Debye Temperature θD Research Articles

Vibrational and cohesive properties in 4d and 5d transition metals: systematics and interrelations.

Motivated by the striking regularities between experimental quantities related to the vibrational properties of the 4d and 5d series early noted by Fernández Guillermet and Grimvall [Phys. Rev. B, 1989, 40(3), 1521], a systematic theoretical study has been made of the vibrational density of states (VDOS) of Ag, Au, Pd, Pt, Rh, Ir, Ru, Os, Tc, Re, Mo, W, Nb, Ta, Zr and Hf. The frequency moments ωD(j), expressed as Debye temperatures θD(j) for 0 ≤ j ≤ 100 were evaluated. Various remarkable correlations between these quantities are reported, which have their roots in the relations of homology between the VDOS. From the θD(j), several k(j) quantities with dimensions of force constants are calculated. For the elements 4d and 5d transition series, these quantities are shown to be strongly correlated, with independence of the value of j. It is suggested that the various interrelations between the θD(j) and k(j) parameters arrived at in the current work have their roots in the homologous variations of the cohesion forces across the 4d and 5d transition series, as revealed by thermophysical properties such as the bulk modulus and the cohesive energy.

Ultralow thermal conductivity of W-Janus bilayers (WXY: X, Y = S, Se, and Te) for thermoelectric devices.

Lattice thermal conductivity (κ) in tungsten dichalcogenide Janus (WXY, where X, Y = S, Se, and Te) monolayers and heterostructures (HSs) have been investigated using ab initio DFT simulations. Tungsten-based Janus monolayers show semiconducting behavior with the bandgap in the semiconducting range for WSSe (1.70 eV), WSTe (1.26 eV), and WSeTe (1.34 eV). When Janus monolayers are stacked to form HSs with weak van der Waals (vdW) interactions, the bandgap reduces to 0.19 eV, 0.40 eV, and 0.24 eV, respectively, for WSeTe/WSTe, WSSe/WSTe, and WSSe/WSeTe HSs. Thermal vibrational characteristics of Janus monolayers are modified when these are stacked in 2D HSs with the introduction of interlayer hybrid phonon modes. Large longitudinal-transverse optical (LO-TO) splitting is noticed at the Brillouin zone-center (Γ-point): 135 cm-1, 140 cm-1, and 150 cm-1 for WSeTe/WSTe, WSSe/WSeTe and WSSe/WSTe HSs, respectively. Thermal conductivity calculations show ultra-low κ values for WSeTe/WSTe (0.01 W m-1 K-1), WSSe/WSTe (0.02 W m-1 K-1) and WSSe/WSeTe HS (0.004 W m-1 K-1) at 300 K. The results can be attributed to the hybrid phonon modes with frequencies very close to acoustic modes at the gamma point, low Debye temperature (θD) and specific heat capacity. Our results highlight the possible applications of these HSs in designing thermoelectric interfaces at the nanoscale.

Structural, thermodynamic, and magnetic properties of SrFe12O19 hexaferrite modified by co-substitution of Cu and Gd.

A hard magnetic system of SrFe12O19 nanomaterial was modified according to the composition of Sr0.95Gd0.05Fe12-xCuxO19 with x = 0.0, 0.30, and 0.60 using the sol-gel technique. The structures of the samples were evaluated using X-ray diffraction (XRD) along with Rietveld refinement, and an M-type hexaferrite with a hexagonal structure was confirmed with a trace amount of the α-Fe2O3 phase. In addition, transmission electron microscopy (TEM) analysis revealed polycrystalline nanoplates in all samples. Furthermore, the bond structures of the octahedral and tetrahedral sites along with the thermodynamic properties of these ferrites were extracted from the FTIR spectra at room temperature. The Debye temperature (θD) decreased from 755.9 K to 749.3 K due to the co-substitution of Gd3+ at Sr2+ and Cu2+ at Fe3+. The magnetic hysteresis (M-H) measurements revealed that the coercivity decreased from 5.3 kOe to 1.5 kOe along with the highest magnetization saturation (Ms) of 65.2 emu g-1 for the composition Sr0.95Gd0.05Fe11.7Cu0.3O19, which is suitable for industrial application. The effect of local crystalline anisotropy in magnetization was explored using the law of approach to saturation (LAS). Finally, thermo-magnetization was recorded in the range from 400 K to 5 K for cooling under zero field and in the presence of a 100 Oe field, and magnetic transitions were tracked due to the introduction of the foreign atoms of Gd and Cu into SrFe12O19.

