Structural Evolution in BiNbO₄

Abstract

The sequence of transitions between different phases of BiNbO 4 has been thoroughly investigated and clarified using thermal analysis, high-resolution neutron diffraction, and Raman spectroscopy. The theoretical optical phonon modes of the α-phase have been calculated. Based on thermoanalytical data supported by density functional theory (DFT) calculations, the βphase is proposed to be metastable, while the αand γ-phases are stable below and above 1040 °C, respectively. Accurate positional parameters for oxygen positions in the three main polymorphs (α, β, and γ) are presented and the structural relationships between these polymorphs are discussed. Even though no significant changes, only relaxation phenomena, are observed in the dielectric behavior of α-BiNbO 4 below 1000 °C, evidence of two further subtle transitions at ∼350 and 600 °C is presented through careful analysis of structural parameters from variable temperature neutron diffraction measurements. Such phase variations are also evident in the phonon modes in Raman spectra and supported by changes in the thermoanalytical data. These subtle transitions may correspond to the previously proposed antiferroelectric to ferroelectric and ferroelectric to paraelectric phase transitions, respectively.

Combined experimental and DFT approach to BiNbO4polymorphs

“…Aer a number of trials for determining the optimum sintering temperature, the a-BNO and b-BNO phases were stabilized at 1000 °C and 1070 °C respectively. 16,24,26,28,29,29,31,40,[78][79][80][81][82][83]…”

“…[23][24][25][26][27] The occurrence and the evolution of phase transition among these BNO polymorphs are intimately linked with experimentally controllable parameters such as temperature, pressure, pH level, and extrinsic defects. 23,26,26,[28][29][30][31] Moreover, the electronic and optical properties of these different polymorphs of BNO vary, making its detailed study a topic of great scientic interest. The common crystallographic building blocks of all these BNO polymorphs are the BiO 6 and NbO 6 octahedra, but the difference in their orchestration germinates the observed variation in physical properties.…”

“…23 For example, both a-BNO and b-BNO consist of corner-sharing niobate octahedral sheet stack separated by Bi 3+ ions, with the difference being the latter having corrugation in the stacking layer. 28 This affects the relevant orbital energies of Bi-6s and O-2p orbitals relevant to the valence band (VB) of the BNO, making the electronic band gap of a-BNO smaller than that of b-BNO. 11,23,32 The corrugation also yields higher crystallographic density in b-BNO compared to that of a-BNO.…”

Pressure-induced phase transition in α- and β-BiNbO4

“…14 Because of its rich polymorphism and excellent physical properties, structural stability and structural evolution in BiNbO 4 is an interesting topic and has been extensively investigated. 15 Using the solid state reaction, a-BiNbO 4 can be synthesized through Bi 2 O 3 and Nb 2 O 5 at 900 1C while b-BiNbO 4 can be obtained above 1150 1C. 8 This result has been attributed to the a-to-b irreversible transition at 1020 1C for BiNbO 4 .…”

“…14 Because of its rich polymorphism and excellent physical properties, structural stability and structural evolution in BiNbO 4 is an interesting topic and has been extensively investigated. 15…”