Sergey V. Rashchenko

Thermal equation of state and thermodynamic properties of molybdenum at high pressures

A comprehensive P-V-T dataset for bcc-Mo was obtained at pressures up to 31 GPa and temperatures from 300 to 1673 K using MgO and Au pressure calibrants. The thermodynamic analysis of these data was performed using high-temperature Birch-Murnaghan (HTBM) equations of state (EOS), Mie-Gruneisen-Debye (MGD) relation combined with the room-temperature Vinet EOS, and newly proposed Kunc-Einstein (KE) approach. The analysis of room-temperature compression data with the Vinet EOS yields V0 = 31.14 ± 0.02 A3, KT = 260 ± 1 GPa, and KT′ = 4.21 ± 0.05. The derived thermoelastic parameters for the HTBM include (∂KT/∂T)P = −0.019 ± 0.001 GPa/K and thermal expansion α = a0 + a1T with a0 = 1.55 ( ± 0.05) × 10−5 K−1 and a1 = 0.68 ( ± 0.07) × 10−8 K−2. Fitting to the MGD relation yields γ0 = 2.03 ± 0.02 and q = 0.24 ± 0.02 with the Debye temperature (θ0) fixed at 455-470 K. Two models are proposed for the KE EOS. The model 1 (Mo-1) is the best fit to our P-V-T data, whereas the second model (Mo-2) is derived by including the shock compression and other experimental measurements. Nevertheless, both models provide similar thermoelastic parameters. Parameters used on Mo-1 include two Einstein temperatures ΘE10 = 366 K and ΘE20 = 208 K; Gruneisen parameter at ambient condition γ0 = 1.64 and infinite compression γ∞ = 0.358 with β  = 0.323; and additional fitting parameters m = 0.195, e0 = 0.9 × 10−6 K−1, and g = 5.6. Fixed parameters include k = 2 in Kunc EOS, mE1 = mE2 = 1.5 in expression for Einstein temperature, and a0 = 0 (an intrinsic anharmonicity parameter). These parameters are the best representation of the experimental data for Mo and can be used for variety of thermodynamic calculations for Mo and Mo-containing systems including phase diagrams, chemical reactions, and electronic structure.A comprehensive P-V-T dataset for bcc-Mo was obtained at pressures up to 31 GPa and temperatures from 300 to 1673 K using MgO and Au pressure calibrants. The thermodynamic analysis of these data was performed using high-temperature Birch-Murnaghan (HTBM) equations of state (EOS), Mie-Gruneisen-Debye (MGD) relation combined with the room-temperature Vinet EOS, and newly proposed Kunc-Einstein (KE) approach. The analysis of room-temperature compression data with the Vinet EOS yields V0 = 31.14 ± 0.02 A3, KT = 260 ± 1 GPa, and KT′ = 4.21 ± 0.05. The derived thermoelastic parameters for the HTBM include (∂KT/∂T)P = −0.019 ± 0.001 GPa/K and thermal expansion α = a0 + a1T with a0 = 1.55 ( ± 0.05) × 10−5 K−1 and a1 = 0.68 ( ± 0.07) × 10−8 K−2. Fitting to the MGD relation yields γ0 = 2.03 ± 0.02 and q = 0.24 ± 0.02 with the Debye temperature (θ0) fixed at 455-470 K. Two models are proposed for the KE EOS. The model 1 (Mo-1) is the best fit to our P-V-T data, whereas the second model (Mo-2) is derived by including...

Preparation of a macrocrystalline pressure calibrant SrB4O7:Sm 2+ suitable for the HP-HT powder diffraction

A new simplified synthesis of monocrystalline chips of SrB4O7: Sm2+ pressure calibrant, well-suited for the diamond anvil cell (DAC) powder diffraction experiments, is proposed. It consists of ordinary solid-state synthesis of fine-grained SrB4O7: Sm2+ and subsequent annealing near melting temperature. The obtained material was characterized and tested in HP-HT DAC experiment.

