Structural phase stability,electronic structure and mechanical properties of alkali metal hydrides AMH4 (A=Li,Na; M=B,AL)

The structural stability of Alkali metal hydrides AMH₄ (A=Li, Na; M=B, Al) is analyzed among the various crystal structures, namely hexagonal (P6₃mc), tetragonal (P4₂/nmc), tetragonal (P-42₁c), tetragonal (I4₁/a), orthorhombic (Pnma) and monoclinic (P2₁/c). It is observed that, orthorhombic (Pnma) phase is the most stable structure for LiBH₄, monoclinic (P2₁/c) for LiAlH₄, tetragonal (P4₂/nmc) for NaBH₄ and tetragonal (I4₁/a) for NaAlH₄ at normal pressure. Pressure induced structural phase transitions are observed in LiBH₄, LiAlH₄, NaBH₄ and NaAlH₄ at the pressures of 4 GPa, 36.1 GPa, 26.5 GPa and 46 GPa respectively. The electronic structure reveals that these metal hydrides are wide band gap insulators. The calculated elastic constants indicate that these metal hydrides are mechanically stable at normal pressure.     

High volumetric hydrogen density phases of magnesium borohydride at high-pressure: A first-principles study

The previously proposed theoretical and experimental structures,bond characterization,and compressibility of Mg(BH 4) 2 in a pressure range from 0 to 10 GPa are studied by ab initio density-functional calculations.It is found that the ambient pressure phases of meta-stable I4 1 /amd and unstable P-3m1 proposed recently are extra stable and cannot decompose under high pressure.Enthalpy calculation indicates that the ground state of F 222 structure proposed by Zhou et al.[2009 Phys.Rev.B 79 212102] will transfer to I4 1 /amd at 0.7 GPa,and then to a P-3m1 structure at 6.3 GPa.The experimental P 6 1 22 structure(α-phase) transfers to I4 1 /amd at 1.2 GPa.Furthermore,both I4 1 /amd and P-3m1 can exist as high volumetric hydrogen density phases at low pressure.Their theoretical volumetric hydrogen densities reach 146.351 g H 2 /L and 134.028 g H 2 /L at ambient pressure,respectively.The calculated phonon dispersion curve shows that the I4 1 /amd phase is dynamically stable in a pressure range from 0 to 4 GPa and the P-3m1 phase is stable at pressures higher than 1 GPa.So the I4 1 /amd phase may be synthesized under high pressure and retained to ambient pressure.Energy band structures show that they are both always ionic crystalline and insulating with a band-gap of about 5 eV in this pressure range.In addition,they each have an anisotropic compressibility.The c axis of these structures is easy to compress.Especially,the c axis and volume of P-3m1 phase are extraordinarily compressible,showing that compression along the c axis can increase the volumetric hydrogen content for both I4 1 /amd and P-3m1 structures.     

Phase transitions in PbTe under quasi-hydrostatic pressure up to 50 GPa

PbTe has been investigated using synchrotron X-ray diffraction (XRD) in a diamond anvil cell under quasi-hydrostatic pressures up to 50 GPa. Upon compression to 6.6 GPa, the initial NaCl phase transforms to an intermediate phase, which is confirmed to be an orthorhombic structure with a space group Pnma. At 18.4 GPa, the intermediate Pnma phase undergoes a phase transition to the CsCl structure. The systemic analysis of the crystal structures between the NaCl and intermediate phases indicates that the structure of the Pnma phase could be derived from the distortion of the NaCl structure. The bulk modulus of the CsCl phase is B₀=52(2) GPa with V₀=60.8(4) ų and B′₀=4.0 (fixed), slightly larger than the NaCl phase (B₀=44(1) GPa) and the intermediate phase (B₀=49(3) GPa).     

