scholarly journals Effects of Vacancy Cluster Defects on Electrical and Thermodynamic Properties of Silicon Crystals

2014 ◽  
Vol 2014 ◽  
pp. 1-8 ◽  
Author(s):  
Pei-Hsing Huang ◽  
Chi-Ming Lu
Keyword(s):  
Heat Capacity ◽  
Debye Temperature ◽  
Density Functional ◽  
Phonon Dispersion ◽  
Vacancy Cluster ◽  
Temperature Limit ◽  
Phase Changes ◽  
Hexagonal Ring ◽  
High Temperature Limit ◽  
Silicon Crystals

A first-principle plane-wave pseudopotential method based on the density function theory (DFT) was employed to investigate the effects of vacancy cluster (VC) defects on the band structure and thermoelectric properties of silicon (Si) crystals. Simulation results showed that various VC defects changed the energy band and localized electron density distribution of Si crystals and caused the band gap to decrease with increasing VC size. The results can be ascribed to the formation of a defect level produced by the dangling bonds, floating bonds, or high-strain atoms surrounding the VC defects. The appearance of imaginary frequencies in the phonon spectrum of defective Si crystals indicates that the defect-region structure is dynamically unstable and demonstrates phase changes. The phonon dispersion relation and phonon density of state were also investigated using density functional perturbation theory. The obtained Debye temperatureθDfor a perfect Si crystal had a minimum value of 448 K atT= 42 K and a maximum value of 671 K at the high-temperature limit, which is consistent with the experimental results reported by Flubacher. Moreover, the Debye temperature decreased with increases in the VC size. VC defects had minimal effects on the heat capacity (Cv) value when temperatures were below 150 K. As the temperature was higher than 150 K, the heat capacity gradually increased with increasing temperature until it achieved a constant value of 11.8 cal/cell·K. The heat capacity significantly decreased as the VC size increased. For a 2 × 2 × 2 superlattice Si crystal containing a hexagonal ring VC (HRVC10), the heat capacity decreased by approximately 17%.

Study of thermo-elastic and lattice dynamics properties of half-Heusler compounds XMgAl (X = Li, Na) by computational investigations

2019 ◽  
Vol 33 (08) ◽  
pp. 1950093 ◽  
Author(s):  
A. Afaq ◽  
Abu Bakar ◽  
M. Rizwan ◽  
M. Aftab Fareed ◽  
H. Bushra Munir ◽  
...  
Keyword(s):  
Debye Temperature ◽  
Elastic Constants ◽  
Density Functional ◽  
Phonon Dispersion ◽  
Dynamic Properties ◽  
Mechanical Parameters ◽  
Heusler Compounds ◽  
Generalized Gradient ◽  
Text Type ◽  
Quantum Espresso

In this study, thermo-elastic and lattice dynamic properties of XMgAl (X = Li, Na) half-Heusler compounds are investigated using density functional theory implemented in WIEN2k and Quantum ESPRESSO codes. Generalized gradient approximation (GGA) as an exchange correlation function has been used in Kohn–Sham equations. Firstly, the structure of these Heusler compounds is optimized and then these optimized parameters are used to find three elastic constants [Formula: see text], [Formula: see text] and [Formula: see text] for [Formula: see text] type structures. Three elastic constants are then used to determine different elastic moduli like bulk modulus, shear modulus, Young’s modulus and other mechanical parameters like Pugh’s ratio, Poisson’s ratio, anisotropic ratio, sound velocities, Debye temperature and melting temperature. On behalf of these mechanical parameters, the brittle/ductile nature and isotropic/anisotropic behavior of the materials has been studied. Different regions of vibrational modes in the materials are also discussed on behalf of Debye temperature calculations. The vibrational properties of the half-Heusler compounds are computed using Martins–Troullier pseudo potentials implemented in Quantum ESPRESSO. The phonon dispersion curves and phonon density of states in first Brillion zone are obtained and discussed. Reststrahlen band of LiMgAl is found greater than NaMgAl.

scholarly journals Lattice Dynamics and thermodynamic Responses of XNbSn Half-Heusler Semiconductors: A First-Principles Approach

2021 ◽  
pp. 121-130
Author(s):  
O. E. Osafile ◽  
O. N. Nenuwe
Keyword(s):  
Debye Temperature ◽  
Lattice Dynamics ◽  
Density Functional ◽  
Phonon Dispersion ◽  
Lattice Structure ◽  
Heusler Alloys ◽  
Experimental Simulation ◽  
Thermodynamic Evaluation ◽  
Optical Modes ◽  
Quantum Espresso