Lattice thermal conductivity of ZnO: experimental and theoretical studies.

ZnO has been explored using different approaches such as doping, nanostructuring, 2D confinement, and introduction of interface effects for improving thermoelectric performance and lowering thermal conductivity. Herein, the lattice thermal conductivity (κL) of ZnO determined from Raman thermometry, the 3ω-method and simulation using three-phonon scattering is presented. The average Debye temperature (θD) of A1(TO) and E2(high) modes estimated utilizing bond-order-length-strength correlation with local bond averaging effects is ∼422 K. The average κL of ZnO calculated using the theoretical coefficient of Slack's equation, θD, and Grüneisen parameter (γ) in Slack's equation is 2.75 W m-1 K-1, which is significantly lower than the κL ∼ 50.9 W m-1 K-1 simulated by considering the Perdew-Burke-Ernzerhof functional with Hubbard and non-analytical corrections. The coefficient of Slack's equation is determined from the κL ∼ 31 W m-1 K-1 of ZnO measured using the 3ω-method for the accurate estimation of the κL. The experimental coefficient of Slack's equation with Raman thermometry data yields κL ∼ 50.6 W m-1 K-1, which is higher than the value obtained using the 3ω-method but consistent with the theoretical value. Thus, three-phonon anharmonicity describes the κL of ZnO associated with Raman scattering. The method adopted to calculate κL will help for the in-depth analysis of phonon dynamics and the design of wurtzite-ZnO-related power electronics.

A study on the magnetothermal properties and magnetocaloric effect in [formula omitted] ([formula omitted

In this study, we investigate the Gd1−xCexMn2Ge2 system with x=0.17 and x=0.33 using the mean field theory. Ab-initio based calculations are used to determine the density of states at Fermi energy, D(Ef), and Debye temperature θD for GdMn2Ge2 main compound. Our aim is to determine key properties such as magnetization, magnetic heat capacity, and the magnetocaloric effect main aspects including both isothermal entropy change and adiabatic temperature change. We examine these parameters under various conditions by applying different field changes up to 6 T and at temperatures spanning a range up to 400 K. The two systems have a Curie temperature, that is around room temperature (around 330 K). Using the trapezoidal method, the Gd1−xCexMn2Ge2 systems exhibit a maximum ΔSm value of 0.79 J/mol K for ΔH=6 T, and a ΔT value of an average 1.3 K/Tesla. In our study, the relative cooling power (RCP) was investigated and moreover, we examined the nature of phase transitions using universal curves and Arrott plots to confirm that the nature of phase transition at Tc in this system corresponds to a second-order phase transition (SOPT). The influence of high magnetic fields on the magnetization was also examined, and it showed first-order phase transition (FOPT) at low temperatures and high fields. Additionally, due to FOPT, a change in magnetic order from ferrimagnetism to ferromagnetism took place.

Numerical simulation studies of the new quaternary MAX phase as future engineering applications: The case study of the Nb2ScAC2 (A = Al, Si) compounds

