Cs 2 CuSi 5 O 12 phase transition? A. M. T. Bell 1* , C. M. B. Henderson 2 . 1 HASYLAB/DESY, Notkestraße 85, 22607 Hamburg, Germany. 2 SEAES, University of Manchester, Manchester M13 9PL, UK. * Contact author; e-mail: tony.bell@desy.de. Keywords: Mineral chemistry, phase transitions, Rietveld refinement. Abstract. We have recently revised the ambient temperature crystal structure of the hydro- thermally synthesized leucite analogue Cs 2 CuSi 5 O 12 [1] using high-resolution synchrotron X- ray and neutron powder diffraction. This has the Pbca crystal structure with Si and Cu com- pletely ordered over the tetrahedral T-sites (T1 = Cu, T2-6 = Si). The Cs 2 CuSi 5 O 12 lattice parameters are: - a = 13.58943(6)Å, b = 13.57355(5)Å and c = 13.62296(4)Å. As a, b and c are so close it was thought that if this sample were to be heated the crystal structure may undergo a phase transition to a higher symmetry space group and/or to a structure with some T-site disorder. High temperature synchrotron X-ray powder diffraction measurements show a significant lattice parameter shift in the temperature range 329-335K. Above this tempera- ture range the crystal structure becomes less distorted but the ordered Pbca structure is re- tained. This “phase transition” is reversible on cooling but takes place in the temperature range 319-317K. 1. Introduction Synthetic analogues of the tetrahedral silicate framework mineral leucite (KAlSi 2 O 6 ) have the general formulae of A 2 BSi 5 O 12 or ACSi 2 O 6 (A = K + , Rb + , Cs + ; B = Mg 2+ , Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , Cd 2+ ; and C = Al 3+ , Fe 3+ ). A cations are extraframework, B and C cations are incorporated into the silicate framework. By varying the synthesis conditions, stoichiometry and framework cation ordering the crystal structures of these leucite analogues can be P2 1 /c monoclinic [2], Pbca orthorhombic [3], I4 1 /a tetragonal [4] or Ia3d cubic [1, 2, 5]. Phase transitions from tetragonal to cubic [6, 7, 8] and monoclinic to orthorhombic [9] have also been observed on heating leucite analogues. The hydrothermally synthesized leucite analogue Cs 2 CuSi 5 O 12 [1] has the Pbca crystal struc- ture with Si and Cu completely ordered over the tetrahedral T-sites (T1 = Cu, T2-6 = Si). The room temperature lattice parameters are: - a = 13.58943(6)Å, b = 13.57355(5)Å and c = 13.62296(4)Å. As a, b and c are so close it was thought that if this sample were to be heated the crystal structure may undergo a phase transition to a higher symmetry space group and/or to a structure with some T-site disorder.