Aluminate socialite, Ca8[Al12O24](CrO4)2, with tetragonal and orthorhombic superstructures
European Journal of Mineralogy Volume 8 Number 3 (1996), p. 477 - 486
published: Jun 17, 1996
manuscript accepted: Jan 31, 1996
manuscript received: Jun 22, 1995
ArtNo. ESP147050803002, Price: 29.00 €
Abstract Transmission electron microscopy (TEM) has provided evidence for ordering of [Ca4CrO4]6+ clusters in an aluminate sodalite, Ca8[Al12O24](CrO4)2. At high temperature (probably > 610 K), the [Ca4CrO4]6+ clusters in the cubic aluminate sodalite are disordered together with the framework oxygen atoms. As the temperature is lowered (probably between 453 and 610 K), the [Ca4CrO4]6+ clusters are ordered and are accompanied by positional modulations of the framework oxygen atoms on the Ol and O2 positions. As the temperature is lowered, different ordering of the [Ca4CrO4]6+ clusters produces tetragonal (probably between 432 and 453 K) and orthorhombic phases (probably < 432 K). Remnants of transitions from an unmodulated cubic structure, to a modulated pseudo-cubic structure, to an orthorhombic superstructure are directly observed in a crystal fragment. The cell parameters for the tetragonal and orthorhombic superstructures, with the cubic cell parameter of 9.222 Å, are (a+b) x (-a+b) x c, and 3a x 2b x c, respectively.