Thiti Bovornratanaraks

Revealing an unusual transparent phase of superhard iron tetraboride under high pressure

Significance Solids have been mainly studied at ambient conditions (i.e., at room temperature and zero pressure). However, it was realized early that there is also a fundamental relation between volume and structure and that this dependence could be most fruitfully studied by means of high-pressure experimental techniques. From a theoretical point of view this is an ideal type of experiment, because only the volume is changed, which is a very clean variation of the external conditions. In the present study we show a hard superconducting material, iron tetraboride, transforms into a novel transparent phase under pressure. Further, this phase is the first system in this class, to our knowledge, and opens a new route to search for and design new transparent materials. First principles–based electronic structure calculations of superhard iron tetraboride (FeB4) under high pressure have been undertaken in this study. Starting with a “conventional” superconducting phase of this material under high pressure leads to an unexpected phase transition toward a semiconducting one. This transition occurred at 53.7 GPa, and this pressure acts as a demarcation between two distinct crystal symmetries, metallic orthorhombic and semiconducting tetragonal phases, with Pnnm and I41/acd space groups, respectively. In this work, the electron–phonon coupling-derived superconducting Tc has been determined up to 60 GPa and along with optical band gap variation with increasing pressure up to 300 GPa. The dynamic stability has been confirmed by phonon dispersion calculations throughout this study.

High pressure orthorhombic structure of CuInSe2

The structural behaviour of CuInSe(2) under high pressure has been studied up to 53 GPa using angle-dispersive x-ray powder diffraction techniques. The previously reported structural phase transition from its ambient pressure tetragonal structure to a high pressure phase with a NaCl-like cubic structure at 7.6 GPa has been confirmed. On further compression, another structural phase transition is observed at 39 GPa. A full structural study of this high pressure phase has been carried out and the high pressure structure has been identified as orthorhombic with space group Cmcm and lattice parameters a = 4.867(8) Å, b = 5.023(8) Å and c = 4.980(3) Å at 53.2(2) GPa. This phase transition behaviour is similar to those of analogous binary and trinary semiconductors, where the orthorhombic Cmcm structure can also be viewed as a distortion of the cubic NaCl-type structure.

The effects of Na on high pressure phases of CuIn0.5Ga0.5Se2 from ab initio calculation

The effects of Na atoms on high pressure structural phase transitions of CuIn(0.5)Ga(0.5)Se(2) (CIGS) were studied by an ab initio method using density functional theory. At ambient pressure, CIGS is known to have chalcopyrite (I42d) structure. The high pressure phase transitions of CIGS were proposed to be the same as the order in the CuInSe(2) phase transitions which are I42d → Fm3m → Cmcm structures. By using the mixture atoms method, the Na concentration in CIGS was studied at 0.1, 1.0 and 6.25%. The positive mixing enthalpy of Na at In/Ga sites (Na(InGa)) is higher than that of Na at Cu sites (Na(Cu)). It confirmed previous studies that Na preferably substitutes on the Cu sites more than the (In, Ga) sites. From the energy-volume curves, we found that the effect of the Na substitutes is to reduce the hardness of CIGS under high pressure. The most significant effects occur at 6.25% Na. We also found that the electronic density of states of CIGS near the valence band maximum is increased noticeably in the chalcopyrite phase. The band gap is close in the cubic and orthorhombic phases. Also, the Na(Cu)-Se bond length in the chalcopyrite phase is significantly reduced at 6.25% Na, compared with the pure Cu-Se bond length. Consequently, the energy band gap in this phase is wider than in pure CIGS, and the gap increased at the rate of 31 meV GPa(-1) under pressure. The Na has a small effect on the transition pressure. The path of transformation from the cubic to orthorhombic phase was derived. The Cu-Se plane in the cubic phase displaced relatively parallel to the (In, Ga)-Se plane by 18% in order to transform to the Cmcm phase. The enthalpy barrier is 0.020 eV/atom, which is equivalent to a thermal energy of 248 K. We predicted that Fm3m and Cmcm can coexist in some pressure range.

The hcp to fcc transformation path of scandium trihydride under high pressure

We used density functional theory to calculate the phase stability of the hcp (hexagonal close packed) and the fcc (face centered cubic) structures of ScH3. The hcp form is stable up to 22 GPa according to the generalized gradient approximation calculation. Then the fcc form becomes energetically more stable. In order to provide insight into the phase transition mechanism, we modeled the hcp to fcc transition by sliding the hcp basal planes, i.e. (001)h planes, in such a way that the ABABAB sequence of the hcp form is altered into the ABCABC sequence of the fcc form. This sliding was suggested by the experiment. The configurations of these sliding steps are our proposed intermediate configurations, whose symmetry group is the Cm group. By using the Cm crystallography, we can match the d-spacings from the lattice planes of the hcp and fcc forms and the intermediate planes measured from the experiment. We also calculated the enthalpy per step, from which the energy barrier between the two phases at various pressures was derived. The barrier at 35 GPa is 0.370 eV per formula or 0.093 eV/atom. The movements of the hydrogen atoms during the hcp to intermediate phase transition are consistent with the result from the Raman spectra.

