scholarly journals Investigation of the structural and electronic properties of CdS under high pressure: an ab initio study

2018 ◽  
Vol 96 (2) ◽  
pp. 216-224 ◽  
Author(s):  
C. Yamcicier ◽  
Z. Merdan ◽  
C. Kurkcu
Keyword(s):  
Phase Transition ◽  
Ab Initio ◽  
Density Functional ◽  
Structural Phase ◽  
Type Structure ◽  
Ambient Conditions ◽  
Space Group ◽  
Space Groups ◽  
Intermediate States ◽  
Calculation Results

An ab initio constant pressure study is carried out to explore the behaviour of cadmium sulfide (CdS) under high hydrostatic pressure. We have studied the structural properties of CdS using density functional theory (DFT) under pressure up to 200 GPa. CdS crystallizes in a wurtzite (WZ)-type structure under ambient conditions. CdS undergoes a structural phase transition from the hexagonal WZ-type structure with space group P63mc to cubic NaCl-type structure with space group [Formula: see text]. Another phase transition is obtained from NaCl-type structure to the orthorhombic CdS-III-type structure with space group Pmmn. The first transformation proceeds via seven intermediate states with space group Cmc21, P21, Pmn21, P21/m, Pmmn, I4/mmm, and Cmcm. The latter transformation is based on two intermediate states with space groups Immm and P21/m. These phase transitions are also studied by total energy and enthalpy calculations. According to these calculations, the phase transformations occur at about 3 and 51 GPa, respectively. Calculation results on the other basic properties, such as lattice constant, volume, and bulk modulus are also compared with those of other recent theoretical and experimental data, and generally, good agreement with the available data are obtained.

scholarly journals High-pressure structural phase transitions, electronic properties, and intermediate states of CaSe

2019 ◽  
Vol 97 (7) ◽  
pp. 797-802
Author(s):  
Cihan Kürkçü
Keyword(s):  
Crystal Structure ◽  
High Pressure ◽  
Density Functional ◽  
Structural Phase ◽  
Ambient Conditions ◽  
Generalized Gradient ◽  
Electronic Band ◽  
Space Group ◽  
Structure Space ◽  
Intermediate States

In this study, ab initio calculations have been carried out to understand the effect of extreme external pressure on the crystal structure of CaSe. The crystal structure of CaSe, a calcium chalcogen, is studied using density functional theory (DFT) with the generalized gradient approximation (GGA) up to 250 GPa under high hydrostatic pressure. Structurally CaSe crystallizes in cubic NaCl-type (B1) structure (space group: [Formula: see text]) at ambient conditions. The results indicated that CaSe undergoes a structural phase transition from this cubic structure to another cubic CsCl-type (B2) structure (space group: [Formula: see text]) at high pressure. This transformation is based on two intermediate states with space group [Formula: see text] and C2/m. Additionally, the electronic band structures and density of states for the obtained B1 and B2 structures of CaSe have been calculated. According to these calculations, obtained band gap values are in good agreement with the values reported in the literature.

Structural phase transition, electronic, elastic, and vibrational properties of LiAl intermetallic compound: insights from first-principles calculations

2017 ◽  
Vol 95 (8) ◽  
pp. 691-698
Author(s):  
Y. Mogulkoc ◽  
Y.O. Ciftci ◽  
G. Surucu
Keyword(s):  
Phase Transition ◽  
Density Of States ◽  
Structural Phase Transition ◽  
First Principles ◽  
Density Functional ◽  
Structural Phase ◽  
Type Structure ◽  
Vibrational Properties ◽  
First Principles Calculations ◽  
Electronic Band

Using the first-principles calculations based on density functional theory (DFT), the structural, elastic, electronic, and vibrational properties of LiAl have been explored within the generalized gradient approximation (GGA) using the Vienna ab initio simulation package (VASP). The results demonstrate that LiAl compound is stable in the NaTl-type structure (B32) at ambient pressure, which is in good agreement with the experimental results and there is a structural phase transition from NaTl-type structure (B32) to CsCl-type structure (B2) at around 22.2 GPa pressure value. The pressure effects on the elastic properties have been discussed and the elastic property calculation indicates that the elastic instability could provide a phase transition driving force according to the variations relation of the elastic constant versus pressure. To gain further information about this, we also have investigated the other elastic parameters (i.e., Zener anisotropy factor, Poisson’s ratio, Young’s modulus, and isotropic shear modulus). The electronic band structure, total and partial density of states, phonon dispersion curves, and one-phonon density of states of B2 and B32 phases are also presented with results.

