Liwei Deng

Solid solutions between lead fluorapatite and lead fluorvanadate apatite: compressibility determined by using a diamond-anvil cell coupled with synchrotron X-ray diffraction

The synthetic solid solutions between lead fluorapatite and lead fluorvanadate apatite, Pb10[(PO4)6−x(VO4)x]F2 with x equal to 0, 1, 2, 3, 4, 5, and 6, were compressed up to about 9 GPa at ambient temperature by using a diamond-anvil cell coupled with synchrotron X-ray radiation. A second-order Birch–Murnaghan equation of state was used to fit the data. As the substitution of the PO43− cations by the VO43− cations progresses, the isothermal bulk modulus steadily decreases, with a maximum reduction of about 16% (from 68.4(16) GPa for Pb10(PO4)6F2 to 57.2(28) GPa for Pb10(VO4)6F2). For the entire composition range, the a-axis dimension remains more compressible than the c-axis dimension, with the ratio of the axial bulk moduli (KT−c:KT−a) larger than 1. The ratio of KT−c to KT−a increases from about 1.04(4) to 1.23(14) as the composition parameter x increases from 0 to 6, suggesting that the apatite solid solutions Pb10[(PO4)6−x(VO4)x]F2 become more elastically anisotropic.

A first-principles study of the phase transition from Holl-I to Holl-II in the composition KAlSi3O8

Abstract The phase relation and structural evolution of Holl-I and Holl-II in the composition KAlSi3O8 at 0 K have been investigated by the first-principles method up to 130 GPa. Holl-I and Holl-II are polymorphs of KAlSi3O8 stable at low pressures and high pressures, respectively. The transition pressure is determined at -23(5) GPa, in agreement with recent experimental observations. All experimentally observed major changes associated with this phase transition such as the deviation of the γ-angle from 90°, splitting of the a- and b-axes, as well as its P-V evolution, are successfully simulated. By evaluating the effect of different Al/Si substitution mechanisms on the computing cell of Holl-I, we have found: (1) different Al/Si substitution mechanisms do not result in apparent difference in the minimized cohesive energies, suggesting a possible random distribution of Al and Si; (2) different Al/ Si substitution mechanisms lead to different powder X‑ray diffraction features, which, compared to the experimentally observed powder X‑ray diffraction data, implies that local non-random distribution of Al and Si exists to some extent in the Holl-I structure; and (3) the phase transition from Holl-I to Holl-II might be associated with a change in the distribution pattern of Al and Si in the structure. From the simulated compression data, we have derived K0 = 174 GPa and V0 = 244.82 Å3 for Holl-I, and K0 = 168 GPa and V0 = 244.8 Å3 for Holl-II (K0′ fixed at 4). The larger K0 of Holl-I is probably related to the more stable squared open tunnel delimited by the rigid tetragonal octahedral framework, which is gradually deformed by compression in Holl-II after the phase transition from Holl-I to Holl-II.

PRESSURE-RELATED PHASE STABILITY OF MgSiO3and(Mg0.75, Fe0.25)SiO3 AT LOWER MANTLE CONDITION

Relative stability of different phases for MgSiO3 and (Mg0.75, Fe0.25)SiO3 within 0–110 GPa are investigated using first-principles method. For MgSiO3, the computed equation of state for orthorhombic phase of Pbnm space group agrees well with experimental results. The relative stability reduces from observed Pbnm orthorhombic phase to intermediated tetragonal P4mbm phase, and then to hypothetical cubic phase. For (Mg0.75, Fe0.25)SiO3, the same sequence of relative phase stability is observed. Thus, the low-symmetric orthorhombic MgSiO3 should be favored in the lower mantle condition, while adding Fe into MgSiO3 will make it less stable at the same depth.

Variations of Thermal Pressure for Solids along the Principal Hugoniot

The behavior of thermal pressure PTH for all kinds of solid materials was investigated using the lattice dynamics theory up to 500GPa. The results show that for most metals, ionic crystal and minerals, the thermal pressure is approximately independent on volume, whereas the thermal pressure of a few solids has strong dependence on volume. The volume dependence of thermal pressure has no relation with the chemical bonding type and crystal structure of materials, but is correlated with the Debye temperature ΘD and the second Gruneisen parameter q. The ratio of the thermal pressure to the total pressure (PTH /PTotal) along the Hugoniot keeps constant over a wide compression range, not only for non‐porous materials but also for porous materials within certain porosity, which could explain the existence of material constant parameter β along solid Hugoniot.

A New Evidence of the Stability of (Mg, Fe)SiO3 Perovskite at Lower Mantle Conditions: Shock Recovery Experiments

By using a two‐stage light gas gun, 9 experiments of shock recovery experiments with initial sample of (Mg0.92, Fe0.08) SiO3 Enstatite (En) (7 experiments) and MgO+SiO2 (2 experiments) were conducted between 65 and 110 GPa shock pressure (the corresponding temperature is estimated as 2500∼5000K). The analysis of X‐Ray Diffraction (XRD) and Infrared (IR) for the shock recovered samples indicate that there is no possibility for the chemical decomposition of (Mg0.92, Fe0.08) SiO3 perovskite into SiO2 plus (Mg0.92, Fe0.08) O during shock compression. Our experiments support that (Mg, Fe) SiO3‐perovskite is stable at lower mantle P&T conditions.