A tem study of defect ordering in a calcium yttrium sulfide solid solution with an average NaCl-type structure

Abstract

A detailed transmission electron microscope and powder X-ray diffraction study has been made of Ca5Y4S11, which is one limiting composition of the widely substoichiometric, NaCl-related, solid solution (1 - x)CaS + (x) Y2S3, 0 ≤ x ≤ 2/7, in the CaS-Y2S3 system. Long-range vacancy ordering and associated structural relaxation characterized by the modulation wave vector q = 1 2(111) is shown to lower the space group symmetry from cubic to rhombohedral. Twinning between rhombohedral domains and antiphase boundaries within a particular rhombohedral domain are observed as well as a spectacular diffuse intensity distribution which shows many similarities but also significant differences to that previously reported in substoichiometric transition metal carbides and nitrides. Show more