Recently, MAX phases have attained considerable technological interest owing to their two inherent properties metallic and ceramic properties. This study extensively examined Nb2ScAC2 MAX phases using DFT, to assess the structural, mechanical, electronic, and Thermal characteristics. Firstly, the stability of these two compounds was confirmed through the formation energy, elastic constants (Cij), and phonon band structure, which confirmed their thermodynamic, mechanical, and dynamical stability. The optimized lattice parameters of these compounds were examined and then utilized to calculate the physical properties of the Nb2ScAC2 compound. Our compounds are brittle due to their Pugh’s ratio of less than 1.75. The covalent bonding of the structure revealed by the Poisson ratio is less than 0.25 for the two compounds. The Nb2ScAC2 material is anisotropic, and Nb2ScAlC2 is harder than Nb2ScSiC2.The metallic character of the materials was affirmed by the electronic band structure analysis. Calculated thermal properties such as Debye temperature and minimum and lattice thermal conductivity reveal that both compounds have the potential to enhance their deployment in thermal barrier coating materials. On the other hand, the high melting temperatures indicate that our compounds could potentially be utilized in demanding or severe conditions. Finally, the thermodynamic characteristics, comprising the isochoric heat capacity (Cv) and Debye temperature (ϴD) were analyzed subjected to high temperatures and pressures. The optical constants such as real and imaginary parts of the dielectric function, refractive index and reflectivity, are investigated. The current study recognizes these two compounds as promising candidates for utilization in modern technologies and diverse industries.

Investigating the thermal properties of CrN: Theoretical insights and real ceramics

Ab-initio calculations of lattice heat capacity and phonon thermal conductivity have been performed for CrN of rock-salt structural type (paramagnetic) and for the orthorhombic antiferromagnetic (AFM) phase below TN ≈ 285 K. The comparison with experimental data below ≈30 K unveils that in contrary to the theoretically calculated heat capacity following the βT3 term with Debye temperature ΘD = 830 K, a large excess of experimental heat capacity in a form of δT2 is observed. This additional heat capacity is ascribed to magnons in the specific layered-type AFM arrangement of CrN. The experimental thermal conductivity shows a similar low-temperature excess of T2-dependence. Considering that mean free path of heat carriers is essentially limited by the grain size and calculated phonon conductivity itself is very low below ≈30 K, such additional contribution can be ascribed again to AFM magnons though theoretical model of such conductivity is still to be assessed. In spite of this complexity, our model of phonon thermal conductivity reproduces well the maximum of 7.2 Wm−1K−1 at 150 K followed by a decline tied to phonon-phonon scattering (Umklapp process). Nonetheless at higher temperatures, the experimental data evidence sudden drop at TN to ≈2 Wm−1K−1 with only slightly varying thermal conductivity at elevated temperatures, which reminds rather a behavior of amorphous or defective solids. It is proposed that atomic displacements that stabilize spin-parallel and spin-antiparallel Cr pairs in the orthorhombic AFM phase persist partially as fluctuating ones in the cubic paramagnetic state above TN, which gives rise to dynamic strain fields that cause strong phonon scattering.

Structural, Electronic, and Mechanical Properties of Ag‐Cu‐Ga Intermetallic Compounds: First‐Principles Calculations

AbstractIn this paper, the mechanical properties, electronic structures, and thermodynamic properties of Ag2Ga and CuGa2 under pressure are studied using the first‐principles method based on Density functional theory (DFT). With the increase in pressure, the equilibrium lattice constants (a/a0 and c/c0) and volume ratio (V/V0) of Ag2Ga and CuGa2 continuously decrease. The elastic constants show that Ag2Ga and CuGa2 are mechanically stable under pressures of 0–20 GPa, and the increase in pressure improves their deformation resistance. The results of Young's modulus (E) show that the hardness of CuGa2 is higher than that of Ag2Ga. The results of Cauchy pressure (C12 ‐ C44) and B/G ratio show that the plasticity of Ag2Ga and CuGa2 can be maintained with the increase in pressure. By analyzing the density of states (DOS), charge density difference (CDD), and population, the influence of pressure on electronic structure is studied. The calculated Debye temperature (θD) shows that the covalent bond of CuGa2 is stronger.

