Crystal Structures and Electronic States of High-Pressure-Synthesized (1-x)PbVO3-xBiCrO3 Solid Solutions

ABSTRACT Electronic states in solid-solution transition metal oxides may differ from those in their parent compounds, and this results in interesting electronic properties. In this study, the valence states and electronic properties of solid solutions of (1 − x)PbVO3− xBiCrO3 are reported. The solid solutions are successfully synthesized under high-pressure and high-temperature conditions of 5 GPa or 7 GPa and 1273 K, respectively. A change in the crystal structure from centrosymmetric monoclinic (C2/c), as in BiCrO3, to polar tetragonal (P4mm), as in PbVO3, is observed. A tetragonal-to-cubic phase transition, which results in a negative thermal expansion, was observed in 7/8PbVO3-1/8BiCrO3 at approximately 700 K. X-ray absorption spectroscopy and magnetic studies reveal that the Cr and V atoms are in the valence states 3+ and 4+, respectively, which are the same as those of the parent compounds. Thus, it was concluded that the valence state of V4+/Cr3+ and the electron localization in the (1 − x)PbVO3− xBiCrO3 solid solutions are considerably robust.


Introduction
Valence states in solid solutions are sometimes different from those in their parent compounds, because of the difference in the energy of the valence orbitals of the constituent ions, that is, the electronegativity. High-pressure synthesis is a powerful tool for preparing solid solutions because the applied pressure prevents ion desorption due to Le Chatelier's principle. Recently, we reported a high-pressure synthesis and elucidated the valence states of perovskite-type PbVO 3 − BiCoO 3 solid solutions [1]. An intermetallic charge transfer between vanadium and cobalt ions, that is, V 4+ + Co 3+ → V 5+ + Co 2+ , was observed, which resulted in a polar-to-nonpolar structural transition and a large volume shrinkage.
Perovskite-type PbVO 3 possesses a PbTiO 3 -type tetragonal distortion [2,3]. The c/a ratio (tetragonality) reaches 1.23, which is considerably higher than that of PbTiO 3 . Both the divalent lead ion (A-site) and the tetravalent vanadium ion (B-site) contribute to the enhancement of the tetragonal distortion. This tetragonal structure is stabilized by the strong covalency of the Pb-O bonds and the stereochemical activity of the 6s 2 lone pairs [4]. Furthermore, the Jahn-Teller effect of V 4+ (3d 1 ) contributes to strengthening the distortion [5,6]. The t 2g orbital degeneracy is lifted in the square pyramidal geometry, splitting into stabilized 3d xy and destabilized 3d yz and 3d zx orbitals, as shown in Figure 1. A single d-electron occupies the 3d xy orbital, which stabilizes the distortion.
At room temperature, perovskite-type BiCrO 3 has a centrosymmetric monoclinic symmetry with the space group C2/c ( Figure 1) [7][8][9], which is transformed into a GdFeO 3 -type orthorhombic structure with the Pnma space group at 420 K [8,10,11]. The valence state is Bi 3+ Cr 3+ O 3 , which is not Jahn-Teller active [12]. BiCrO 3 undergoes an antiferromagnetic transition below the Néel temperature, T N , of ~110 K [7]. Spin reorientation occurs at approximately 80 K, increasing the magnetization caused by the Dzyaloshinskii-Moriya interaction [8]. Notably, a related compound PbCrO 3 , which also possesses a trivalent chromium ion [13] Based on this context, we investigated the valence state of (1 − x)PbVO 3 − xBiCrO 3 solid solutions for the following reasons. First, an intermetallic charge transfer and a complex valence state can be expected because of the close energy of the valence orbitals of chromium, vanadium, and lead ions. Second, an insulator-to-metal transition similar to that of PbV 1-x Cr x O 3 may occur [14]. Herein, the evolution of the crystal structure, valence state, and electronic properties of (1 − x)PbVO 3 − xBiCrO 3 (x = 1/8 − 7/8) solid solutions were studied. Solid solutions were synthesized using a high-pressure and high-temperature (HP-HT) method. The crystal structure was observed to change from polar tetragonal (PbVO 3 side) to centrosymmetric monoclinic (BiCrO 3 side). X-ray absorption spectroscopy (XAS) and magnetic susceptibility measurements confirmed that all the compounds had the valence states V 4+ and Cr 3+ , which were the same as those of the end members. The semiconductive behavior of these compounds is attributed to these valence states. The weak ferromagnetism was systematically suppressed by increasing the PbVO 3 content, which may be attributed to a change in the spin orientation. The pressure dependence of electrical resistivity in the 4/8PbVO 3 -4/8BiCrO 3 sample demonstrates the robustness of the V 4+ -Cr 3+ valence state and the electron localization. A tetragonal-to-cubic phase transition, which may result in an insulator-to-metal transition, was observed in 7/8PbVO 3 -1/8BiCrO 3 . This transition was accompanied by a negative thermal expansion.

