Crystal and Band Structures, Bonding, and Optical Properties of Solid Compounds of Alkaline Indium (III) Pyrophosphates MInP2O7 (M = Na, K, Rb, Cs)

Ternary indium (III) pyrophosphates with threedimensional frameworks of MInP2O7 (M = Na, K, Rb, Cs) have been found, and the crystal structures have been first reported in this paper. The optical and bonding properties were investigated in terms of the absorption and emission spectra, as well as the calculated band structures and density of states. The crystal band structures calculated by the DFT method show that the solid compounds of MInP2O7 are insulators with direct band gaps, and the calculated average third-order susceptibilities are increasing in the order of K < Rb < Cs in the MInP2O7 crystal (M = K, Rb, Cs).