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.
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