Diffuse scattering in silver hypodiphosphate, Ag₄(P₂O₆), probed by 3D ED

The crystal structure of silver hypodiphosphate, Ag₄(P₂O₆), was determined using 3D ED. The average structure is hexagonal, described in the space group P6₃/mcm, and is isomorphous with the average structure of known Li₄(P₂S₆). The silver cations form hexagonal atomic ring layers and the hypodiphosphate anions occupy channels that centre every hexagonal ring, with the P-P bond oriented along the unique c axis. The hypodiphosphate P₂O₆^4- anions are disordered, with the P atoms occupying two positions having 50% occupancy each and with O atoms common for both positions of the anion. The O atoms form an octahedral coordination environment for the silver cations. The hypodiphosphate anions are stacked into columns along the unique axis direction. Neighbouring hypodiphosphate columns may have P-P bonds on the same (ferro-type) or on different (antiferro-type) levels. This correlated disorder manifests itself in diffuse scattering observed on hkl layers with uneven l values. Simulations based on an Ising-type model with geometric frustration align well with the experimental data, providing insight into the short-range antiferro-like arrangement of disordered hypodiphosphate anions.