Thermal properties of 2:1 bismuth borate ceramics: microscopic contribution of lone electron pairs for lattice expansion tensors

MURSHED M.M.; CURTI M.; SENYSHYN, A. ; FISCHER M.; BARAN, V.; PETERSEN, H.; MENDIVE C.

Boston

Congreso; Materials Research Society - Fall Meeting & Exhibit; 2018

Materials Research Society

The applications of bismuth-borates requireknowledge of thermal expansion coefficients for optical, mechanical andelectronic properties. However, to understand the microscopic features leadingto macroscopic thermal expansion of bismuth borates is not straightforward asborates mostly show high anisotropic thermal expansion even with negativeexpansion tensors. Herein we report the thermal expansion of Bi4B2O9 between 4K and 900 K using temperature-dependent neutron and X-ray powder diffractionsand Raman spectroscopy. Thermal expansion was modeled using theDebye-Einstein-Anharmonicity [1,2] fit. The calculated phonon density of states(PDOS) shows two broad continua with a sharp feature at 890(1) cm-1. Comparingthe highest frequency of the PDOS the model simulation results in a Debyefrequency that represents only the acoustic lower frequency parabolic spectrum.The model also follows six independent thermal expansion tensors of themonoclinic system. The 11B-enriched sample showed a mean B-O bond distance of138(1) pm within the whole investigated temperature range. Due tostereochemical activity of the 6s2 lone electron pair (LEP) of Bi3+, fourdistinctive BiOx polyhedra showed different distortion indices. The Wang-Liebaueccentricity (WLE) parameter [3] for each Bi-atom decreases with increasingtemperature, indicating the electron deformation density associated with LEP isa function of temperature as well as the thermal expansion tensors. Thesephenomena are of fundamental interest to understand temperature-dependentnon-linear optical and ferroelectric behaviors of relevant centrosymmetric systems.