferroelastic transition between rutile-type and CaCl 2 -type SiO 2 at high pressures[END_REF][START_REF] Togo | Evolution of crystal structures in metallic elements[END_REF][START_REF] Akamatsu | Inversion Symmetry Breaking by Oxygen Octahedral Rotations in the Ruddlesden-Popper NaRTiO 4 Family[END_REF][START_REF] Togo | First principles phonon calculations in materials science[END_REF] We used 2×2×2 rhombohedral supercell for LiNbO 3 -type CuNbO 3 .

We also performed Cu-K XANES calculations with the full-potential linearized augmented plane wave plus local orbitals method and the PBEsol + U (U eff = 3 eV) functional available in the WIEN2k code. [START_REF] Anisimov | Density-functional theory and NiO photoemission spectra[END_REF][START_REF] Schwarz | Electronic structure calculations of solids using the WIEN2k package for material sciences[END_REF] A core-hole was directly treated for final state calculations. The cut-off parameter of the plane wave, R MT K MAX , was set to 6.0 bohr Ry 1/2 . Calculated spectra were broadened by Lorenz functions with a natural width of Cu-K shell. [START_REF] Krause | Natural widths of atomic K and L levels, Kα X-ray lines and several KLL Auger lines[END_REF] To correct the calculated transition energies, the calculated Cu-K XANES spectra in the present study were shifted by -29.0 eV, which corresponds to a transition-energy difference between the theoretical and experimental Cu-K XANES for the reference material, Cu 2 O.

RESULTS

3.1. Crystal Structure: HP-CuNbO 3 was previously prepared at 6.5 GPa and 1273 -1473 K and reported to adopt a monoclinic symmetry with a = 3.836 Å (≈ a pc ), b = 10.432 Å (≈ 2 a pc ), c = 5.546 Å (≈ a pc ), and β = 94.66°. [START_REF] Sleight | Preparation of CuNbO 3 and CuTaO 3 at high pressure[END_REF] 2 2 In this study, we found that the higher-purity samples tend to be obtained by the use of the higher synthesis pressure (~12 GPa) (see Figure S1 in Supporting Information).

First, we performed ED for the high-purity samples to deduce the crystal symmetry of HP-CuNbO 3 . Figure 1 shows the ED patterns at room temperature along the [001], [0 0], and [100] zone axes. In contrast to the previous 1 report, [START_REF] Sleight | Preparation of CuNbO 3 and CuTaO 3 at high pressure[END_REF] all reflections can be indexed with another monoclinic cell with a ≈ 5.5 Å (≈ a pc ), b ≈ 5.2 Å (≈ a pc ), 2 2 c ≈ 7.7 Å (≈ 2a pc ), and β ≈ 95°. The observations of 120 reflection at [001] and 012 reflection at [100] (see red circles in Figure 1) are indicative of a primitive monoclinic unit cell. The absence of 00l reflections with l = odd (e.g., 001 reflection) in the [0 0] pattern reveals the presence of 1 the c-glide symmetry. The finite intensity of the forbidden 001 reflection along [100] is due to multiple scattering effects. Given the 0k0 reflections with k = odd (e.g., 010 reflections) in the [001] and [100] zone axes, one will expect the absence of the 2 1 screw axis. However, when the multiple scattering effects are significant, the 010 reflections are observed even in the presence of the 2 1 screw axis. Namely, the presence or absence of the 2 1 screw axis cannot determined by the ED analysis alone. We thus suggest possible space groups to be Pc, P2/c, or P2 1 /c.

Next, we carried out SXRD and NPD experiments at 300 K to identify the crystal symmetry of HP-CuNbO 3 . The main reflections seem at first sight to be indexed with a R3c (LiNbO 3 -type) structure (a ≈ 5.2 Å and c ≈ 14.0 Å in a hexagonal setting), but the indexing of SXRD and NPD patterns with the successive dichotomy method [START_REF] Louër | Powder pattern indexing and the dichotomy algorithm[END_REF] indicates a monoclinic cell a ≈ 5.5 Å (≈ a pc ), b ≈ 5.2 Å (≈ a pc ), c ≈ 2 2 7.7 Å (≈ 2a pc ) and β ≈ 95°, in good agreement with the ED analysis. Consistent with the symmetry lowering from the rhombohedral R3c structure, the peak splitting (e. g., 01 h 4 and 110 h ) and superstructure reflections (e. g., 012 m and 102 m ) are detected in the SXRD pattern (Figure 2a), where the subscript h and m represent the hexagonal and monoclinic unit cells, respectively. The symmetry lowering from the R3c structure is also found from the peak splitting (e.g., 02 h and 024 h ) in the NPD pattern (Figure 2b). The 2 reflection conditions in the SXRD and NPD data, as well as in the ED data, leave Pc, P2/c, or P2 1 /c as possible space groups. The Pc structure is noncentrosymmetric-polar, A c c e p t e d M a n u s c r i p t while the P2/c and P2 1 /c structures are centrosymmetric. To identify the space group of HP-CuNbO 3 , we additionally measured the second-order nonlinear optical properties. Our experiments confirmed an SHG activity (Figure S2 in Supporting Information), showing the noncentrosymmetric character. Therefore, the space group of HP-CuNbO 3 is found to be Pc.

