Optical properties of $$\hbox {TlGa}(\hbox {S}_{{x}}\hbox {Se}_{1-x})_{2}$$ TlGa ( S x Se 1 - x ) 2 layered mixed crystals $$(0 \le x \le 1)$$ ( 0 ≤ x ≤ 1 ) : Absorption edge and photoluminescence study at $$T = 10\hbox { K}$$ T = 10 K