Structural evolution of a Ge-substituted SnSe thermoelectric material with low thermal conductivity

2018 ◽  
Vol 51 (2) ◽  
pp. 337-343 ◽  
Author(s):  
Federico Serrano-Sánchez ◽  
Norbert M. Nemes ◽  
José Luis Martínez ◽  
Oscar Juan-Dura ◽  
Marco Antonio de la Torre ◽  
...  
Keyword(s):  
Phase Transition ◽  
Thermal Conductivity ◽  
Structural Phase Transition ◽  
Structural Phase ◽  
Structural Evolution ◽  
Type Structure ◽  
Scanning Calorimetry ◽  
Space Group ◽  
Low Thermal Conductivity ◽  
Structure Space

Thermoelectric materials are expected to become new alternative sources of sustainable energy. Among them, the SnSe intermetallic alloy has been described as an excellent thermoelectric compound, characterized by an extremely low thermal conductivity with maximum performance at the onset of a structural phase transition at 800 K. Recently, novel SnSe derivatives with Ge substitution have been synthesized by a direct arc-melting technique. This produces nanostructured polycrystalline samples that exhibit a record high Seebeck coefficient, anticipating an excellent performance above room temperature. Here, the structural phase transition from a GeS-type structure (space groupPnma) to a TlI-type structure (space groupCmcm) is investigatedin situ vianeutron powder diffraction (NPD) in the temperature range 298–853 K for the selected composition Sn0.8Ge0.2Se. This transition takes place at 803 K, as shown by differential scanning calorimetry. The analysis from the NPD data shows a non-monotonic behaviour of the anisotropic displacement parameters upon entering the domain of theCmcmstructure. The energies of the atomic vibrations have been quantitatively analysed by fitting the temperature-dependent mean-square displacements to Einstein oscillators. The thermal conductivity of Sn0.8Ge0.2Se is as low as 0.35 W m−1 K−1at 773 K, which mostly represents the lattice thermal contribution.

Structural phase transition in polycrystalline SnSe: a neutron diffraction study in correlation with thermoelectric properties

2016 ◽  
Vol 49 (6) ◽  
pp. 2138-2144 ◽  
Author(s):  
F. Serrano-Sánchez ◽  
N. M. Nemes ◽  
O. J. Dura ◽  
M. T. Fernandez-Diaz ◽  
J. L. Martínez ◽  
...  
Keyword(s):  
Phase Transition ◽  
Structural Phase ◽  
Neutron Diffraction Study ◽  
Type Structure ◽  
Scanning Calorimetry ◽  
Space Group ◽  
Temperature Range ◽  
Seebeck Coefficients ◽  
Structure Space ◽  
Arc Melting

SnSe has been recently reported as a promising and highly efficient thermoelectric intermetallic alloy. The present material has been prepared by arc melting, as mechanically robust pellets, consisting of highly oriented polycrystals. The evolution of its orthorhombic GeS-type structure (space groupPnma) and phase transition to TlI-type structure (space groupCmcm) at high temperature has been studiedin situby neutron powder diffraction (NPD) in the temperature range 295–873 K. This transition has been identified by differential scanning calorimetry measurements, yielding sharp peaks at 795 K. In addition, thermal transport properties were measured in a similar temperature range, and large Seebeck coefficients, as high as 1050 µV K−1at 625 K, were found. The analysis from NPD data demonstrates an almost perfect stoichiometry, Sn0.998(8)Se, that does not evolve with temperature, and a progressive decrease of the anharmonicity of the chemical bonds upon entering the domain of theCmcmstructure.

STRUCTURAL PHASE TRANSITION INDUCED BY PRESSURE IN THE ORDERED ALLOY FeRh

2005 ◽  
Vol 19 (21) ◽  
pp. 3389-3395 ◽  
Author(s):  
M. RAJAGOPALAN
Keyword(s):  
Phase Transition ◽  
Structural Phase Transition ◽  
First Principles ◽  
Density Functional ◽  
Structural Phase ◽  
Ferromagnetic State ◽  
Ambient Conditions ◽  
Density Functional Calculation ◽  
Antiferromagnetic Ground State ◽  
The One

First-principles density functional calculation of the total energy as a function of volume has been performed by the TB-LMTO approach for the ordered alloy FeRh in the anti-ferromagnetic state. We find that FeRh undergoes a structural phase transition from NaCl -type to tetragonal-type structure around 20.3 GPa which is in best agreement with the recent experimental observation. The calculations show that the energy of the antiferromagnetic ground state is lower than the one for the ferromagnetic state at ambient conditions.

scholarly journals Pressure-induced Pb–Pb bonding and phase transition in Pb2SnO4

2020 ◽  
Vol 76 (6) ◽  
pp. 979-991
Author(s):  
Dominik Spahr ◽  
Michał Stękiel ◽  
Dominik Zimmer ◽  
Lkhamsuren Bayarjargal ◽  
Katja Bunk ◽  
...  
Keyword(s):  
Phase Transition ◽  
Density Functional Theory ◽  
High Pressure ◽  
Powder Diffraction ◽  
Density Functional ◽  
Structural Phase ◽  
Irreducible Representations ◽  
Ambient Conditions ◽  
Functional Theory ◽  
Electron Pairs

