Synthesis, crystal structures and optical properties of two congruent-melting isotypic diphosphates: LiM3P2O7 (M=Na, K)

Two new isotypic diphosphates, LiNa3P2O7 (1) and LiK3P2O7 (2), have been synthesized by conventional solid-state reaction. The single-crystal X-ray structural analyses have shown that they crystallize in the orthorhombic space group C2221 (No. 20) with the unit cells: a=5.4966(2) Å, b=9.1365(4) Å, c=12.2764(5) Å for compound 1 and a=6.0373(14) Å, b=9.339(2) Å, c=13.292(3) Å for compound 2. The LiM3P2O7 (M=Na, K) consist of two-dimensional [LiP2O7]3− layers, which are composed by LiO4 tetrahedral and diphosphate groups, the Na or K atoms are filled in the interlayers and balance the charge. Second harmonic generation (SHG) on powder samples have been measured using Kurtz and Perry techniques. Thermal analyses, IR spectroscopy, UV–vis–NIR diffuse reflectance spectra, and band structure calculations are performed on the reported compounds.