Effect of halogen ions on low thermal conductivity of cesium halide perovskite

The lattice dynamics of CsSnX3 (X = Cl, Br, and I) and CsPbI3, which are low-thermal-conductivity materials, are investigated using first-principles phonon calculations. Because of the strong lattice anharmonicity and the accompanying instability of high-temperature cubic phases, the self-consistent phonon theory, which can incorporate the effect of lattice anharmonicity at a mean-field level, is applied in this study. The calculated lattice thermal conductivity reproduced a low thermal conductivity, as shown experimentally, owing to the short phonon lifetime due to the incoherent scattering contribution of Cs atoms. The halogen ion dependence on thermal conductivity reveals that CsSnCl3 exhibits an anomalous lattice thermal conductivity that is as low as that of CsSnBr3. This indicates that the lattice dynamics cannot be explained merely in terms of the atomic mass of the compounds. The low thermal conductivity of CsSnCl3 is caused by the exceptionally short phonon lifetime; further, a bonding analysis suggests that covalent bonding contributes significantly to the unusual anharmonicity of CsSnCl3

Ab initio calculation for electronic structure and optical property of tungsten carbide in a TiCN-based cermet for solar thermal applications

We present an ab initio calculation to understand electronic structures and optical properties of a tungsten carbide WC being a major component of a TiCN-based cermet. We found that the WC has a fairly low-energy plasma excitation $\sim$0.6 eV (2 $\mu$m) and therefore can be a good constituent of a solar selective absorber. The evaluated figure of merit for photothermal conversion is prominently high compared to those of the other materials included in the TiCN-based cermet. The imaginary part of the dielectric function is considerably small around the zero point of the real part of the dielectric function, corresponding to the plasma excitation energy. Therefore, a clear plasma edge appeared, ensuring the high performance of the WC as the solar absorber.Comment: 13pages, 8 figures, 2table

First Observation of Superlattice Reflections in the Hidden Order at 105 K of Spin–Orbit Coupled Iridium Oxide Ca5Ir3O12

We report the inelastic X-ray scattering (IXS) experimental results of iridium oxide Ca5Ir3O12 with a strong spin–orbit interaction, showing the hidden order at 105 K where no superlattice reflections were observed so far. We measured the IXS spectra of Ca5Ir3O12 along Γ–A, Γ–M, Γ–K–M, M–L, and K–H directions in the Brillouin zone of a hexagonal lattice down to 20 K. The obtained phonon spectra show almost no change on cooling; there are no soft phonon modes. However, the superlattice reflections specified by wavevector q=(1/3,1/3,1/3) are observed below 105 K. For the order parameter in the hidden order, the characteristic on intensity for observed superlattice reflections can lead to the irreducible representation A2 order parameter in the point group 31m. Furthermore, the theoretical study indicates that the hidden order at 105 K comes from an electric toroidal dipole or higher-order multipole ordering

Anisotropic Optical Conductivity Accompanied by a Small Energy Gap in One-Dimensional Thermoelectric Telluride Ta4SiTe4

We investigated the optical properties of single crystals of one-dimensional telluride Ta4SiTe4, which shows high thermoelectric performance below room temperature. Optical conductivity estimated from reflectivity spectra indicates the presence of a small energy gap of 0.1-0.15 eV at the Fermi energy. At the lowest energy, optical conductivity along the Ta4SiTe4 chain is an order of magnitude higher than that perpendicular to this direction, reflecting the anisotropic electron conduction in Ta4SiTe4. These results indicate that coexistence of a very small band gap and anisotropic electron conduction is a promising strategy to develop a high-performance thermoelectric material for low temperature applications.Comment: 6 pages, 4 figure

