Crystal Structure of γ-Ca2P2O7

Abstract

Three polymorphs of calcium pyrophosphate, Ca2P2O7, are known. Here the crystal structure of γ-Ca2P2O7 is described for the first time. The crystal structure could be solved from powder X-ray data. The results were confirmed by 31P high-resolution solid-state NMR spectroscopy measurements and by quantum chemical calculations under period boundary conditions. With the help of spin-echo experiments the 2J(31P,31P) coupling for the P−O−P bonds could be determined. The different polymorphs can unambiguously be identified from 31P magic-angle-spinning NMR spectroscopy. The 31P NMR chemical shift parameters of α-, β- and γ-Ca2P2O7 were determined from magic-angle-spinning experiments at slow spinning frequencies, while a peak assignment was achieved on the basis of gipaw-calculations and confirmed by the observed J-coupling values. The polymorph γ-Ca2P2O7 belongs to the structure type of Cd2P2O7 in the space group (Formula presented.) (a=6.6660 (2) Å, b=6.7220 (2) Å, c=6.7374 (2) Å, α=65.113 (2)°, β=87.763 (2)°, γ=85.076 (2)°).