Crystal Structure Determination of SbSI

Abstract

A change of atomic arrangement of SbSI between the paraelectric phase and the ferroelectric phase is investigated. Analyses are made on Weissenberg photographs taken at 35°C (paraelectric phase) and at 5°C (ferroelectric phase). In the paraelectric phase (space group D 2 h 16 - Pnam , lattice constants, a =8.52Å b =10.13Å c =4.10Å), the atomic positions are: Sb( x =0.118 9 , y =0.124 3 , z =1/4), S( x =0.84 0 , y =0.05 0 , z =1/4), I( x =0.508 0 , y =0.827 6 , z =1/4); the thermal vibration of Sb ions along the c axis is relatively large with the mean square amplitude \bar U z 2 =(0.16) 2 Å 2 . In the ferroelectric phase ( C 2 v 9 - Pna 2 1 ), the x and y parameters are substantially the same as those determined in the paraelectric phase. Major changes take place in the z parameters: Sb( z =1/4+0.04 8 ) and S( z =1/4+0.01) shift along the ferroelectric c axis with respect to iodine by 0.20Å and 0.05Å respectively. Evidences are shown that the phase transition is of displacive type.