Synthesis and crystal structure of the alkaline-earth thallates M_nTl_2O_(3+n) (M=Ca,Sr)

摘要

The phases present in the Tl_2O_3–MO system, where M=Ca and Sr, have been synthesised and characterised using powder neutron diffraction data. The structures of the phases known to exist in the Ca_nTl_2O_(3+n) system with n=1, 1.5, 2 and 3 have been refined and the oxycarbonate phase with composition Ca_4Tl_2O_6CO_3 has been identified. In the Sr_nTl_2O_(3+n) system, the structures of the phases with n=1, 2 and 3 have also been refined. The MnTl_2O_(3+n) phases with M=Ca and Sr and n=1–3 are related to the lillianite structure while the n=4 compound is an oxycarbonate that has been found to crystallise in I4/mmm symmetry for M=Ca and in three different modifications with space groups P4/mmm, I4/mmm and Pmmm for M=Sr, depending on the ordering of the carbonate groups. The relationships between these structures are discussed and comparisons are made to the previously published structures determined from X-ray powder diffraction data.