Non-ambient crystal chemistry of stillwellite-like BaBPO<SUB>5</SUB> from single crystal XRD data

Abstract

Stillwellite-(Ce) and some its synthetic analogues tend to undergo phase transition from polar (ferroelectric) to nonpolar (paraelectric) modification on heating. However, the reasons for the transition and phase stability remain the subject of scientific debate. Here we present detail studies (scanning electron microscopy, energy-dispersive X-ray spectroscopy, Raman spectroscopy and single-crystal X-ray diffraction (SCXRD)) of hydrothermally grown BaBPO5 isostructural with stillwellite-(Ce). Its thermal behavior was studied by an in situ low- (from –173 to +25 °C) and high-temperature (HT; 25–800 °C) SCXRD. Fully ordered crystal structure of BaBPO5 (at T = 25 °C: trigonal, P3221, a = 7.1166(1) Å, c = 7.0011(1) Å, V = 307.07(1) Å3, R1 = 1.42 %) does not exhibit any change of symmetry upon cooling / heating unlike natural stillwellite-(Ce). Thermal expansion of BaBPO5 is almost isotropic (αmin = 8.4, αmax = 8.7⋅10-6 °C-1) despite the chain nature of borophosphate anion. The comparative crystal chemical analysis of HT behavior of cationic polyhedra in the stillwellite-family members is presented.