The structural relationship between the Pc and R3c phases is a maximal non-isomorphic group/subgroup with the following transformation matrix: ,

( 𝑎 𝑚 𝑏 𝑚 𝑐 𝑚 ) = ( -1/3 -2/3 1/3 -1 0 0 -2/3 -4/3 -1/3 )( 𝑎 ℎ 𝑏 ℎ 𝑐 ℎ )
where a m , b m , and c m are the lattice parameters of the monoclinic unit cell, and a h , b h , and c h are the lattice parameters in the hexagonal setting of the rhombohedral R3c unit cell. To the best of our knowledge, simple ABO 3 perovskites crystallizing in Pc with a m ≈ a pc , b m ≈ a pc , 2 2 and c m ≈ 2a pc unit cell have not been discovered yet.

Rietveld refinements were performed against NPD and SXRD patterns of HP-CuNbO 3 at 300 K by assuming as an initial model a Pc unit cell with a a pc  a pc  2a pc unit 2 2 cell. The initial structural model was derived from the group-subgroup relationship with an R3c (LiNbO 3 -type) model using the program ISODISTORT of the ISOTROPY Software Suite, [START_REF] Campbell | ISODISPLACE: a web-based tool for exploring structural distortions[END_REF] and confirmed by structure solution against the SXRD data using direct methods. The Nb1 position is fixed to (0, 0, 0). Figure 3a, b displays the refinement results for SXRD and NPD data, respectively. For the SXRD data, the presence of heavy Nb and Cu atoms leads to a very low contrast for oxygen atoms. The B iso of oxygen atoms were accordingly fixed to the B iso value of Nb2. The stoichiometric composition models provide good overall fits to the observed patterns for both SXRD (weighted profile R factor R wp = 11.60 % and Bragg R factor R B = 6.62 % ) and NPD data (R wp = 10.34 % and R B = 3.66 %). The cation site occupancies were refined to check deviations in the stoichiometry, but the refined occupancies remained within 1 or 2 % of the expected full site values. This result is in agreement with complementary ICP-OES analysis in which the cationic ratio is almost stoichiometric (Cu/Nb = 1.05) within standard uncertainties.

We also examined the possibility of cation mixing on the A-site. According to the previous structural analyses on LiNbO 3 , the cation distribution was estimated to be (Li 1-5x Nb x ☐ 4x )NbO 3 , where ☐ represents a vacancy. [START_REF] Abrahams | Defect Structure Dependence on Composition in Lithium Niobate[END_REF][START_REF] Iyi | Comparative study of defect structures in lithium niobate with different compositions[END_REF] In the course of our refinements against SXRD and NPD data, the introduction of a small amount of Nb (a few percent) at the Cu1 or Cu2 sites did not improve the fitting quality. The refined structural parameters from SXRD and NPD data are listed in Table 1 and Table 2, respectively. The selected bond lengths and bond angles are tabulated in Table S1. The calculated density of HP-CuNbO 3 [6.12 g cm -3 (SXRD) and 6.14 g cm -3 (NPD)] is significantly larger than that of AP-CuNbO 3 (5.02 g cm -3 ), [START_REF] Joshi | Photoelectrochemical Investigation and Electronic Structure of a p-Type CuNbO 3 Photocathode[END_REF] consistent with the expectation that the high-pressure synthesis provides a higher-density phase. Bond-valence-sum calculations [START_REF] Brese | Bond-Valence Parameters for Solids[END_REF] (+4.76 (8)) for Cu1, Cu2, Nb1, and Nb2, respectively, confirming the Cu + Nb 5+ O 3 ionic model. This result conforms to those of XANES and temperature-dependent magnetic susceptibility experiments, which will be separately described below. Figure 4 depicts the crystal structures of HP-CuNbO 3 refined against the NPD data at 300 K. The monoclinic distortion of HP-CuNbO 3 highlights the tilting distortion of NbO 6 octahedra and the off-center displacements of Nb 5+ ions. The octahedral tilt system is a -a -c -. Glazer [START_REF] Glazer | The Classification of Tilted Octahedra in Perovskites[END_REF] has shown that this tilt system in an aristotype cubic Pm m perovskite leads to a 3 centrosymmetric monoclinic C2/c structure. In HP-CuNbO 3 perovskite, the symmetry is lowered to the space group Pc by the two inequivalent off-center displacements of Nb 5+ ions (see arrows in Figure 4b).

