Diffuse scattering in silver hypodiphosphate, Ag 4 (P 2 O 6 ), probed by 3D ED

The crystal structure of silver hypodiphosphate, Ag 4 (P 2 O 6 ), was determined using 3D ED. The average structure is hexagonal, described in the space group P 6 3 / mcm , and is isomorphous with the average structure of known Li 4 (P 2 S 6 ). 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 P 2 O 6 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.