Crystal Structure of γ‐Ca 2 P 2 O 7

Three polymorphs of calcium pyrophosphate, Ca 2 P 2 O 7 , are known. Here the crystal structure of γ‐Ca 2 P 2 O 7 is described for the first time. The crystal structure could be solved from powder X‐ray data. The results were confirmed by 31 P high‐resolution solid‐state NMR spectroscopy measurements and by quantum chemical calculations under period boundary conditions. With the help of spin‐echo experiments the 2 J ( 31 P, 31 P) coupling for the P−O−P bonds could be determined. The different polymorphs can unambiguously be identified from 31 P magic‐angle‐spinning NMR spectroscopy. The 31 P NMR chemical shift parameters of α‐, β‐ and γ‐Ca 2 P 2 O 7 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‐Ca 2 P 2 O 7 belongs to the structure type of Cd 2 P 2 O 7 in the space group ( 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)°).