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 сij 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.