In situ observations of temperature- and pressure-induced phase transitions in TiH2: Angle-dispersive and synchrotron energy-dispersive X-ray diffraction studies

We investigated the behavior of the structure of titanium hydride (TiH₂), an important compound in hydrogen storage research, at elevated temperatures (0-120 °C) and high pressures (1 bar-34 GPa). Temperature-induced changes of TiH₂ as indicated in the alteration of the ambient X-ray demonstrated a cubic to tetragonal phase transition occurring at about 17 °C. The main focus of this study was to identify any pressure-induced structural transformations, including possible phase transitions, in TiH₂. Synchrotron X-ray diffraction studies were carried out in situ (diamond anvil cell) in a compression sequence up to 34 GPa and in subsequent decompression to ambient pressure. The pressure evolution of the diffraction patterns revealed a cubic (Fm-3m) to tetragonal (I4/mmm) phase transition at 2.2 GPa. The high-pressure phase persisted up to 34 GPa. After decompression to ambient conditions the observed phase transition was completely reversible. A Birch-Murnaghan fit of the unit cell volume as a function of pressure yielded a zero-pressure bulk modulus K₀=146(14) GPa, and its pressure derivative K′₀=6(1) for the high-pressure tetragonal phase of TiH₂.     

Structural,electronic and optical properties of LiBeP in its normal and high pressure phases

An investigation on the structural stabilities, electronic and optical properties of LiBeP under high pressure was conducted using the all-electron density functional theory within the local density approximation. Our results show that the sequence of the pressure induced phase transition of LiBeP is the Cu₂Sb-type structure (P4/nmm), the MgSrSi-type structure (Pnma) and the LiGaGe-type structure (P6₃mc). The first transition (P4/nmm to Pnma) takes place at 2.95 GPa and the second (Pnma to P6₃mc) at 6.65 GPa. In the three phases, the bandgap is indirect and the valence band maximum is at the zone center. With increasing pressure LiBeP in the LiGaGe structure becomes a direct gap semiconductor at 19.75 GPa. The assignments of the structures in the optical spectra and the band structure transitions are discussed. The mean value of the optical dielectric constant for the Cu₂Sb phase is smaller than that for the MgSrSi and the LiGaGe ones. This compound has a positive uniaxial anisotropy in the LiGaGe structure. The absorption coefficient along the z   direction, _αzz${\alpha }_{\mathit{zz}}$, for the MgSrSi structure is higher than that in the other two structures in the visible regime.     

High pressure study of phase transitions inα-FePO4

High pressure behaviour of FePO₄ in berlinite form has been investigated up to 10 GPa using vibrational Raman spectroscopy and energy dispersive x-ray diffraction. Combination of these techniques along with studies on pressure quenched samples reveal structural transitions in this material from its room pressure trigonal phase to a disordered and a crystalline phase near 3±0.5 GPa. The latter is the Cmcm phase which is the equilibrium structure at high pressures. These high pressure phases do not revert back to its initial structure after release of pressure. Irreversibility of these transformations indicates that FeO₄ tetrahedra do not regain their initial coordination. These high pressure transitions can be rationalized in terms of the three level free energy diagram for such systems.     

Structural studies of the P-T phase diagram of sodium niobate

The crystal structure of sodium niobate (NaNbO₃) has been investigated by energy-dispersive X-ray diffraction at high pressures (up to 4.3 GPa) in the temperature range 300–1050 K. At normal conditions, NaNbO₃ has an orthorhombic structure with Pbcm symmetry (antiferroelectric P phase). Upon heating, sodium niobate undergoes a series of consecutive transitions between structural modulated phases P-R-S-T(1)-T(2)-U; these transitions manifest themselves as anomalies in the temperature dependences of the positions and widths of diffraction peaks. Application of high pressure leads to a decrease in the temperatures of the structural transitions to the R, S, T(1), T(2), and U phases with different baric coefficients. A phase diagram for sodium niobate has been build in the pressure range 0–4.3 GPa and the temperature range 300–1050 K. The dependences of the unit-cell parameters and volume on pressure and temperature have been obtained. The bulk modulus and the volume coefficients of thermal expansion have been calculated for different structural modulated phases of sodium niobate. A phase transition (presumably, from the antiferroelectric orthorhombic P phase to the ferroelectric rhombohedral N phase) has been observed at high pressure (P = 1.6 GPa) and room temperature.     

