High-pressure crystal chemistry of scheelite-type tungstates and molybdates.docx

《High-pressure crystal chemistry of scheelite-type tungstates and molybdates.docx》由会员分享，可在线阅读，更多相关《High-pressure crystal chemistry of scheelite-type tungstates and molybdates.docx（3页珍藏版）》请在淘文阁 - 分享文档赚钱的网站上搜索。

1、ResearchGate See discussions, stats, and author profiles for this publication at: http:/www.rGsearchgatG.net/publication/222248489 High-Pressure Crystal Chemistry of Scheelite-Type Tungstates and Molybdates ARTICLE in JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS * JANUARY 1985 Impact Factor: 1.59 DOI:

2、 10.1016/0022-3697(85)90039-3 CITATIONS 154 3 AUTHORS, INCLUDING: Robert M. Hazen Carnegie Institution for Science 233 PUBLICATIONS 6,865 CITATIONS SEE PROFILE L. W. Finger Carnegie Institution for Science 135 PUBLICATIONS 6,822 CITATIONS SEE PROFILE Available from: Robert M. Hazen Retrieved on: 30

3、August 2015 J. Phys. Chem. Solids Vol. 46, No. 2, pp. 253-263, 1985 Printed in the U.S.A. Papers from the GEOPHYSICAL LABORATORY 0022-3697/85 $3.00 + .00 Carnegie Institution of Washington p Press Ltd No. 1943 HIGH-PRESSURE CRYSTAL CHEMISTRY OF SCHEELITE- TYPE TUNGSTATES AND MOLYBDATES ROBERT M. HAZ

4、EN and LARRY W. FINGER Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C. 20008, U.S.A. and JOSEPH W. E. MARIATHASAN Clarendon Laboratory, Oxford University, Parks Road, Oxford 0X1 3PU, England (Received 30 May 1984; accepted in revised form 25 July 1984) AbstractUnit-cell

5、parameters and crystal structures of CaW04 (scheelite) and CaMo04 (powellite) have been determined at several pressures to 5.8 GPa; and unit-cell parameters of PbMo04 (wulfenite), PbW04 (stolzite) and CdMo04 have been measured at pressures to 6.0 GPa. All five tetragonal scheelite-type compounds com

6、press anisotropically, with the c axis 1.2 to 1.9 times more compressible than a. In both CaW04 and CaMo04 the cation tetrahedra (with W6+ or Mo6+) behave as rigid structural elements with no observed cation-oxygen compression (WO and M O bond compression 0.001 GPa-1). Compression of the eight-coord

7、inated calcium polyhedron, on the other hand, is comparable to bulk compression of the compounds (CaO bond compression = 0.005 0.001 GPa_1). Anisotropies in the pressure response of the calcium polyhedron, which is more compressible parallel to c than perpendicular to Cy result in the anisotropic un

8、it-cell compression. Bulk moduli of the five compounds (with K assumed to be 4) are CaW04 (68 9 GPa), CaMo04 (81.5 0.7 GPa), PbW04 (64 2 GPa), PbMo04 64 2 GPa), and CdMo04 104 2 GPa). No reversible transitions to the monoclinic (fergusonite) distortion of scheelite were observed in these compounds.

9、Pressure-volume data for PbW04, however, display strong positive curvature (Xa|C = 23 2) up to about 5 GPa, at which pressure crystals appear to undergo a first-order phase transition. The relatively large curvature may be a premonitory effect prior to a reconstructive transition. Structural changes

10、 in these compounds with increasing pressure are qualitatively similar to changes that result from isobaric cooling or substitution of a smaller cation in the eight- coordinated site. INTRODUCTION Scheelite-type ABO A, compounds, with eight-coordinated A cations and tetrahedral B cations, are common

11、 binary oxides in both natural and synthetic systems. The scheelite structure is very versatile and occurs with -H, +2, +3 and +4 A cations in combination with +7, +6, +5 and +4 B cations, respectively. Solid solutions based on coupled, mixed- valence substitution and nonstoichiometric varieties suc

12、h as La 7M0O4 are also known. Tungstates, molybdates, niobates and vanadates with the scheelite structure have been the focus of recent studies at high temperature and high pressure because of the identification of several phase transitions in these compounds. Nicol and Durana 1 measured high-pressu

13、re Raman spectra of CaW04 and CaMo04 in a solid-media apparatus in which NaCl was used as the pressure medium. Single-crystal specimens were aligned with the tetragonal c axis both parallel and perpendicular to the uniaxial stress of the experimental system. In both orientations, splitting of EK mod

14、es was observed at pressures above 2 GPa. This behavior was interpreted as evidence for a transition from the tetragonal scheelite structure (space group 14)/a) to the closely related, but topologically distinct, wolframite structure (monoclinic, P2/c). A similar high-pressure Raman study of SrW04 b

15、y Ganguly and Nicol 2 also revealed splitting of Efr modes above 2 GPa. Subsequent high-pressure Raman spectroscopy with polycrystalline CaW04 and CaMo04 in opposed-anvil devices by Breitinger et al. 3 and Jayaraman et ai 4, however, showed no evidence for such transitions. It appears from the oppos

16、ed-anvil experiments that there are no high- pressure transitions in hydrostatically compressed CaW04 or CaMo04 below 3 GPa. Raman mode splittings of the type reported by Nicol and Durana could result from a reversible transition from tetragonal scheelite to the monoclinic fergusonite structure (spa

