Carbonate-Metal Reactions in the Lower Mantle

Carbonates are important carbon-bearing phases in the mantle. While their role in upper mantle petrologic processes has been well studied, their effect on phase relations, melting, and transport properties in the lower mantle is less understood. The stability of carbonates in the mantle depends on a host of factors, including pressure, temperature, oxygen fugacity, and reactions with surrounding mantle phases. To understand the stability of carbonates in the presence of metal in the lower mantle, carbonate-metal reaction experiments on the Fe-Si-Ca-Mg-C-O system were conducted up to 124 GPa and 3200 K. We find that carbonates react with iron alloys to form silicates, iron carbides, and oxides. However, the temperature at which these reactions occur increases with pressure, indicating that along a geotherm in the lowermost mantle carbonates are the stable carbon-bearing phase. Carbon is found to be less siderophilic at high-pressure compared to silicon.

Melting line of calcium characterized by in situ LH-DAC XRD and first-principles calculations

In this work, the melting line of calcium has been characterized both experimentally, using synchrotron X-ray diffraction in laser-heated diamond-anvil cells, and theoretically, using first-principles calculations. In the investigated pressure and temperature range (pressure between 10 and 40 GPa and temperature between 300 and 3000 K) it was possible to observe the face-centred phase of calcium and to confirm (and characterize for the first time at these conditions) the presence of the body-centred cubic and the simple cubic phase of calcium. The melting points obtained with the two techniques are in excellent agreement. Furthermore, the present results agree with the only existing melting line of calcium obtained in laser-heated diamond anvil cells, using the speckle method as melting detection technique. They also confirm a flat slope of the melting line in the pressure range between 10 and 30 GPa. The flat melting curve is associated with the presence of the solid high-temperature body-centered cubic phase of calcium and to a small volume change between this phase and the liquid at melting. Reasons for the stabilization of the body-centered face at high-temperature conditions will be discussed.

Recent Insights Into Electronic Performance, Magnetism and Exchange Splittings in the Cr-substituted CaO

The first-principles computations of density functional theory are employed to characterize the structural properties, electronic structures, and ferromagnetism induced by Cr impurities in Ca_1-xCr_xO compounds at concentrations x = 0. 25, 0.5, and 0.75. The dynamic stability is performed by the phonon spectra calculations. The structural parameters are computed by using Wu-Cohen generalized gradient approximation, while the electronic and magnetic properties are determined by the accurate Tran–Blaha-modified Becke–Johnson exchange potential. The crystal field, direct and indirect exchange splittings were investigated to determine the origin and stability of ferromagnetic state configuration. The Ca_1-xCr_xO systems have right half-metallicities, which are verified by the spin polarization of 100% and the integer values of total magnetic moments. The Ca_0.75Cr_0.25O, Ca_0.5Cr_0.5O, and Ca_0.25Cr_0.75O are half-metallic ferromagnetic with flip-gaps of 1.495, 0.888, and 0.218 eV, respectively. Therefore, the Ca_1-xCr_xO materials are suitable candidates for possible applications of spin-injection in future semiconductors spintronics.

Contributed Review: Culet diameter and the achievable pressure of a diamond anvil cell: Implications for the upper pressure limit of a diamond anvil cell

Recently, static pressures of more than 1.0 TPa have been reported, which raises the question: what is the maximum static pressure that can be achieved using diamond anvil cell techniques? Here we compile culet diameters, bevel diameters, bevel angles, and reported pressures from the literature. We fit these data and find an expression that describes the maximum pressure as a function of the culet diameter. An extrapolation of our fit reveals that a culet diameter of 1 μm should achieve a pressure of ∼1.8 TPa. Additionally, for pressure generation of ∼400 GPa with a single beveled diamond anvil, the most commonly reported parameters are a culet diameter of ∼20 μm, a bevel angle of 8.5°, and a bevel diameter to culet diameter ratio between 14 and 18. Our analysis shows that routinely generating pressures more than ∼300 GPa likely requires diamond anvil geometries that are fundamentally different from a beveled or double beveled anvil (e.g., toroidal or double stage anvils) and culet diameters that are ≤20 μm.