In this work, we have evaluated how CoNbSn, IrNbSn, and RhNbSn half Heusler alloys respond to temperature change and the accompanying lattice vibrations as a cubic crystal. There are reports in the literature for CoNbSn with which we compared our result; there are, however, no reports for the other two alloys except for their Debye temperature obtained via machine learning, and our results compare well. Considering that results in the literature for IrNbSn and RhNbSn are scanty, we first computed the alloys' structural and electronic properties to establish their structural stability using the density functional theory and generalised gradient approximation as implemented in the quantum espresso computational suite. We confirmed the equilibrium lattice structure by exploring the three possibilities for a half Heusler alloy and fitting the results to the state's Murnaghan equation. The negative formation energies obtained supports experimental simulation of the alloys. Results from the lattice dynamics and thermodynamic evaluation show that the alloys favour ionic bonding and are ductile. The Debye temperature positions IrNbSn to be the most promising material for thermoelectric application because it has the least Debye temperature; hence it is supposed to have the lowest thermal conductivity. The Dulong-Petit law is obeyed at high temperature as expected. The phonon dispersion and density of states show that the d orbitals of Co and Nb are the significant contributors to the dispersions at both the acoustic and optical modes of the alloys.

scholarly journals Efficient Debye Function Interpolation Formulae: Sample Applications to Diamond

2021 ◽  
Vol 3 (4) ◽  
pp. 1-1
Author(s):  
Roland Pässler ◽  
Keyword(s):  
Heat Capacity ◽  
Debye Temperature ◽  
Phonon Dispersion ◽  
Cubic Boron Nitride ◽  
Wide Band ◽  
Interpolation Formula ◽  
Heat Capacities ◽  
Debye Function ◽  
Present Sample ◽  
Capacity Data

The well-known classical heat capacity model developed by Debye proposed an approximate description of the temperature-dependence of heat capacities of solids in terms of a characteristic integral, the T-dependent values of which are parameterized by the Debye temperature, Θ D . However, numerous tests of this simple model have shown that within Debye’s original supposition of approximately constant, material-specific Debye temperature, it has little chance to be applicable to a larger variety of non-metals, except for a few wide-band-gap materials such as diamond or cubic boron nitride, which are characterized by an unusually low degree of phonon dispersion. In this study, we present a variety of structurally simple, unprecedented algebraic expressions for the high-temperature behavior of Debye’s conventional heat-capacity integral, which provide fine numerical descriptions of the isochoric (harmonic) heat capacity dependences parameterized by a fixed Debye temperature. The present sample application of an appropriate high-to-low temperature interpolation formula to the isobaric heat capacity data for diamond measured by Desnoyers and Morrison [17], Victor [24], and Dinsdale [25] provided a fine numerical simulation of data within a range of 200 to 600 K, involving a fixed Debye temperature of about 1855 K. Representing the monotonically increasing difference of the isobaric versus isochoric heat capacities by two associated anharmonicity coefficients, we were able to extend the accurate fit of the given heat capacity ( C p ( T ) ) data up to 5000 K. Furthermore, we have performed a high-accuracy fit of the whole C p ( T ) dataset, from approximately 20 K to 5000 K, on the basis of a previously developed hybrid model, which is based on two continuous low-T curve sections in combination with three discrete (Einstein) phonon energy peaks. The two theoretical alternative curves for the C p ( T ) dependence of diamond were found to be almost indistinguishable throughout the interval from 200 K to 5000 K.

Exact calculations of the thermal properties of two-electron GaAs quantum dots with inverse-square interactions

2021 ◽  
pp. 1-8
Author(s):  
F.S. Nammas ◽  
Eyad Hasan Hasan ◽  
A.N. Alnowafa
Keyword(s):  
Quantum Dots ◽  
Heat Capacity ◽  
Thermal Properties ◽  
Low Temperature ◽  
Three Dimensional ◽  
Interaction Strength ◽  
Temperature Limit ◽  
Oscillator System ◽  
High Temperature Limit ◽  
Impressive Result