High-pressure single-crystal to 20 GPa and powder diffraction measurements to 50 GPa, show that the structure of Pb2SnO4 strongly distorts on compression with an elongation of one axis. A structural phase transition occurs between 10 GPa and 12 GPa, with a change of space group from Pbam to Pnam. The resistivity decreases by more than six orders of magnitude when pressure is increased from ambient conditions to 50 GPa. This insulator-to-semiconductor transition is accompanied by a reversible appearance change from transparent to opaque. Density functional theory-based calculations show that at ambient conditions the channels in the structure host the stereochemically-active Pb 6s 2 lone electron pairs. On compression the lone electron pairs form bonds between Pb2+ ions. Also provided is an assignment of irreducible representations to the experimentally observed Raman bands.

Group actions as applied to symmetry breaking in hexagonal crystals with space group P63/m

1984 ◽  
Vol 62 (11) ◽  
pp. 1152-1173
Author(s):  
Rose M. Morra ◽  
Robin L. Armstrong
Keyword(s):  
Phase Transition ◽  
Structural Phase ◽  
Group Actions ◽  
Order Parameters ◽  
High Symmetry ◽  
Space Group ◽  
Space Groups ◽  
Symmetry Space ◽  
Low Symmetry ◽  
Symmetry Space Group

A procedure is outlined that allows one to predict the possible low-symmetry space groups for a commensurate phase transition associated with a broken symmetry. By considering each irreducible representation of the high-symmetry space group and using a subduction procedure involving the theory of group actions, one may determine a complete set of representative order parameters and their little groups. Each little group can be identified as a possible space group for the low-symmetry phase. The method is used to deduce all possible broken symmetries of space group P63/m that can result from a phase transition associated with one of the symmetry points in the Brillouin zone. As a specific application of the resulting tables of possible order parameters and associated low-symmetry space groups, changes in the X nuclear quadrupole resonance spectrum due to structural phase transitions in hexagonal AX3 crystals with the high-symmetry space group P63/m are considered.

scholarly journals Struktur- und Phasenübergangsuntersuchungen an β-Li2ZnGe/ Structure and Phase Transition Investigations on β-Li2ZnGe

1981 ◽  
Vol 36 (8) ◽  
pp. 917-921 ◽  
Author(s):  
Hans-Otto Cullmann ◽  
Heinz-Walter Hinterkeuser ◽  
Hans-Uwe Schuster
Keyword(s):  
Phase Transition ◽  
Thermal Analysis ◽  
Differential Thermal Analysis ◽  
Single Crystal ◽  
Ternary Compound ◽  
Type Structure ◽  
Crystal Data ◽  
Space Group ◽  
Cell Parameters ◽  
Structure Space

Abstract The ternary compound β-Li2ZnGe was prepared and its structure determined from powder and single crystal data. The compound crystallizes in a modified Na3As type structure, space group Ṗ̇̇̇̇̇̇̇̇̇̇̇̇̇̇3̄m 1 - D33d.The cell parameters are: a = 432.6 pm, c = 1647.0 pm, c/a= 3.83.A phase transition between a-and β-Li2ZnGe was found and the reaction of the elements lithium, zinc and germanium to a-Li2ZnGe was followed by differential thermal analysis. The temperatures and the enthalpies of transition and fusion were determined.

Crystal structure of mechanochemically prepared Ag2FeGeS4

2021 ◽  
Vol 0 (0) ◽  
Author(s):  
Eva M. Heppke ◽  
Shamini Mahadevan ◽  
Thomas Bredow ◽  
Martin Lerch
Keyword(s):  
Density Functional ◽  
Type Structure ◽  
Rietveld Refinements ◽  
Space Group ◽  
X Ray ◽  
Structure Space ◽  
Solid State Route ◽  
Wyckoff Position ◽  
Structure Evaluation ◽  
Hybrid Density Functional

Abstract Ag2FeGeS4 was synthesized as a phase-pure and highly crystalline product by mechanochemical milling from the binary sulfides and iron metal, followed by annealing in H2S atmosphere. The structure evaluation was carried out using X-ray powder diffraction with subsequent Rietveld refinements. As Fe and Ge atoms are not distinguishable using conventional X-ray methods, the chalcopyrite-type structure (space group I 4 ‾ 2 d $I‾{4}2d$ ), exhibiting a statistical distribution of Fe and Ge on Wyckoff position 4b, was considered. However, quantum-chemical calculations at hybrid density-functional level indicate that mechanochemically prepared Ag2FeGeS4 crystallizes in the kesterite-type structure (space group I 4 ‾ $I‾{4}$ ) where the cations are arranged in an ordered way. Ag2FeGeS4 is a further example of a mechanochemically prepared compound differing structurally from the commonly known polymorph exhibiting the stannite type (solid-state route).

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