Anna Yu. Likhacheva

Giant improvement of thermoelectric power factor of Bi2Te3 under pressure

The pressure (P) dependencies of both the thermopower (Seebeck effect) S and the electrical resistance (R) for p-type single crystals of Bi2Te3 and indium-doped bismuth telluride (InxBi2−xTe3,0.04≤x≤0.10) are reported on a pressure range of 0–8.5 GPa. The thermoelectric power factor (efficiency) (ae=S2/R) exhibits two maxima: the first one near ∼1 GPa and the second near ∼2.5–4.5 GPa. These features evidence a giant increase in the power factor by a factor of ∼10. Possible values of the dimensionless figure of merit under pressure are also estimated. The maxima are explained in terms of pressure-driven changes in an electron structure. The second feature may be also addressed to an intermediate high-pressure phase detected in x-ray diffraction studies.

Pressure-induced over-hydration of thomsonite: A synchrotron powder diffraction study

Abstract The structural behavior of thomsonite compressed in aqueous medium up to 3 GPa was studied by means of in situ synchrotron powder diffraction with a diamond anvil cell. In the range between 0.0001 and 2 GPa, the compressibility of thomsonite is markedly lower than that reported previously, where a non-penetrating medium (with only 6% H2O) was used. This indicates a pressure-induced hydration (PIH), which results in the transition to an over-hydrated phase observed at 2 GPa. The structure of over-hydrated thomsonite contains one additional, half-occupied H2O position, coordinated by the calcium at the Ca2 site, with a scolecite-like coordination [CaO4(H2O)3]. The appearance of new H2O position causes a 4.5% volume expansion through the cooperative rotation of [T2O5]∞ chains, leading to the enlargement of the cross-section of the main channels parallel to c axis. The observed deformation mechanism is similar to that found in high-hydrated and super-hydrated natrolite, although only a half of the channels are affected by PIH. The present data indicate that the over-hydration effect in fibrous zeolites strongly depends on the partial water pressure in compressing medium.

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.

Enhanced power factor and high-pressure effects in (Bi,Sb)2(Te,Se)3 thermoelectrics

We investigated the effects of applied high pressure on thermoelectric, electric, structural, and optical properties of single-crystalline thermoelectrics, Bi2Te3, BixSb2−xTe3 (x = 0.4, 0.5, 0.6), and Bi2Te2.73Se0.27 with the high thermoelectric performance. We established that moderate pressure of about 2–4 GPa can greatly enhance the thermoelectric power factor of all of them. X-ray diffraction and Raman studies on Bi2Te3 and Bi0.5Sb1.5Te3 found anomalies at similar pressures, indicating a link between crystal structure deformation and physical properties. We speculate about possible mechanisms of the power factor enhancement and suppose that pressure/stress tuning can be an effective tool for the optimization of the thermoelectric performance.

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.

Compressibility of Gas Hydrates

Experimental data on the pressure dependence of unit cell parameters for the gas hydrates of ethane (cubic structure I, pressure range 0-2 GPa), xenon (cubic structure I, pressure range 0-1.5 GPa) and the double hydrate of tetrahydrofuran+xenon (cubic structure II, pressure range 0-3 GPa) are presented. Approximation of the data using the cubic Birch-Murnaghan equation, P=1.5B(0)[(V(0)/V)(7/3)-(V(0)/V)(5/3)], gave the following results: for ethane hydrate V(0)=1781 Å(3) , B(0)=11.2 GPa; for xenon hydrate V(0)=1726 Å(3) , B(0)=9.3 GPa; for the double hydrate of tetrahydrofuran+xenon V(0)=5323 Å(3) , B(0)=8.8 GPa. In the last case, the approximation was performed within the pressure range 0-1.5 GPa; it is impossible to describe the results within a broader pressure range using the cubic Birch-Murnaghan equation. At the maximum pressure of the existence of the double hydrate of tetrahydrofuran+xenon (3.1 GPa), the unit cell volume was 86% of the unit cell volume at zero pressure. Analysis of the experimental data obtained by us and data available from the literature showed that 1) the bulk modulus of gas hydrates with classical polyhedral structures, in most cases, are close to each other and 2) the bulk modulus is mainly determined by the elasticity of the hydrogen-bonded water framework. Variable filling of the cavities with guest molecules also has a substantial effect on the bulk modulus. On the basis of the obtained results, we concluded that the bulk modulus of gas hydrates with classical polyhedral structures and existing at pressures up to 1.5 GPa was equal to (9±2) GPa. In cases when data on the equations of state for the hydrates were unavailable, the indicated values may be recommended as the most probable ones.