Influence of pressure on the structural, elastic and thermodynamic properties of α- and β- PtAl high temperature alloys

Although PtAl alloy is a promising high-temperature material, the mechanical properties and thermodynamic behavior of PtAl alloy under high pressure is entirely unknown. To solve this problem, here, we apply the first-principles calculations to study the influence of high pressure on the structural stability, mechanical and thermodynamic properties of PtAl alloy. Here, the α-PtAl and β-PtAl alloys under high pressure are selected. The calculated results show that α-PtAl has better thermodynamic stability in comparison to β-PtAl under pressure. Whether zero pressure or high pressure, this α-PtAl is a dynamical stability, in while the β-PtA is a dynamical instability. Although the elastic constants and elastic modulus of two PtAl alloys increase with increasing pressure, the ductility of two PtAl becomes better. Importantly, it is found that the calculated shear modulus and Young's modulus of α-PtAl are higher than the β-PtAl under high-pressure. Naturally, the high elastic modulus of PtAl is demonstrated by the change of Pt–Al bond under high-pressure. With increasing pressure, the bond length of Pt–Al bond becomes shorter whether α-PtAl or β-PtAl. Finally, it is found that the calculated Debye temperature (θD) of α-PtAl is higher than the β-PtAl under high pressure.

First-principles screening of structural, electronic, optical and elastic properties of Cu-based hydrides-perovskites XCuH3 (X=Ca and Sr) for hydrogen storage applications

In the present study, density functional theory is used to conduct an in-depth investigation of the physicochemical features of the perovskite XCuH3 (X = Ca and Sr). The CaCuH3 and SrCuH3, with lattice parameters of 3.66 Å and 3.77 Å, are synthesizable and thermodynamically stable, according to structural simulations. Analysis of the electronic band structure (0 eV) and density of states (DOS) shows that XCuH3 (X = Ca and Sr) perovskites exhibit metallic behaviour. For novel polycrystalline materials, the shear and Young's moduli as well as the Poisson's ratio are computed and the findings suggested that compounds with the formula XCuH3 (X = Ca and Sr) are ductile. Hydrogen storage qualities have been analyzed and it has been shown that the CaCuH3 and SrCuH3 have gravimetric hydrogen storage capacities of 2.85 wt% and 1.97 wt%, respectively. The SrCuH3 has the highest Debye temperature (θD) 344.41 K. The findings, taken as a whole, provide a practical strategy for developing innovative, potentially useful perovskite-type hydrides for hydrogen storage.

Designing Quaternary and Quinary Refractory-Based High-Entropy Alloys: Statistical Analysis of Their Lattice Distortion, Mechanical, and Thermal Properties

The rapid evolution in materials science has resulted in a significant interest in high-entropy alloys (HEAs) for their unique properties. This study focuses on understanding both quaternary and quinary body-centered cubic (BCC) of 12 refractory-based HEAs, and on analysis of their electronic structures, lattice distortions, mechanical, and thermal properties. A comprehensive assessment is undertaken by means of density functional theory (DFT)-based first principles calculations. It is well known that multiple constituents lead to notable lattice distortions, especially in quinary HEAs. This distortion, in turn, has significant implications on the electronic structure that ultimately affect mechanical and thermal behaviors of these alloys such as ductility, lattice thermal conductivity, and toughness. Our in-depth analysis of their electronic structures revealed the role of valence electron concentration and its correlation with bond order and mechanical properties. Local lattice distortion (LD) was investigated for these 12 HEA models. M1 (WTiVZrHf), M7 (TiZrHfW), and M12 (TiZrHfVNb) have the highest LD whereas the models M3 (MoTaTiV), M5 (WTaCrV), M6 (MoNbTaW), and M9 (NbTaTiV) have the less LD. Furthermore, we investigated the thermal properties focusing on Debye temperature (ΘD), thermal conductivity (κ), Grüneisen parameter (γα), and dominant phonon wavelength (λdom). The NbTaTiV(M9) and TiVNbHf(M10) models have significantly reduced lattice thermal conductivities (κL). This reduction is due to the mass increase and strain fluctuations, which in turn signify lattice distortion. The findings not only provide an understanding of these promising materials but also offer guidance for the design of next-generation HEAs with properties tailored for potential specific applications.

