Bismuth niobate fourfold oxide (BiNbO₄) belongs to the family of ABO₄ compounds. The structure of ABO₄ compound is versatile with A and B cations, and has been studied at high temperature and high pressure.^[1]

In general, BiNbO₄ possess two polymorphs: a low-temperature orthorhombic (α) phase^[2–4] and a high-temperature triclinic (β) phase.^[5–7] Recently, a new phase of BiNbO₄, called HP-BiNbO₄, has been obtained above 3 GPa and 800 °C. The phase transitions of α-BiNbO₄ and β-BiNbO₄ were investigated at 0–5 GPa and 300–1800 °C.^[8] It has been shown that the transition temperature decreases with increasing pressure and the phase transition is irreversible after cooling and decompressing. Furthermore, the in-situ x-ray diffraction, differential thermal analysis, thermal expansion and dielectric properties of the novel HP-BiNbO₄ ceramic were studied in detail by Zhou et al.^[9] The results indicate that the energy density of HP-BiNbO₄ ceramic is four times higher than that of the α-BiNbO₄ ceramic and the dielectric permittivity increases obviously.^[9] However, to the best of our knowledge, the crystal structure of HP-BiNbO₄ has not been determined, and the behaviors of the novel HP- BiNbO₄ in a diamond anvil cell (DAC) have not yet been studied by using the angle-dispersive x-ray diffraction (ADXRD) measurements under high pressure with synchrotron radiation either. The differences in bulk modulus from those of other ABO₄ compounds remain to be studied.

In our work, we apply the in-situ synchrotron x-ray diffraction method to HP-BiNbO₄ under high pressures up to 24.1 GPa, and investigate the compression behavior and the phase stability. In addition, the equation of state (EOS) of HP-BiNbO₄ is determined. Here, we for the first time report on the new phase of HP-BiNbO₄ in DAC by using in-situ high pressure ADXRD with synchrotron radiation.

The HP-BiNbO₄ was synthesized at 5 GPa and 1300 °C for 10 min through an HPHT method, using α-BiNbO₄ powders as the raw material. BiNbO₄ ceramics were prepared by a solid-state reaction method from starting materials of Bi₂O₃ and Nb₂O₅ mixed in 1:1 stoichiometric ratio. The powders were compressed into cylinders, and then were sintered at 950 °C for 24 h to obtain the α-BiNbO₄ ceramic. X-ray diffraction analysis (XRD; DX-2500, Dandong, China) using a Cu-Ka radiation source with λ = 1.5404 Å and scanning electron microscopy (SEM; JSM-6490, JEOL, Akishima, Japan) were performed on raw materials to detect the microstructures and compositions. The HP-BiNbO₄ powders with a small ruby chip were loaded into a diamond anvil cell (DAC) with a pair of 300 μm culets, and a stainless steel gasket was used with a sample hole of 100 μm diameter.^[10–15] Methanol-ethanol (4:1) was used as a pressure-transmitting medium to minimize the uniaxial stress effect.^[10] The pressures were determined by the ruby fluorescence shifts.^[16]

The high-pressure ADXRD experiments were performed at 4W2 beamline, Beijing Synchrotron Radiation Facility (BSRF, China) with a wavelength of 0.6199 Å. The monochromatic x-ray beam was focused into a spot with a size of approximately 38 μm × 16 μm. The diffraction patterns at various pressures were recorded with a Mar345 imaging plate detector. High-purity CeO₂ powder was used to calibrate the geometrical parameters of the detector. The collected two-dimensional (2D) diffraction patterns were integrated into one-dimensional (1D) profiles with software FIT2D.^[15,17] The GASA program was applied to the Rietveld refinements of sample x-ray diffraction data.^[18]

X-ray diffraction patterns of the α-BiNbO₄ samples prepared via a solid-state reaction method and the HP-BiNbO₄ samples synthesized through an HPHT method are shown in Figs. 1(a) and 1(b). The results of the synthesized α-BiNbO₄ and HP-BiNbO₄ are demonstrated to be consistent with previous results.^[8] The insets in Figs. 1(a) and 1(b) show the scanning electron microscopy (SEM) images of α-BiNbO₄ and HP-BiNbO₄, respectively, and reveal their different microstructures.

Fig. 1.

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	Fig. 1. X-ray diffraction patterns of BiNbO4. (a) x-ray diffraction patterns of the α-BiNbO4 prepared via a solid-state reaction. (b) x-ray diffraction patterns of the HP-BiNbO4 sintered at 5 GPa and 1100 °C. Insets show the SEM micrographs of (a) α- BiNbO4 and (b) HP-BiNbO4.

Rietveld refinement is employed to solve the crystal structure of the HP-BiNbO₄ phase. Cubic structure (A₂B₂O₇) is chosen as a starting model,^[19] since this compound has a formula unit potentially similar to that of BiNbO₄. Figure 2 displays the powder x-ray diffraction data of the synthesized HP-BiNbO₄ and the results of Rietveld refinements, with Sm₂Sn₂O₇ and Dy₃NbO₇ used as starting model, by using the program GASA,^[18] the agreement factors are R_P = 1%, W_RP = 1.6%, R(F²) = 5.4%, and CHI² = 6.745; R_P = 1.5%, W_RP = 2.5%, R(F²) = 6.1%, and CHI² = 15.64, respectively. However, the Dy₃NbO₇ structure cannot explain the two peaks at 14–16 angles. The cubic Sm₂Sn₂O₇ structure fitting is better for HP-BiNbO₄ structure than for Dy₃NbO₇ structure. The crystal HP-BiNbO₄ is found to have a cubic microstructure.

Fig. 2.

