Structure comparison and optical properties of Na7Mg4.5(P2O7)4: a sodium magnesium phosphate with isolated P2O7 units

Abstract

Herein, a sodium magnesium diphosphate, Na₇Mg_4.5(P₂O₇)₄, was synthesized via a standard solid-state reaction method, and a single crystal X-ray diffraction method was adopted to determine its crystal structure. It was found that it crystallizes in the triclinic system with the space group P. The crystal structure of Na₇Mg_4.5(P₂O₇)₄ features a parallelogram channel composed of corner/edge-sharing isolated [P₂O₇] dimers and a Na(2)O₅ polyhedra to form a 3D framework. Thermal behaviors indicate that Na₇Mg_4.5(P₂O₇)₄ congruently melts and IR spectrum shows the existence of the P₂O₇ groups. Moreover, Na₇Mg_4.5(P₂O₇)₄ possesses an experimental band gap of 5.90 eV and an ultraviolet (UV) cut-off edge of about 210 nm, which was confirmed via diffuse reflection spectrum. To analyze the contribution of the atomic orbitals to the energy band, band structures and partial density of states (PDOS) of Na₇Mg_4.5(P₂O₇)₄ have also been reported.