1,720,960 research outputs found
Melting Behavior of CaO at High Temperature and Pressure: A Molecular Dynamics Study
No full textThe thermodynamic behavior of calcium oxide (CaO) under high temperature and pressure conditions is critical for understanding the physics of planetary interiors. This study employs molecular dynamics (MD) simulations, including both classical and ab initio approaches, to investigate the melting behavior of CaO. We calculate the melting temperature of CaO by the void-nucleated melting and two-phase coexistence techniques, aiming to resolve discrepancies in experimental data on the melting point, which range from 2843 to 3223 K in different studies due to the high reactivity and vapor pressure of the substance. The obtained results are Tf = 3066 ± 12 K and Tf = 2940 ± 65 K using the void-nucleated melting and the two-phase coexistence method, respectively. Additionally, we calculate the enthalpy of fusion and the high-pressure melting curve for the first time without making any assumption on the Clapeyron slope. This is extremely important since in experiments, the Clapeyron slope of the melting curve is estimated from low pressure measurements and the overheating ratio [i.e., η = (Ts/Tf) -1, where Ts represents the thermal instability limit corresponding to the homogeneous melting temperature of the solid] is often assumed to be constant in simulations. Our MD results show that Ts increases more rapidly with pressure than Tf and, thus, that the overheating ratio sensibly depends upon pressure. These findings contribute to accurate modeling of the CaO phase diagram, which is essential for geochemistry, cosmochemistry, and materials science
Comparative Analysis of DFT+U, ACBN0, and Hybrid Functionals on the Spin Density of YTiO<sub>3</sub> and SrRuO<sub>3</sub>
Full text linkWe present a quantitative analysis of the theoretical spin density map of two ferromagnetic perovskites, YTiO3 and SrRuO3. We calculated the spin density using the standard density functional theory (DFT)+U method, where the Hubbard U correction is applied to the Ti and Ru ions, and with the pseudo-hybrid ACBN0 method, where the Hubbard U parameters are determined self-consistently. The ACBN0 calculations yielded a large value of the Hubbard U of the oxygen 2p orbitals. We also used the screened hybrid HSE06 functional, which is widely used to describe the electronic structure of oxides. We used the Quantum Theory of Atoms in Molecules (QTAIM) theory and integrated the spin density in the atomic basins instead of projecting on atomic orbitals. This way, our results can be compared to experimental reports as well as to other DFT calculations
Ab initio thermal expansion and thermoelastic properties of ringwoodite ( γ-Mg2SiO4) at mantle transition zone conditions
Full text linkThermal convection in the Earth's mantle is driven by lateral variations in temperature and density, which are substantially controlled by the local volume thermal expansion of the constituent mineral phases. Ringwoodite is a major component of the lower mantle transition zone, but its thermal expansivity and thermoelastic properties are still affected by large uncertainties. Ambient thermal expansion coefficient (αV0ggggggg), for instance, can vary as much as 100g% according to different experimental investigations available from the literature. In this work, we perform ab initio density functional theory calculations of vibrational properties of spinel-structured Mg2SiO4 ringwoodite in order to provide reliable thermophysical data up to mantle transition zone conditions. Temperature- and pressure-dependent thermal expansivity has been obtained by phonon dispersion calculations in the framework of quasi-harmonic approximation (QHA) up to 25gGPa and 2000gK. Theoretical analysis of vibrational spectra reveals that accurate prediction of IR and silent modes, along with their relative mode Grüneisen parameters, is crucial to define thermal expansivity. A six-parameter analytical function is able to reproduce ab initio values fairly well in the whole investigated P-T range, i.e., αV(P,T)Combining double low line(1.6033×10-5+8.839×10-9T+11.586×10-3T-1-6.055T-2+804.31T-3)g×expĝ (-2.52×10-2P), with temperature in kelvin and pressure in gigapascal. Ab initio static and isothermal bulk moduli have been derived for ringwoodite along with their P, T and cross derivatives, i.e., K0gCombining double low lineg184.3gGPa, KT,300KgCombining double low lineg176.6gGPa, K0′gCombining double low lineg4.13, KT,300K′gCombining double low lineg4.16, g KTg TPgCombining double low lineg-0.0233gGPagK-1 and g 2KTg Pg T0Combining double low line1.0×10-4gK-1. Computed thermal expansivity and thermoelastic properties support the evidence that QHA performs remarkably well for Mg2SiO4 ringwoodite up to mantle transition zone temperatures. Since volume thermal expansion of ringwoodite is strongly pressure-dependent and its pressure dependence becomes more marked with the increasing temperature, internally consistent assessments and empirical extrapolation of thermoelastic data to deep mantle conditions should be taken with care to avoid inaccurate or spurious predictions in phase equilibrium and mantle convection numerical modeling
High pressure structure studies of three SrGeO3 polymorphs – Amorphization under pressure
No full textWe report on the synthesis and high pressure behavior of three polymorphs of SrGeO3. At ambient pressure, SrGeO3 crystallizes in the monoclinic structure pseudo-wollastonite. Two high pressure polymorphs, triclinic walstromite, and cubic perovskite were synthesized using a large volume multi-anvil press. The crystal structures of the three polymorphs were investigated with powder X-ray diffraction as a function of pressure using diamond anvil cells. It was found that the pseudo-wollastonite polymorph becomes amorphous at 10 GPa and equation of state fitting of the volume data yielded a bulk modulus of K0 = 47(4) GPa, reported for the first time. Compression of the walstromite structure showed the structure to be very compressible with two distinct phase transitions at around 10–12 GPa and 35–38 GPa. The data suggest that the structure then becomes amorphous although it retains a small degree of long-range order to the highest pressure studied. The perovskite polymorph was very incompressible and equation of state fitting of the volume data yielded a high bulk modulus of K0 = 194(3) GPa. All the experimental data was compared to density functional theory calculations, which were observed to fit well with the experiments