Experimental section
Polycrystalline samples of (1 − x)PbVO 3 − xBiCrO 3 (x = 2/8, 3/8, 4/8, 5/8, 6/8, and 7/8) were synthesized from stoichiometric mixtures of PbO, Bi 2 O 3 , V 2 O 3 , V 2 O 5 , and Cr 2 O 3 packed in a Pt capsule at 5 GPa and 1273 K for 30 min using a cubic anvil cell-type high-pressure apparatus. A 7/8PbVO 3 -1/8BiCrO 3 (x = 1/8) sample was synthesized at 7 GPa and 1273 K for 30 min using a Walker-type high-pressure apparatus. Synchrotron X-ray diffraction (SXRD) measurements were conducted on BL19B2 and BL02B2, SPring-8, Japan, at a wavelength of λ = 0.42 Å. The SXRD data were analyzed via the Rietveld method using the RIETAN-FP program [15]. Negligible amounts of impurities, including Pb 3 V 2 O 8 and Bi 2 O 3 , were observed, except in case of the 6/8PbVO 3 -2/8BiCrO 3 sample. The crystal structures were drawn using the VESTA program [16]. Soft XAS measurements of the L 2,3 edge of Cr for all the samples and BiCrO 3 were conducted at room temperature using the total electron yield method at the BL12, SAGA-LS, Kyushu Synchrotron Light Research Center. The temperature dependence of the magnetic susceptibility of all samples with x = 1/8-5/8 was measured using a superconducting quantum interference device magnetometer (Quantum Design, MPMS). The temperature (two-probe method) and pressure dependence (quasi-four-probe method) of the electrical resistivity of the 4/8PbVO 3 -4/8BiCrO 3 sample was measured using a multimeter (Keithley 2100), a cryostat, and a high-pressure apparatus. Perovskite-type phases were obtained for all the samples. Space groups and crystal structures were successfully determined for 1/8PbVO 3 -7/8BiCrO 3 and 7/8PbVO 3 -1/8BiCrO 3 samples, as shown in Figure 3. However, in the other composites, peak overlaps, as explained below, and the occurrence of peak broadening, which  may result from local disorder [17], hindered good agreement from crystal structure refinement. The lattice parameters and crystal structure parameters are shown in Figure S1 and Tables S1-S4. A centrosymmetric monoclinic (C2/c) phase, which is the same as that in BiCrO 3 , was observed in the x = 7/ 8-6/8 samples. The monoclinic phase seems to be preserved in the samples with x = 5/8-3/8; however, because of peak overlaps, their lattice parameters could not be precisely determined. No superlattice reflection indicating cation ordering was observed, even in the 4/8PbVO 3 -4/8BiCrO 3 sample. A polar tetragonal (P4mm) phase was also observed in the 4/8PbVO 3 -4/8BiCrO 3 and 3/8PbVO 3 -5/8BiCrO 3 samples. The fraction of the tetragonal phase was found to increase with increasing PbVO 3 content, and the 7/8PbVO 3 -1/8BiCrO 3 sample possessed only the tetragonal structure. Figure 4 shows the evolution of the lattice parameters of the tetragonal phase, and Table  S2 summarizes the corresponding numerical values. The c/a ratio systematically increased toward the PbVO 3 edge, which can be ascribed to an increase in the fraction of the Jahn-Teller active tetravalent vanadium ion [18]. The 6/8PbVO 3 -2/8BiCrO 3 samples showed an abnormal peak broadening, even the samples synthesized at 7 GPa ( Figure S2). This broadening may result from composition fluctuations owing to phase separation. While an additional phase was observed in the (1 − x)PbVO 3 − xBiCoO 3 solid solutions, no additional changes were observed in the present system [1]. Figure 5 shows the XAS spectra of the Cr L 2,3 edge of BiCrO 3 and all the synthesized (1 − x)PbVO 3 − xBiCrO 3   solid solutions. White lines at approximately 576 eV (L 3 edge) and 584 eV (L 2 edge), confirming the presence of the Cr 3+ state, were observed for the BiCrO 3 sample. Similar spectra were obtained for the (1 − x)PbVO 3 − xBiCrO 3 samples, indicating that trivalent chromium ions were preserved even after the solid solutions were prepared. To completely determine the valence state and reveal the magnetic properties, the temperature dependence of the magnetic susceptibility was measured for the (1 − x)PbVO 3 -xBiCrO 3 samples with 3/ 8 ≤ x ≤ 7/8; results are shown in Figure 6 (a)-(e). For each sample, an antiferromagnetic transition occurred at approximately 110 K. Clear Curie-Weiss behaviors were observed above 200 K, with an obvious linear behavior in the reciprocal plot (insets of Figure 6 (a)-(e)). The data in the temperature range of 250-300 K in the reciprocal plots were fitted using the Curie-Weiss law, χ −1 = H/M = (T − θ W )/C, where C is the Curie constant and θ W is the Weiss temperature. Table 1 summarizes the obtained and theoretical values. The value of the Curie constant for 1/8PbVO 3 -7/8BiCrO 3 was larger than that expected for Pb 2+ V 4+ O 3 -Bi 3+ Cr 3+ O 3 . A similar trend, attributed to the presence of a short-range order, was reported for BiCrO 3 [19,20].

