arXiv subjects

K. C. Bhamu

Publications and source records attributed to K. C. Bhamu.

11 recordsLinked to original sources

arXiv2026

Predicted High $n$-Type $zT$ and Ultralow Lattice Thermal Conductivity in A$_2$AgIrCl$_6$ (A = Cs, Rb)

A useful thermoelectric device must impede heat flow without impeding charge transport. Here, we examine how closely cubic Cs$_2$AgIrCl$_6$ and Rb$_2$AgIrCl$_6$ approach this balance using first-principles calculations of structural stability, chemical bonding, elastic response, lattice dynamics, and scattering-resolved carrier transport. Both materials satisfy the cubic elastic-stability criteria, and neither harmonic phonon spectrum contains an imaginary mode. Replacing Cs with Rb mainly exerts chemical pressure: the lattice contracts by 1.34\% and the Ag--Cl and Ir--Cl bonds strengthen, whereas the band-edge topology changes little. HSE06 calculations including spin--orbit coupling yield direct X-point gaps of 1.597 and $1.637\,\mathrm{eV}$ for Cs$_2$AgIrCl$_6$ and Rb$_2$AgIrCl$_6$, respectively. The three symmetry-equivalent X valleys have light electron masses of $0.43$--$0.57\,m_0$, whereas the hole masses span $2.10$--$4.68\,m_0$. For Cs$_2$AgIrCl$_6$ and Rb$_2$AgIrCl$_6$, respectively, the modified Debye--Callaway model gives lattice thermal conductivities of 0.346 and $0.428\,\mathrm{W\,m^{-1}\,K^{-1}}$ at 300 K, decreasing to 0.118 and $0.150\,\mathrm{W\,m^{-1}\,K^{-1}}$ at 800 K. Treating acoustic-deformation-potential, ionized-impurity, and polar-optical-phonon scattering with AMSET gives peak $n$-type $zT$ values of 2.81 and 2.36 at 800 K near $6\times10^{19}\,\mathrm{cm^{-3}}$. This response arises from the convergence of light, valley-degenerate electrons, intermediate doping, and weak lattice heat transport rather than from a single exceptional coefficient. The predicted values are experimentally testable targets, contingent on retaining the cubic phases and controlled electron doping at elevated temperatures.

cond-mat.mtrl-sci↗

arXiv2026

Anisotropic Phonon Heat Flow and Thermoelectric Response in Tetragonal GeS$_2$ and GeSe$_2$

The electronic structure, lattice dynamics, bonding, elastic response, and anisotropic thermoelectric transport properties of tetragonal GeS$_2$ and GeSe$_2$ were investigated using density functional theory, density functional perturbation theory, Wannier interpolation, and scattering-aware Boltzmann transport. The relaxed structures are mechanically and dynamically stable within the calculated harmonic description. The HSE03/Wannier band gaps are 2.48 eV for GeS$_2$ and 1.23 eV for GeSe$_2$, while substitution of S by Se lowers the upper phonon frequency from approximately 13.6 to 10.3 THz. The phonon Boltzmann transport calculations reveal pronounced lattice-transport anisotropy. Within the relaxation-time approximation, the 300 K in-plane and cross-plane lattice thermal conductivities are 26.86 and 1.19 W m$^{-1}$ K$^{-1}$ for GeS$_2$, and 18.74 and 1.52 W m$^{-1}$ K$^{-1}$ for GeSe$_2$, respectively. At 800 K, these values decrease to 10.22 and 0.46 W m$^{-1}$ K$^{-1}$ for GeS$_2$, and 7.25 and 0.58 W m$^{-1}$ K$^{-1}$ for GeSe$_2$. Frequency-resolved analysis shows that low-frequency phonons carry most of the heat, whereas the small cross-plane values reflect restricted out-of-plane phonon transport. Combining the ShengBTE RTA lattice tensors with AMSET electronic coefficients gives $zT=0.257$ for n-type cross-plane GeS$_2$ at 800 K and $10^{19}$ cm$^{-3}$. The corresponding PBE-AMSET estimate for GeSe$_2$ is $zT=0.066$ for p-type cross-plane transport at 800 K and $3\times10^{20}$ cm$^{-3}$. LOBSTER analysis identifies mixed covalent--ionic Ge--X bonding, with Ge--S bonds having a larger stabilizing ICOHP magnitude than Ge--Se bonds ($-5.27$ versus $-4.74$ eV per bond). These results identify tetragonal GeX$_2$ compounds as strongly anisotropic thermoelectrics with moderate calculated $zT$ values whose cross-plane response benefits from suppressed lattice heat transport.

