Optical phonon modes and structure of ZnGa2Se4 and ZnGa2S4

Abstract We present the infrared and Raman study of the optical phonon modes of the defective compounds ZnGa 2 Se 4 and ZnGa 2 S 4 . Most of the A II B 2 III C 4 VI compounds have been found to crystallize in the thiogallate structure (defect chalcopyrite) with space group I 4 ¯ 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 I 4 ¯ 2 m (defect stannite), has been reported. For ZnGa 2 Se 4 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 I 4 ¯ for the investigated crystals of ZnGa 2 Se 4 . Regarding ZnGa 2 S 4 we have found three modes exclusively showing Raman activity (2 A ⊕1 B 1 ), and only eight modes showing infrared as well as Raman activity (3 B 2 ⊕5 E ). The assignment of the modes has been derived by analyzing the spectral positions of the vibrational modes in comparison to a number of A II B 2 III C 4 VI compounds. From the number and symmetry assignment of the optical phonon modes we confirm that ZnGa 2 S 4 most likely crystallizes in space group I 4 ¯ 2 m .