An X-ray diffraction study of the pressure-induced hydration in cordierite at 4–5 GPa

Abstract The elastic and structural behavior of natural cordierite compressed in aqueous medium up to 6 GPa was studied by means of in situ synchrotron powder diffraction with a diamond-anvil cell. In the range between 1-4 GPa the elastic behavior is regular and slightly anisotropic, with linear compressibilities βa:βb:βc = 4:4:5, the most rigid a-b plane coinciding with the orientation of 6-membered rings. A distinct decrease of compressibility in the range of 4-5 GPa indicates a pressure-induced hydration (PIH), which is confirmed by the structure refinements. The addition of about 60% of the initial water content into the cordierite channels proceeds through positional disordering of the H2O sites inside the channel cavity and a stepwise filling of the H2O position inside the 6-membered rings, leading to the phase transition at about 4.7 GPa. The appearance of H2O molecules inside 6-membered rings prevents their contraction and even causes their slight enlargement along the a direction, apparently related to the orientation of H-bonds. This results in an anisotropic deformation of the unit cell and an increase of the a parameter in the HP phase at 4.9 GPa, as well as a decrease of linear compressibility along a upon the further compression up to 6 GPa (βa:βb:βc = 5:9:10).

Non-hydrostatic compression of zeolite NaA in water medium: connection to anomalous conductivity

Abstract High-pressure synchrotron X-ray powder diffraction measurements of a synthetic zeolite NaA were carried out up to 2.5 GPa using pure water as pressure-transmitting (P) medium to provide non-hydrostatic (wet) conditions in a diamond anvil cell. The compressibility of wet zeolite NaA is similar to that measured at hydrostatic compression in water within the P-range 0–0.8 GPa, whereas between 1–2 GPa the zeolite becomes slightly more compressible and progressively amorphizes due to the non-hydrostatic conditions. Rietveld refinement at 0.37 GPa reveals a selective additional filling of the H2O sites in α- and β-cage, leading to about 30% increase of the total water content. The over-hydrated state of the zeolite is partially preserved after the pressure release. The over-hydration of zeolite pores, combined with a partial disordering at the onset of amorphization, apparently provides necessary conditions for the P-induced enhancement of water-cationic diffusion and the corresponding increase of ionic conductivity observed in zeolite NaA in non-hydrostatic water medium.

Pyrometamorphic osumilite : occurrence, paragenesis, and crystal structure as compared to cordierite

A detailed mineralogical study and single-crystal X-ray analysis were carried out on K-Mg osumilite from high temperature pyrometamorphic rocks. These rocks were generated during spontaneous combustion of coal-bearing spoil-heaps in the South Urals, Russia. The osumilite-bearing metapelitic rocks – clinkers – contain K-Na- and K-Na-Ca feldspars, tridymite, mullite as main phases and magnesian K-bearing cordierite, corundum, native iron, pyrrhotite, cohenite, graphite, black carbon, and iron phosphide as minor phases. The composition of pyrometamorphic osumilites approaches the synthetic compound KMg 2 Al 3 (Al 2 Si 10 )O 30 . Pyrometamorphic alterations of sedimentary protolith during coal combustion proceeded at atmospheric pressure. The osumilite-bearing parageneses are formed under the conditions of low f O2 and water activity, at temperatures above 900 °C and below the melting point for the osumilite-bearing associations. The crystal structure of osumilite was refined from X-ray single-crystal data. Its crystal-chemical formula is K 0.83 C Na 0.10 B′ (Mg 1.78 Fe 0.16 Mn 0.03 Ti 0.03 ) A (Al 2.88 Fe 0.12 ) T2 (Al 1.91 Si 10.09 ) T1 O 30 . K cations occupy the standard 12-coordinated C site, whereas Na cations populate the B′position, located between three 6-membered double rings above the A position. The comparative crystal-chemical analysis shows that in the structure of osumilite, in contrast to cordierite, potassium not only compensates for the framework charge, but also stabilizes the structure. The role of sodium in cordierite and osumilite is similar and consists in a charge compensating function.

In situ spectroscopic study of water intercalation into talc: New features of 10 Å phase formation

Abstract The synthesis of 10 Å phase via the reaction of talc plus water at 8 GPa and 500 °C was studied by in situ Raman spectroscopy using a diamond-anvil cell. The initial fast (2 h) incorporation of interlayer H2O molecules into the talc structure is traced by gradual growth of new OH stretching bands at 3592 and 3621 cm-1 and the shift of several framework bands. Further monitoring at HP-HT conditions over 7 h reveals gradual weakening of the 3592 cm-1 band, which can probably be related to the onset of the formation of “long-run” 10 Å phase through the appearance of silanol groups following the model proposed by Pawley et al. (2010), influencing the interlayer hydrogen bonding.