DFT based computational investigations of the physical properties of perovskites-structure SrXO3 (X = Si, Tb, Th) for optoelectronic and thermo-mechanical applications

Density Functional Theory (DFT) method is employed for investigating the structural, elastic, electronic, optical, and thermal properties of Perovskite structure SrXO3 (X = Si, Tb, and Th). The studied lattice parameters show good agreement with the previous theoretical and experimental results. The elastic stiffness constants, (Cij) of SrSiO3, SrTbO3, and SrThO3 satisfied the born stability criteria which confirm the mechanical stability of these phases. It is observed that when Tb or Th is replaced with Si, the brittleness behaviours of SrTbO3 or SrThO3 change to ductility, which improves its machinability for industrial applications. The investigation of anisotropic factors of SrSiO3, SrTbO3, and SrThO3 ensured that SrSiO3 is more anisotropic than SrTbO3 and SrThO3. By evaluating the electronic properties, it can be advantageous for applying the topological insulators (TI), solar cells, or PV (Photovoltaic) cell fabrication. Furthermore, the optical properties are studied and detail discussed with SrXO3 (X = Si, Tb, and Th). The reason of low Debye temperature (ΘD) and low minimum thermal conductivity (Kmin) has been perfectly explicated through the consideration of the mean atomic weight (M/n) of the compounds. Temperature-dependent of heat capacities (Cv, Cp) and linear thermal expansion coefficient (α) are also estimated using the quasi-harmonic Debye model and discussed. The SrTbO3 and SrThO3 compounds can be employed as potential thermal barrier coating (TBC) materials for high-temperature applications, according to the low values of Kmin, ΘD and α.

Exploring the potential of inorganic cubic halide perovskites RbSrM3 (M=Cl, Br) for advanced optoelectronic applications: A DFT study

Halide perovskites, being a large family attained researchers focus for multiple targets because of their outstanding properties and flexible chemistry. In this study, density functional theory (DFT) based on WIEN2K code were utilized, to thoroughly explore the structural, electronic, optical, and mechanical properties of inorganic cubic halide perovskites, i.e.; RbSrM3 (M = Cl, Br) for optoelectronic applications. Variation in the lattice parameters and unit cell volume were observed, as a result of halogen atom substitution from Cl with Br. Subsequently, the electronic characteristics certify the direct bandgap nature, with bandgap values of 7.73 eV for RbSrCl3 and 6.79 eV for RbSrBr3, respectively. Optical properties proposed the high optical absorption, high conductivity, and low reflectivity. Further analysis into the mechanical properties, including bulk modulus (B), shear modulus (G), young modulus (E), anisotropic factor (A), Poison’s ratio (ν), and Pugh’s ratio (B/G), comply the near ductility, stiffness, and elastically anisotropic behavior, confirmed the stability and reliability of the designed compound. Debye temperature (θD), showing high capability to withstand against heat produced by lattice vibrations, further associated the thermodynamic stability that support the formation energy calculations. Based on the findings, it is confidently suggested that investigated halide perovskite materials i.e.; RbSrM3 (M = Cl, Br), are promising candidates for next-generation photodetectors and optoelectronic devices.

Mechanical stability criterions of cubic Na2He up to 500 GPa

The Cauchy pressure Cp was usually used to explain the atomic bonding character in a compound, while the mechanical stability criterions are usually used to study the stability of the structure under external compression. Using the elastic constants reported recently by Zahidur Rahaman et al. in Chinese Journal of Physics, 56 (2018) 231-237, as well as different mathematical formula, we reported on the Cauchy violation and the generalized mechanical stability criterions of cubic Na2He compound under high hydrostatic compression from 100 GPa to 500 GPa. For pressures ranging from 100 to 500 GPa, all values of both Cauchy pressure Cp and the generalized mechanical stability criteria G are negative. In addition we calculated the Debye temperature θD of Na2He compound using Siethoff’s approach. The results obtained are slightly lower than those obtained from the Debye approach reported by Zahidur Rahaman et al.

