Elastic Properties and Regularities in Frequencies of Optical Phonons of \({{{\mathbf{A}}}^{{{\mathbf{II}}}}}{\mathbf{B}}_{2}^{{{\mathbf{III}}}}{\mathbf{C}}_{4}^{{{\mathbf{VI}}}}\) Compounds

Elastic constants с_ij of thiogalates CdGa₂S₄, CdGa₂Se₄, CdGa₂Te₄, and ZnGa₂Sе₄ are calculated using the density functional theory (DFT). The elastic moduli B have been calculated. The regularities have been found in the dependences of the optical phonon frequencies on the atomic masses in \({{{\text{A}}}^{{{\text{II}}}}}{\text{B}}_{2}^{{{\text{III}}}}{\text{C}}_{4}^{{{\text{VI}}}}\) compounds. The force constants of interatomic bonds in CdGa₂Te₄ and ZnGa₂Sе₄ compounds are determined.