Pressure-induced first-order phase transition in (NH₄)₂V₃O₈ fresnoite: a double coordination change for V⁴⁺ and V⁵⁺

The high-pressure behaviour of (NH₄)₂V₃O₈ with the fresnoite structure (P4bm, Z = 2) has been studied at room temperature with single-crystal X-ray diffraction in diamond anvil cells using laboratory and synchrotron facilities. At ambient conditions, the crystal structure is composed of layers of corner-sharing V⁵⁺O₄ tetrahedra and V⁴⁺O₅ square pyramids separated by layers of the NH₄⁺ cations. At about 3 GPa, there occurs a reversible first-order phase transition to a three-dimensional structure (P4/mbm, Z = 2) built of corner-sharing V⁵⁺O₅ trigonal bipyramids and V⁴⁺O₆ octahedra. The NH₄⁺ cations fill up the interstitial sites in the tunnels formed by the vanadate framework. Up to the phase transition, the a lattice parameter of the low-pressure polymorph does not change while the contraction perpendicular to the stacking of the V₃O₈ slabs accounts entirely for the bulk compressibility. Above the phase transition, the a lattice parameter slightly expands. The structural features of the high-pressure phase of (NH₄)₂V₃O₈ are compared to those of other vanadium oxides.