Optical phonon modes and structure of ZnGa2Se4 and ZnGa2S4

We present the infrared and Raman study of the optical phonon modes of the defective compounds ZnGa2Se4 and ZnGa2S4. Most of the AIIB2IIIC4VI compounds have been found to crystallize in the thiogallate structure (defect chalcopyrite) with space group I4¯ where all cations and vacancies are ordered. For some Zinc compounds a partially disordered cationic sublattice with various degrees of cation and vacancy statistical distribution, which lead to the higher symmetry I4¯2m (defect stannite), has been reported. For ZnGa2Se4 we have found three modes of A symmetry, showing Raman activity only. In addition, we have observed each five modes of B and E   symmetry, showing infrared as well as Raman activity. The number of modes and their symmetry assignment, based on polarized measurements, clearly indicate space group I4¯ for the investigated crystals of ZnGa2Se4.Regarding ZnGa2S4 we have found three modes exclusively showing Raman activity (2A⊕1B1), and only eight modes showing infrared as well as Raman activity (3B2⊕5E  ). The assignment of the modes has been derived by analyzing the spectral positions of the vibrational modes in comparison to a number of AIIB2IIIC4VI compounds. From the number and symmetry assignment of the optical phonon modes we confirm that ZnGa2S4 most likely crystallizes in space group I4¯2m.