In ABO 3 perovskite oxides, the decrease in the A-site ionic radius, i.e., the reduction in the tolerance factor t, results in enhanced octahedral tilting, leading to the decrease of the B-O-B bond angle. As expected from the small t value (t = 0.89), the Pc phase of HP-CuNbO 3 possesses a notable magnitude of NbO 6 octahedral tilts, with the Nb-O-Nb angles deviating significantly from the ideal value of 180° by up to ∼36° (see Table S1). The Nb-O-Nb bond angles of HP-CuNbO 3 (144-148°) are smaller than those of NaNbO 3 (157-162°), [START_REF] Johnston | The Polar Phase of NaNbO 3 : A Combined Study by Powder Diffraction, Solid-State NMR, and First-Principles Calculations[END_REF] but are larger than that of LiNbO 3 (140°). [START_REF] Abrahams | Ferroelectric Lithium Niobate. 3. Single Crystal X-Ray Diffraction study at 24°C[END_REF] Namely, the average Nb-O-Nb bond angle decreases in the order NaNbO 3 (t = 0.97) > CuNbO 3 (t = 0.89) > LiNbO 3 (t = 0.85), demonstrating the A-site-cation-size-dependent octahedral tilting. In Pc CuNbO 3 , the coordination environments of the two independent Nb sites, Nb1 and Nb2, are strongly asymmetric, due to their off-center displacements along [001] (=<100> pc ) within the octahedra (see Figure 4b). The NbO 6 octahedral distortion Δ of HP-CuNbO 3 was calculated from the NPD-refined bond lengths; (see Table S1). We obtained Δ(Nb1O 6 ) = 1.5(2)  10 -3 and Δ(Nb2O 6 ) = 2.9(1)  10 -3 .The magnitude of these octahedral distortions is comparable to that for other perovskite niobates ANbO 3 with second-order Jahn-Teller active Nb 5+ (4d 0 ) ions (e.g., 3.11(1)  10 -3 for LiNbO 3 [START_REF] Abrahams | Ferroelectric Lithium Niobate. 3. Single Crystal X-Ray Diffraction study at 24°C[END_REF] and 4.0(3)  10 -3 for KNbO 3 ). [START_REF] Kawamura | Electronic Polarization in KNbO 3 Visualized by Synchrotron Radiation Powder Diffraction[END_REF] The large octahedral distortions for HP-CuNbO 3 are thus ascribed to the second-order Jahn-Teller effects.

Δ = 1/6Σ i [(d i -⟨d⟩)/⟨d⟩] 2 ,
Asymmetric environments are also observed for the Cu coordination in Pc CuNbO 3 : Cu atoms are displaced approximately along [20 ] (=[111] pc ) with respect to the 3 oxygen-triangle planes, occupying two independent Cu sites, Cu1 and Cu2 (see Figure 4b). Each of Cu1 and Cu2 atoms forms three shorter and nine longer bonds with the oxygen atoms (see also Table S1), although the A-site cations in aristotype perovskites are surrounded by 12 Interestingly, we notice that the perovskite-type HP-CuNbO 3 has the same crystal symmetry and unit-cell metrics as the lowest-temperature monoclinic phase of WO 3 , ε-WO 3 . [START_REF] Salje | Crystal structure and paramagnetic behaviour of ε-WO 3-x[END_REF] Namely, both of them crystallize in Pc symmetry with a m ≈ a pc , b m ≈ a pc , and c m ≈ 2a pc . Note 2 2 that the structure of WO 3 can be regarded as an ABO 3 perovskite-type structure with empty A-sites. In the Pc structure, the B1 and B2 atoms (B = Nb in HP-CuNbO 3 or W in ε-WO 3 ) undergo almost antiparallel off-center displacements along [001] (=<100> pc ) within the octahedra. For ε-WO 3 , the antiparallel displacements of W1 and W2 atoms with different amplitudes produce "collinear ferrielectricity". For HP-CuNbO 3 , the antiparallel displacements of Nb1 and Nb2 with different magnitudes are accompanied by the parallel displacements of Cu1 and Cu2 atoms approximately along [20 ] (=[111] pc ).

3 Consequently, a noncollinear ferrrielectric structure appears in the Pc phase of HP-CuNbO 3 . The spontaneous polarization calculated from the NPD-refined crystal structure using the formal charges of Cu + and Nb 5+ is 9.7 μC cm -2 .

It is worth underlining that ferrielectricity is observed in AgNbO 3 , but it stems from collinear electric dipole order. On the other hand, HP-CuNbO 3 is a rare example of a ferrielectric material with noncollinear electric dipole order, representing a new type of polar phase in the perovskite structure.

