Advances in data reduction of high-pressure x-ray powder diffractiondata from two-dimensional detectors: a case study of schafarzikite(FeSb2O4)

Abstract

Methods have been developed to facilitate the data analysis of multiple two-dimensionalpowder diffraction images. These include, among others, automatic detection andcalibration of Debye–Scherrer ellipses using pattern recognition techniques, andsignal filtering employing established statistical procedures like fractile statistics.All algorithms are implemented in the freely available program package Powder3Ddeveloped for the evaluation and graphical presentation of large powder diffraction datasets.As a case study, we report the pressure dependence of the crystal structure of iron antimony oxideFeSb2O4 (p≤21 GPa, T = 298 K) using high-resolution angle dispersive x-ray powder diffraction.FeSb2O4 shows two phase transitions in the measured pressure range. Thecrystal structures of all modifications consist of frameworks ofFe2+O6 octahedra and irregularSb3+O4 polyhedra. At ambientconditions, FeSb2O4 crystallizesin space group P42/mbc (phase I). Between p = 3.2 GPa and 4.1 GPa it exhibits a displacive second order phase transition to a structure of space groupP 21/c (phaseII, a = 5.7792(4) Å, b = 8.3134(9) Å,c = 8.4545(11) Å,β = 91.879(10)°,at p = 4.2 GPa). A second phase transition occurs betweenp = 6.4 GPa and 7.4 GPa to a structure of space groupP42/m (phaseIII, a = 7.8498(4) Å, c = 5.7452(5) Å,at p = 10.5 GPa). A nonlinear compression behaviour over the entire pressure range isobserved, which can be described by three Vinet equations in the ranges fromp = 0.52 GPa top = 3.12 GPa,p = 4.2 GPa top = 6.3 GPa andfrom p = 7.5 GPa to p = 19.8 GPa. The extrapolated bulk moduli of the high-pressure phases were determined toK0 = 49(2) GPa for phaseI, K0 = 27(3) GPa forphase II and K0 = 45(2) GPa for phase III. The crystal structures of all phases are refinedagainst x-ray powder data measured at several pressures betweenp = 0.52 GPa, and 10.5 GPa.