High-pressure Structural Transition Research Articles

High-pressure phase transition of cesium chloride and cesium bromide.

The high-symmetry cubic cesium chloride (CsCl) structure with a space group of Pm3¯m (Z = 1) is one of the prototypical AB-type compounds, which is shared with cesium halides and many binary metallic alloys. The study of high-pressure evolution of the CsCl phase is of fundamental importance in helping to understand the structural sequence and principles of crystallography. Here, we have systematically investigated the high-pressure structural transition of cesium halides up to 200 GPa using an effective CALYPSO algorithm. Strikingly, we have predicted several thermodynamically favored high-pressure phases for cesium chloride and cesium bromide (CsBr). Further electronic calculations indicate that CsCl and CsBr become metallic via band-gap closure at strong compression. The current predictions have broad implications for other AB-type compounds that likely harbor similar novel high-pressure behavior.

The high-pressure structural transition in MnS: an ab initio study

The structural phase transition between B1 (α-MnS) and B3 (β-MnS) is investigated using a density functional theory method. The structural phase transition pressure Pt from α-MnS to β-MnS, which is determined on the basis of the third-order Birch–Murnaghan equation of states, is 30.75 GPa. Also, the lattice parameters a, the bulk modulus B and pressure derivative of bulk modulus B′, which are generally in good agreement with experiments and other theoretical values, are obtained under zero pressure. For further investigation of the structural phase transition pressure of MnS, the relative volumes V/V 0, the bulk modulus B, first and second pressure derivatives (B′ and B″) of bulk modulus for the two structures of MnS have been calculated under various pressures.

Characterisation of the high-pressure structural transition and elastic properties in boron arsenic

This paper carries out the First principles calculation of the crystal structures (zinc blende (B3) and rocksalt (B1)) and phase transition of boron arsenic (BAs) based on the density-functional theory. Using the relation between enthalpy and pressure, it finds that the transition phase from the B3 structural to the B1 structural occurs at the pressure of 113.42GPa. Then the elastic constants C11, C12, C44, bulk modulus, shear modulus, Young modulus, anisotropy factor, Kleinman parameter and Poisson ratio are discussed in detail for two polymorphs of BAs. The results of the structural parameters and elastic properties in B3 structure are in good agreement with the available theoretical and experimental values.

A first principles study of the thermal stability ofAm(MH4)n light complex hydrides

From the physical point of view, the cohesive energy of a reactant is preferable toits formation energy for characterizing its influence on the reaction processesfrom the reactants to the products. In fact it has been found that there is acertain correlation between the experimental hydrogen desorption temperatureand the cohesive energy calculated by a first principles method for a series ofAm(MH4)n (A = Li, Na, Mg;M = Be, B, Al) light complexhydrides (including Na2BeH4, Li2BeH4,NaAlH4,LiAlH4,Mg(AlH4)2,LiBH4 andNaBH4), which suggests that cohesive energy may be a useful physical quantity for evaluatingthe hydrogen desorption ability of complex hydrides, especially in cases whendehydrogenation products have unknown crystal structures, or may even be unknown. Tounderstand this correlation more deeply, the ionic interaction between A and theMH4 complex and the covalent interaction between M and H were calculatedand their contributions to the cohesive energy evaluated quantitatively.The calculated results show that the covalent M–H interaction in theMH4 complex is the dominant part of the cohesive energyEcoh (up to more than 75%) and hardly changes during high-pressure structural transitions ofAm(MH4)n. It was also found that low electronegativity of M or high electronegativity of A is responsiblefor the weak covalent M–H interaction and finally leads to the low thermodynamic stability ofAm(MH4)n, suggesting thatcomplex hydrides Am(MH4)n can be destabilized by partial substitution of M (A) with an element with electronegativitylower (higher) than Ms (As). This conclusion has been confirmed by lots of experimentalresults and may be a useful guideline for the future design of new complex hydrides of the typeAm(MH4)n.