cond-mat.mtrl-sci↗

arXiv2026

Distorted polyhedral architecture enabled high thermoelectric performance of columnar double halide perovskites Cs2AgPdCl5 and Cs2AgPtCl5

We investigate the thermoelectric properties of two newly synthesized columnar double halide perovskites Cs$_2$AgPdCl$_5$ and Cs$_2$AgPtCl$_5$. These materials accommodate a distorted local polyhedral architecture with tetrahedral symmetry compared to traditional double halide perovskites. By employing density functional theory along with the semiclassical transport model, we have analyzed the electronic and transport properties of these materials. Our results show that at 800 K, the largest figure of merit ($zT$) is 1.30 (0.86) for p-type (n-type) Cs$_2$AgPdCl$_5$ and 0.87 for n-type Cs$_2$AgPtCl$_5$ at doping concentrations of $1.94 \times 10^{20}$ ($3.76 \times 10^{19}$) cm$^{-3}$ and $3.52 \times 10^{19}$ cm$^{-3}$, respectively. Remarkably, a very low doping concentration is required to achieve a high $zT$, setting these materials apart from others in this field. Our calculations demonstrate that Cs$_2$AgPdCl$_5$ benefits from the presence of conduction and valence band valleys near the band edges; however, the flat bands present in the valence band of Cs$_2$AgPtCl$_5$ do not improve its thermoelectric performance. Among these systems, hole doping in Cs$_2$AgPdCl$_5$ has shown remarkable thermoelectric performance. Interestingly, the local octahedral distortions present in these perovskites contribute to a marked reduction in the lattice thermal conductivity to 0.27 W/mK in Cs$_2$AgPtCl$_5$ and 0.20 W/mK in Cs$_2$AgPdCl$_5$ by causing enhanced phonon scattering, further improving the thermoelectric figure of merit. This drop in thermal conductivity, combined with the favorable electronic properties, underscores the potential use of these materials for applications in highly efficient thermoelectric devices.

cond-mat.mtrl-sci↗

arXiv2026

Synergistic effect of the electronic band delocalization and bond anharmonicity on the thermoelectric performance of Cs2TeX6(X=Cl, Br, I)

We investigate the structural, mechanical, and thermoelectric properties of lead-free double halide perovskites Cs2TeX6 (X = Cl, Br, I) using first-principles calculations and semiclassical Boltzmann transport theory. The HSE06 band gap is incorporated using the scissor correction method along with PBE calculated electronic band structures including spin orbit coupling to accurately predict transport properties. The band gap values are 3.27, 2.50, and 1.55 eV for Cs2TeX6 (X = Cl, Br, I), respectively. The coexistence of heavy and light bands in the Cs2TeI6 band structure helps mitigate the trade-off between the Seebeck coefficient and electrical conductivity. Among these systems, Cs2TeI6 exhibits superior performance with a ZT of 1.97 at 800 K and an electronic concentration of 3.35 x 10^19 cm^-3. Such a high ZT at relatively low carrier concentration arises from high electrical conductivity combined with low lattice thermal conductivity. The lattice thermal conductivity of Cs2TeI6 is found to be 0.41 W m^-1 K^-1 at room temperature. This low lattice thermal conductivity is attributed to weak Te-I bonding and non-uniform out-of-phase displacement of Cs atoms. The presence of local TeX6 units together with weak bonds strongly resists heat conduction, leading to significant suppression of lattice thermal conductivity. In particular, transverse acoustic phonons and optical phonons play a key role in limiting lattice thermal conductivity. These results identify Cs2TeI6 as a promising candidate for high performance thermoelectric applications.

cond-mat.mtrl-sci↗

arXiv2025

Cu2XSiS4 (X = Ge, Sn, and Pb) materials for solar-cell applications: A DFT+SCAPS-1D simulation

