Ba 3.5 Cu 7.55 In 1.15 Se 9 : A Wide-Bandgap Copper Indium Selenide Reveals Strong Luminescence and Third-Harmonic Generation.

A new copper indium selenide, Ba 3.5 Cu 7.55 In 1.15 Se 9 , was synthesized by the KBr flux reaction at 800 °C. The compound crystallizes with orthorhombic Pnma , a = 46.1700(12) Å, b = 4.26710(10) Å, c = 19.8125(5) Å, and Z = 8. The structural framework mainly consists of four sites of cubane-type defective M 4 Se 3 (M = Cu, Cu/In) units with disordered Cu + /In 3+ ions present at the part corner of each unit. The single crystal emits intense photoluminescence at 657 nm with a relative quantum yield (RQY) 0.2 times that of rhodamine 6G powder. The compound belongs to a direct band gap at 1.91 eV, analyzed by Tauc's plot, and the energy is close to the PL position. The Hall effect measurement on a pressed pellet reveals an n-type conductivity with a carrier concentration of 3.358 × 10 17 cm -3 and a mobility of 24.331 cm 2 V -1 s -1 . Furthermore, the compound produces a strong nonlinear third-harmonic generation (THG), with an χ S (3) value of 1.3 × 10 5 pm 2 /V 2 comparable to 1.6 × 10 5 pm 2 /V 2 for AgGaSe 2 measured at 800 nm.