THE ROLE OF THE Sb3+ LONE-ELECTRON PAIRS AND Fe2+ COORDINATION IN THE HIGH-PRESSURE BEHAVIOR OF BERTHIERITE

Abstract

A single crystal of natural berthierite, FeSb 2 S 4 , was investigated at high pressure by means of X-ray diffraction using a diamond-anvil cell equipped with diamond backing plates. No phase transitions were indicated up to 8 GPa. The third-order Birch–Murnaghan equation of state, calculated using high-accuracy volume–pressure data up to 8.05 GPa, gave the following coefficients: V 0 = 608.78(7) A 3 , K T0 = 37.2(2) GPa and K ′ = 7.0(1). The evolution of the structure as a function of pressure has been determined at seven different pressures up to 7.41 GPa. As in other structures with stereochemically active lone-electron pairs (LEP), the Sb 3+ LEP, influencing long Sb–S bonds in berthierite, accommodate most of the compression. The Fe octahedron, which is the stiffest coordination polyhedron in berthierite, increases its distortion until approximately 5 GPa, but shows pronounced stiffening at higher pressures. This deformation at high pressures can be related to an increase in the Jahn–Teller effect on the Fe 2+ coordination. The influence of Fe on compressional behavior makes a distinct difference between the compression of berthierite and that of stibnite, Sb 2 S 3 . The bridging Fe coordination between the structural rods in berthierite makes it stiffer. This, together with the direct structural relation to the analogous PbBi 2 S 4 , galenobismutite, makes the compressional characteristics of berthierite more akin to those of galenobismutite, despite quantitative differences in the stereochemical expression of the LEP of Sb 3+ and Bi 3+ .