In this study, we theoretically scrutinize the effect of the inverse-square interaction on the thermal properties of two electrons trapped in a parabolic GaAs quantum dot. The analytical energy spectrum was used to calculate the thermal properties of the system using the canonical ensemble formalism. It was found that the thermal energy increased with the increase in temperature, while it remained almost constant for sufficiently low temperatures; it was also demonstrated that the inverse-square interaction increased the thermal mean energy. Moreover, the heat capacity increased sharply within a low-temperature window and saturated to the value of 2kB in the high-temperature limit. As expected, entropy increased linearly with increasing temperature. It was also shown that both entropy and heat capacity decreased rapidly when the confinement strength increased (or the dot size decreased) in the low-temperature limit, regardless of the influence of the interaction between the electrons. We also show that the number of allowed states of the system decreased as the interaction strength increased (Z(λ = 0) > Z(λ ≠ 0)). Finally, the stability of the system was investigated through F–T curves. The three-dimensional surface for the temperature-dependent mean energy and heat capacity was also plotted. It should be noted that, for the thermal mean energy, partition function, and Helmholtz free energy, the normal physical behavior of the two-oscillator system with Fermi statistics is recovered for λ → 0. However, heat capacity and entropy show exact two-fermion oscillator system behavior. The most impressive result found in this work is that the inverse-square interaction does not affect the heat capacity and entropy at all despite its noticeable effects on the thermal mean energy. This, in turn, facilitates theoretical studies related to finding the distinctive parameters of quantum dots without going into the heavy calculations resulting from the effects of interactions.

Theoretical study of thermodynamic properties for SmAlO3 under the effects of pressure and temperature

2018 ◽  
Vol 32 (23) ◽  
pp. 1850247 ◽  
Author(s):  
Ghulam Mustafa ◽  
Ahmad Afaq ◽  
Najm Ul Aarifeen ◽  
Muhammad Asif ◽  
Jamil Ahmad ◽  
...  
Keyword(s):  
Free Energy ◽  
Thermodynamic Properties ◽  
Bulk Modulus ◽  
Internal Energy ◽  
Debye Temperature ◽  
Density Functional ◽  
Helmholtz Free Energy ◽  
Coefficient Of Thermal Expansion ◽  
Temperature Limit ◽  
Debye Model

In the present paper, we have investigated SmAlO3 for their thermodynamic properties under effect of pressure and temperature by employing density functional theory (DFT) and quasi-harmonic Debye model. The various thermodynamic properties like Bulk Modulus, entropy, internal energy, Helmholtz free energy, Debye temperature, coefficient of thermal expansion, Grüneisen parameter and heat capacities of the ternary alloy are calculated. We found that Bulk Modulus, Debye temperature and Helmholtz free energy have decreasing trend with rise of temperature while their values have increasing behavior with rise of pressure. The internal energy of the system almost remains same with variation in pressure but temperature effectively increasing it. Our results are in good agreement with available data at low-temperature limit.

Comparison of Two Computing Method on Thermal Conductivity of Aluminum Nitride

2014 ◽  
Vol 989-994 ◽  
pp. 3509-3512 ◽  
Author(s):  
Xue Mei Cai ◽  
Qian Neng Zhou ◽  
Jing Mei Wang
Keyword(s):  
Thermal Conductivity ◽  
Heat Capacity ◽  
Aluminum Nitride ◽  
Debye Temperature ◽  
Density Functional ◽  
Harmonic Approximation ◽  
Relative Difference ◽  
Debye Theory ◽  
Debye Temperatures ◽  
Density Functional Perturbation Theory

Thermal conductivity of aluminum nitride (AlN) has been calculated by density functional perturbation theory (DFPT) and quasi-harmonic approximation (QHA) combined with Debye theory in the paper. Debye temperature is evaluated respectively from sound velocity and heat capacity. From 300K up to 1000K, the predicted thermal properties in pure crystal AlN based on these two Debye temperatures are compared with each other and the latter shows excellent agreement with Slack’s experimental data. The relative difference based on Debye temperature from heat capacity is within the limits of ±5.5%. This agreement with experiment is due to the Debye temperature derived from capacity contains the temperature effect while describe the three phonon process.

The mechanical and thermodynamic properties of ZrAl2 under pressure from first-principles investigation

2016 ◽  
Vol 27 (01) ◽  
pp. 1650001
Author(s):  
Ning Wei ◽  
Xuefei Wang ◽  
Xuzhong Zuo
Keyword(s):  
Heat Capacity ◽  
High Pressure ◽  
Thermodynamic Properties ◽  
Debye Temperature ◽  
First Principles ◽  
Density Functional ◽  
Applied Pressure ◽  
Debye Model ◽  
Effect Of Temperature ◽  
Expansion Formula

