arXiv subjects

S. C. Rakesh Roshan

Publications and source records attributed to S. C. Rakesh Roshan.

4 recordsLinked to original sources

arXiv2022

Effect of Hydrostatic Pressure on Lone Pair Activity and Phonon Transport in Bi$_2$O$_2$S

Dibismuth dioxychalcogenides, Bi$_2$O$_2$Ch (Ch = S, Se, Te) are emerging class of materials for next generation electronics and thermoelectrics with an ultrahigh carrier mobility and excellent air stability. Among these, Bi$_2$O$_2$S is fascinating because of stereochemically active 6$s^2$ lone pair of Bi$^{3+}$ cation, heterogeneous bonding and high mass contrast of constituent elements. In this work, we systematically investigate the effect of hydrostatic pressure and its implications on lattice dynamics and phonon transport properties of Bi$_2$O$_2$S by employing first principles calculations along with the Boltzmann transport theory. The ambient $Pnmn$ phase exhibits a low average lattice thermal conductivity ($κ_l$) of 1.71 W-m/K at 300 K. We also find that Bi$_2$O$_2$S undergoes a structural phase transition from low symmetry ($Pnmn$) to a high symmetry ($I4/mmm$) structure around 4 GPa due to the Bi$^{3+}$ cation centering. Upon compression the lone pair activity of Bi$^{3+}$ cation is suppressed, which increases $κ_l$ by nearly 3 times to 4.92 W-m/K at 5 GPa for $I4/mmm$ phase. The calculated phonon lifetimes and Grüneisen parameters show that anharmonicity reduces with increasing pressure due to further suppression of lone pair, strengthening of intra and inter molecular interactions, which raises the average room temperature $κ_l$ to 12.82 W-m/K at 20 GPa. Overall, the present study provides a comprehensive understanding of hydrostatic pressure effects on stereochemical activity of the Bi$^{3+}$ cation lone pair and its consequences on phonon transport properties of Bi$_2$O$_2$S.

cond-mat.mtrl-sci↗

arXiv2022

Lattice Instability and Ultralow Lattice Thermal Conductivity of Layered PbIF

Understanding the interplay between various design strategies (for instance, bonding heterogeneity and lone pair induced anharmonicity) to achieve ultralow lattice thermal conductivity ($κ_l$) is indispensable for discovering novel functional materials for thermal energy applications. In the present study, we investigate layered PbXF (X = Cl, Br, I), which offers bonding heterogeneity through the layered crystal structure, anharmonicity through the Pb$^{2+}$ $6s^2$ lone pair, and phonon softening through the mass difference between F and Pb/X. The weak inter-layer van der Waals bonding and the strong intra-layer ionic bonding with partial covalent bonding result in a significant bonding heterogeneity and a poor phonon transport in the out-of-plane direction. Large average Grüneisen parameters ($\geq$ 2.5) demonstrate strong anharmonicity. The computed phonon dispersions show flat bands, which suggest short phonon lifetimes, especially for PbIF. Enhanced Born effective charges are due to cross-band-gap hybridization. PbIF shows lattice instability at a small volume expansion of 0.1$\%$. The $κ_l$ values obtained by the two channel transport model are 20-50$\%$ higher than those obtained by solving the Boltzmann transport equation. Overall, ultralow $κ_l$ values are found at 300 K, especially for PbIF. We propose that the interplay of bonding heterogeneity, lone pair induced anharmonicity, and constituent elements with high mass difference aids the design of low $κ_l$ materials for thermal energy applications.

cond-mat.mtrl-sci↗

arXiv2022

Lattice instability, anharmonicity and Raman spectra of BaO under high pressure: A first principles study

Alkaline-earth metal oxides, in particular MgO and CaO dominate Earths lower mantle, therefore, exploring high pressure behavior of this class of compounds is of significant geophysical research interest. Among all these compounds, BaO exhibits rich polymorphism in the pressure range of 0-1.5 Mbar. Static enthalpy calculations revealed that BaO undergoes a pressure induced structural phase transition from NaCl-type (B1) $\rightarrow$ NiAs-type (B8) $\rightarrow$ distorted CsCl-type (d-B2) $\rightarrow$ CsCl-type (B2) at 5.1, 19.5, 120 GPa respectively. B1 $\rightarrow$ B8 $\&$ B8 $\rightarrow$ d-B2 transitions are found to be first order in nature whereas d-B2$\rightarrow$ B2 is a second order or weak first order phase transition. Interestingly, d-B2 phase shows stability over a wide pressure range, $\sim$19.5-113 GPa. Mechanical and dynamical stabilities of ambient and high pressure phases are demonstrated through computed elastic constants and phonon dispersion curves, respectively. Under high pressure, significant phonon softening and soft phonon mode along M-direction are observed for B8, d-B2 and B2 phases, respectively. Pressure dependent Raman spectra suggest a phase transition from d-B2 to Raman inactive phase under pressure. Overall, the present study provides a comprehensive understanding of underlying mechanisms behind pressure-induced structural phase transitions in BaO.

cond-mat.mtrl-sci↗

arXiv2021

Anomalous Lattice Thermal Conductivity in Rocksalt IIA-VIA Compounds

Materials with an intrinsic (ultra)low lattice thermal conductivity (k$_L$) are critically important for the development of efficient energy conversion devices. In the present work, we have investigated microscopic origins of low k$_L$ behavior in BaO, BaS and MgTe by exploring lattice dynamics and phonon transport of 16 iso-structural MX (Mg, Ca, Sr, Ba and X = O, S, Se and Te) compounds in the rocksalt (NaCl)-type structure by comparing their lattice transport properties with the champion thermoeletric iso-structural material, PbTe. Anomalous trends are observed for k$_L$ in MX compounds except the MgX series in contrast to the expected trend from their atomic mass. The underlying mechanisms for such low k$_L$ behavior in relatively low atomic mass systems namely BaO, BaS and MgTe compounds are thoroughly analyzed. We propose the following dominant factors that might be responsible for low k$_L$ behavior in these materials: 1) softening of transverse acoustic (TA) phonon modes despite low atomic mass, 2) low lying optic (LLO) phonon modes fall deep into acoustic mode region which enhances overlap between longitudinal acoustic (LA) and LLO phonon modes which increases scattering phase space, 3) short phonon lifetimes and high scattering rates, 4) relatively high density (\r{ho}) and large Grüneisen parameter. Moreover, tensile strain also causes a further reduction in k$_L$ for BaO, BaS and MgTe through phonon softening and near ferroelectric instability. Our comprehensive study on 16 binary MX compounds might provide a pathway for designing (ultra)low k$_L$ materials even with simple crystal systems through phonon engineering.

cond-mat.mtrl-sci↗