High-Pressure Computational Search of Trivalent Lanthanide Dinitrides
Full text linkTransition-metal nitrides have attracted much interest of the scientific community for their intriguing properties and technological applications. Here, we focus on yttrium dinitride (YN2) and its formation and structural transition under pressure. We employed a fixed composition USPEX search to find the most stable polymorphs. We choose yttrium as a proxy for the lanthanide series because it has only +3 oxidation state, contrary to most transition metals. We then computed the thermodynamic and dynamical stabilities of these structures compared to the decomposition reactions, and we found that the compound undergoes two structural transitions, the latter showing the formation of N-4 chains. A closer look into the nature of the nitrogen bonding showed that in the first two structures, where nitrogen forms dimers, the bond length is intermediate between that of a single bond and that of a double bond, making it hard to rationalize the proper oxidation state configuration for YN2. In the latter structure, where there is formation of N-4 chains, the bond lengths increase significantly up to a value that can be justified as a single bond. Finally, we also studied the electronic structure and dynamical stability of the structures we found
High-Pressure, High-Temperature Studies of Phase Transitions in SrOsO3 - Discovery of a Post-Perovskite
No full textUsing a recently developed method for in situ high-pressure, laser heating experiments in diamond anvil cells, we obtained a novel post-perovskite phase of SrOsO3. The phase transition from perovskite SrOsO3 was induced at 44 GPa and 1350 K in a diamond anvil cell and characterized with synchrotron powder X-ray diffraction. The newly obtained post-perovskite is quenchable and Le Bail refinements under ambient conditions yielded the unit cell parameters: a = 3.152(9) Å, b = 10.82(2) Å, c = 7.27(1) Å, V = 248.1(1) Å3. In addition, the compression of perovskite SrOsO3 at ambient temperature was investigated up to 66 GPa in a diamond anvil cell using synchrotron powder X-ray diffraction. The compression at ambient temperature showed that pressure alone does not induce the first-order phase transition to the post-perovskite structure. However, at 36 GPa, a continuous phase transition to monoclinic (P21/n) symmetry was detected, persistent up to 58 GPa, where the perovskite transitioned back to orthorhombic (Pbnm) symmetry. Fitting a third-order Birch-Murnaghan equation of state to the obtained P-V data for perovskite SrOsO3 yielded a bulk modulus of K0 = 187.4(15) GPa. Density functional theory calculations were performed to support the experimental findings in the compression study at ambient temperature. This work shows that transformations to the post-perovskite structure can be obtained for a wider range of perovskites than simple empirical rules otherwise suggest
High-pressure, low-temperature studies of phase transitions in SrRuO3 – Absence of volume collapse
No full textWe report powder diffraction data measured on SrRuO3 to pressures of 88 GPa at ambient temperature. The perovskite structure is found to persist up to the highest pressure measured, but the system undergoes a continuous transition consistent with a 2nd order phase transition to the monoclinic space group P21/n near 25 GPa. Fitting the P–V data with the 2nd order Birch-Murnaghan EoS yielded a bulk modulus of K0 = 177.4(10) GPa. Four high-pressure low-temperature experiments have been performed to resolve discrepancies in previous studies. SrRuO3 cooled to 6 K at near-constant pressures of 10 GPa and 40 GPa showed no volume collapse during cooling. A compression experiment conducted at 6 K up to 65 GPa, showed a structural transition to the P21/n space group at 28 GPa, and fitting the P–V data with a 3rd order Birch-Murnaghan EoS yielded a bulk modulus of K0 = 154(6) GPa. The experimental data have been compared to DFT calculations. In good agreement with experiments, calculations including ferromagnetic interactions show a structural transition to the P21/n space group at 30–40 GPa. Surprisingly, calculations show that the SrRuO3 returns to Pbnm symmetry above 90 GPa. The magnetic moment on Ru was predicted to disappear above 70 GPa and at the same time the RuO6 octahedra was found to become distorted. The present studies provide a solid ground for further studies of phase relations of SrRuO3 and related perovskites
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Structure Evolution of Ge-Doped CaTiO3 (CTG) at High Pressure: Search for the First 2:4 Locked-Tilt Perovskite by Synchrotron X-ray Diffraction and DFT Calculations
Full text linkThis research investigates the high-pressure behavior of the Ca(Ti0.95Ge0.05)O3 perovskite, a candidate of the locked-tilt perovskite family (orthorhombic compounds characterized by the absence of changes in the octahedral tilt and volume reduction under pressure controlled solely by isotropic compression). The study combines experimental high-pressure synchrotron diffraction data with density functional theory (DFT) calculations, complemented by the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), to understand the structural evolution of the perovskite under pressure. The results show that CTG undergoes nearly isotropic compression with the same compressibility along all three unit-cell axes (i.e., Ka0 = Kb0 = Kc0, giving a normalized cell distortion factor with pressure dnorm(P) = 1). However, a modest increase in octahedral tilting with pressure is revealed by DFT calculations, qualifying CTG as a new type of GdFeO3-type perovskite that exhibits both isotropic compression and nonlocked tilting. This finding complements two existing types: perovskites with anisotropic compression and tilting changes and those with isotropic compression and locked tilting. The multimethod approach provides valuable insights into the structural evolution of locked-tilt perovskites under high pressure and establishes a protocol for the efficient study of complex high-pressure systems. The results have implications for the design of new functional materials with desirable properties
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