Pressure dependence of the crystal structure of lead hydrogen phosphate (PbHPO4)

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Abstract
Lead monohydrogen phosphate (PbHPO4) undergoes a ferroelectric phase transition at Tc=310 K at atmospheric pressure. Tc falls to room temperature at a pressure of about 0.125 GPa. The crystal structure of PbHPO4 has been determined by X-ray diffraction at 2.0 and 2.3 GPa and the lattice parameters have been measured at a number of pressures between 0.1 MPa and 3.8 GPa. The unit-cell dimensions change anisotropically under high pressure: the compressibility of b is approximately twice that of a and c. The principal structural changes with pressure are shown to be compression of the distance between the sheets (parallel to the ac plane) of electrostatically interacting Pb2+ and HPO2- ions, and compression of the hydrogen bond connecting the PO4 anions of the neighbouring sheets.