Structural transition of NaBH4 under high pressure: Ab initio calculations

The pressure induced structural transition of NaBH₄ from β-NaBH₄ (tetragonal-P42_1c) to γ-NaBH₄ (orthorhombic-Pnma) is investigated by ab initio plane-wave pseudopotential density functional theory method (DFT). The BaSO₄-type structure of orthorhombic high-pressure phase is testified theoretically for the first time. The calculated transition pressure of β-NaBH₄ (tetragonal-P42_1c) to γ-NaBH₄ (orthorhombic-Pnma) is 9.66 GPa and the orthorhombic high-pressure phase is stable up to 30 GPa. Our results agree well with previous experimental results and demonstrate that high-pressure phase transition from β-NaBH₄ to γ-NaBH₄ may occur at low temperature. At last, the pressure effects on the electronic structures of α-, β- and γ-NaBH₄ are discussed.     

A DFT study on the electronic structure for iridium nitride under high pressure

We present density functionary theory (DFT) calculations on the structural parameters and electronic structure for iridium nitride by using the generalized gradient approximation (GGA) and the Perdew–Burke–Ernserhof (PBE) exchange-correlation functional. The lattice parameters and bulk modulus (B ₀) for the ground state are obtained, and the energy band structure and electron densities of states (DOS) of IrN₂ are presented. It is found that IrN₂ has a very close indirect energy gap. There is a strong covalent bond between the two nearest N atoms. This gives rise to a very high elastic modulus of IrN₂ and reveals the quasimolecular nature of the N₂ in IrN₂ crystal. Lattice parameters, bulk modulus, and the electronic structure of IrN₂ under high pressure have also been investigated based on DFT. The compressibility along three cell vectors is very close to each other. The band gap increases a little with the pressure even when the pressure is up to 100 Gpa.     

First-principles investigations on structural,electronic and elastic properties of BeSe under high pressure

The structural, electronic and elastic properties of BeSe in both B3 and B8 structures have been studied by first-principles calculations within the generalized gradient approximation (GGA). The calculated lattice parameters and bulk modulus of BeSe are in reasonable agreement with previous results. The predicted value of phase transition pressure from B3 to B8 is 50.24 GPa, which is well in line with the experimental data (56 ± 5 GPa). The calculation of the electronic band structure shows that the energy gap is indirect for B3 and B8 phases. Especially, the elastic constants of B8 BeSe under high pressure were studied for the first time. The bulk modulus, shear modulus, compressional and shear wave velocities of B8 BeSe evaluated from elastic constants as a function of pressure were investigated. In addition, Poisson's radio, elastic anisotropy and Debye temperature were analyzed successfully.     

Prediction of stable BC3N2 monolayer from first-principles calculations: Stoichiometry,crystal structure,electronic and adsorption properties

In this paper, a novel BC₃N₂ monolayer has been found with a graphene-like structure using the developed particle swarm optimization algorithm in combination with ab initio calculations. The predicted structure meets the thermodynamical, dynamical, and mechanical stability requirements. Interestingly, the BC₃N₂ plane shows a metallic character. Importantly, BC₃N₂ has an in-plane stiffness comparable to that of graphene. We have also investigated the adsorption characteristics of CO₂ on pristine monolayer and Mo functionalized monolayer using density functional theory. Subsequently, electronic structures of the interacting systems (CO₂ molecule and substrates) have been preliminarily explored. The results show that Mo/BC₃N₂ has a stronger adsorption capacity towards CO₂ comparing with the pristine one, which can provide a reference for the further study of the CO₂ reduction mechanism on the transition metal-functionalized surface as well as the new catalyst’s design.     

Quasi-hydrostatic X-ray powder diffraction study of the low- and high-pressure phases of CaWO4 up to 28 GPa

We have studied CaWO₄ under compression using Ne as pressure-transmitting medium at room temperature by means of synchrotron X-ray powder diffraction. We have found that CaWO₄ beyond 8.8 GPa transforms from its low-pressure tetragonal structure (scheelite) into a monoclinic structure (fergusonite). The high-pressure phase remains stable up to 28 GPa and the low-pressure phase is totally recovered after full decompression. The pressure dependence of the unit-cell parameters, as well as the pressure–volume equation of state, has been determined for both phases. Compared with previous studies, we found in our quasi-hydrostatic experiments a different behavior for the unit-cell parameters of the fergusonite phase and a different transition pressure. These facts suggest that deviatoric stresses influence on the high-pressure structural behavior of CaWO₄ as previously found in related compounds. The reported experiments also provide information on the pressure dependence of interatomic bond distances, shedding light on the transition mechanisms.     