A new mechanism of anionic substitution in fluoride borates

A comprehensive study of the BaF2-Ba3(BO 3)2 phase diagram has revealed a significant difference between the two intermediate phases Ba5(BO3)3F and Ba 7(BO3)4-y F2+3y. The latter exhibited (BO 3)3- ↔ 3F- anionic substitution which, unusually, strongly influences the solidus temperature. A comparison of the Ba5(BO3)3F and Ba7(BO3)4-y F2+3y crystal structures, along with consideration of other compounds demonstrating (BO3)3- ↔ 3F- isomorphism, allows for the disclosure of the mechanism of (BO3)3- ↔ 3F - heterovalent anionic substitution in fluoride borates via [(BO 3)F]4- tetrahedral groups being replaced by four fluoride anions. No exception to this mechanism has been discovered among all known phases with (BO3)3- ↔ 3F- substitution.

Phase formation in the BaB2O4–BaF2–BaO system and new non-centrosymmetric solid-solution series Ba7(BO3)4−xF2+3x

Detailed study of the BaB2O4–BaF2–BaO system resulted in the discovery of the new Ba7(BO3)4−xF2+3x solid solution belonging to the BaF2–Ba3(BO3)2 section. The distinguishing feature of the crystal structure of Ba7(BO3)4−xF2+3x phase is its extensive (BO3)3− ↔ 3F− anionic isomorphic substitution, confirmed by X-ray diffraction study of Ba7(BO3)3.51F3.47 (x = 0.49) single crystals (space group P63; a = 11.18241(11) A, c = 7.23720(8) A). The area of homogeneity for Ba7(BO3)4−xF2+3x solid solution spans between Ba7(BO3)3.35F3.95 and Ba7(BO3)3.79F2.63 compositions (0.21 < x < 0.65). Also, a new orthorhombic phase with a tentative composition of Ba5(BO3)3F has been identified in XRD powder patterns and indexed with cell parameters a = 7.605 A, b = 14.843 A and c = 10.291 A.

Revised calibration of the Sm:SrB4O7 pressure sensor using the Sm-doped yttrium-aluminum garnet primary pressure scale

The pressure-induced shift of Sm:SrB4O7 fluorescence was calibrated in a quasi-hydrostatic helium medium up to 60 GPa using the recent Sm-doped yttrium-aluminum garnet primary pressure scale as a reference. The resulting calibration can be written as P = −2836/14.3 [(1 + Δλ/685.51)−14.3 − 1]. Previous calibrations based on the internally inconsistent primary scales are revised, and, after appropriate correction, found to agree with the proposed one. The calibration extended to 120 GPa was also performed using corrected previous data and can be written as P = 4.20 Δλ (1 + 0.020 Δλ)/(1 + 0.036 Δλ).

High‐pressure structural properties of naphthalene up to 6 GPa

The elastic and structure behavior of solid naphthalene (C10H8) was studied by in situ synchrotron powder X-ray diffraction up to 5.6 GPa in a diamond anvil cell. Rietveld refinements were applied to obtain atomic coordinates and lattice parameters as a function of pressure. Within the studied pressure range, the intramolecular C—C and C—H distances decrease by 3.5%, whereas intermolecular distances within and between the herringbone layers decrease by 18 and 10%, respectively. Noindication of a symmetry change (space group P21/a) was observed. A markedly anisotropic compression is preserved up to 5.6 GPa, with linear compressibilities βa:βb:βc:β[200] ≃ 7:5:3:10. Fitting of the P–V experimental data to the Vinet equation of state yields K0 = 7.9 (3) GPa and K′ = 7.5 (3) at V0 = 361 A3. At the highest pressure of 5.6 GPa, the unit-cell volume is decreased by 23%. The overall regularity of compression confirms the absence of major structural transformations in naphthalene up to 5.6 GPa. However, a distinct bend of the pressure dependence of the interlayer C⋯C distances observed at 2 GPa suggests a minor structural irregularity, which is apparently associated with spectroscopic anomalies observed previously.