Thermodynamic stability of Mg-based laves phases

To investigate the stability of various Mg-based Laves phases, the formation enthalpy and phonon dispersion were obtained by first-principles calculation. The calculated formation enthalpy and phonon dispersion indicate that MgX2 (X = Al, Co, Ni, Cu, Zn) and Mg2X (X = Ca, Sr, Y, Ba, La) are stable both statically and dynamically. These results are consistent with the experimental results except for MgAl2 and Mg2La. These compounds are considered to be in a metastable state in each binary system. We also used the cluster expansion method to examine the possibility of adding a third element to MgZn2. Our theoretical investigations suggest attractive interaction between Zn and a third element such as Ag, Ca, and Zr in the MgZn2 lattice. However, Ca and Zr replace a small amount of Zn in MgZn2 owing to the instability of MgCa2 and MgZr2, in agreement with the experimental result. Furthermore, it is suggested that Zr becomes stable at the Mg site in the MgZn2 lattice owing to the stability of ZrZn2

Ab initio calculation for electronic structure and optical property of tungsten carbide in a TiCN-based cermet for solar thermal applications

Abstract We present an ab initio calculation to understand electronic structures and optical properties of a tungsten carbide WC being a major component of a TiCN-based cermet. The TiCN-based cermet is widely used as a cutting tool, and is discarded as usual after use. On the other hand, cermet itself is also a famous ingredient of a solar absorption film. We found that the WC has a fairly low-energy plasma excitation $$\sim$$ ∼ 0.6 eV (2 \upmu μ m) and therefore can be a good constituent of a solar selective absorber. The evaluated figure of merit for photothermal conversion is prominently high compared to those of the other materials included in the TiCN-based cermet. The imaginary part of the dielectric function is considerably small around the zero point of the real part of the dielectric function, corresponding to the plasma excitation energy. Therefore, a clear plasma edge appeared, ensuring the high performance of the WC as the solar absorber. This is a fascinating aspect, because the wasted TiCN-based cermet cutting tool can be recycled as the solar absorption film after proper treatments and modifications

Role of substituted atoms in stacking fault formation in long-period stacking ordered system

To study the formation mechanism of the long-period stacking ordered (LPSO) structures, the reaction pathways of solid–solid transformations from a hexagonal close-packed (HCP) structure to LPSO structures in Mg-Y-Zn alloys were calculated using the generalized solid-state nudged elastic band method. The energy increases along the transition from HCP to 18R, and the peak positions represent the activation energy for the transition. Y substitution hardly changes the activation energy but makes the 18R-type LPSO structure more stable than HCP. In contrast, Zn or Y + Zn substitution results in higher activation energy and makes the 18R-type LPSO structure less stable than HCP. The calculated results for 14H and 24R LPSO structures also show similar activation energy and LPSO stability to the HCP-18R transition. Therefore, Y substitution plays an important role in stabilizing the stacking faults in LPSO systems. For the microscopic mechanism, the volume dependence of the total energy in pure FCC and HCP Y were examined, and the result suggests that FCC-Y is stable than HCP-Y under pressure. Therefore, the effect of substitution of Y in HCP Mg can be explained by the characteristics of Y under the chemical pressure exerted by the small size of Mg lattice

Thermodynamic Stability of Mg-Based Laves Phases

To investigate the stability of various Mg-based Laves phases, the formation enthalpy and phonon dispersion were obtained by first-principles calculation. The calculated formation enthalpy and phonon dispersion indicate that MgX2 (X = Al, Co, Ni, Cu, Zn) and Mg2X (X = Ca, Sr, Y, Ba, La) are stable both statically and dynamically. These results are consistent with the experimental results except for MgAl2 and Mg2La. These compounds are considered to be in a metastable state in each binary system. We also used the cluster expansion method to examine the possibility of adding a third element to MgZn2. Our theoretical investigations suggest attractive interaction between Zn and a third element such as Ag, Ca, and Zr in the MgZn2 lattice. However, Ca and Zr replace a small amount of Zn in MgZn2 owing to the instability of MgCa2 and MgZr2, in agreement with the experimental result. Furthermore, it is suggested that Zr becomes stable at the Mg site in the MgZn2 lattice owing to the stability of ZrZn2