Structural distortions and polymorphic behaviour in ABC2 and AB2C4 tetrahedral compounds

We examine the connections between thermal polymorphism and structural properties in the families of normal ABC₂ and defect AB₂C₄ tetrahedral structure compounds. We show that the thermal stability of these ternary compounds in the ordered tetragonal structure is strongly determined by the values of the lattice distortion parameters. Separate analysis is presented in terms of the room temperature lattice compression η_o=c_o /2a_o (tetragonal distortion) and of the anion displacement σ_o from the ideal position (internal distortion). We find that for the ABC₂ compounds the chalcopyrite structure remains stable up to the melting temperature for η_o values below $\text{η}{}^{\mathrm{\ast }}{}_{\mathrm{o}}\text{= 0.977 ± ± 0.001}$, while no critical value is obtained for the AB₂C₄ compounds. The exceptions of ZnGeP₂, AgInS₂, and AgInSe₂ are examined separately. Conversely, by considering the internal distortion, it turns out that the order-disorder behaviour occurs, for both families of compounds and without exceptions, for anion displacement values below $\text{σ}{}^{\mathrm{\ast }}{}_{\mathrm{o}}\text{= 0.0145 ± 0.0005}$. The effect on the symmetry changes due to the temperature dependence of the structural distortions is also considered and discussed.