Effects of pressure on the two polymorphs of [Co(NH3)5NO2]I2: The anisotropy of lattice distortion and a phase transition

Abstract

The effect of pressure on the two polymorphs of [CO(NH₃)₅NO₂]I₂ (phase I-orthorhombic, S.G. Pnma; phase II-monoclinic, S.G. C2/m) was studied by X-ray powder diffraction in a diamond anvil cell (DAC). In the presence of the ethanol-methanol-water mixture used as a pressure-transmitting liquid polymorph I was shown to undergo a phase transition at pressures between 0.45 GPa and 0.65 GPa. The diffraction pattern of the high-pressure phase (phase III) could be indexed as tetragonal with lattice parameters similar to those, which were previously reported for polymorph II in a 'pseudotetragonal setting'. The lattice distortions of phases II and III were studied at pressures up to 3.2 GPa and 3.7 GPa, correspondingly, and were shown to be very similar. Phases II and III were supposed to be very closely related. If poly(chlortrifluorethylen)-oil was used as a pressure-transmitting medium, no phase transitions were observed in phase I of [CO(NH₃)₅NO₂I₂ at least up to 1.8 GPa (the point when poly(chlortrifluorethylen)-oil becomes solid), and the anisotropy of lattice distortion could be measured.     

Phase transition of cadmium fluoride under high pressure

We investigated the high pressure phases of CdF₂ by a joint theoretical and experimental study. The structural and electronic properties of CdF₂ were extensively explored to high pressure by ab initio calculations based on the density functional theory. A structural phase transition from the fluorite-type  (Fm-3m, Z=4) structure to the cotunnite-type (Pnma, Z=4) structure was estimated below 8 GPa, and this phase transition was examined by the high pressure experiments up to 35 GPa at room temperature. Both high pressure angle dispersive X-ray diffraction and Raman spectroscopy experiments provided convincing evidence to verify the phase transition. Our work makes clear pressure-induced phase transitions and structural information of CdF₂ under high pressure.     

Structural and electronic transitions in gadolinium iron borate GdFe<Superscript>3</Superscript>(BO<Superscript>3</Superscript>)<Superscript>4</Superscript> at high pressures

The optical properties and structure of gadolinium iron borate GdFe₃(BO₃)₄ crystals are studied at high pressures produced in diamond-anvil cells. X-ray diffraction data obtained at a pressure of 25.6 GPa reveal a firstorder phase transition retaining the trigonal symmetry and increasing the unit cell volume by 8%. The equation of state is obtained and the compressibility of the crystal is estimated before and after the phase transition. The optical spectra reveal two electronic transitions at pressures ～26 GPa and ～43 GPa. Upon the first transition, the optical gap decreases jumpwise from 3.1 to ～2.25 eV. Upon the second transition at P=43 GPa, the optical gap deceases down to ～0.7 eV, demonstrating a dielectric-semiconductor transition. By using the theoretical model developed for a FeBO₃ crystal and taking into account some structural analogs of these materials, the anomalies of the high-pressure optical spectra are explained.     

Structural phase transition in Bi2Te3 under high pressure

Structural change in Bi₂Te₃ under high pressure up to 16.6 GPa has been studied by powder x-ray diffraction. We observed two times of phase transitions at room temperature at the pressures of 8 and 14 GPa, respectively. According to our preliminary result on electrical resistance, it is reasonable to suppose that superconducting transition with T _c =2.8 K at the pressures of 10.2 GPa is observed in phase II. On the other hand, we found anomalies of the pressure dependences of lattice parameters and volume at around 2 GPa, which probably means the change in electrical structure on the Fermi surface.     

First-Principles Study of Structural Stabilities, Electronic and Optical Properties of SrF2 under High Pressure

An investigation of structural stabilities, electronic and optical properties of SrF2 under high pressure is conducted using a first-principles calculation based on density functional theory （DFT） with the plane wave basis set as implemented in the CASTEP code. Our results predict that the second high-pressure phase of SrF2 is of a Ni2In- type structure, and demonstrate that the sequence of the pressure-induced phase transition of SrF2 is the fluorite structure （Fm3m） to the PbC12-type structure （Pnma）, and to the Ni2In-type phase （P63/mmc）. The first and second phase transition pressures are 5. 77 and 45.58 GPa, respectively. The energy gap increases initially with pressure in the Fm3m, and begins to decrease in the Pnma phases at 30 GPa. The band gap overlap metallization does not occur up to 210 GPa. The pressure effect on the optical properties is discussed.     