In-depth analysis of ternary NaCuX (X = Se and Te) chalcogenides: Bridging structural elastic, electronic, and optical properties

This study employs density functional theory (DFT) using the full potential linearized augmented plane wave plus local orbital (FP-LAPW+1o) approach to investigate the structural, elastic, electronic, and optical properties of NaCuX (X = Se and Te) compounds. Additionally, the modified Becke-Johanson (TB-mBJ) potential is applied to ensure accurate band gap results. Both compounds exhibit semiconducting behavior with a direct band gap at the Γ point. The computed elastic constants confirm mechanical stability for NaCuSe and NaCuTe, as evidenced by their respective bulk modulus, Young modulus, shear modulus, Poisson's ratio, B/G ratio, and elastic anisotropy. The materials display ductile characteristics, indicated by their B/G ratio and Poisson's ratio. The Debye temperature (θD) reveals that NaCuSe possesses the highest thermal conductivity. A decrease in θD is observed from NaCuSe to NaCuTe due to declining elastic constants. Additionally, anisotropy in acoustic velocities is assessed and discussed. The study's equilibrium structural properties align well with experimental data. Optical properties, including frequency-dependent dielectric function, refractive index, extinction coefficient, absorption coefficient, reflectivity, and energy loss function, are calculated and comprehensively analyzed.

Zn-Mo-O system: Insights into the thermodynamic stability and applicability in lithium ion batteries

A detailed experimental investigation on the thermochemical stability of Zn2Mo3O8(s) in the Zn-Mo-O system was reported for the first time using high temperature oxide melt solution calorimetry in conjunction with solid oxide based electrochemical measurements. The temperature dependence of molar heat capacity of this compound was determined employing thermal relaxation calorimetry and differential scanning calorimetry. The values of standard molar enthalpy of formation (∆fH298.15°), standard molar entropy (S298.15°), molar heat capacity (Cp) and Debye temperature (θD) for Zn2Mo3O8(s) were determined for the first time in this study. The present study also focuses on the investigation of electrochemical performance of α-ZnMoO4(s) for lithium ion battery applications which demonstrated the effective utilization of this compound as anode material due to its remarkable cycling stability and rate capability.

Mass density, sound velocity and Debye temperature of CaTe semiconductor under high compression

Using the structural parameters and the elastic stiffness constants reported in the literature (by Guo et al. in Solid State Commun., 340, 114488 (2021)) as well as different appropriate analytical and semi-empirical expressions, we reported the high compression effect up to pressure of 27.8 GPa on the average sound velocity and Debye temperature θD of cubic rocksalt calcium telluride (CaTe) semiconducting compound. The pressure dependence of the Debye temperature θD for CaTe compound was obtained using two different approaches. We found that both the average sound velocity and Debye temperature of CaTe compound increase monotonously and non-linearly with enhanced pressure up to 27.8 GPa. We note that similar behavior on the average sound velocity and Debye temperature θD was also observed in the literature for the same compound and for other materials with different crystallographic structures. The deviations on θD between the two different approaches and to those of the literature are quite large at high pressures.

Ab initio study of mechanical and thermal properties of GeTe-based and PbSe-based high-entropy chalcogenides