3.2. Valence state: Figure 5a, b displays the experimental and calculated XANES spectra at the Cu K-edge, respectively. The experimental Cu absorption edge position of HP-CuNbO 3 is close to that of Cu 2 O, but different from that of CuO. The contrasting coordination environment between HP-CuNbO 3 (perovskite A-site) and Cu 2 O (linear coordination), however, makes it difficult to estimate clearly the Cu valency in HP-CuNbO 3 . So, we performed XANES calculations for HP-CuNbO 3 as well as for a reference compound, Cu 2 O (Figure 5b). The experimental spectra for HP-CuNbO 3 and Cu 2 O matches qualitatively the calculated spectra based on Cu + Nb 5+ O 3 and Cu + 2 O models, respectively, revealing that the Cu cations are present as Cu + in both compounds. On the other hand, the Nb valency in HP-CuNbO 3 can be easily estimated to be +5 from the similarity of its experimental spectrum to that of Nb 2 O 5 (Figure 6). Thus, our XANES analyses unambiguously evidence that the valence states in CuNbO 3 are +1 for Cu and +5 for Nb.

Figure 7 gives the temperature dependence of magnetic susceptibility, χ(T), of HP-CuNbO 3 measured at 100 Oe after zero-field cooling. The fitting was carried out on the basis of the Curie-Weiss law described by the following equation:

𝜒(𝑇) = 𝜒 0 + 𝑁𝜇 2 eff 3𝑘 𝐵 (𝑇 -𝜃 w )
, where χ 0 is a temperature-independent term, N the Avogadro constant, μ eff the effective magnetic moment, k B the Boltzmann constant, and Θ w the Weiss temperature. The fitting results are χ 0 = -2.6(1) × 10 -4 emu Oe -1 mol -1 , Θ w = -1.8(2) K, and μ eff = 0.301(6) μ B . The calculated effective magnetic moment is significantly smaller than that for Cu 2+ (1.732 μ B ). The paramagnetic behavior is thus attributed to a small amount of magnetic impurities. In addition, χ(T) is negative above 50 K, with the absolute values being larger shows temperature-variable SXRD patterns upon heating HP-CuNbO 3 above 300 K under ambient pressure. The heating rate was set to 20 K min -1 and the waiting time at each temperature was 1 min. One can see that the peak splittings observed at 300 K, which arise from the monoclinic distortion, disappear at 470 K. This fact is indicative of the phase transition from a monoclinic Pc phase into a higher-symmetry rhombohedral phase. Considering the reflection conditions for the hightemperature phase in the hexagonal setting (-h + k + l = 3n for hkl, h + l = 3n and l = 2n for hh̅ l, l = 3n for hhl, and l = 6n for 00l), the possible space groups are either R3c (noncentrosymmetric polar) or R c (centrosymmetric 3 nonpolar).

To deduce the space group of the high-temperature rhombohedral polymorph, we examined the temperature variation of the lattice parameters between 470 and 510 K, and found that the a h parameter increases with increasing temperature, while the temperature rise results in a decrease in the c h parameter (see Figure S3 in Supporting Information). Similar behavior has been observed for R3c ferroelectric compounds, like LiNbO 3 , LiTaO 3 , [START_REF] Sugii | Temperature variations of lattice parameters of LiNbO 3 , LiTaO 3 and Li(Nb 1-y Ta y )O 3 solid-solutions[END_REF] and LiOsO 3 , [START_REF] Shi | A ferroelectric-like structural transition in a metal[END_REF] where the relaxation of the spontaneous ferroelectric strains along the c axis upon heating up to the Curie temperature (T C ) leads to a decrease of the c h parameters and an increase of the a h parameters; in the absence of spontaneous strains (i.e., in the hightemperature R c phases at T > T C ), both a h and c h 3 parameters increase with increasing temperature, as naturally expected. Judging from the thermal evolution of the c h parameter for the high-temperature phase of HP-CuNbO 3 (i.e., decrease of the c h parameter upon heating), its space group is determined as a polar R3c. In other words, HP-CuNbO 3 transforms upon heating at 470 K into R3c (LiNbO 3 -type) CuNbO 3 . The pseudo-cubic lattice volumes, The Rietveld plot of the R3c CuNbO 3 at 470 K is shown in Figure 9a. The refined structure is displayed in Figure 9b and the refined structural parameters are given in Table 3. The Nb1 position is fixed to (0, 0, 0). We refined the cation site occupancies to confirm deviations in the stoichiometry, and detected a small Cu defect (g(Cu)=0.929 (9)). This nonstoichiometric composition model (Cu 0.93 NbO 3 ) leads to better overall fits than the stoichiometric composition model (CuNbO