Study of the P-T phase diagram of pyridinium perchlorate by X-ray diffraction and Raman spectroscopy

The crystal structure and vibrational spectra of deuterated pyridinium perchlorate (d ₅PyH)ClO₄ (C₅D₅NHClO₄) are studied by means of neutron diffraction in ambient conditions, X-ray diffraction at high pressures up to 3.5 GPa in the temperature range 297–420 K, and Raman spectroscopy at high pressures up to 5.7 GPa. Deuterated pyridinium perchlorate at ambient conditions has rhombohedral structure with the R3m symmetry (paraelectric phase I). Over the pressure range of 0.5–1.2 GPa, the phase II with monoclinic symmetry Cm exists. At pressure P ～ 1.2 GPa, the phase transition to monoclinic phase III with the Pm symmetry is observed at ambient temperature. The lattice parameters, unit cell volume, and frequencies of internal vibrational modes as functions of pressure are obtained for different phases of deuterated pyridinium perchlorate. The P-T phase diagram of (d ₅PyH)ClO₄ over the extended pressure and temperature range is discussed.     

Structural aspects of the antiferroelectric-paraelectric phase transition in double perovskite Pb2MgWO6 at high pressures and temperatures

The crystal structure of antiferroelectric Pb₂MgWO₆ has been studied using neutron diffraction at high pressures to 5.4 GPa at room temperature and energy-dispersive X-ray diffraction at high pressures to 4 GPa in the temperature range 300–400 K. At normal conditions, in Pb₂MgWO₆, there is an antiferroelectric phase with the crystal structure described by the orthorhombic symmetry with space group Pnma. At temperature T = 313 K and normal pressure or at room temperature and pressure P ～ 0.9 GPa, the crystal under-goes a structural phase transition to the cubic phase with space group $Fm\bar 3m$ (paraelectric phase). The temperature and pressure dependences of the lattice parameters, unit cell volume, and interatomic bond lengths have been obtained, and the thermal expansion coefficients and the bulk moduli have been calculated for the antiferroelectric and paraelectric phases of Pb₂MgWO₆.     

High-pressure X-ray diffraction study of 1T-TaS2

We present a room temperature high-pressure X-ray diffraction study of the layered compound 1T-TaS₂ up to 20 GPa. This material is known to exhibit a variety of structural phase transitions that are ascribed to the stabilization of charge density wave states. It has been recently shown that at pressures larger than 3 GPa and up to 25 GPa, 1T-TaS₂ becomes superconductor below about 5 K. It was suggested that this superconductivity coexists with different CDW states, an hypothesis that can be tested by X-ray diffraction. Our first results at room temperature show that at around 1.9 GPa, the nearly-commensurate (NCCDW) phase transforms into a phase similar to the high temperature incommensurate phase (ICCDW). Above 9 GPa, we show the existence of another IC phase, still discernable up to 20 GPa despite the pressure-induced crystal damage above 13 GPa. These results are consistent with resistivity measurements, but call for a complete exploration of the P–T phase diagram of 1T-TaS₂.     

Reversible hydrogen storage in LiBH<Subscript>4</Subscript> destabilized by milling with Al

LiBH₄ possesses a high hydrogen content, and though it is highly stable, its restoration from LiH+B+H₂ can only be accomplished under unacceptable high temperature and pressure conditions (650 °C and 15 MPa). Recently, it has been reported that destabilizing LiBH₄ by, i.e., MgH₂, transition metal oxides and chlorides presents a promising approach to exert its potential for hydrogen storage. In the present study, we find that simple mechanical milling with Al in a mole ratio of 2:1, markedly improves the reversible dehydrogenation performance of LiBH₄. The system possesses a theoretical capacity of 8.5 wt. % and could be reversibly operated at 400–450 °C. The combined property and phase examinations suggest that the observed property improvement should be associated with the formation of AlB₂ in the dehydriding process. Further cyclic examination found that the system suffered from a serious capacity loss in the dehydriding/rehydriding cycles. A better understanding of the degradation mechanism may provide a means for further material property improvement. PACS 81.05.Zx; 84.60.Ve; 82.30.-b; 82.33.Pt; 82.65.+r     

Structural transitions of NaAlH4 under high pressure by first-principles calculations

First-principles calculations have been performed on NaAlH₄ using the generalized gradient approximation pseudopotential method. The predicted β-NaAlH₄ (α-LiAlH₄-type) structure is energetically more favorable than α-NaAlH₄ for pressures over 15.9 GPa, which is apparently correlated with the experimental transition pressure 14 GPa. This transition is identified as first-order in nature with volume contractions of 1.8%. There is no pressure-induced softening behavior from our calculated phonon dispersion curves near the phase transition pressure. Based on the Mulliken population analysis, the β-NaAlH₄ structure is expected to be the most promising candidate for hydrogen storage.     