Ab-initio study on the stability,electronic and mechanical properties of transition metal nitrides under external pressure

Structures of transition metal nitrides (TMNs) were optimized using the plane-wave pseudopotential method based on density functional theory. Energy as a function of volume curves were calculated to predict the phase transition pressures. Density of states (DOS), charge density difference, and charge transfers were calculated. The elastic constant (C₁₁) and modulus (G) as a function of pressure were computed. Results showed that TMNs in the WC structure was most stable at normal pressure. All TMNs exhibited metallic, covalent and ionic property. Metallic character increased and covalent property reduced with increasing atomic number of TM atom. The elastic constant (C₁₁) and modulus (G) increased linearly with increasing pressure due to stronger hybridization, bonding and covalent property. Thus, mechanical property enhanced under external pressure.     

Investigation of crystal structure and associated electronic structure of Sr6BP5O20

Strontium borophosphate phosphate (Sr₆BP₅O₂₀, SrBP), activated by divalent europium ions is a bluish-green phosphor emitting in a broad band with the emission peak near 480 nm. In this paper, we report the crystal structure of SrBP determined from an analysis of the X-ray diffraction pattern of a prismatic single crystal (size 60 μm×50 μm×40 μm). This crystal was chosen from undoped phosphor powder samples prepared for this purpose by solid-state reaction. SrBP is observed to crystallize in a body-centered tetragonal lattice with the lattice parameters and , the associated space group being (space group 120). Using the structural data from this study, we have also calculated its electronic structure using the augmented spherical wave method and the local density approximation (LDA). We show the ordering of the electronic states by the density of states (DOS) and the partial DOS plots. The LDA gives a direct optical band gap at the Γ point of about 5 eV. The significance of the crystal structure and associated electronic structure is discussed with respect to maintenance of this phosphor in Hg-discharge lamps.     

Crystal structure predictions: the crystal and electronic structure of Zr1−δV1+δAs

Zr_1−δV_1+δAs is accessible via arc-melting of different mixtures of ZrAs, VAs, Zr, and V. It crystallizes in the tetragonal La₂Sb type, with a phase range of −0.43(4)?δ?0.15(1). The lattice dimensions (a=382.4(1) pm, c=1486.8(6) pm for Zr_1.43(4)V_0.57As; and a=375.77(7) pm, c=1400.2(3) pm for Zr_0.85(1)V_1.15As) strongly depend on δ because of the different sizes of the Zr and V atoms. The ZrVAs structure comprises sheets of (empty) M₆ octahedra, whose triangular faces are capped by the main group atoms Q. The sheets are interconnected via M−Q bonds to a truly three-dimensional structure. Like in the isostructural ZrTiAs, the smaller 3d M atom prefers the site in the densely packed square planes. In addition to the dominating M−As bonds, the structure comprises strong M−M bonding. Independent of the exact Zr:V ratio, Zr_1−δV_1+δAs is calculated to have three-dimensional metallic properties.     

Space group and crystal structure of the Perovskite CaTiO3 from 296 to 1720 K

The structural phase transitions of the calcium titanate perovskite CaTiO₃ were investigated by the Rietveld analysis of high-temperature neutron and X-ray powder diffraction data in the temperature range of 296-1720 K. The present work demonstrates that the compound exhibits two reversible phase transitions of orthorhombic Pbnm-tetragonal I4/mcm and of I4/mcm-cubic at 1498±25 K and 1634±13 K, respectively. No evidence of Cmcm phase is observed between the Pbnm and I4/mcm structures. The lattice parameters discontinuously change at the Pbnm-I4/mcm transition point, while a continuous change is observed for the transformation. These results indicate that the Pbnm-I4/mcm transition is of first order and that the transformation is of either second or higher order.     