DISCUSSION

As described above, we find two new polar polymorphs of CuNbO 3 . One is the monoclinic Pc perovskite-type phase (HP-CuNbO 3 ) stabilized via the high-pressure and hightemperature synthesis, and the other the rhombohedral R3c perovskite-type (or LiNbO 3 -type) phase obtained by heating the Pc phase in ambient pressure. The former is the first example of a simple ABO 3 perovskite crystallizing in monoclinic Pc symmetry with a ≈ a pc , b ≈ a pc , and c ≈ 2 2 2a pc unit cell. It also represents a rare example of "noncollinear ferrielectric" dipole order. Here, it should be noted that, in contrast to the Pc phases, the highersymmetry R3c phases have been often observed in ABO 3 compounds by the use of high-temperature and highpressure conditions; examples include CuTaO 3 , 32 AFeO 3 (A = Sc, [START_REF] Kawamoto | Roomtemperature polar ferromagnet ScFeO3 transformed from a high-pressure orthorhombic perovskite phase[END_REF] Ga, [START_REF] Niu | Room Temperature Magnetically Ordered Polar Corundum GaFeO 3 Displaying Magnetoelectric Coupling[END_REF] and In 64 ), ATiO 3 (A = Mg, [START_REF] Linton | Complete Fe-Mg solid solution in lithium niobate and perovskite structures in titanates at high pressures and temperatures[END_REF] Mn, [START_REF] Ko | High-pressure phase transition in MnTiO3 from the ilmenite to the LiNbO3 structure[END_REF][START_REF] Aimi | High-Pressure Synthesis and Correlation between Structure, Magnetic, and Dielectric Properties in LiNbO 3 -Type MnMO 3 (M = Ti, Sn)[END_REF] Fe, [START_REF] Linton | Complete Fe-Mg solid solution in lithium niobate and perovskite structures in titanates at high pressures and temperatures[END_REF][START_REF] Leinenweber | Unquenchable High-Pressure Perovskite Polymorphs of MnSnO 3 and FeTiO 3[END_REF] Zn, [START_REF] Inaguma | High-pressure synthesis, crystal structure, and phase stability relations of a LiNbO3-type polar titanate ZnTiO3 and its reinforced polarity by the secondorder Jahn-Teller effect[END_REF] and Hg [START_REF] Sleight | High-Pressure HgTiO 3 and HgPbO 3 : Preparation, Characterization, and Structure[END_REF][START_REF] Shan | Synthesis and Phase Transition of HgTiO 3[END_REF] ), AGeO 3 (A = Mg, [START_REF] Leinenweber | An unquenchable perovskite phase of MgGeO 3 and comparison with MgSiO 3 perovskite[END_REF][START_REF] Akaogi | High-pressure transitions and thermochemistry of MGeO 3 (M=Mg, Zn and Sr) and Sr-silicates: systematics in enthalpies of formation of A 2+ B 4+ O 3 perovskites[END_REF] Fe, [START_REF] Hattori | Clinopyroxene-perovskite phase transition of FeGeO 3 under high pressure and room temperature[END_REF] and Zn 73 ), ASnO 3 (A = Mn 68 and Zn 74 ), LiBO 3 (B = Sb, [START_REF] Inaguma | High-Pressure Synthesis, Crystal Structure, Chemical Bonding, and Ferroelectricity of LiNbO3 -Type LiSbO3[END_REF] W, [START_REF] Ishida | H High-Pressure Synthesis of Non-Stoichiometric LixWO3 (0.5 ≤ x ≤ 1.0) with LiNbO3 Structure[END_REF] and Os 59 ), Pb-and Bicontaining perovskites [START_REF] Belik | High-Pressure Synthesis, Crystal Structures, and Properties of Perovskitelike BiAlO 3 and Pyroxene-like BiGaO 3[END_REF][START_REF] Inaguma | High-pressure synthesis of novel lithium niobate type oxides[END_REF][START_REF] Inaguma | Structural Transformation, Thermal Stability, Valence State, and Magnetic and Electronic Properties of PbNiO 3 with Perovskite and LiNbO 3 -Type Structures[END_REF][START_REF] Yu | A new LiNbO3-type polar oxide with closed-shell cations: ZnPbO3[END_REF][START_REF] Mori | Direct Formation under High Pressure, Structure, and Electronic Properties of LiNbO3-type Oxide PbZnO3[END_REF][START_REF] Sakai | Polar-Nonpolar Phase Transition Accompanied by Negative Thermal Expansion in Perovskite-Type Bi 1-x Pb x NiO 3[END_REF] , and cation mixed perovskites. [START_REF] Sakai | Polar-Nonpolar Phase Transition Accompanied by Negative Thermal Expansion in Perovskite-Type Bi 1-x Pb x NiO 3[END_REF][START_REF] Belik | LiNbO 3 , Corundum, and Hexagonal Polymorphs of (In 1-x M x )MO 3[END_REF][START_REF] Belik | LiNbO 3 -Type Oxide (Tl 1-x Sc x )ScO 3 : High-Pressure Synthesis, Crystal Structure, and Electronic Properties[END_REF][START_REF] Li | Polar and magnetic Mn2FeMO6 (M=Nb, Ta) with LiNbO3-type structure: High-pressure synthesis[END_REF][START_REF] Li | Designing Polar and Magnetic Oxides: Zn 2 FeTaO 6 -in Search of Multiferroics[END_REF][START_REF] Han | Universal A-Cation Splitting in LiNbO3-Type Structure Driven by Intrapositional Multivalent Coupling[END_REF] To examine the stable structure of HP-CuNbO 3 , we subsequently performed phonon calculations using the polar R3c phase as the parent structure. The crystal structure was optimized in the R3c symmetry. Phonon dispersion curves for the R3c structure are shown in Figure 10. We see an imaginary mode at the F point of the Brillouin zone. Within the framework of a second-order phase transition, freezing of the F-point softmode yields a monoclinic polar Pc structure with a ≈ a pc , b ≈ a pc , c ≈ 2 2 2a pc and β ≈ 95°, leading to the antiparallel off-center displacements of Nb 5+ and the a -a -c --type tilts of NbO 6 octahedra. Our calculations demonstrate that the Pc structure has a lower energy than the R3c structure and is dynamically stable, consistent with the experimental observation that the low-temperature and hightemperature phases of HP-CuNbO 3 crystallize in the space groups Pc and R3c, respectively.

