A Raman scattering study of pressure-induced phase transitions in nanocrystalline Bi₂MoO₆

Abstract

Lattice dynamics calculations and a high-pressure Raman scattering study of nanocrystalline Bi(2)MoO(6), a member of the bismuth-layered Aurivillius family of ferroelectrics, are presented. These studies showed the onset of two reversible second-order or weakly first-order phase transitions near 2.5 and 4.5 GPa as well as some subtle structural changes at 8.2 GPa. Symmetry increases upon application of pressure and the first phase transition involves, most likely, the loss of the MoO(6) tilt mode around a pseudo-tetragonal axis. The second phase transition is associated with the instability of a low wavenumber mode, which behaves as a soft mode. This soft mode most likely corresponds to the polar E(u) mode of the tetragonal I4/mmm aristotype and Bi(2)MoO(6) transforms at 4.5 GPa into the centrosymmetric orthorhombic phase. The sequence of the pressure-induced phase transitions in nanocrystalline Bi(2)MoO(6) is similar to that observed for bulk Bi(2)WO(6) but the critical pressures are significantly lower for the molybdenum compound. Our results also show that the critical pressure of the first phase transition is slightly lower for the nanocrystalline Bi(2)MoO(6) (2.5 GPa) than for the microcrystalline (bulk) Bi(2)MoO(6) (2.8 GPa).

An X-ray diffraction study of pressure-induced phase transitions in Bi2MoO6

“…Based upon the equation of state, it was observed that Bi 2 MoO 6 undergoes a phase transformation at $ 6.8 GPa which confirms the previous reports from Raman experiments [14,15]. This high-pressure phase continues to be indexed to an orthorhombic structure and upon decompression from $47 GPa reverts to the low-pressure structure at $1.7 GPa.…”

“…In contrast to this behavior, distortion of the WO 6 octahedra significantly increases in Bi 2 WO 6 with increasing pressure [18]. It was also shown that the high-pressure phases of Bi 2 WO 6 and Bi 2 MoO 6 are orthorhombic and most likely centrosymmetric [14,15,18].…”

“…For instance, Bi 2 MoO 6 has been found to undergo phase transitions at 2.8 GPa and 7.0 GPa while Bi 2 WO 6 transforms at 3.4 GPa and 6.2 GPa [14,18]. Raman data also indicated that the pressureinduced structural changes in Bi 2 MoO 6 are mainly related to the rigid rotations of MoO 6 octahedra and changes in the Bi 2 O 2 layers [14,15]. In contrast to this behavior, distortion of the WO 6 octahedra significantly increases in Bi 2 WO 6 with increasing pressure [18].…”

“…Pressure dependent studies of this family of compounds are scarce and only high-pressure Raman spectroscopy has been undertaken for Bi 2 MoO 6 [14,15]. Raman scattering studies under pressure have also been reported for a few other crystals belonging to this family of compounds such as Na 0.…”

Hydrothermal Synthesis, Characterization, and Optical Properties of Ce Doped Bi2MoO6 Nanoplates

“…Additional weak spin-orbit doublet peaks with binding energy of 157.92 eV for Bi 4f 7/2 and 163.40 eV for Bi 4f 5/2 are also detected, suggesting that some of bismuth exist as the (+3 − ) valence state [16]. Probably, the Bi (+3− ) formal oxidation state could be attributed to the substoichiometric phase within the microsized plates [16] [23][24][25].…”