Rubidium chromium fluoride structures of the hexagonal tungsten bronze type

Abstract

Rb x CrF 3 samples with x = 0.20, 0.25, 0.275 and 0.30 have been studied by selected-area electron diffraction and high-resolution electron microscopy. Four different structure types related to the hexagonal tungsten bronze structure were observed: type (i) with a =2√3 a HTB, b = b HTB and c = c HTB; type (ii) with a = a HTB, b = b HTB and c = 3 c HTB; type (iii) with a = a HTB, b = b HTB and c = 6 c HTB; and type (iv) with a = √3 a HTB, b = b HTB and c=3c HTB. The HREM images reveal an ordering of Rb atoms in the hexagonal tunnel sites. Images of models assuming certain ordered occupancies have been calculated and matched with those observed.