GeTe-based and PbSe-based high-entropy compounds have outstanding thermoelectric (TE) performance and crucial applications in mid and high temperatures. Recently, the optimization of TE performance of high-entropy compounds has been focused on reducing thermal conductivity by strengthening the phonon scattering process to improve TE performance. We report a first-principles investigation on nine GeTe-based high-entropy chalcogenide solid solutions constituted of eight metallic elements (Ag, Pb, Sb, Bi, Cu, Cd, Mn, and Sn) and 13 PbSe-based high-entropy chalcogenide solid solutions: Pb0.99-ySb0.012SnySe1-2xTexSx (x = 0.1, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, and y = 0) and Pb0.99-ySb0.012SnySe1-2xTexSx (y = 0.05, 0.1, 0.15, 0.2, 0.25 and x = 0.25). We have investigated the mechanical properties focusing on Debye temperature (ΘD), thermal conductivity (κ), Grüneisen parameter (γα), dominant phonon wavelength (λdom), and melting temperature (Tm). We find that the lattice thermal conductivity is significantly reduced when GeTe is alloyed into the following compositions: Ge0.75Sb0.13Pb0.12Te, Ge0.61Ag0.11Sb0.13Pb0.12Bi0.01Te, and Ge0.61Ag0.11Sb0.13Pb0.12Mn0.05Bi0.01Te. This reduction is due to the mass increase and strain fluctuations. The results also show that Ge0.61Ag0.11Sb0.13Pb0.12Bi0.01Te solid solution has the lowest Young’s modulus (30.362 GPa), bulk and shear moduli (18.626 and 12.359 GPa), average sound velocity (1653.128 m/sec), Debye temperature (151.689 K), lattice thermal conductivity (0.574 W.m–1.K–1), dominant phonon wavelength (0.692 Å), and melting temperature (535.91 K). Moreover, Ge0.61Ag0.11Sb0.13Pb0.12Bi0.01Te has the highest Grüneisen parameter with a reduced and temperature-independent lattice thermal conductivity. The positive correlation between ΘD and κ is revealed. Alloying of PbSe-based high-entropy by Sb, Sn, Te, and S atoms at the Se and Pb sites resulted in much higher shear strains resulted in the reduction of phonon velocity, a reduced ΘD, and a lower lattice thermal conductivity.

Investigating the phase transition and properties of CaSiN2 under pressure based on first-principles calculations

In this study, we apply first-principles calculations to examine the pressure-induced phase transformation of CaSiN2 in a range of pressure of 0–100 GPa. Its pressure-induced transitions at 1.3 GPa, 15.3 GPa, and 55.8 GPa followed the order of α- CaSiN2→β- CaSiN2→α- CaSiN2→γ- CaSiN2, for α- CaSiN2→β- CaSiN2, β- CaSiN2→α- CaSiN2, and α- CaSiN2→γ- CaSiN2, respectively. The stability of the phases of CaSiN2 was confirmed based on calculations of the Born criterion of elastic stability. Its behavior transitioned in the sequence of brittle (0–1.3 GPa) → ductile (1.3–55.8 GPa) → brittle (55.8–100 GPa). The structure of its projected orbital band reflected insulating behavior by CaSiN2 under a range of pressure of 0–55.8 GPa with a direct band gap, which transformed into metallic behavior by the γ- CaSiN2 phase under pressures higher than 55.8 GPa, due to a shift in energy to higher levels around the Γ point of the N p orbitals and Si p orbitals. The Si-N bonds in CaSiN2 were found to be covalent, while ionic bonding dominated the Ca-Si and Ca-N bonds in the range of pressure of 0–100 GPa. We also investigate and discuss its mechanical properties, Vickers hardness (Hv), average sound velocity vm, and Debye temperature (θD).

A comprehensive investigation on CdO-SnO2 system: Structure, thermal expansion and energetics

An extensive investigation into the thermochemical stability of two ternary oxides, ilmenite-type CdSnO3(s) and orthorhombic Cd2SnO4(s), in the Cd-Sn-O system was reported for the first time using high temperature oxide melt solution calorimetry in conjunction with differential scanning calorimetry and thermal relaxation calorimetry. The values of standard molar enthalpy of formation (∆fH298.15°), standard molar entropy (S298.15°), molar heat capacity and Debye temperature (θD) for ilmenite-type CdSnO3(s) and orthorhombic Cd2SnO4(s) were determined. Using the experimentally obtained thermochemical data, the standard molar Gibbs energy of formation of these compounds were derived and the corresponding expressions are given as follows:∆fGm°<CdSnO3>±1.3kJmol−1=−847.9+0.2781(T/K)(T:298.15−1000K)∆fGm°<Cd2SnO4>±3.6kJmol−1=−1138.1+0.3831(T/K)(T:298.15−1000K)Moreover, high temperature XRD experiments were conducted to evaluate their thermal expansion behaviour as well as for the in-situ identification of phase evolution resulting from decomposition of cadmium stannates.