Ab initio investigation of CaO-ZnO alloys under high pressure

Ca_xZn_1–xO alloys are potential candidates to achieve wide band-gap, which might significantly promote the band gap engineering and heterojunction design. We performed a crystal structure search for CaO-ZnO system under pressure, using an ab initio evolutionary algorithm implemented in the USPEX code. Four stable ordered Ca_xZn_1–xO structures are found in the pressure range of 8.7–60 GPa. We further constructed the pressure vs. composition phase diagram of CaO-ZnO alloys based on the detailed enthalpy calculations. With the increase in Ca concentration, the CaO-ZnO alloy first undergoes a hexagonal to monoclinic transition, and then transforms back to a hexagonal phase. At Above 9 GPa, there is no cubic structure in the alloys, in contrast to the insostructural components (B1-B1). The band gap of the Ca_xZn_1–xO alloy shows an almost linear increase as a function of the Ca concentration. We also investigated the variation regularity of the band gap under pressure.

The vibrational and configurational entropy of α-brass

The heat capacities of two samples of a fcc Cu-Zn alloy with the composition CuZn15 and CuZn34 were measured from T = 5 K to 573 K using relaxation and differential scanning calorimetry. Below ∼90 K, they are characterised by negative excess heat capacities deviating from ideal mixing by up to -0.20 and -0.44 J · mol-1 · K-1 for CuZn15 and CuZn34, respectively. The excess heat capacities produce excess vibrational entropies, which are less negative compared to the excess entropy available from the literature. Since the literature entropy data contain both, the configurational and the vibrational part of the entropy, the difference is attributed to the excess configurational entropy. The thermodynamics of different short-range ordered samples was also investigated. The extent of the short-range order had no influence on the heat capacity below T = 300 K. Above T = 300 K, where the ordering changed during the measurement, the heat capacity depended strongly on the thermal history of the samples. From these data, the heat and entropy of ordering was calculated. The results on the vibrational entropy of this study were also used to test a relationship for estimating the excess vibrational entropy of mixing.

Phonon and elastic instabilities in rocksalt calcium oxide under pressure: a first-principles study

The lattice and elastic instabilities of rocksalt (RS) calcium oxide CaO under pressure are extensively studied to reveal the physically driven mechanism of the phase transition from RS to CsCl structure by using the pseudopotential plane-wave method within density-functional theory. The predicted phase transition pressure is 66.38 GPa, employing the total energy method, which falls in the experimental transition pressure range of 60-70 GPa. A pressure-induced soft transverse acoustic (TA) phonon mode is identified at the zone boundary X point in the Brillouin zone, signifying a structural instability. A predicted charge transfer from Ca to O with pressure might be attributable to the phonon softening. Moreover, a softening behavior in the C(44) shear modulus with pressure is predicted. Analysis of the calculated results suggested that, with increasing pressure, the predicted TA phonon softening behavior, instead of C(44) shear modulus instability, is mainly responsible for the pressure-induced structural phase transition. We also find that the bond between Ca and O becomes more ionic under compression from Mulliken population analysis.

A first-principle analysis on the phase stabilities, chemical bonds and band gaps of wurtzite structure A(x)Zn(1-x)O alloys (A = Ca, Cd, Mg)

The phase stabilities and structural and electronic properties of three zinc-based oxide alloy systems (Ca(x)Zn(1-x)O, Cd(x)Zn(1-x)O and Mg(x)Zn(1-x)O) are studied by first-principle methods. We examine all alloy configurations in three 16-atom supercells (1 × 1 × 2 B1 phase structure, 2 × 2 × 1 and 2 × 1 × 2 B4 phase structures) and utilize symmetry of the bulk materials to reduce the amount of calculation. Taking into account the contribution of the alloy statistics, we have drawn the regions of phase stability for Ca(x)Zn(1-x)O (0.25<x<0.375), Mg(x)Zn(1-x)O (0.375<x<0.5) and Cd(x)Zn(1-x)O (0.75<x<0.875). We have also analyzed lattice constants (a and c), structural parameter u and the bond lengths in the wurtzite phases. We found that the averaged lattice constants of Mg(x)Zn(1-x)O and Ca(x)Zn(1-x)O do not follow the Vegard rule and this is related to the degree of instability of the wurtzite MgO and CaO. Wurtzite CaO is not stable and turns into hexagonal CaO upon geometry optimization. The calculated band gaps are found to be consistent with the experimental values for alloys Cd(x)Zn(1-x)O and Mg(x)Zn(1-x)O. The bowing parameters for alloys Mg(x)Zn(1-x)O and Cd(x)Zn(1-x)O are estimated to be 0.87 and 1.30 eV, respectively.