Monitoring phase transition of aqueous biomass model substrates by high‐pressure and high‐temperature microfluidics

Aqueous‐Phase Reforming (APR) is a promising hydrogen production method, where biomass is catalytically reformed under high pressure and high temperature reaction conditions. To eventually study APR, in this paper, we report a high‐pressure and high‐temperature microfluidic platform that can withstand temperatures up to 200°C and pressures up to 30 bar. As a first step, we studied the phase transition of four typical APR biomass model solutions, consisting of 10 wt% of ethylene glycol, glycerol, xylose or xylitol in MilliQ water. After calibration of the set‐up using pure MilliQ water, a small increase in boiling point was observed for the ethylene glycol, xylitol and xylose solutions compared to pure water. Phase transition occurred through either explosive or nucleate boiling mechanisms, which was monitored in real‐time in our microfluidic device. In case of nucleate boiling, the nucleation site could be controlled by exploiting the pressure drop along the microfluidic channel. Depending on the void fraction, various multiphase flow patterns were observed simultaneously. Altogether, this study will not only help to distinguish between bubbles resulting from a phase transition and/or APR product formation, but is also important from a heat and mass transport perspective.     

Characterization and optical properties of mechanochemically synthesized molybdenum-doped rutile nanoparticles and their electronic structure studies by density functional theory

The optical and electronic properties of molybdenum (Mo) doped rutile TiO₂ prepared by the mechanochemical method were studied both experimentally and using density functional theory (DFT). The synthesized nanoparticles were characterized by XRD, TEM, EDS-MAP, and XPS. The XRD results showed the successful incorporation of Mo in the rutile crystal lattice. High-resolution TEM images illustrated a decreasing trend in the (110) d-spacing for samples doped up to 3 at%. The shift toward higher binding energies in the XPS spectra was due to the higher oxidization tendencies of Mo⁵⁺ and Mo⁶⁺ substituted in Ti⁴⁺ sites. The optical behavior of samples was examined by UV–Vis and photoluminescence spectroscopy. The bandgap energy value of rutile was reduced from 3.0 eV to 2.4 eV by 2 at% Mo doping. The DFT calculations showed a reduction of bandgap energy value of rutile to 2.35 eV with 2 at% Mo, which is in harmony with the experimental results. The creation of energy states below the conduction band because of Mo doping was identified as the reason for reducing the bandgap energy and photoluminescence emission of rutile.     

Single crystal X-ray diffraction study of a mixed-valence gold compound, Cs2AuAuCl6 under high pressures up to 18 GPa: Pressure-induced phase transition coupled with gold valence transition

We performed the single-crystal X-ray diffraction study of a perovskite-type gold mixed-valence compound, Cs₂Au^IAu^IIICl₆, under high pressures up to 18 GPa by using a diamond-anvil-cell with helium gas as an ideal hydrostatic pressure-transmitting medium. The lattice parameters and the variable atomic positional parameters were obtained with reasonable accuracy at various pressures. A structural phase transition at ca. 12.5 GPa from I4/mmm to Pm3m was found. The lattice parameters a₀ and c₀, denoted in the tetragonal cell setting, result in the relationship 2^1/2a₀=c₀, and the superstructure reflections h k l (l is odd), caused by the shift of the Cl ions from the midpoint of the Au ions, disappeared at pressures above the phase transition. Both elongated [Au^IIICl₆] and compressed [Au^ICl₆] octahedra in the low-pressure phase smoothly approach regular octahedra with increasing pressure. Above the structural phase transition at 12.5 GPa, all the [AuCl₆] octahedra are crystallographically equivalent, which shows that the tetragonal-to-cubic phase transition accompanies the valence transition from the Au^I/Au^III mixed-valence state to the Au^II single-valence state.     

