L. C. Ming

Decomposition of phase D in the lower mantle and the fate of dense hydrous silicates in subducting slabs

AbstractHigh pressure, high temperature quench-type experiments were carried out on serpentine to pressures of 53 GPa andtemperatures between 800–1800oC. X-ray analyses show that recovered phase assemblages varied considerably for thedifferent high pressure and high temperature studies. The dense hydrous magnesium silicate phases decompose sequentiallyas the pressure increases and appear to serve as reservoirs for H 2 O in the mantle, eventually releasing it at the highestpressure. Superhydrous phase B is stable up to pressure equivalent to the boundary between the transition zone and lowermantle. Phase D decomposes at pressures of 44 GPa, equivalent to about 1250 km depth. This may define the lower depthlimit for the presence of dense hydrous magnesium silicates. Beyond this pressure, only nominally anhydrous phases havebeen recovered in high-pressure experiments. This may indicate a lack of stoichiometric-hydrogen bearing silicate phasesat higher pressure. 1998 Elsevier Science B.V. All rights reserved.Keywords: X-ray diffraction analysis; lower mantle; serpentine; high pressure

Pressure-Induced Phase Transitions in KDy(MoO4)2 and KY(MoO4)2: a High-Pressure Raman Study

High-pressure Raman studies on potassium–rare earth double molybdates, KDy(MoO4)2 and KY(MoO4)2, up to 35 GPa are reported. The Raman data indicate the occurrence of a reversible pressure-induced phase transition near 3 GPa, from the orthorhombic Pbcn phase. High-pressure x-ray diffraction measurements suggest that the new phase is also orthorhombic and has a smaller volume. The phase transition is structurally similar to that observed in KTb(MoO4)2 near 3 GPa and seems to be characteristic of the layer-type orthorhombic, potassium–heavy rare earth dimolybdates. Changes in optical transmission observed in all three dimolybdates are compared and discussed.

Pressure-induced structural and electronic transition in KTb(MoO4)2 through Raman and optical studies

Raman and optical absorption studies under pressure have been conducted on KTb(MoO4)2 up to 35.5 GPa. A phase transformation occurs at 2.7 GPa when the crystal is pressurized at ambient temperature in a hydrostatic pressure medium. The sample changes to a deep yellow color at the transition and visibly contracts in theα-axis direction. The color shifts to red on further pressure increase. The Raman spectral features and the X-ray powder pattern change abruptly at the transition indicating a structural change. The pressure-induced transition appears to be a property of the layer-type alkali rare earth dimolybdates. However, the color change at the transition in KTb(MoO4)2 is rather unusual and is attributed to a valence change in Tb initiated by the structural transition and consequent intervalence charge transfer between Tb and Mo.In situ high pressure X-ray diffraction data suggest that phase II could be orthorhombic with a unit cell having 3 to 4% smaller volume than that of phase I.

High pressure Raman studies on ABO3 perovskite type structure compound PbHfO3 to 40 GPa: pressure-induced phase transitions

Lead hafnate (PbHfO3), an ABO3 perovskite family antiferroelectric (AFE), has been investigated up to 40 GPa by high pressure Raman scattering in the diamond anvil cell. In the range 0 to 40 GPa three pressure-induced phase transitions are found to occur, near 2, 7 and 15 GPa, as revealed by changes in the Raman spectra features at the above pressures. At pressures above 20 GPa the Raman peaks become very weak and the background scattering becomes very strong. Optical observations under a microscope show that the color of the samples changes from colorless to deep orange by about 40 GPa. We believe that this is connected with the downward motion of the d-band of the transition metal ion. The possible role of the electronic structure in the pressure-induced phase transitions is discussed. The phase transitions are subtle, and optical observations of the domain patterns (AFE domains) indicate that the high pressure phases II and III and possibly also IV are all AFE. X-Ray diffraction studies on PbHfO3 up to 52 GPa reported in the accompanying paper are in broad agreement with the high pressure Raman study.

An in-situ high pressure x-ray diffraction study of PbHfO3 to 52.5 GPa at room temperature and pressure-induced phase transformations

Abstract In-situ high pressure X-ray diffraction studies have been carried out on lead hafnate PbHfO 3 up to 52.5 GPa at room temperature using the diamond anvil cell and a synchrotron X-ray source in the energy dispersive mode. The X-ray data show that three pressure-induced phase transitions occur in this range in the following sequence: from an orthorhombic phase I ( Z = 8) to a tetragonal phase II ( Z = 1) at 8 GPa; from a tetragonal phase II to an orthorhombic phase III ( Z = 4) near 15 GPa, and from an orthorhombic phase III to phase IV with an undetermined structure near 45 GPa. The volume changes at these transitions are insignificantly small. Fits to pressure-volume data up to 8 GPa using the Birch-Murnaghan equation of state and K ′ 0 = 4 yields K 0 as 142 (12) GPa for orthorhombic PbHfO 3 (I); however, a similar fit over the range to 31 GPa yields 169 (6) GPa for PbHfO 3 I, II and III all taken together and assuming K ′ 0 = 4 again. Fitting for PbHfO 3 III yields 188 GPa for the bulk modulus K 0 obtained using a finite strain analysis method recently developed for high pressure phases. The results are in agreement with the pressure-induced phase transitions observed in a high pressure Raman study up to 40 GPa.