Accurate crystal structure studies at high pressure

Neutron-diffraction studies have been carried out on KH2P0 9 and other H-bonded materials exhibiting a similar H-ordering phase transition, just above the transition temperature Tc 9 at atmospheric pressure and under applied hydrostatic pressures up to 19 kbar. The results show a systematic relationship between Tc and 6, where 6 is the separation between the two sites over which the H atoms are 50:50 disordered above T0. T MTP (TT T('T T f\1T KH2PO (KDP) undergoes an H-ordering transition on cooling through Tc = 122 K to its ferroelectric phase. Above Tc the H atoms are 50:50 disordered over two off-centre sites in short 0—H—O bonds linking the PD X, groups. Below Tc the H atoms order onto one of these sites. The deuterated form, KD2P0 (DKDP), exhibits the same behaviour at the substantially higher Tc of 229 K. The application of hydrostatic pressure decreases Tc in both KDP and DKDP'. In KDP, Tc falls to 0 K at 17 kbar; it is estimated that a pressure of —65 kbar is needed to reduce Tc to 0 K in DKDP. Pressure also reduces the 0—H(D)--0 bondlength, 2R, and the H-site separation, 6; previous neutron-diffraction studies of KDP and DKDP by us have established that the changes in 2R and 6t under pressure are linearly related to the changes in T0 under pressure, up to at least 12 kbar'. Similar results have been obtained for H2C,O (squaric acid or H2SQ) which also has an H-ordering transition. But H2SQ has a much tNote that 6 is the separation of the maxima of the H (or D) distribution, as determined from diffraction data. This is to be distinguished from the separation, A, of the minima of the double-minimum H(D) potential well. Because of the asymmetry of each minimum of the well, 6 is less than A more so for H than for D, because the H energy levels are higher in the well. [277]/1883 [278}/1884 M.I. McMAHON, R.O. PILTZ AND R.J. NELMES higher Tc (= 3714 K) than KDP, and the ordering below Tc is anti- ferroelectric5 (unlike KDP). Furthermore, the structure of H2SQ is two- dimensionally linked by the H-bonds forming H2C04 sheets which are not H-bonded to one another whereas KDP has a three-dimensionally linked H2PO 4 framework. Despite these substantial differences, not only do 2R and 6 again decrease linearly with Tc under pressure, but the relationship is such that compression of 6 in H2SQ to the same value as in KDP (at atmospheric pressure) removes most of the large difference in Tc6 These results suggest that Tc is strongly determined just by the magnitude of 6 in any system. To explore this relationship further, we have now made structural studies of two other H-ordering systems - namely, PbHP0 (LHP) and its deuterated form LDP, and NHH2PO 4 (ADP) and its deuterated form DADP. In LHP for which the atmospheric-pressure 7 results have already been obtained the PO groups are linked only one-dimensionally by the 0—H-0 bonds, forming HPO, chains; Tc is 310 K for LHP and rises to 1452 K for LDP; and the ordering below Tc is ferroelectric. The structure of ADP is isomorphous with KDP above Tc, but the ordering below Tc is antiferroelectric. Tc is 151 K for ADP and rises to 235 K for DADP.