It is worth emphasizing that HP-CuNbO 3 (Pc) has a lowersymmetry structure than LiNbO 3 (R3c), despite the very close t values between CuNbO 3 (t = 0.89) and LiNbO 3 (t = 0.85). Comparing the crystal structure of CuNbO 3 and LiNbO 3 (see Figure 11) reveals that A-site cations (Cu + in To evaluate comparatively the bonding character for HP-CuNbO 3 and LiNbO 3 , we also calculated their electronic structures. In the calculations, we utilized the same R3c structure as that used in the phonon calculations. Figure 12a, b depicts the calculated total and partial density of states (DOS) for R3c CuNbO 3 and LiNbO 3 , respectively. For CuNbO 3 , the valence band mainly consists of Cu 3d and O 2p states, and the conduction band is composed of the Nb 4d state. The Cu 3d state contributes to the formation of the top of valence band through the orbital overlap with the O 2p state (see Figure 12a). This is in striking contrast to LiNbO 3 , where the valence band is composed of the O 2p state, without any significant participation of Li orbitals (see Figure 12b). Thus, the covalent bond character of A-site cations is obvious for CuNbO 3 . Note that the Nb 4d states of CuNbO 3 and LiNbO 3 , which are completely empty in the ionic limits, are slightly occupied through orbital overlap with the O 2p states, forming the bottom of the valence band. This is indicative of the Nb-O covalency that drives the second-order Jahn-Teller effect. We believe that the covalent character of Cu-O and Nb-O bonds will be associated with the appearance of the noncollinear ferrielectric cation displacements as observed for HP-CuNbO 3 .

CONCLUSION

A novel perovskite-type oxide, CuNbO 3 , has been synthesized at 12 GPa and 1273 K. It forms a monoclinic Pc structure at 300 K, representing a rare example of a "noncollinear ferrielectric" dipole order. The Pc phase transforms to a "collinear ferroelectric" R3c structure upon heating at 470 K. The noncollinear ferrielectric (Pc)collinear ferroelectric (R3c) transition is of displacive origin, and the condensing soft mode responsible for the phase transition is observed at the F point of the rhombohedral Brillouin zone, which corresponds to the structural distortion mainly due to the antiparallel off-center Nb displacements and the a -a -c -NbO 6 octahedral tilting. The original distortion within the monoclinic Pc perovskite phase, in particular with respect to LiNbO 3 , is explained on the basis of bonding character, as highlighted by electronic structure calculations.

Figure 1 .

 1 Figure 1. ED patterns of HP-CuNbO 3 at room temperature taken along [001], [0 0], and [100] zone axes. Representative indices 1 are shown just above the corresponding reflection spots. The reflections marked in red are indicative of a primitive cell.

  using the NPD (SXRD)-refined bond lengths give +0.93(3) (+0.85(2)), +0.99(2) (+0.84(2)), +4.85(7) (+5.04(8)), and +4.84(8) A c c e p t e d M a n u s c r i p t

Figure 2 .

 2 Figure 2. Enlarged portions of (a) SXRD (λ = 0.75003 Å) and (b) NPD (λ = 1.594 Å) patterns of HP-CuNbO 3 at 300 K. Some reflections in the observed patterns are split by a monoclinic distortion from the rhombohedral R3c structure; the purple ticks correspond to the positions of the allowed Bragg reflections for the R3c structure. The subscript h represents the rhombohedral unit cell in a hexagonal setting, and the subscript m indicates the monoclinic unit cell.

Figure 3 .

 3 Figure 3. Rietveld refinements against (a) SXRD (λ = 0.75003 Å) and (b) NPD (λ = 1.594 Å) patterns of HP-CuNbO 3 at 300 K. These refinements were performed on the basis of the monoclinic Pc perovskite-type structure model with a a pc  𝟐 a pc  2a pc unit cell, showing the observed (red circles) and 𝟐 calculated (black solid lines) profiles. The blue solid lines represent the difference between the observed and calculated profiles. The green ticks correspond to the positions of the allowed Bragg reflections.

  where d i is the individual Nb-O bond length and ⟨d⟩ is the average Nb-O bond length

Figure 4 .