Pressure induced phase transition in tin: Ab-initio calculations

First principles total energy calculations as a function of unit cell volume have been performed on α (diamond structure), β (double bct), bct, bcc and hcp structures of tin. The comparison of total energies of these structures at various volumes suggests high pressure structural transitions of α → β → bet → bct at pressures of ~ 0.15 GPa, 2.7 GPa and 28 GPa. Also, 300 K isotherm upto maximum pressures of ~ 400 GPa has been determined for this metal. Using theoretical isotherm, shock Hugoniot of tin has been derived. Further, theoretically evaluated volume dependent Gruneisen parameter in conjunction with Lindeman melting law has been used to determine the pressure dependent melting of tin. The intersection of the theoretical melting curve with theoretically determined temperature rise with shock pressure reveals that this metal will melt at shock pressure of ~ 38 GPa (temperature ~ 1428 K) in good agreement with experimental value of 39 GPa (1550+100 K).

High-Pressure Structural Transitions of Sc2O3 by X-ray Diffraction, Raman Spectra, and Ab Initio Calculations

The high-pressure behavior of scandium oxide (Sc(2)O(3)) has been investigated by angle-dispersive synchrotron powder X-ray diffraction and Raman spectroscopy techniques in a diamond anvil cell up to 46.2 and 42 GPa, respectively. An irreversible structural transformation of Sc(2)O(3) from the cubic phase to a monoclinic high-pressure phase was observed at 36 GPa. Subsequent ab initio calculations for Sc(2)O(3) predicted the phase transition from the cubic to monoclinic phase but at a much lower pressure. The same calculations predicted a second phase transition at 77 GPa from the monoclinic to hexagonal phase.

Characterization of the High-Pressure Structural Transition and Thermodynamic Properties in Sodium Chloride: A Computational Investigation on the Basis of the Density Functional Theory

Using first-principles calculations, the elastic constants, the thermodynamic properties, and the structural phase transition between the B1 (rocksalt) and the B2 (cesium chloride) phases of NaCl are investigated by means of the pseudopotential plane-waves method. The calculations are performed within the generalized gradient approximation to density functional theory with the Perdew-Burke-Ernzerhof exchange-correlation functional. On the basis of the third-order Birch-Murnaghan equation of states, the transition pressure Pt between the B1 phase and the B2 phase of NaCl is determined. The calculated values are generally speaking in good agreement with experiments and with similar theoretical calculations. From the theoretical calculations, the shear modulus, Young's modulus, rigidity modulus, and Poisson's ratio of NaCl are derived. According to the quasi-harmonic Debye model, we estimated the Debye temperature of NaCl from the average sound velocity. Moreover, the pressure derivatives of elastic constants, partial differentialC11/partial differentialP, partial differentialC12/partial differentialP, partial differentialC44/partial differentialP, partial differentialS11/partial differential P, partial differentialS12/partial differentialP, and partial differentialS44/partial differentialP, for NaCl crystal are investigated for the first time. This is a quantitative theoretical prediction of the elastic and thermodynamic properties of NaCl, and it still awaits experimental confirmation.

High-pressure phase transition inRb6C60

High-pressure and high-temperature studies have been carried out on the alkali metal--intercalated fullerene system ${\text{Rb}}_{6}{\text{C}}_{60}$ by performing x-ray diffraction (XRD), x-ray absorption, and Raman spectroscopy measurements. All of these techniques allow the obtaining of complementary information on the high-pressure behavior and evolution of the studied system. In particular, the occurrence of a reversible transition has been clearly identified at around 35 GPa both at 600 K and at room temperature by XRD and Raman spectroscopy, respectively. The XRD data show that the high-pressure phase is compatible with a hexagonal unit-cell structure with $a=8.360(2)\text{ }\text{\AA{}}$ and $c=14.830(7)\text{ }\text{\AA{}}$ at 43 GPa and 600 K. In correspondence with such transition, the Raman spectra exhibit an abrupt change in the frequency of the normal vibrations. By coupling the information obtained with the different experimental techniques, we suggest that the high-pressure structural transition is accompanied by the formation of two-dimensional polymers of ${\text{C}}_{60}$.

Elastic constants and high pressure structural transitions in yttrium pnictides

We present first-principles calculations on the structural, elastic and high pressure properties of yttrium pnictide compounds, using the pseudo-potential plane-waves approach based on density functional theory, within the generalized gradient approximation. Results are given for lattice constant, bulk modulus and its pressure derivative. The pressure transition at which these compounds undergo structural phase transition from NaCl-type (B1) to CsCl-type (B2) structure is calculated and compared with previous calculations and available experimental data. The elastic constants and their pressure dependence are calculated using the static finite strain technique. We derived the bulk and shear moduli, Young’s modulus and Poisson’s ratio of the B1 phase for YN, YP, YAs and YSb compounds. We estimated the Debye temperature of these compounds from the average sound velocity. This is the first quantitative theoretical prediction of the elastic properties of YN, YP, YAs and YSb compounds, and it still awaits experimental confirmation.

