Ultrahigh-pressure disordered eight-coordinated phase of Mg2GeO4: Analogue for super-Earth mantles.

Mg2GeO4 is important as an analog for the ultrahigh-pressure behavior of Mg2SiO4, a major component of planetary interiors. In this study, we have investigated magnesium germanate to 275 GPa and over 2,000 K using a laser-heated diamond anvil cell combined with in situ synchrotron X-ray diffraction and density functional theory (DFT) computations. The experimental results are consistent with the formation of a phase with disordered Mg and Ge, in which germanium adopts eightfold coordination with oxygen: the cubic, Th3P4-type structure. DFT computations suggest partial Mg-Ge order, resulting in a tetragonal [Formula: see text] structure indistinguishable from [Formula: see text] Th3P4 in our experiments. If applicable to silicates, the formation of this highly coordinated and intrinsically disordered phase may have important implications for the interior mineralogy of large, rocky extrasolar planets.

Phase transitions beyond post-perovskite in NaMgF3 to 160 GPa.

Neighborite, NaMgF3, is used as a model system for understanding phase transitions in ABX3 systems (e.g., MgSiO3) at high pressures. Here we report diamond anvil cell experiments that identify the following phases in NaMgF3 with compression to 162 GPa: NaMgF3 (perovskite) → NaMgF3 (post-perovskite) → NaMgF3 (Sb2S3-type) → NaF (B2-type) + NaMg2F5 (P2 1 /c) → NaF (B2) + MgF2 (cotunnite-type). Our results demonstrate the existence of an Sb2S3-type post-post-perovskite ABX3 phase. We also experimentally demonstrate the formation of the P2 1 /c AB2X5 phase which has been proposed theoretically to be a common high-pressure phase in ABX3 systems. Our study provides an experimental observation of the full sequence of phase transitions from perovskite to post-perovskite to post-post-perovskite followed by 2-stage breakdown to binary compounds. Notably, a similar sequence of transitions is predicted to occur in MgSiO3 at ultrahigh pressures, where it has implications for the mineralogy and dynamics in the deep interior of large, rocky extrasolar planets.

High-Pressure Study of Perovskites and Postperovskites in the (Mg,Fe)GeO3 System.

The effect of incorporation of Fe2+ on the perovskite (Pbnm) and postperovskite (Cmcm) structures was investigated in the (Mg,Fe)GeO3 system at high pressures and temperatures using laser-heated diamond anvil cell and synchrotron X-ray diffraction. Samples with compositions of Mg# ≥ 48 were shown to transform to the perovskite (∼30 GPa and ∼1500 K) and postperovskite (>55 GPa, ∼1600-1800 K) structures. Compositions with Mg# ≥ 78 formed single-phase perovskite and postperovskite, whereas those with Mg# < 78 showed evidence for partial decomposition. The incorporation of Fe into the perovskite structure causes a decrease in octahedral distortion as well as a modest decrease in bulk modulus (K0) and a modest increase in zero-pressure volume (V0). It also leads to a decrease in the perovskite-to-postperovskite phase transition pressure by ∼9.5 GPa over compositions from Mg#78 to Mg#100.

Electronic structure, optical properties and the mechanism of the B3-B8 phase transition of BeSe: insights from hybrid functionals, lattice dynamics and NPH molecular dynamics.

We have investigated the electronic structure and the mechanism of the pressure induced phase transition of beryllium selenide (BeSe) by employing a first-principles pseudopotential method within the framework of density functional theory. Our study demonstrates that use of the hybrid PBE0 functional (PBE stands for Perdew, Burke and Ernzerhof) leads to significant improvement in the band gap calculations, compared to those using either of the common density functionals (local density approximation (LDA) and generalized gradient approximation (GGA)), which severely underestimate the band gap of BeSe. The band gap obtained from the hybrid PBE0 functional shows excellent agreement with available experimental data. A constant-pressure (NPH) first-principles molecular dynamics (FPMD) approach has been adopted to characterize the first-order pressure induced phase transition from the zinc blende (ZB) to the nickel arsenide (NiAs) structure. We have shown that the FPMD simulation overestimates the transition pressure P(T) (compared to static enthalpy and experimental data) due to overpressure in the simulation box. The MD simulation reveals the structural pathway (cubic → orthorhombic → monoclinic → hexagonal), leading from the ZB phase to the NiAs phase. To find an explanation for the phase transition we calculated the vibrational and elastic properties under pressure. Negative Grüneisen parameters were obtained for the transverse acoustic phonon modes at the X and L high symmetry points. However, no mechanical instability or imaginary frequencies were found at pressures near P(T). Thus the transition results from a thermodynamic instability rather than an elastic/dynamical one. We have also calculated the optical properties of both the B3 and B8 phases, such as the real and imaginary parts of the dielectric constant, reflectivity, loss function and refractive index, and compared them with the existing experimental and theoretical data. An abrupt decrease is obtained from the reflectivity spectrum of the NiAs phase at P(T), which is supported from the peaks in the loss function.