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.