 4 Figure 4. The refined crystal structures of HP-CuNbO 3 from the NPD data at 300 K. The structural views in (a) bc and (b) ac planes are shown. The arrows in (b) indicate the cation displacements from the closed-packed oxygen planes and those from the centroids of NbO 6 octahedra.

  Space group: Pc (No. 7), Z = 4. CuNbO 3 (fw = 204.45 g mol -1 ): λ = 0.75003 Å, 0.595 Å < d < 20.2 Å; lattice parameters: a = 5.54535(4) Å, b = 5.21029(3) Å, c = 7.67491(6)Å, β = 94.6924(6)°, and V = 221.007(3) Å 3 , R wp = 11.60 % and R B = 6.62 %; R wp = [w i (y ioy ic ) 2 / w i y io 2 ] 1/2 , where y io and y ic are the observed and calculated intensities, respectively, and w i is the weighting factor. R B =  |I o (h K ) -I c (h K )| /  | I o (h K )|, where I o (h K ) and I c (h K ) are the observed and calculated integrated intensities for reflection K, respectively.

A c c e p t e d M a n u s c r i p t

Figure 5 .

 5 Figure 5. (a) The experimental Cu K-edge XANES spectrum of HP-CuNbO 3 (blue) in comparison with those of Cu 2 O (black) and CuO (red). (b) The calculated Cu K-edge XANES spectra of HP-CuNbO 3 (blue) and Cu 2 O (black).

Figure 6 .

 6 Figure 6. The experimental Nb K-edge XANES spectrum of HP-CuNbO 3 (green) in comparison with those of Nb foil (black), NbO 2 (red), and Nb 2 O 5 (blue).

Figure 7 .

 7 Figure 7. Temperature dependence of magnetic susceptibility, χ = M/H, of HP-CuNbO 3 measured at H = 100 Oe after zero-field cooling. The dots represent the observed data, while the red solid line is the fitted curve according to the Curie-Weiss law.

3. 3 .

 3 Temperature-driven phase transition: Figure8a,b

Figure 8 .

 8 Figure 8. (a) Temperature evolution of SXRD patterns (λ = 0.75565 Å) of HP-CuNbO 3 in the temperature range of 300 to 470 K. (b) Comparison between SXRD patterns at 300 K (Pc) and at 470 K (R3c). The subscript h represents the rhombohedral unit cell in a hexagonal setting, and the subscript m indicates the monoclinic unit cell.

  3 ); R wp = 7.91% and R B = 7.11% for Cu 0.93 NbO 3 and R wp = 7.92% and R B = 11.4% for CuNbO 3 . The formation of Cu deficiency is related to the onset of the decomposition of the R3c phase as mentioned below, which is driven by the release of A-site Cu. The calculated spontaneous polarization along [001] (=[111] pc ) for R3c Cu 0.93 NbO 3 is 15 μC cm -2 , which is larger than that of the lower-symmetry Pc CuNbO 3 . The counterintuitive result is due to the fact that the almost antiparallel off-center displacements of Nb 5+ ions in Pc CuNbO 3 cancel each other's polarization. It should be mentioned that the R3c phase starts to decompose at around 500 K to produce Cu 2 O and CuNb 2 O 6 , according to the temperature-variable SXRD experiments. The former compound, Cu 2 O, is formed by the release of A-site Cu in the R3c phase and its subsequent oxidation by atmospheric oxygen. The latter compound, CuNb 2 O 6 , is isostructural to a perovskite-type CuTa 2 O 6 (cubic or pseudo-cubic). 60, 61 The thermal stability temperature range of R3c phase is so narrow that the Pc-R3c phase transition almost concurs with the decomposition reaction of the R3c phase. Given the nominal cation valences of the final decomposition product (Cu 2+ Nb 5+ 2 O 6 ), we believe that a partial oxidation of Cu + to Cu 2+ (Cu + 0.86 Cu 2+ 0.07 Nb 5+ O 3 ) will occur in R3c Cu 0.93 NbO 3 to compensate for its Cu vacancies.

Figure 9 .

 9 Figure 9. (a) Rietveld refinements against SXRD patterns (λ = 0.77565 Å) of HP-CuNbO 3 at 470 K. The refinement was performed on the basis of the rhombohedral R3c perovskite-type structure model with A-site deficiency (i.e., LiNbO 3 -type Cu 0.93 NbO 3 ), showing the observed (red circles) and calculated (black solid lines) profiles. The blue solid lines represent the difference between the observed and calculated profiles. The green ticks correspond to the positions of the allowed Bragg reflections. Peaks due to Cu 2 O and CuNb 2 O 6 were excluded in the refinement. (b)The refined crystal structure of R3c (LiNbO 3 -type) Cu 0.94 NbO 3 at 470 K. The arrows indicate the cation displacements from the closed-packed oxygen planes and those from the centroids of NbO 6 octahedra.

