Crystal Growth and Optical Properties of Cs 2 TeX 6 (X = Cl, Br)

High-quality single-crystal ingots of Cs₂TeX₆ (X = Cl, Br) with dimensions of Ø10 × 40 mm and a mass of 9 g were grown using the vertical Bridgman-Stockbarger method. The solution synthesis method improves the purity of the resulting Cs₂TeX₆ powders to 99.995% compared to the starting materials (99%) and yields more than 95%. The photoluminescence of Cs₂TeX₆ was studied in the temperature range 80-280 K. A broad luminescence with a wavelength maximum at 595 nm for Cs₂TeCl₆, and at 693 nm for Cs₂TeBr₆, was observed at 80 K. With the temperature increase, the emission maximum of Cs₂TeBr₆ exhibits a red-shift of 11 nm and the luminescence intensity is completely quenched above 210 K. For Cs₂TeCl₆, the emission maximum exhibits a red-shift of 14 nm and weak luminescence persists up to room temperature. The proposed luminescence mechanism via self-trapped exciton (STE) emission is supported by strong electron-phonon coupling, as evidenced by the high Huang-Rhys parameters (S = 25 for Cs₂TeCl₆ and S = 21 for Cs₂TeBr₆). Raman spectra reveal only four active modes, T_2g(1), T_2g(2), E_g, and A_1g, identified between 49 and 292 cm^-1, as well as a four-phonon decay through the E_g mode of Cs₂TeCl₆.