A high pressure Raman study of TeO2 to 30 GPa and pressure-induced phase changes

The effect of pressure on the Raman modes in TeO₂ (paratellurite) has been investigated to 30GPa, using the diamond cell and argon as pressure medium. The pressure dependence of the Raman modes indicates four pressure-induced phase transitions near 1 GPa, 4.5 GPa, 11 GPa and 22 GPa. Of these the first is the well studied second-order transition fromD ₄ ⁴ symmetry toD ₂ ⁴ symmetry, driven by a soft acoustic shear mode instability. The remarkable similarity in the Raman spectra of phases I to IV suggest that only subtle changes in the structure are involved in these phase transitions. The totally different Raman spectral features of phase V indicate major structural changes at the 22GPa transition. It is suggested that this high pressure-phase is similar to PbCl₂-type, from high pressure crystal chemical considerations. The need for a high pressure X-ray diffraction study on TeO₂ is emphasized, to unravel the structure of the various high pressure phases in the system.     

High-pressure powder diffraction study of TaO2F

TaO₂F, with a ReO₃-type structure, has been studied at up to 12.8 GPa using monochromatic synchrotron powder diffraction and diamond anvil cells. Two-phase transitions at ∼0.7 and 4 GPa were observed on compression. Below ∼0.7 GPa the cubic material was found to have a bulk modulus (K₀) of 36(3) GPa (K_p fixed at 4.0), similar to that reported for NbO₂F but much smaller than that of ReO₃. Immediately above 0.7 GPa on compression, the diffraction data were not fully consistent with a VF₃-type structure as previously proposed for NbO₂F. On decompression, the data between 8 and 4 GPa could be satisfactorily attributed to a single R-3c phase with a VF₃-type structure and an average bulk modulus of 60(2) GPa.     

Ternary LiBH4–MgH2–NaAlH4 hydride confined into nanoporous carbon host for reversible hydrogen storage

Ternary hydride of LiBH₄–MgH₂–NaAlH₄ confined into carbo n aerogel scaffold (CAS) via melt infiltration for reversible hydrogen storage is proposed. Nanoconfinement of hydrides into CAS is obtained together with surface occupation of some phases, such as Al and/or LiH. Regarding nanoconfinement, not only multiple-step decomposition of LiBH₄–MgH₂–NaAlH₄ hydride reduces to about single step, but also reduction of dehydrogenation temperature is significantly observed, for example, ∆T up to 70 °C regarding last dehydrogenation step. Moreover, decomposition of NaBH₄ in nanoconfined sample can be done at 360 °C (dehydrogenation temperature in this study), which is 115 and 180 °C lower than that of NaBH₄ in milled LiBH₄–MgH₂–NaAlH₄ and bulk NaBH₄, respectively. The reaction of LiBH₄+NaAlH₄→LiAlH₄+NaBH₄ takes place during nanoconfinement and the decomposition of LiAlH₄ is observed, resulting deficient hydrogen content liberated. However, hydrogen content released (1st cycle) and reproduced (2nd–4th cycles) from this ternary hydride enhances up to 11% and 22% of full hydrogen storage capacity due to nanoconfinement. After rehydrogenation (T=360 °C and P(H₂)=50 bar H₂ for 12 h), NaBH₄, MgH₂, and Li₃AlH₆ are reversible, whereas Li₃AlH₆ and NaBH₄ in milled sample cannot be recovered due to deficient hydrogen pressure (T=360 °C and P(H₂)=80 bar) and probably evaporation of molten sodium during dehydrogenation, respectively. The latter results in inferior hydrogen content reproduced from milled sample to nanoconfined sample.