Xinzhuan Guo

Pressure‐induced enhancement of proton conduction in brucite

The electrical conductivity of single-crystal brucite was measured as a function of pressure and temperature (P-T) using impedance spectroscopy (IS). IS measurements demonstrated that electrical conductivity perpendicular to the c axis is nearly half order of magnitude higher than that parallel to the c axis under the same P-T conditions. Electrical conductivity increased by 2 orders of magnitude during compression from 3.7 to 11 GPa, irrespective of crystallographic direction. However, the conductivity increase with pressure became less significant upon further compression from 11 to 13 GPa. The pressure effect is closely related to the interactions between neighboring hydroxyls. The ratio of free protons to total hydrogen increases from 2% to 33% when pressure increases from 3.7 to 13 GPa at 950 K. This indicates that most of the protons are bound within the crystal structure at low pressures, whereas more protons become free and mobile at higher pressures.

H-D interdiffusion in brucite at pressures up to 15 GPa

Abstract Proton diffusion in brucite was investigated by conducting hydrogen-deuterium (H-D) exchange experiments using multi-anvil high-pressure apparatuses at pressures from 3 to 15 GPa and temperatures in the range of 750-1050 K. The diffusion couple was composed of a natural proton-dominated brucite single crystal surrounded by synthesized D-doped brucite polycrystals. Micro-Raman spectroscopy was used to determine the diffusion profiles of the samples. The D/H diffusion profile across the boundary between single crystal and polycrystalline D-doped brucite showed an asymmetric pattern characterized by faster diffusion in aggregates. The D/H interdiffusion rate determined from the analysis of the single crystal side indicates that the interdiffusion rate increases with increasing H/D ratio. The H-D interdiffusion rate in the direction perpendicular to the c-axis is about 0.5 orders of magnitude higher than that in the direction parallel to the c-axis. At 3 GPa, the H-D interdiffusion coefficients [D(m2/s)] along and perpendicular to the c-axis of brucite at compositions of CODnorm = 0.2 in the single-crystal region were determined to be 3.30(±1.77) ×10−11exp[-48.2(±5.8) (kJ/mol)/ RT] and 1.43(±1.33) ×10−9exp[-67.5(±23.2) (kJ/mol)/RT], respectively. The H-D interdiffusion rate perpendicular to and along the c-axis increased about one order of magnitude by compression from 3 to 10 GPa, but the pressure enhancement became weaker above 10 GPa. From 10 to 15 GPa, there is almost no pressure dependence of proton diffusion for both directions. As pressure increases up to 10 GPa, enhancement of the proton migration is strongly correlated with the activation of the atomic interaction and decrease of O…O′ distance induced by compression. The positive pressure effect on the proton diffusion in brucite suggests that proton diffusion in higher-pressure hydrous phase becomes faster because of the shorter O…O′ distance.

Phase boundary between perovskite and post-perovskite structures in MnGeO3 determined by in situ X-ray diffraction measurements using sintered diamond anvils

Abstract To determine the phase boundary between the perovskite and post-perovskite structures in MnGeO3, in situ X-ray observations were carried out at pressures of 57-68 GPa and temperatures of 1000-1900 K using the Kawai-type high-pressure apparatus equipped with sintered diamond anvils interfaced with synchrotron radiation. The phase boundary was determined to be P (GPa) = 39.2 + 0.013T (K) based on Tsuchiya’s (2003) gold pressure scale. The Clapeyron slope, dP/dT, of 13(+12/-5) MPa/K, determined in the present study is larger that of MgGeO3 and MgSiO3.

P-V-T relations of γ-Ca3(PO4)2 tuite determined by in situ X-ray diffraction in a large-volume high-pressure apparatus

Abstract Tuite, γ-Ca3(PO4)2, is regarded as an important phosphate mineral in the deep mantle playing a crucial role as a host for rare earth elements, large ion lithophile elements, and phosphorus. In this study we report the thermoelastic properties of synthetic γ-Ca3(PO4)2 at simultaneously high pressures and temperatures of up to 35.4 GPa and 1300 K, respectively, as determined by means of in situ energydispersive X‑ray diffraction in a large-volume multi-anvil apparatus. The pressure-volume-temperature data obtained for γ-Ca3(PO4)2 were fitted by the high-temperature Birch-Murnaghan equation of state to yield V2 = 447.4(4) Å3, KT0 = 100.8(18) GPa, K′T0 = 5.74(13), (∂KT/∂T)P = -0.020(1) GPa/K, and αT = 3.26(18) × 10-5 + 1.76(24) × 10-8 T. In addition, fitting the present data to the Mie-Grüneisen-Debye equation of state gives γ0 = 1.35(6), Θ0 = 944(136) K, and q = 0.37(29). Based on the thermoelastic properties obtained in our study, the density profiles of γ-Ca3(PO4)2 tuite along typical cold and hot slab geotherms were calculated and are compared with those of the coexisting silicate minerals in subducting mid-ocean ridge basalt.

High pressure generation and investigation of the spin transition of ferropericlase (Mg0.83Fe0.17)O

High pressure generation has been tried by using the Kawai-cell equipped with sintered diamond cubes in conjunction with investigation of the spin transition in Fe2+ of (Mg0.83Fe0.17)O (ferropericlase, Fp). The Kawai-cell was squeezed in the DIA type press SPEED mkII installed at SPring-8. The volumes of the Fp and Au pressure standard were simultaneously determined by in situ X-ray diffraction using the synchrotron radiation. The maximum attainable pressure has reached 90 GPa at 300 K based on Anderson et al.s Au scale [4]. The P-V data of (Mg0.83Fe0.17)O were acquired at 300 K and 700 K up to 90 GPa. From detailed analysis of the compression data, it is suggested that the spin transition proceeds over pressure ranges from 50 to 70 GPa at 300 K and from 50 to 75 GPa at 700 K.