Effect of isomorphic atom substitution on lattice parameters of $TlBX_2$ -type mixed crystals

Özet

The variation of the lattice parameters of $TIBX_2$-type mixed crystals with composition have been investigated by X-ray diffraction technique. The lattice anisotropy (c/a) of the mixed crystals changes linearly depending on substitution of the atoms located both at the centre and at the vertices of the $BX_4$ tetrahedra. For some of the mixed crystals studied, structural phase transition (monoclinic $\rightleftharpoons$ tetragonal) is observed due to atom substitution when the sum of tetrahedral covalent radii of the B and X atoms in $BX_4$ tetrahedra reaches a critical value of about 0.255 nm.