The Crystal Structures of High‐ and Low‐Temperature Silver Pyrophosphate: Structural Instability and Its Consequences

The crystal structures of the low‐temperature (LT) and high‐temperature (HT) modifications of silver pyrophosphate, Ag 4 P 2 O 7 , were determined from single‐crystal X‐ray data. Thermal analysis, vibrational and 31 P‐MAS‐NMR spectroscopy studies, and density functional theory calculations complement the results. The crystal structure of HT‐Ag 4 P 2 O 7 ( T = 487 °C, , Z = 2, a = 5.5734(5) Å, c = 13.7613(18) Å) is very similar to that of the aristotype, γ ‐K 4 P 2 O 7 , whereas the crystal structure of LT‐Ag 4 P 2 O 7 ( T = 25 °C, P 3 1 21/ P 3 2 21, Z = 6, a = 5.5128(1) Å, c = 40.8723(11) Å) differs significantly from LT‐K 4 P 2 O 7 . The experimental results and those of DFT structure optimization yield bent pyrophosphate groups (bridging angle ∠(P−O b −P) ≈ 129°) for both LT‐ and HT‐Ag 4 P 2 O 7 , while their conformations change from eclipsed (HT) to staggered (LT). For one half of the Ag + cations in the HT modification, the unusual ninefold coordination in a truncated hexagonal pyramid changes to a distorted tetrahedral coordination in the LT form. Structural/dynamic instability expresses itself by formation of multinary compounds in the systems A 4 P 2 O 7 /Ag 4 P 2 O 7 ( A = Li, Na). Thus, the crystal structures of Li 3 AgP 2 O 7 and LiAg 3 P 2 O 7 show no similarity at all to that of Ag 4 P 2 O 7 , while the closely related crystal structure of Na 2 Ag 2 P 2 O 7 shows distinct ordering of Na + /Ag + despite similar ionic radii.