J. Phys: Condens. Matter zyxwvutsr 4 (1992) 8131-8140. Printed in the UK A Raman spectroscopic and energy-dispersive x-ray diffraction study of the high-pressure phase transitions of sodium hydrogen fluoride Andrew G Christyt, Julian Hainest and Simon M Clark$ t Department of Chemistry, University of Leicester, Leicester zyxw LEI 7RH, UK zyx $ Daresbury Laboratory, Daresbury, Cheshire WA4 4AD, UK Received 22 May 1992 Abslrrtcl Under ambient conditions, NaHF2 adopts a rhombohedral Structure zyx (R?m, zy Z = 3. n = 3.474( I) & c = 13.788( 10) & 2 = 3). Two high-pressure phase transitions have been noted previously, which we have located at 5 kbar and 41 kbar. We have studied all three phases zyxwvut by Raman spectroscopy and energy-dispersive x-ray diffraction. llie symmetric [FHF] stretching vibration is at a significantly higher frequency in phase I (631 an-' at ambient) than in phase I1 (623 cm-l and 612 cm-' at 10.6 kbar). The phase I structure is also incompressible down the triad axis, along which the [FHF) groups are oriented. The implication is that the [FHF] anions act as rigid braces in the phase I structure. The phase II x-ray data were fitted by a monoclinically distorted marcasite-like cell (P2/m, n = 4.825(12) if, 6 = 3.188(13) if, c = 5.128(25) A, zyx fl = 91.16(29)', Z = 2 at 27.0 kbr). This structure is easily derived from that of phase I by a shear, with accompanying rotation of the [FHF] groups into two distinct orientations, mnsistent with the observed splitting of the Raman modes. Although displacive, the transformation is first order, with a 13.3% volume reduction at the transition pressure. A further 5% volume discontinuity occurs at the 11-111 transition. The diffraction data for phase 111 index on a tetragonal cell whose dimensions suggest a variant of the sIructure adopted by KHF2 at ambient pressure (P4/ncc, a = 7.193(6) A, c = 5.657(22) if, Z = 8 at 41.6 kbar). The [FHF] groups arc canted in four or eight different directions. Simple diffusionless transformation pathways between the proposed phase II and phase 111 structures are suggested. 1. Introduction The linear anions (FHFI- and ["NI- form sa..s with the heavier alkali metals and TI+ whose structures under ambient conditions are simple tetragonally or trigonally distorted variants of the cubic NaCl and CsCl structure types [l]. We have found elsewhere [2] that when there is a polarizable or intrinsically non-spherical species present, the structures adopted with increasing pressure and coordination number are not necessarily analogues of the B3 (four-coordinated)-B 1 (skcoordinated)-B2 (eight-coordinated) progression. Even when the principal cation-anion bonds are highly ionic, the increased relative importance of other interactions at pressure results in the stabilization of structural topologies that are unusual at atmospheric pressure, such as the TI1 type in the case of the alkali hydroxides and hydrosulphides [2, 31. 0953-8984/92/418131+10$04.50 @ 1992 IOP Publishing Ltd 8131