A Mixed Metal Phosphate Containing Two Types of Phosphoric Anionic Groups: Cs_2Ga_4(P_2O_7)_2(P_4O_(13))

A new mixed metal phosphate of Cs_2Ga_4P_8O_(27), which also can be written as Cs_2Ga_4(P_2O_7)_2(P_4O_(13)), was synthesized by high temperature solid state syntheses and structurally characterized by X-ray single-crystal diffraction for the first time. The title compound crystallizes in monoclinic system with space group P2_1/c(No.14), and features a 3D framework which can be considered as alternating layers of {Ga2(P_4O_(13))}_n and {Ga_2(P_2O_7)_2}_n~(2n-) parallel to the bc plane further connected by Ga-O-P linkages, where Cs~+ cations are located in the free space between two adjacent layers to charge the valence. The Ga~(3+) cations in the compound contain two kinds of coordination models(4 and 6). Furthermore, the title compound coexists of two phospho-ric anionic groups which are non-condensed horseshoe-shaped(P_4O_(13)) and two(P_2O_7) with different symmetries. The density functional theory calculations indicate that Cs_2Ga_4P_8O_(27) is a direct band gap insulator with flat valence and dispersive conduction bands and a band gap of 4.13 eV.