Displacive mechanisms and order-parameter symmetries for the A7-incommensurate-bcc sequences of high-pressure reconstructive phase transitions in Group Va elements

The high-pressure structural transitions observed in the Group Va elements P, As, Sb, and Bi are described by displacive atomic mechanisms. A theoretical analysis of the proposed mechanisms shows that two different structural paths are followed relating the low-pressure A7 structure to the highest-pressure bcc structure. One structural path gives rise to the host-guest structures found in As, Sb, and Bi, whereas another path yields the incommensurate P IV structure recently disclosed in P. The incommensurate character of the host-guest and P IV structures is shown to be imposed by the energetically most favorable path relating the A7 and bcc structures.

High-pressure structural, electronic and spin transitions in new multiferroic GdFe3(BO3)4

High-pressure structural, electronic and spin transitions in new multiferroic GdFe₃(BO₃)₄

Structural mechanisms of the high-pressure phase transitions in the elements of groupIVa

Displacive mechanisms are proposed for the high-pressure structural transitions which takeplace in the elements of group IVa. The mechanisms are analysed in the framework of theLandau theory of phase transitions. In carbon the polymorphs are shown to correspond tolimit states resulting from critical fractional displacements and critical strains, thecorresponding order parameters being defined as periodic functions of the displacements. InSi, Ge, Sn and Pb a number of unifying features are discussed in connection with theelectronic structures of these elements.

Elastic constants and high-pressure structural transitions in lanthanum monochalcogenides from experiment and theory

The high-pressure structural behavior of lanthanum monochalcogenides is investigated by theory and experiment. Theory comprises density-functional calculations of LaS, LaSe, and LaTe with the general gradient approximation for exchange and correlation effects, as implemented within the full-potential linear muffin-tin orbital method. The experimental studies consist of high-pressure angle dispersive x-ray-diffraction investigations of LaS and LaSe up to a maximum pressure of $41\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. A structural phase transition from the NaCl-type to CsCl-type crystal structure is found to occur in all cases. The experimental transition pressures are 27--28 and $19\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ for LaS and LaSe, respectively, while the calculated transition pressures are 29, 21, and $10\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ for LaS, LaSe, and LaTe, respectively. The calculated ground-state properties such as equilibrium lattice constant, bulk modulus and its pressure derivative, and Debye temperatures are in good agreement with experimental results. Elastic constants are predicted from the calculations.

High-pressure structural transitions in Cm and Am0.5Cm0.5 binary alloy

The high-pressure behaviour of Cm and Am0.5Cm0.5 binary alloy is investigated theoretically using ab initio electronic structure methods. Our calculations reproduce the structural phase transitions, which are observed in recent experiment performed by Heathman et al. [S. Heathman, R.G. Haire, T. Le Bihan et al., Science 309 110 (2005)] and Lindbaum et al. [A. Lindbaum, S. Heathman, T. Le Bihan et al., J. Phys: Condens. Matter 15 S2297 (2003)]. Calculated transition pressures are in reasonable agreement with the experimental values. Calculations performed for an antiferromagnetic state are essential to reproduce the stability of Cm-III phase.

Theory of the high-pressure structural phase transitions in Si, Ge, Sn, and Pb

Displacive mechanisms are proposed for the high-pressure structural transitions which take place in Si, Ge, Sn, and Pb. The mechanisms are analyzed in the framework of the Landau theory of phase transitions. It reveals a number of unifying features which are discussed in connection with the electronic structures of these elements, leading to a more precise understanding of the diversity of phase diagrams found for the elements of Group IVa.