High-pressure synthesis and crystal structure of the lithium borate HP-LiB3O5

The new lithium borate HP-LiB₃O₅ was synthesized under high-pressure/high-temperature conditions of 6 GPa and 1050 °C in a multianvil press with a Walker-type module. The compound crystallizes in the space group Pnma (no. 62) with the lattice parameters a=829.7(2), b=759.6(2), and c=1726.8(4) pm (Z=16). The high-pressure compound HP-LiB₃O₅ is built up from a three-dimensional network of BO₄ tetrahedra and BO₃ groups, which incorporates Li⁺ ions in channels along the b-axis. Band assignments of measured IR- and Raman spectra were done via quantum-mechanical calculations. Additionally, the thermal behavior of HP-LiB₃O₅ was investigated.     

Twisted molecular geometry and localized electronic structure of the triplet excited gem-diphenyltrimethylenemethane biradical: substituent effects on thermoluminescence and related theoretical calculations

Substituent effects on the energies of electronic transitions (ETs) between the triplet excited and ground states of gem-diphenyltrimethylenemethane biradicals (³2a) were explored by using thermoluminescence (TL) spectroscopy and density functional theory (DFT) including time-dependent (TD) DFT. Linear free energy (Hammett) analyses of TL energies of a variety of para-substituted aryl derivatives of ³2* gave reasonable correlations with the substituent constant, σ. The slope of Hammett plots of the data are nearly identical to one obtained from a similar analysis of the photoluminescence (PL) energies of the structurally-related 1,1-diarylethyl radicals (3*). The results suggest that TL of ³2* and PL of 3* derive from a common diarylmethyl radical fluorophore. This interpretation is also supported by the DFT and TDDFT calculated electronic structures and ET energies of ³2 and 3. Thermodynamic and kinetic analyses of the charge recombination (CR) process between 2⁺ and 1⁻, which generates ³2*, revealed that substituents not only alter the TL energies but also the TL intensities of ³2*. The observations made in this effort demonstrate that ³2* as well as ³2 and 2⁺ have greatly twisted molecular geometries and highly localized electronic structures.     

The electronic structure of 2-chloro-2′-deoxyadenosine (drug) studied by Cl-NQR spectroscopy and ab initio calculations

Electronic structure and conformation of 2-chloro-2′-deoxyadenosine (2CldAdo) and 2-chloroadenosine (2ClAde) have been studied by ³⁵Cl-NQR spectroscopy and ab initio calculations by the B3LYP/6-31G^* method.

The usefulness of NQR spectroscopy and ab initio calculations for the investigation of redistribution of electron density and structural changes in 2CldAdo and 2ClAde was assessed and a comparison of the electron distribution in the unsubstituted 2ClAde (monomer, dimer) and the substituted 2ClAdo was made.     

Optimizing the gas chromatographic separation and detection of polychlorinated biphenyls by use of electronic pressure programming and experimental design

The applicability of electronic pressure control and Taguchi L₂₇ experimental design to the optimization of the gas chromatographic separation and detection of polychlorinated biphenyls has been evaluated. The influence of several experimental variables, column temperature program, carrier gas pressure program, on-column injector temperature program, and make-up gas pressure program, was studied using analysis of variance. Simultaneous optimization of sample introduction, column efficiency, and detector performance could be achieved without compromising system performance. The relationships between system performance and experimental variables were established using regression analysis. Agreement between the simulated and experimental results obtained using suggested optimum conditions demonstrated the applicability of the technique developed in this study. The improvement achieved in the chromatographic separation of PCBs is presented.     

Synthesis, crystal and electronic structure, and physical properties of the new lanthanum copper telluride La3Cu5Te7

The new lanthanum copper telluride La₃Cu_5−xTe₇ has been obtained by annealing the elements at 1073 K. Single-crystal X-ray diffraction studies revealed that the title compound crystallizes in a new structure type, space group Pnma (no. 62) with lattice dimensions of a=8.2326(3) Å, b=25.9466(9) Å, c=7.3402(3) Å, V=1567.9(1) Å³, Z=4 for La₃Cu_4.86(4)Te₇. The structure of La₃Cu_5−xTe₇ is remarkably complex. The Cu and Te atoms build up a three-dimensional covalent network. The coordination polyhedra include trigonal LaTe₆ prisms, capped trigonal LaTe₇ prisms, CuTe₄ tetrahedra, and CuTe₃ pyramids. All Cu sites exhibit deficiencies of various extents. Electrical property measurements on a sintered pellet of La₃Cu_4.86Te₇ indicate that it is a p-type semiconductor in accordance with the electronic structure calculations.     

