Neutron diffraction investigation of the hydrogen positions in the crystal structure of monetite,CaHPO4

An occupation-factor analysis using single-crystal neutron diffraction data has shown that the hydrogen atoms in the structure of the mineral monetite, CaHPO₄, occupy hydrogen-bonding sites closely similar to those predicted in a previous X-ray refinement. Least-squares refinement in space groupP¯1 of the potential coordinates and isotropic temperature parameters for the three distinct proton sites led to a resultantR of 0·10 over the 371 independent neutron structure factors. Detailed analysis showed that the distribution of the hydrogen atoms was almost certainly ordered in symmetryP1 with only half of the O(6) atoms (in space groupP¯1) carrying hydrogen atoms; the short O(7)-P(1)... O(7) hydrogen bond of 2·44 Å across what would be a symmetry centre inP¯1 is probably unsymmetrical. Minute piezo- and pyroelectric effects confirmedP1 as the space group. Further refinement of the X-ray data in space groupP1 indicated that the deviations from centrosymmetry must be small and, so far as can be detected, confined to the hydrogen atoms.We thank Mr James R. Lehr for the crystal, Prof. W. G. Perdok for the piezoelectric measurement, and Dr. G. Ferraris for a useful discussion. We are also grateful to Mr. D. H. C. Harris and the staff of the University Support Group for help at AERE, and to the S.R.C. for providing access to neutron and computational facilities and for financial support to WAD.