Raman scattering studies on mercuric iodide at high pressures and at low temperatures

Raman scattering measurements have been performed on single crystal red mercuric iodide under high-pressures up to ∼5.5 GPa and at low-temperatures down to 40 K. Our low-temperature measurements help clarify the issues related to the accidental degeneracy of the E g 1 and B 1 g 2 modes in the tetragonal phase. Raman modes stiffen and their widths reduce with the lowering of the temperature. All the Raman active mode frequencies increase with pressure up to 1.3 GPa, beyond which the modes of the tetragonal phase disappear and new modes appear supporting the earlier reported red → high-pressure yellow structural transition at this pressure. Knowledge of both temperature and pressure dependences has allowed us to separate, for all the Raman active modes, the explicit phonon-excitation contribution and the implicit volume-driven contribution to the observed temperature coefficient d ω/d T for both the phases.

Size effects on the structure and phase transition behavior of baddeleyite TiO 2

High-pressure structural transitions in nanocrystalline systems are of significant interest as models of first-order phase transitions. We demonstrate size-induced lattice expansion and significant atomic rearrangements in the crystal structure of nanocrystalline high-pressure baddeleyite-TiO 2. The α-PbO 2 structured TiO 2 recovered after dozens of pressure cycles in the α-PbO 2-baddeleyite pressure field displayed elongate 25–35 nm crystallites, compared to starting 34-nm anatase crystallites, suggesting crystallite coherency across anatase, baddeleyite, and α-PbO 2 structures and ‘single structural domain’ behavior of the nanocrystalline system.

High Pressure Structural transitions in Cm metal

AbstractThe high pressure behaviour of Cm metal is investigated theoretically using ab initio electronic structure methods. Our calculations reproduce the structural phase transitions which are obsereved in recent experiment performed by Heatman et al. [Science 309, 110 (2005)]. Calculated transition pressures are in reasonable agreement with the experiment. Calculations performed in antiferromagnetic state is essential to reproduce the stability of Cm-III phase. Thus stability of Cm-III phase is related to the spin polarization of 5f electrons.

Single-crystal elastic constants of fluorite (CaF 2 ) to 9.3 GPa

The second-order elastic constants of CaF2 (fluorite) have been determined by Brillouin scattering to 9.3 GPa at 300 K. Acoustic velocities have been measured in the (111) plane and inverted to simultaneously obtain the elastic constants and the orientation of the crystal. A notable feature of the present inversion is that only the density at ambient condition was used in the inversion. We obtain high-pressure densities directly from Brillouin data by conversion to isothermal conditions and iterative integration of the compression curve. The pressure derivative of the isentropic bulk modulus and of the shear modulus determined in this study are 4.78 ± 0.13 and 1.08 ± 0.07, which differ from previous low-pressure ultrasonic elasticity measurements. The pressure derivative of the isothermal bulk modulus is 4.83 ± 0.13, 8% lower than the value from static compression, and its uncertainty is lower by a factor of 3. The elastic constants of fluorite increase almost linearly with pressure over the whole investigated pressure range. However, at P ≥ 9 GPa, C 11 and C 12 show a subtle structure in their pressure dependence while C 44 does not. The behavior of the elastic constants of fluorite in the 9–9.3 GPa pressure range is probably affected by the onset of a high-pressure structural transition to a lower symmetry phase (α-PbCl2 type). A single-crystal Raman scattering experiment performed in parallel to the Brillouin measurements shows the appearance of new features at 8.7 GPa. The new features are continuously observed to 49.2 GPa, confirming that the orthorhombic high-pressure phase is stable along the whole investigated pressure range, in agreement with a previous X-ray diffraction study of CaF2 to 45 GPa. The high-pressure elasticity data in combination with room-pressure values from previous studies allowed us to determine an independent room-temperature compression curve of fluorite. The new compression curve yields a maximum discrepancy of 0.05 GPa at 9.5 GPa with respect to that derived from static compression by Angel (1993). This comparison suggests that the accuracy of the fluorite pressure scale is better than 1% over the 0–9 GPa pressure range.

New high-pressure structural transition of oxygen at 96 GPa associated with metallization in a molecular solid.

By using a high-energy and high-brilliance synchrotron radiation source in the European Synchrotron Radiation Facility and the Photon Factory, an angle-dispersive powder x-ray diffraction study of solid oxygen has been carried out up to 116 GPa at room temperature. A new high-pressure structural transition was observed at 96 GPa and explained as an isostructural transition with a discontinuous change in lattice constants in a monoclinic cell. Metallization of oxygen reported by Desgreniers, Vohra, and Ruoff [J. Phys. Chem. 94, 1117 (1990)] is ascribed to the transition.