Polymorphism of TlLn(MoO4)2, Ln=La-Ho

Type TlLn(MoO4)2 compounds, where In are rare earths elements from La to Dy, were investigated. It is found that all the compounds are polymorphic, except for the Ho ones. TlLa(MoO4)2 crystallizes according to a scheelite type structure: at high temperatures (above 750 deg C) the structure is ''purely'' scheelite, and at lower temperatures a distorted low-temperature modification of rare earth element compounds from Ce to Ho, an isostructure similar to the rhombic type KY(MoO4)2. Heating results in the transformation into a phase of an unknown structure through a modification of the pseudotetragonal RbPr(MoO4)2 type, which, as regards Sm and the heavier rare earth elements, possesses a very narrow temperature interval of stability. All the compounds melt at about 900 deg C, except for TbCe(MoO4)2 which decomposes at 720 deg C. The parameters of the elementary cells have been determined