Orthorhombic to cubic high temperature phase transition in synthetic Rb2CoSi5O12 leucite analogue

Orthorhombic to cubic high temperature phase transition in synthetic Rb2CoSi5O12 leucite analogue. A.M.T.Bell (Materials and Engineering Research Institute, Sheffield Hallam University, Sheffield, S1 1WB, UK). The previously unknown crystal structure of the Rb2CoSi5O12 leucite analogue has been determined to be Pbca orthorhombic [1]. This is isostructural with the crystal structure of the Cs2CdSi5O12 leucite analogue [2], which has ordered tetrahedrally coordinated sites (T-sites). High temperature X-ray powder diffraction on Rb2CoSi5O12 shows a phase transition [1] from Pbca orthorhombic to Pa-3 cubic at 457 K. This is a similar phase transition to that observed at 566 K in Cs2ZnSi5O12 leucite analogue [3]. In the Pbca leucite structure for Rb2CoSi5O12 there are 6 T-sites, 5 fully occupied by Si and 1 fully occupied by Co. However, in the corresponding Pa-3 structure there are only 2 T-sites, 1 fully occupied by Si and 1 partially ordered by Si (2/3 occupancy) and Co (1/3 occupancy). The central channel of the Pbca leucite structure is more distorted than for the Pa-3 structure. The decrease in distortion of this central channel means that there is a unit cell decrease on passing through the transition! [1] Bell, A.M.T. Minerals 13, (2023) 210. [2] Bell, A.M.T. et al. Acta Cryst. B50, (1994) 560-566. [3] Bell, A.M.T. & Henderson, C.M.B. Mineralogical Magazine, 76, (2012) 1257-1280.