No abstract

Abstract. Cryolite, Na3A1F6[=2Na+(Nag.sAlg.~)F3] is a mixed fluoride perovskite, in which the corner-sharing octahedral framework is formed by alternating [NaF6] and [A1F6] octahedra and the cavities are occupied by Na § ions. At 295 K, it is monoclinic (~ phase), space group P21/n with a=5.4139(7), b=5.6012(5) and c = 7.7769 (8) A and fl = 90.183 (3), Z = 2. A high temperature single crystal X-ray diffraction study in the range 295-900 K indicates a fluctuation-induced first-order phase transition from monoclinic to orthorhombic symmetry at TO~885 K, in contrast to a previous report that it becomes cubic at ,-~ 823 K. The space group of the high temperature 1~ phase is Immm with a = 5.632 (4), b= 5.627 (3) and c= 7.958 (4)/~, Z=2 at 890 K. Above To, the coordination number of the Na § ion in the cavity increases from eight to twelve and the zigzag Nal-A1 octahedral chains parallel to c become straight with the Nal -F--A1 angle = 180 ~ The phase transition is driven by two coupled primary order parameters. The first corresponds to the rotation of the nearly rigid [A1F6] group and transforms according to the F4 + irreducible representation of Immm. Coupled to the [A1F6] rotation is a second primary order parameter corresponding to the displacement of the Na2 + ion in the cavity from its equilibrium position. This order parameter transforms according to the X~ irreducible representation of Immm. Following Immm ~ P21/n phase transition, four equivalent domains of P21/n are determined relative to Immm, which are in an antiphase and/or twin relationship. The abrupt shortening of the octahedral A1-F and Na--F bonds and a sudden change in orientations of the atomic thermal vibration ellipsoids above To indicate a crossover from displacive to an order-disorder mechanism near the transition temperature. The fl phase is interpreted as a dynamic average of four micro-twin and -antiphase domains of the ~ phase. This view is consistent with the entropy of phase transition, ASt .... (11.43 JK-1 mol-1) calculated from heat capacity measurements (Anovitz et al. 1987), which corresponds closely to R ln4 (11.53 JK -1 tool-l), where 4 is the number of domains formed during the phase transition. The dynamic nature of the/~ phase is independently confirmed from a considerable narrowing of the 27A1 nuclear magnetic resonance (NMR) line-shape above To .

Key indicatorsSingle-crystal X-ray study T = 273 K Mean (e-O) = 0.001 Å R factor = 0.019 wR factor = 0.052 Data-to-parameter ratio = 21.0 For details of how these key indicators were automatically derived from the article, see

Liu (1986Liu ( , 1987 reported the diffraction pattern of a hydrous magnesium silicate resulting from the breakdown of serpentine at 22 GPa and 1000 ЊC and named this material ''phase D.'' Since that time there have been several reports of the synthesis of phase D, but its confirmed composition and crystal structure have not yet been reported. We synthesized a new dense hydrous magnesium silicate at 20 GPa and 1200 ЊC and solved its crystal structure (R w ϭ 0.015 and R ϭ 0.014). The single crystal has composition Mg 1.11 Si 1.89 H 2.22 O 6 (ideal formula: MgSi 2 H 2 O 6 ), cell parameters a ϭ 4.7453(4), c ϭ 4.3450(5) Å , and V ϭ 84.74(2) Å 3 , and space group P31m. The crystal structure is relatively simple with all the Si occupying octahedral sites in a layer similar to that of brucite, but with one of every three octahedra vacant. The MgO 6 octahedra are located above and below each vacant octahedral site. All O-H bonding occurs between SiO 6 octahedral layers. This is the only high-pressure hydrous magnesium silicate structure reported to date that contains all octahedrally coordinated Si. The calculated density of phase D (d cal ϭ 3.50 g/cm 3 ) is substantially greater than any other high-pressure hydrous magnesium silicate phase.