The system Na2CO3-FeCO3 at 6 GPa and its relation to the system Na2CO3-FeCO3-MgCO3

Abstract The phase relations in the Na2CO3-(Fe0.87Mn0.06Mg0.07)CO3 system have been studied in Kawai-type multi-anvil experiments using graphite capsules at 6.0 GPa and 900-1400 °C. Subsolidus assemblages comprise the stability fields of Na2CO3 + Na2Fe(CO3)2 and Na2Fe(CO3)2 + siderite with the transition boundary at X(Na2CO3) = 50 mol%. Intermediate Na2Fe(CO3)2 compound has rhombohedral R3̅ eitelite structure with cell parameters a = 4.9712(16), c = 16.569(4) Å, V = 354.61(22). The Na2CO3- Na2Fe(CO3)2 eutectic is established at 1000 °C and 66 mol% Na2CO3. Na2Fe(CO3)2 disappears between 1000 and 1100 °C via incongruent melting to siderite and a liquid containing about 55 mol% Na2CO3. Siderite remains a subliquidus phase at 1400 °C at X(Na2CO3) ≤ 30 mol%. The ternary Na2CO3-FeCO3-MgCO3 system can be built up from the corresponding binary systems: two systems with intermediate Na2(Mg,Fe)(CO3)2 phase, which melts congruently at the Mg-rich side and incongruently at the Fe-rich side, and the (Mg,Fe)CO3 system with complete solid solution. The phase relations suggest that the maximum contribution of FeCO3 component into the lowering solidus temperatures of Na-bearing carbonated mantle domains could not exceed several tens of degrees Celsius.

Thermal equation of state to 33.5 GPa and 1673 K and thermodynamic properties of tungsten

A comprehensive P-V-T dataset for bcc-tungsten was obtained for pressures up to 33.5 GPa and temperatures 300–1673 K using MgO and Au pressure scales. The thermodynamic analysis of these data was performed using high-temperature (HT) and Mie-Gruneisen-Debye (MGD) relations combined with the Vinet equations of state (EOS) for room-temperature isotherm and the newly proposed Kunc-Einstein (KE) EOS. The KE EOS allowed calibration of W thermodynamic parameters to the pressures of at least 300 GPa and temperatures up to 4000 K with minor uncertainties (<1% in calculated volume of W). A detailed analysis of room-temperature compression data with Vinet EOS yields V0 = 31.71 ± 0.02 A3, KT = 308 ± 1 GPa, and KT′  = 4.20 ± 0.05. Estimated thermoelastic parameters for HT include (∂KT/∂T)P = −0.018 ± 0.001 GPa/K and thermal expansion α = a0 + a1T with a0 = 1.35 (±0.04) × 10−5 K−1 and a1 = 0.21 (±0.05) × 10−8 K−2. Fitting to the MGD relation yielded γ0 = 1.81 ± 0.02 and q = 0.71 ± 0.02 with the Debye temperature (θ0,)...

Incommensurately modulated twin structure of nyerereite Na1.64K0.36Ca(CO3)2

The incommensurately modulated twin structure of nyerereite Na1.64K0.36Ca(CO3)2 has been first determined in the (3 + 1)-dimensional symmetry group Cmcm(α00)00s with modulation vector q = 0.383a*. Unit-cell values are a = 5.062 (1), b = 8.790 (1), c = 12.744 (1) Å. Three orthorhombic components are related by threefold rotation about [001]. Discontinuous crenel functions are used to describe the occupation modulation of Ca and some CO3 groups. The strong displacive modulation of the O atoms in vertexes of such CO3 groups is described using x-harmonics in crenel intervals. The Na, K atoms occupy mixed sites whose occupation modulation is described in two ways using either complementary harmonic functions or crenels. The nyerereite structure has been compared both with the commensurately modulated structure of K-free Na2Ca(CO3)2 and with the widely known incommensurately modulated structure of γ-Na2CO3.

Search for compounds of the NaBaR(BO3)2 family (R = La, Nd, Gd, and Yb) and the new NaBaYb(BO3)2 orthoborate

A search for compounds of the NaBaR(BO3)2 composition (where R = La3+, Nd3+, Gd3+, or Yb3+) is performed by solid state synthesis and spontaneous crystallization. A new compound, NaBaYb(BO3)2, is found in this series. It crystallizes in space group