17、ce group I2/a which is displayed by several rare earth niobates and BiV 4 5, 6. Raman spectra of monoclinic LaNb04 by Wada et al. 5 display two Bf, modes in the room- temperature, low-symmetry phase; these modes converge to an EH mode at about 480C, where LaNb04 transforms reversibly to the undistor

18、ted scheelite form. Furthermore, under room conditions of temperature and pressure, compounds along the solid solution join LaNb4CaW04 are monoclinic for lanthanum-rich compositions but approach tetragonal symmetry with increasing calcium content and achieve the ideal scheelite structure at about 40

19、% CaW04 component 7. It is not unreasonable to expect, therefore, that CaW04 might undergo a distortional 253 254 ncfpn； rir- transition to the fergusomte structure jkherjiv the presence of shear or at high pressure. Recently Jayaraman 8 has observed the appearance of several new bands in the Raman

20、spectra of PbW04 and PbMo04 at about 5.0 and 9.0 GPa, respectively. These changes in vibration modes, in contrast to the simple splittings of bands in the scheelite-to-fergusonite transition, imply a first-order phase transition. The objectives of this high-pressure crystallographic study of scheeli

21、te-type tungstates and molybdates are to: (1) document and characterize any phase transitions in CaW04 (scheelite), CaMo04 (powellite), PbW04 (wulfenite), PbMo04 (stolzite) and CdMo04 to pressures above 5 GPa; (2) determine pressure-volume equation-of-state parameters for these five compounds; and (

22、3) determine high-pressure crystal structures in order to define relationships between bonding and compression of scheelite-type compounds. EXPERIMENTAL Specimen description Crystals of CaW04, CaMo04 and CdMo04 were provided by John S. White from the synthetic compounds collection of the National Mu

23、seum of Natural History, Smithsonian Institution (specimen numbers 148668, 148773 and 5IS, respectively). Both CaW04 and CdMo04 were synthesized at Bell Laboratories, whereas the CaMo04 was synthesized by Isomet Corporation. Crystal fragments of PbW04 and PbMo04 were supplied by A. Jayaraman from ma

24、terial synthesized at Bell Laboratories. All five synthetic scheelite-type compounds were obtained as fragments from colorless boules of nearideal compositions. Exact conditions of sample preparation were not specified, but room-condition unitcell parameters match those reported for end-member mater

25、ial 9. Data collection at room pressure Single-crystal diffraction data were collected on rectangular cleavage fragments with a 60-/xm maximum dimension for Ca and Cd compounds and a 40-m maximum dimension for the highly absorbing Pb compounds. Room-temperature lattice parameters for each of the fiv

26、e compounds were refined from diffractometer angles of 20 reflections, each of which was measured in eight equivalent positions 10. Unit-cell parameters are recorded in Table 1. Crystal-structure refinements at room and high pressure were performed on CaW04 and CaMo04. All scheelite-type tungstates

27、and molybdates have relatively large linear absorption coefficients and thus may be subject to systematic errors in X-ray data. The calcium compounds were selected for crystal- structure analysis both because of their mineralogical interest and because they have lower X-ray absorption than many othe

28、r scheelite-type crystals. Intensities of all reflections in a sphere with (sin 0)/X S 0.7 were measured by an automated, four-circle diffractometer with Nb-filtered MoATa radiation 11. Refinement conditions and refined structural parameters for CaW04 and CaMo04 appear in Table 2; the refined anisot

29、ropic temperature factors under room conditions are presented in Table 3. Data collection at high pressure Flat cleavage plates from 10 (for Pb compounds) to 50 ixm thick and from 50 to 100 fim in diameter were employed in high-pressure experiments. Crystals were mounted in a diamond-anvil pressure

30、cell for X-ray diffraction 12. An alcohol mixture of 4:1 methanohethanol was used as the hydrostatic pressure medium 13, and ruby crystals were included in each mount for pressure calibration 14. Details of procedures for crystal mounting and high-pressure X-ray diffraction are described elsewhere 1

31、2. Lattice constants of all five scheelite-type compounds were measured at several pressures. From 10 to 20 reflections were measured by the method of Hamilton 15, as modified by King and Finger 10, in order to correct for errors in crystal centering and diffractometer alignment. Each set of angular

32、 data was refined without constraint, and the resulting “triclinic” cell was examined for conformity with the tetragonal symmetry of scheelite. These symmetry conditions (i.e. a = b and a = /3 = 7 = 90) are satisfied within two standard deviations for all compounds at all pressures studied. The unia

33、xial behavior observed is evidence that no deviations from tetragonal symmetry occurred and, furthermore, that hydrostatic conditions were maintained during the experiments. High-pressure unit-cell parameters are recorded in Table 1 and are illustrated for the calcium and lead scheelites in Figs. 1

34、and 2, respectively. Intensity data for three-dimensional structure refinements were collected for the two calcium compounds; all accessible reflections with (sin 0)/X 0.7 were measured.* The fixed-0 mode of data collection was used to maximize reflection accessibility and minimize attenuation by th

35、e diamond cell 16, and a correction was made for X-ray absorption by the diamond and beryllium components of the pressure cell 12. Conditions of high-pressure refinements as well as refined structure parameters are recorded in Table 2. Refined high-pressure structure parameters for CaMo04 were consi

36、stent with those determined at room pressure. High-pressure parameters for CaW04, however, varied systematically from the room-pressure values (Table 2). It was necessary, therefore， to collect a room-pressure data set from the high-pressure crystal in the pressure cell. This structure refinement was used in subsequent calculations of bond compressions. The significant differences in room- * Tabulated observed and calculated structure factors for all refinements are available from the authors on request.