The effect of morphology and confinement on the high-pressure phase transition in ZnO nanostructure

The transition pressure (Pt) of the B4-to-B1 phase transformation of zinc oxide nanoparticle (n-ZnO) structures was investigated in terms of their size and morphology. Nanorods, nanopencils, nanopyramids, nanowires, and nanotubes of the B4 phase in various sizes were directly built up by accounting for the atomic basis of the core and surface regions. The previously proposed transformation path was performed for constructing shapes and sizes compatible with B1 phases. Using systematic density functional theory, the surfaces were cleaved from the optimized crystal structures at different pressures in both the B4 and B1 phases. A method for calculating the surface energy at different pressures is proposed using an asymmetric slab model. Using the proposed model, the transition pressure of n-ZnO structures was found to significantly depend on their morphology and size, which is in good agreement with the available experimental reports.

High pressure structural studies of AgInTe2

The structural phase transformations in the chalcopyrite semiconductor AgInTe2 have been studied up to 10 GPa on both pressure increase and decrease. The experiments were conducted using angle-dispersive X-ray diffraction with synchrotron radiation and an image plate. The diffraction patterns of AgInTe2 at ambient pressure reveal two coexisting phases: the first has the chalcopyrite structure while the second has a zincblende-like structure. On pressure increase both phases transformed at 3-4 GPa to a cation-disordered orthorhombic structure with spacegroup Cmcm. On pressure decrease, the chalcopyrite phase started to reappear at 0.55 GPa, and the Cmcm phase disappeared completely at ambient pressure.

Low temperature preparation of oxygen-deficient tin dioxide nanocrystals and a role of oxygen vacancy in photocatalytic activity improvement

The introduction of oxygen vacancies (Vos) into tin dioxide crystal structure has been found as an effective method to improve its photocatalytic performance. Herein, oxygen-deficient tin dioxide (SnO2-x) nanocrystals were successfully prepared via a facile, one-step hydrothermal method at the temperature lower than those reported previously. The effect of hydrothermal temperature on phase composition and Vos content was also firstly investigated. Due to its high oxygen vacancy concentration, the SnO2-x prepared at 80 °C provides the best photocatalytic degradation of methyl orange under UV-visible light. Scavenger trapping and nitroblue tetrazolium experiments also show that the Vos act as electron trapped sites and molecular oxygen adsorption sites, therefore increasing the production of active O2- radical which is the main species governing the photocatalytic activity of SnO2-x nanocrystals. Raman spectroscopy, X-ray photoelectron spectroscopy, photoluminescence measurement and electron spin resonance investigation clearly indicate that increasing the hydrothermal temperature results in the coexistence of SnO2-x and Sn3O4 phases and the reduction of Vos concentration which are detrimental to the photocatalytic performance. Density functional theory calculations also reveal that the presence of Vos is responsible for the upshift of valence band maximum and an extended conduction band minimum, hence a valence band width broadening and band gap narrowing which consequently enhance the photocatalytic performance of the oxygen-deficient SnO2-x.

Phase stability and superconductivity of strontium under pressure

We have used the ab initio random structure searching method together with density functional theory calculations to find stable structures of strontium under pressures up to 50 GPa. We predict a sequence of structural phase transitions and the stability of an orthorhombic structure of Cmcm symmetry above 25 GPa. Our energy, lattice dynamics, and molecular dynamics calculations confirm the stability of the Cmcm structure. The electron-phonon coupling calculations show that superconductivity arises in the bcc structure of compressed Sr and that it continues to exist in the Cmcm structure. The calculated superconducting transition temperatures are in good agreement with experiment. Our study gives an excellent account of the experimental observations.

Structural prediction of host-guest structure in lithium at high pressure

Ab initio random structure searching (AIRSS) technique is used to identify the high-pressure phases of lithium (Li). We proposed the transition mechanism from the fcc to host-guest (HG) structures at finite temperature and high pressure. This complex structural phase transformation has been calculated using ab initio lattice dynamics with finite displacement method which confirms the dynamical harmonic stabilization of the HG structure. The electron distribution between the host-host atoms has also been investigated by electron localization function (ELF). The strongly localized electron of p bond has led to the stability of the HG structure. This remarkable result put the HG structure to be a common high-pressure structure among alkali metals.

Role of relativity in high-pressure phase transitions of thallium

We demonstrate the relativistic effects in high-pressure phase transitions of heavy element thallium. The known first phase transition from h.c.p. to f.c.c. is initially investigated by various relativistic levels and exchange-correlation functionals as implemented in FPLO method, as well as scalar relativistic scheme within PAW formalism. The electronic structure calculations are interpreted from the perspective of energetic stability and electronic density of states. The full relativistic scheme (FR) within L(S)DA performs to be the scheme that resembles mostly with experimental results with a transition pressure of 3 GPa. The s-p hybridization and the valence-core overlapping of 6s and 5d states are the primary reasons behind the f.c.c. phase occurrence. A recent proposed phase, i.e., a body-centered tetragonal (b.c.t.) phase, is confirmed with a small distortion from the f.c.c. phase. We have also predicted a reversible b.c.t. → f.c.c. phase transition at 800 GPa. This finding has been suggested that almost all the III-A elements (Ga, In and Tl) exhibit the b.c.t. → f.c.c. phase transition at extremely high pressure.