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	Fig. 2. (color online) The x-ray diffraction and Rietveld refinement patterns of HP-BiNbO4 about 1 GPa. (a) The HP-BiNbO4 sample based on Sm2Sn2O7. (b) The HP-BiNbO4 sample based on Dy3NbO7. Empty circle: observed curve; red line: calculated curve; green short vertical lines: all possible Bragg positions; bottom line: difference curve.

In a previous paper,^[8] to study the stability of HP-BiNbO₄, the samples were annealed at different temperatures under atmospheric pressure. It has been found that HP-BiNbO₄ remains stable up to 500 °C. At above 600 °C, the HP-BiNbO₄ is converted into α-BiNbO₄ and transformed into β-BiNbO₄ when the annealed temperature is increased to 1150 °C. However, there has been not enough research of compression behavior of HP-BiNbO₄. The in-situ high-pressure angle-dispersive x-ray diffraction data are shown in Fig. 3(a) for both compression and decompression procedures. The highest pressure applied is 24.1 GPa. Five major diffraction peaks of HP-BiNbO₄ are observed, the other peaks are relatively weak. All the diffraction peaks shift toward larger diffraction angles with pressure increasing, and there no new peaks appear. The pressure dependence of d-spacing of different planes is shown in Fig. 3(b). Under compression, we do not find any evidence of pressure-induced phase transition nor chemical decomposition, indicating that the cubic structure remains stable at pressures up to 24.1 GPa. In addition, as the pressure increases to 24.1 GPa, peak broadening increases. This fact can be attributed to the stress during the experiment. X-ray diffraction data of the decompression sample further prove the result.

Fig. 3.

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	Fig. 3. (color online) (a) X-ray diffraction patterns of HP-BiNbO4 under various pressures, (b) pressure dependence of d-spacing of the mainly five planes.

As starting parameters, the refined cell parameters (a = b = c = 10.6754 Å) with space group Fd-3m (No. 227) are used. The pressure dependence of the unit-cell parameters of HP-BiNbO₄ is plotted in Fig. 4. The present pressure–volume data are fitted to a third-order Birch–Murnaghan equation,^[20,21] 1 where K_o and V_o are the bulk modulus and unit-cell volume in ambient condition, respectively, and K_o′ is the pressure derivative of K_o. The equation of cubic HP-BiNbO₄ is determined: K_o = 185(7) GPa, K_o′ = 2.9(0.8). For comparison, the results obtained in earlier studies of BiNbO₄ and other ABO₄ compounds are collected in Table 1. We can see that the bulk moduli differ widely among these ABO₄ compounds. The bulk modulus of HP-BiNbO₄ (about 185 GPa) is slightly larger than that of tetragonal BiVO₄ and significantly greater than those of the tungstates and molybdates. Hazen et al.^[1] and Mariathasan et al.^[22] reported that ABO₄ compound has bulk modulus that is primarily a function of 8-coordinated site valence, and higher valence, the less compressibility.^[22] Therefore, the compressibility values of divalent cation polyhedrons in these tungstates and molybdates are both greater than that of the Bi³⁺ polyhedron, leading to the difference. For the same A–O bonds, such as HP-BiNbO₄ and tetragonal BiVO₄, the difference in bulk modulus is attributed to the structure. The HP-BiNbO₄ is a dense high pressure phase compared with the tetragonal BiVO₄. Calculated values of α-BiNbO₄ and β-BiNbO₄ are slightly higher than that of HP-BiNbO₄, for which the main reason, we think, is that there exist some discrepancies between the experimental result and theoretical calculation due to the fact that the parameters used in theoretical calculation are obtained from the crystal in ambient conditions.

Table 1.

Table 1.

Table 1. List of the values of bulk modulus (Ko), pressure derivative (Ko′) of BiNbO4, compared with previous results. Bulk moduli of some ABO4 compounds are listed for comparison. . Compounds Ko/GPa Ko′ Techniques Reference HP-BiNbO4 185 2.9 experiment This work α-BiNbO4 201 4.52 calculation [23] β-BiNbO4 206 4.55 calculation [23] CaWO4 68 4 experiment [1] CaMoO4 81.5 4 experiment [1] PbWO4 64 4 experiment [1] PbMoO4 64 4 experiment [1] CdMoO4 104 4 experiment [1] BiVO4 150 4 experiment [22]

Table 1. List of the values of bulk modulus (Ko), pressure derivative (Ko′) of BiNbO4, compared with previous results. Bulk moduli of some ABO4 compounds are listed for comparison. .

Fig. 4.

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	Fig. 4. (color online) Normalized unit cell volumes of HP-BiNbO4 as a function of pressure. The red solid line represents experimental fitting of third-order Birch–Murnaghan equation. The blue dashed line denotes the best fitting of second-order Birch–Murnaghan equation.

In this paper, the structure of HP-BiNbO₄ is revealed to be a cubic structure by XRD refinements, with cubic Sm₂Sn₂O₇ and Dy₃NbO₇ used as starting model. The compressibility and phase stability of HP-BiNbO₄ are investigated at high pressure under quasi-hydrostatic conditions by using synchrotron x-ray diffraction. The HP-BiNbO₄ is found to have a stable cubic structure at pressures up to 24.1 GPa. The derived bulk modulus is 185 GPa, which is about 10% lower than the reported theoretical calculation values for α-BiNbO₄ and β-BiNbO₄ (201 GPa), but it is significantly greater than those of the BiVO₄ (150 GPa), tungstates and molybdates (60 to 105 GPa). The results indicate that the HP-BiNbO₄ ceramic is synthesized via the HPHT method, and the bulk modulus of HP-BiNbO₄ ceramic is improved obviously.