Results and discussion
The same reason could explain the overestimation of the Weiss temperature. By increasing the PbVO 3 content, the Curie constant converged to the theoretical value ( Figure S3). Given that the presence of Cr 3+ was verified by XAS, this result confirms the valence states of Pb 2+ V 4+ O 3 -Bi 3+ Cr 3+ O 3 for the solid solutions.
The preparation of the solid solution clearly affected the antiferromagnetic properties of the compound. Below the T N , an increase in the susceptibility of the 1/8PbVO 3 -7/8BiCrO 3 sample was observed at 90-100 K, which may correspond to a spin reorientation transition [8,17]. The antiferromagnetic orderings possess weak ferromagnetic components because of the Dzyaloshinskii-Moriya interaction [21]. In BiCrO 3 , the magnetic moments align along the b-axis at 90 K and rotate in the bc-plane by ~50° below the spin reorientation temperature (60-80 K) [8]. This reorientation results in a prominent increase in spontaneous magnetization. Interestingly, this anomaly at 90-100 K was suppressed by gradually increasing the PbVO 3 content. The remanent magnetization also decreased systematically ( Figure 6 (f) shows the M-H curves), which may be attributed to a change in the orientation of the magnetic spin [22]. A theoretical study predicted that the combination of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) determines the spin direction [23]. In the case of Cr 3+ and V 4+ , HOMO and LUMO are 3d xy;yz;zx and 3d x 2 À y 2 ;z 2 orbitals, respectively, for Cr 3+ , and both 3d xy;yz;zx orbitals for V 4+ . Neutron diffraction measurements are necessary to reveal the details.
The (1 − x)PbVO 3 -xBiCrO 3 samples exhibited Mott insulator features at ambient pressure. As a representative example, the temperature dependence of the electrical resistivity of the 4/8PbVO 3 -4/8BiCrO 3 sample is shown in Figure 7 (a). The inset shows the lnρ -1000/T plot. A clear semiconductive behavior was observed. The activation energy obtained from the Arrhenius plot was E a = 0.28 eV, which is similar to that observed in perovskite-type tetravalent vanadates and trivalent chromates (Mott insulators) [22,24]. The pressure dependence of the electrical resistivity up to 5 GPa was measured because the external field was expected to induce an insulatorto-metal transition or intermetallic charge transfer, as previously observed for PbVO 3 , PbCrO 3 , PbCoO 3 , and BiNiO 3 [2,13,[25][26][27]; results are shown in Figure 7 (b). A clear resistivity drop was not observed, indicating that the abovementioned phenomena did not occur up to 5 GPa. Taken together, these results suggest that the valence state of V 4+ /Cr 3+ and the electron localization in the (1 − x)PbVO 3 -xBiCrO 3 solid solutions are quite robust.
A tetragonal-to-cubic phase transition was observed in the 7/8PbVO 3 -1/8BiCrO 3 sample, which was accompanied by a negative thermal expansion. Note that a crystal structure transition was not observed in PbVO 3 because the Curie temperature was higher than the decomposition one [2]. Figure 8 shows the SXRD patterns for heating and cooling between 100 and 700 K. A cubic perovskite-type phase appeared at 700 K during heating. The unit cell volume was ~8.0% smaller than that of the tetragonal structure, which could lead to the realization of an excessively high negative thermal expansion. The phase transition was not completed at temperatures up to 700 K. The fraction of the cubic phase decreased with decreasing temperature; however, approximately 16% of the cubic phase remained even at 100 K, as shown in Figure 9. The tetragonal-to-cubic phase transition may be accompanied by an insulator-to-metal transition, as in the case of Cr-substituted PbVO 3 [14]. The presence of trivalent chromium ions weakens the Jahn-Teller effect and may introduce electronic itinerancy, which may trigger phase transition in 7/8PbVO 3 -1/8BiCrO 3 .

Conclusion
In summary, we reported the evolution of the crystal structures, valence states, and electronic properties of (1 − x)PbVO 3 -xBiCrO 3 solid solutions. Solid solutions were synthesized using the HP-HT method. The crystal structures changed from polar tetragonal (P4mm, PbVO 3 side) to centrosymmetric monoclinic (C2/c, BiCrO 3 side). A tetragonal-to-cubic phase transition, which may result in an insulator-to-metal transition, was observed at approximately 700 K in 7/8PbVO 3 -1/8BiCrO 3 . This transition was accompanied by a negative thermal expansion. XAS and magnetic measurements revealed that the solid solutions have a valence state of Pb 2+ V 4+ O 3 -Bi 3+ Cr 3+ O 3 , which is the same as that of the end members. The