By means of the first-principles density functional theory (DFT), I2-II-IV-VI4 type Cu-based quaternary chalcogenides Cu 2 XSiS 4 (X = Ge, Sn, and Pb) have been thoroughly investigated. We report the study of Ge and Sn substitution in the divalent cation site for their potential applications in photovoltaics for the first time. The structural, electronic, optical, and mechanical properties have been calculated. The structural and thermal stability is verified by calculating the elastic constants, formation energy and total potential energy at 300 K from the ab-initio molecular dynamics (MD) simulation. The compounds under our investigation exhibited an indirect band gap in the range of 1.0--1.56 eV, suitable for energy harvesting by trapping the sunlight. The presence of absorption peaks within the visible region complements their potential in photovoltaic applications. For further validation, we have designed a model of a heterostructure (FTO/TiO2/Cu2XSiS4/CuO/Au) solar cell, and a numerical simulation has been performed by solving the Poisson equation and continuity equations to obtain the I-V characteristic by using SCAPS-1D. All the inputs needed for solar- cell simulation in SCAPS-1D have been taken from the DFT results. The corresponding Power Conversion Efficiency (PCE) is denoted by η% and their respective values for X=Ge, Sn and Pb are 23.46%, 23.29% and 22.60%, at room temperature. The Ge-based system exhibits the highest η%, owing to its band gap value in the visible range of the solar spectrum. Thus, we report that Ge-based compounds may act as a promising absorber layer in heterostructure solar-cell applications.

cond-mat.mtrl-sci↗

arXiv2025

Octahedral dynamics and local symmetry in hybrid perovskite FAPbI3 under thermal excitation

Density Functional Theory (DFT) and ab initio molecular dynamics (AIMD) simulations have been employed to investigate the evolution of local motifs within the tetragonal phase of FAPbI3 under thermal excitation. Our results reveal a distinct broadening in the distribution of PbI6 octahedral volumes with increasing temperature, indicating a gradual breakdown of symmetry and emergence of diverse local environments. These octahedral volume distortions are primarily driven by the dynamic behaviour of the FA cation leading to softening of PbI6 octahedra, evident from calculated mean octahedral volume and Pb-I-Pb bond angles. The examination of electronic structure confirmed that this dynamic structural phenomenon is directly responsible for the change in fundamental band gap value, highlighting the role of PbI6 octahedra in modifying and modulating the electronic properties in FAPbI3. The results demonstrate the microscopic origin of thermally induced dynamical behaviour to the macroscopic electronic properties and underscore the pivotal role of local motifs in hybrid perovskites.

cond-mat.mtrl-sci↗

arXiv2025

Half-metallicity and anomalous Slater-Pauling behaviour in half-Heusler CrMnSb

This study provides a first-principles insight into half-Heusler CrMnSb to understand its deviation from the conventional Slater-Pauling semiconducting behavior. CrMnSb, having a valence electron count of 18, has been proposed to exhibit compensated ferrimagnetic character instead of the expected nonmagnetic semiconducting ground state. As half-Heusler systems with a valence electron count of 18 are not known to exhibit magnetic ordering, we have investigated the electronic and magnetic properties of CrMnSb using a combination of density functional theory and Green's function-based multiple-scattering theory. We show that, despite satisfying the 18 valence electron Slater-Pauling rule, CrMnSb does not exhibit ground-state nonmagnetic semiconducting behavior. Instead, it reveals a half-metallic, fully compensated ferrimagnetic ground state. This anomaly originates from the presence of localized sublattice moments, resulting from antiparallel alignment between Cr and Mn sublattices, which enforces half-metallic ferrimagnetism despite its ideal 18 valence electron count.

cond-mat.mtrl-sci↗

arXiv2022

Impressive Electronic Transport in Be$_2$C Monolayer

We present thermoelectric properties of Be$_2$C monolayer based on density functional theory and semi-classical Boltzmann transport theory. Electronic structure calculations predict this material as a semiconductor with a direct bandgap of 2.0 eV computed using Gaussian-attenuating Perdew-Burke-Ernzerhof (Gau-PBE) hybrid functional. The Gau-PBE band structure is used to compute transport properties by solving the Boltzmann transport equation under the constant relaxation time approximation. In this work, we have explicitly determined the relaxation time by studying the electron-phonon interactions in the system to estimate absolute transport coefficients. Our results show that the monolayer possesses a high power factor ($\sim$ 3.44 mW/mK$^2$ @300K), similar to the commercial TE materials doped-Bi$_2$Te$_3$ and PbTe, suggesting that Be$_2$C monolayer is a promising thermoelectric material.