The mechanical and thermodynamic properties of ZrAl2 alloy under high pressure are investigated by first-principles based on the density functional theory. Due to all the elastic constants of ZrAl2 alloy satisfy generalized stabilities criteria, ZrAl2 is mechanically stable under pressure up to 100[Formula: see text]GPa. By analyzing the value of B/G and Poisson’s ratio [Formula: see text] which are correlated with the ductility and brittleness of material, we found that ZrAl2 belongs to brittle material at pressure of 0–70[Formula: see text]GPa and will change from brittleness to ductility at 70[Formula: see text]GPa. Combining with high bulk modulus B and shear modulus G, the mechanical of properties will be improved under high pressure. Moreover, the thermodynamic properties, such as the Debye temperature [Formula: see text], heat capacity [Formula: see text] and thermal expansion [Formula: see text], are discussed using the quasi-harmonic Debye model. We noted that the Debye temperature [Formula: see text] is mainly dependent on the pressure and the effect of temperature on the heat capacity [Formula: see text] is more important than the applied pressure.

scholarly journals First-Principles Study on the Photoelectric Properties of CsGeI3 under Hydrostatic Pressure

2020 ◽  
Vol 10 (15) ◽  
pp. 5055
Author(s):  
Li-Ke Gao ◽  
Yan-Lin Tang ◽  
Xin-Feng Diao
Keyword(s):  
Heat Capacity ◽  
Free Energy ◽  
Hydrostatic Pressure ◽  
Debye Temperature ◽  
Gibbs Free Energy ◽  
First Principles ◽  
Density Functional ◽  
Applied Pressure ◽  
Perovskite Solar Cells ◽  
Photoelectric Properties

CsGeI3 has been widely studied as an important photoelectric material. Based on the density functional theory (DFT), we use first-principles to study the photoelectric properties of CsGeI3 by applying successive hydrostatic pressure. It has been found that CsGeI3 has an optimal optical band gap value of 1.37 eV when the applied pressure is −0.5 GPa, so this paper focuses on the comparative study of the photoelectric properties when the pressure is −0.5 GPa and 0 GPa. The results showed that CsGeI3 has a higher dielectric value, conductivity, and absorption coefficient and blue shift in absorption spectrum when the pressure is −0.5 GPa. By calculating and comparing the effective masses of electrons and holes and the exciton binding energy, it was found that their values are relatively small, which indicates that CsGeI3 is an efficient light absorbing material. CsGeI3 was found to be stable under both pressure conditions through multiple calculations of the Born Huang stability criterion, tolerance factor T, and phonon spectrum with or without virtual frequency. We also calculated the elastic modulus of both pressure conditions and found that they are both soft, ductile, and anisotropic. Finally, the thermal properties of CsGeI3 under two kinds of pressure were studied. It was found that the Debye temperature and heat capacity of CsGeI3 increased with the increase of thermodynamic temperature, and the Debye temperature increased rapidly after pressure, while the heat capacity slowly increased and finally stabilized. Through the calculation of enthalpy, entropy, and Gibbs free energy of CsGeI3, it was found that the Gibbs free energy decreases faster with the increase of temperature without applied pressure, which indicates that CsGeI3 has a higher stability without pressure. Through the comparative analysis of the photoelectric properties of CsGeI3 under pressure, it was found that CsGeI3 after applied pressure is a good photoelectric material and suitable for perovskite solar cells (PSCs) material.

Ab initio investigation of the electronic, lattice dynamic and thermodynamic properties of ScCd intermetallic alloy

2016 ◽  
Vol 30 (24) ◽  
pp. 1650175
Author(s):  
B. I. Adetunji ◽  
A. S. Olayinka ◽  
J. B. Fashae ◽  
V. C. Ozebo
Keyword(s):  
Heat Capacity ◽  
Thermodynamic Properties ◽  
Specific Heat ◽  
Density Of States ◽  
Specific Heat Capacity ◽  
Density Functional ◽  
Phonon Dispersion ◽  
Harmonic Approximation ◽  
Generalized Gradient ◽  
Vibrational Free Energy

The electronic structures, lattice dynamics and thermodynamic properties of rare-earth intermetallic ScCd alloy are studied by the first-principles plane-wave pseudopotential method within the generalized gradient approximation in the framework of density functional pertubation theory. The band structure, density of states, phonon dispersion frequencies, vibrational free energy [Formula: see text], specific heat capacity [Formula: see text] and entropy are studied between 0 K and 1500 K. Finally, using the calculated phonon density of states, the thermodynamic properties are determined within the quasi-harmonic approximation and a value of 47.9 (J/mol⋅K) at 300 K for specific heat capacity of ScCd is predicted.

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