  R3c (No. 161), Z = 6. Cu 0.93 NbO 3 (fw = 199.9 g mol -1 ): λ = 0.77565 Å, 0.615 Å < d < 6.03 Å; lattice parameters: a = 5.2433(2) Å, c = 14.0006(4) Å, and V = 333.34(2) Å 3 , R wp = 7.91%, R B = 7.11%. Cu1 and Nb1 were refined anisotropically.

Figure 10 .

 10 Figure 10. Phonon dispersion curves of HP-CuNbO 3 calculated with the R3c symmetry.

Figure 11 .

 11 Figure 11. Short distance coordination around (a) Cu in HP-CuNbO 3 (this work) and (b) Li in LiNbO 3 (from Ref. 21). The light green, blue, and red spheres correspond to Li, Cu, and O atoms, respectively.

Figure 12 .

 12 Figure 12. The calculated projected total and partial density of states (DOS) of R3c (a) CuNbO 3 and (b) LiNbO 3 . The zero of energy is set to the top of the valence band. The inset of (b) shows an enlarged view of the partial DOS of Li 4s and Li 4p states.

For

  Table of Contents OnlySynopsis: Perovskite-type CuNbO 3 was synthesized at 12 GPa and 1473 K. It crystallizes in a polar monoclinic Pc space group. This Pc phase exhibits unusual noncollinear ferrielectric structure, due to parallel Cu displacements and antiparallel Nb displacements along different axes, representing a new type of polar phase in the perovskite structure. At about 470 K, it transforms into the collinear ferroelectric R3c structure that features parallel Cu and Nb displacements in the same direction.

Table 1 . Structural Parameters of HP-CuNbO 3 at 300 K Obtained from Refinements against the SXRD Data

 1 , and so the effective coordination number of Cu atoms is three. The coordination environments of Cu1O 3 and Cu2O 3 differ in terms of polyhedral distortions and average bond lengths. The displacement from the centers of oxygen triangles is about nine times larger for Cu1 than for Cu2 (see Figure4band Figure11).

	equidistant

Table 2 . Structural Parameters of HP-CuNbO 3 at 300 K Obtained from Refinements against the NPD Data

 2 Pc (No. 7), Z = 4. CuNbO 3 (fw = 204.45 g mol -1 ): λ = 1.594 Å, 0.809 Å < d < 6.11 Å; lattice parameters: a = 5.548088(10) Å, b = 5.208766(9) Å, c = 7.695716(15) Å, β = 94.67983(10)° , and V = 221.6547(7) Å 3 , R wp = 10.34 % and R B = 3.66 %. Cu1 was refined anisotropically.

	atom	site	g	x	y	z	U eq/iso / Å 2
	Cu1	2a	1	0.514(4)	0.988(2)	0.226(2)	0.065(5)
	Cu2	2a	1	0.989(2)	0.498(2)	0.735(2)	0.011(1)
	Nb1	2a	1	0	0	0	0.094(13)
	Nb2	2a	1	0.4830(14)	0.5122(14)	0.4610(8)	0.015(2)
	O1	2a	1	0.184(2)	0.686(2)	0.433(2)	0.007(1)
	O2	2a	1	0.686(3)	0.185(2)	0.935(2)	0.014(2)
	O3	2a	1	0.290(3)	0.205(2)	0.546(2)	0.005(2)
	O4	2a	1	0.795(3)	0.701(2)	0.043(2)	0.002(2)
	O5	2a	1	0.993(3)	0.119(2)	0.233(2)	0.007(1)
	O6	2a	1	0.491(3)	0.613(2)	0.733(2)	0.008(2)
	Space group:

Accession Codes

CCDC 1977808 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 1EZ, UK; fax: +44 1223 336033.

ACKNOWLEDGEMENT

SXRD experiments were performed on BL02B2 (Proposal Nos. 2019A1071 and 2019B1384) at SPring-8 with the approval of JASRI. XANES experiments were carried out on BL14B2 (Proposal Nos. 2017B1900 and 2019A1650). NPD experiments at D2B were supported by a beam time allocation from Institut Laue-Langevin (Proposal No. 5-23-714); doi:10.5291/ILL-DATA.5-23-714). The authors thank Suguru Yoshida and Jinsuke Miyake for their help with the NPD measurements.

Funding Sources

This work was financially supported by JSPS KAKENHI Grantin-Aids for Scientific Research (A) (Grant No. 17H01320) and (B) (Grant No. 19H02433), and Challenging Research (Exploratory) (Grant No. 18K18940). K. F. also thanks a grant from The Murata Science Foundation.

ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXX.

Dependence of sample quality on synthesis pressures, oscilloscope traces of the SHG signals, and detailed structural analysis results (selected bond lengths and angles, and lattice parameters at various temperatures).

AUTHOR INFORMATION