cond-mat.mtrl-sci↗

arXiv2021

Ab-initio investigations for Structural, Mechanical, Optoelectronic, and Thermoelectric properties of Ba2SbXO6 (X=Nb, Ta) compounds

We report the structural, mechanical, electronic, optical, thermoelectric properties and spectroscopic limited maximum efficiency (SLME) of oxide double perovskite structure Ba2SbNbO6 and Ba2SbTaO6 compounds. All the investigations were performed through the first-principles density functional theory (DFT). The obtained values for the elastic constants reveal the mechanical stability of the studied compounds. The calculated data of bulk modulus (B), shear modulus (G), and Young's modulus (E) for Ba2SbTaO6 are found to be greater than those of Ba2SbNbO6. The ratio of Bulk to shear ratio (B/G) shows that Ba2SbNbO6 and Ba2SbTaO6 are ductile. The computed electronic band structure reveals the semiconducting nature of both compounds. We have also studied the electron relaxation time-dependent thermoelectric properties, such as Seebeck coefficient, thermal conductivity, electrical conductivity, thermoelectric power factor, and the figure of merit as a function of chemical potential at various temperatures for p-type and n-type charge carriers. The high absorption spectra and good figure of merit (ZT) reveal that both the studied compounds, Ba2SbXO6 (X = Nb, Ta) are promising materials for photovoltaic and thermoelectric applications. The calculated SLME of 26.8% reveals that Ba2SNbO6 is an appealing candidate for single-junction solar cells.

cond-mat.mtrl-sci↗

arXiv2020

Computational Study of Defect variant Perovskites A2BX6 for Photovoltaic Applications

A comprehensive study of the structural, electronic, and optical properties of lead-free perovskites has been carried out by means of first principles method based on DFT. The calculations are performed for the compound of the type A2BX6 with A=Rb, and Cs; B=Sn, Pd, and Pt; and X=Cl, Br, and I. The calculated structural parameters (lattice constants and bond lengths) agree well with the experiments. The computed band gap reveals a semiconducting profile for all these compounds showing a decreasing trend of the band gap energy by changing the halide ions consecutively from Cl to Br and Br to I. However, for variation in the B-site cation, the band gap increases by changing the cation from Pd to Pt via Sn. The most likely compounds, Rb2PdBr6 and Cs2PtI6, exhibit a band gap within the optimal range of 0.9-1.6 eV for single-junction photovoltaic applications. The optical properties in terms of the optimal value of the dielectric constant, optical conductivity, and absorption coefficient are also investigated upto the photon energy of 10 eV. Our results indicate that upon changing the halogen ions (Cl by Br and Br by I) the optical properties altered significantly. Maximum dielectric constants and high optical absorption are found for Rb2PdI6 and Cs2PtI6. The unique optoelectronic properties such as ideal band gap, high dielectric constants, and optimum absorption of A2BX6 perovskites could be efficiently utilized in designing high performance single and multi-junction perovskite solar cells.

cond-mat.mtrl-sci↗

arXiv2020

Improving the Optical and Thermoelectric Properties of Cs2InAgCl6 with Substitutional Doping: A DFT Insight

New generation Indium based lead-free Cs2InAgCl6 is a promising halide material in photovoltaic applications due to its good air stability and non-toxic behavior. But its wide band gap (>3 eV) is not suitable for solar spectrum and hence reducing the photoelectronic efficiency for device applications. Here we report a significant band gap reduction from 3.3 eV to 0.6 eV by substitutional doping and its effect on opto-electronic and opto-thermoelectric properties from first-principles study. The results predict that Sn/Pb and Ga & Cu co-doping enhance the density of states significantly near the valence band maximum (VBM) and thus reduce the band gap by shifting the VBM upward while the alkali-metals (K/Rb) slightly increase the band gap. A strong absorption peak near Shockley-Queisser limit is observed in co-doped case while in Sn/Pb-doped case, we notice a peak in the middle of the visible region of solar spectrum. The nature of band gap is indirect with Cu-Ga/Pb/Sn doping with a significant reduction in the band gap. We observe a significant increase in the power factor (PF) (2.03 mW/mK2) for n-type carrier in Pb-dpoing, which is ~3.5 times higher than the pristine case (0.6 mW/mK2) at 500 K.

cond-mat.mtrl-sci↗