The disproportionation reaction phase transition,mechanical, and lattice dynamical properties of the lanthanum dihydrides under high pressure: A first principles study

The pressure-induced disproportionation reaction phase transition, mechanical, and dynamical properties of LaH₂ with fluorite structure under high pressure are investigated by performing first-principles calculations using the projector augmented wave (PAW) method. The phase transition of 2LaH₂ → LaH + LaH₃ obtained from the usual condition of equal enthalpies occurs at the pressure of 10.38 GPa for Perdew–Wang (PW91) functional and 6.05 GPa for Ceperly–Adler (CA) functional, respectively. The result shows that the PW91 functional calculations agree excellently with the experimental finding of 11 GPa of synchrotron radiation (SR) X-ray diffraction (XRD) of Machida et al. and 10 GPa of their PBE functional theoretical result. Three independent single-crystal elastic constants, polycrystalline bulk modulus, shear modulus, Young's modulus, elastic anisotropy, Poisson's ratio, the brittle/ductile characteristics and elastic wave velocities over different directions dependences on pressure are also successfully obtained. Especially, the phonon dispersion curves and corresponding phonon density of states of LaH₂ under high pressure are determined systematically using a linear-response approach to density functional perturbation theory (DFPT). Our results demonstrate that LaH₂ in fluorite phase can be stable energetically up to 10.38 GPa, stabilized mechanically up to 17.98 GPa, and stabilized dynamically up to 29 GPa, so it may remain a metastable phase above 10.38 GPa up to 29 GPa, these calculated results accord with the recent X-Ray diffraction experimental finding and theoretical predictions of Machida et al.     

Dependence of the lone pair of bismuth on coordination environment and pressure: An ab initio study on Cu4Bi5S10 and Bi2S3

DFT calculations have been carried out for Cu₄Bi₅S₁₀ and Bi₂S₃ to provide an analysis of the relation between electronic structure, lone electron pairs and the local geometry. The effect of pressure is considered in Bi₂S₃ and the results are compared to published experimental data. Bi³⁺ in Cu₄Bi₅S₁₀ is found at both symmetrically and asymmetrically coordinated sites, whereas the coordination environments of Bi in Bi₂S₃ are asymmetric at room conditions and get more regular with increasing pressure. The charge density maps of the asymmetric sites show the lone pairs as lobes of non-shared charge. These lobes are related to an effective Bi s-Bi p hybridization resulting from coupling to S p orbitals, supporting the modern view of the origin of the stereochemically active lone pair. No effective Bi s-p hybridization is seen for the symmetric site in Cu₄Bi₅S₁₀, whereas Bi s-p hybridization coexists with a much reduced lone pair in Bi₂S₃ at high pressure.     

Investigation of the electronic and structural properties of potassium hexaboride, KB6, by transport, magnetic susceptibility, EPR, and NMR measurements, temperature-dependent crystal structure determination, and electronic band structure calculations

The electronic and structural properties of potassium hexaboride, KB(6), were examined by transport, magnetic susceptibility, EPR, and NMR measurements, temperature-dependent crystal structure determination, and electronic band structure calculations. The valence bands of KB(6) are partially empty, but the electrical resistivity of KB(6) reveals that it is not a normal metal. The magnetic susceptibility as well as EPR and NMR measurements show the presence of localized electrons in KB(6). The EPR spectra of KB(6) have two peaks, a broad ( approximately 320 G) and a narrow (less than approximately 27 G) line width, and the temperature-dependence of the magnetic susceptibility of KB(6) exhibits a strong hysteresis below 70 K. The temperature-dependent crystal structure determination of KB(6) shows the occurrence of an unusual variation in the unit cell parameter hence supporting that the hysteresis of the magnetic susceptibility is a bulk phenomenon. The line width DeltaH(pp) of the broad EPR signal is independent of temperature and EPR frequency. This finding indicates that the line broadening results from the dipole-dipole interaction, and the spins responsible for the broad EPR peak has the average distance of approximately 1.0 nm. To explain these apparently puzzling properties, we examined a probable mechanism of electron localization in KB(6) and its implications.