On the pressure dependence of the crystal structure of PbHPO4

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Abstract
The room-temperature crystal structure of LbHPO4 has been determined by neutron diffraction at 1.5 GPa. Under these conditions, PbHPO4 is in its high-temperature, disordered phase. The principal structural changes with pressure are compression of 2R, the length of the O-H-O bonds connecting the PO4 groups, and of delta , the separation of the two sites over which the H atoms are disordered in these bonds above the phase transition temperature, Tc. The relationship between 2R and delta , their pressure dependence, and the relationship between delta and Tc are compared for PbHPO4, KH2PO4 and squaric acid (H2C4O4).