Umesh V. Waghmare

Jawaharlal Nehru Centre for Advanced Scientific Research

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C. N. R. Rao

Jawaharlal Nehru Centre for Advanced Scientific Research

Kanishka Biswas

Jawaharlal Nehru Centre for Advanced Scientific Research

Sharmila N. Shirodkar

Rice University

Koushik Pal

Jawaharlal Nehru Centre for Advanced Scientific Research

D. V. S. Muthu

Indian Institute of Science Bangalore

Anjali Singh

Center for Study of Science Technology and Policy

A. Sundaresan

Jawaharlal Nehru Centre for Advanced Scientific Research

Raagya Arora

Harvard University Press

A. K. Sood

Indian Institute of Science Bangalore

Karin M. Rabe

Rutgers Sexual and Reproductive Health and Rights

Cooperative Institutions

Jawaharlal Nehru Centre for Advanced Scientific Research

231

Indian Institute of Science Bangalore

176

Bangalore University

Tata Institute of Fundamental Research

Centre National de la Recherche Scientifique

Homi Bhabha National Institute

Northwestern University

Indian Institute of Technology Madras

Indian Institute of Science Education and Research Mohali

Indian Institute of Technology Bombay

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Metavalent Bonding Origins of Unusual Properties of Group IV Chalcogenides

Advanced Materials (2022)

Raagya Arora Umesh V. Waghmare C. N. R. Rao

A distinct type of metavalent bonding (MVB) is recently proposed to explain an unusual combination of anomalous functional properties of group IV chalcogenide crystals, whose electronic mechanisms and origin remain controversial. Through theoretical analysis of evolution of bonding along continuous paths in structural and chemical composition space, emergence of MVB in rocksalt chalcogenides is demonstrated as a consequence of weakly broken symmetry of parent simple-cubic crystals of Group V metalloids. High electronic degeneracy at the nested Fermi surface of parent metal drives spontaneous breaking of its translational symmetry with structural and chemical fields, which open up a small energy gap and mediate strong coupling between conduction and valence bands making metavalent crystals highly polarizable, conductive, and sensitive to bond-lengths. Stronger symmetry-breaking structural and chemical fields, however, transform them discontinuously to covalent and ionic semiconducting states. MVB involves bonding-antibonding pairwise interactions alternating along linear chains of at least five atoms, which facilitate long-range electron transfer in response to polar fields causing unusual properties. The precise picture of MVB predicts anomalous second-order Raman scattering as an addition to set off their unusual properties, and will guide in design of new metavalent materials with improved thermoelectric, ferroelectric and nontrivial electronic topological properties.

Antibonding molecular orbital

10.1002/adma.202208724

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Citations (35)

First-principles model hamiltonians for ferroelectric phase transitions

Ferroelectrics (1992)

Karin M. Rabe Umesh V. Waghmare

Abstract A model-based approach to the first-principles study of ferroelectric phase transitions is described. Formal relationships of the various possible models to the full crystal Hamiltonian and connections to phenomenological models in the literature are examined. Specific considerations for application to the perovskite ferroelectrics BaTiO3 and PbTiO3, including the incorporation of long-range interactions, are discussed. Approaches to the computation of the nonlinear spontaneous polarization are compared and related to Berry's phase arguments and the quantum adiabatic theorem.

Hamiltonian (control theory)

Phenomenological model

10.1080/00150199208016074

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Citations (29)

ChemInform Abstract: Zn₂NF and Related Analogues of ZnO.

ChemInform (2016)

Srinivasa Rao Lingampalli K. Manjunath Sandhya Shenoy Umesh V. Waghmare C. N. R. Rao

Abstract Variation of the band gap of ZnO is achieved by progressive substitution of O by N and F.

10.1002/chin.201642015

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First-principles study of spontaneous polarization in multiferroicBiFeO3

Physical Review B (2005)

Jeffrey B. Neaton Claude Ederer Umesh V. Waghmare Nicola A. Spaldin Karin M. Rabe

The ground-state structural and electronic properties of ferroelectric $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$ are calculated using density functional theory within the local spin-density approximation (LSDA) and the $\mathrm{LSDA}+U$ method. The crystal structure is computed to be rhombohedral with space group $R3c$, and the electronic structure is found to be insulating and antiferromagnetic, both in excellent agreement with available experiments. A large ferroelectric polarization of $90--100\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{C}∕{\mathrm{cm}}^{2}$ is predicted, consistent with the large atomic displacements in the ferroelectric phase and with recent experimental reports, but differing by an order of magnitude from early experiments. One possible explanation is that the latter may have suffered from large leakage currents. However, both past and contemporary measurements are shown to be consistent with the modern theory of polarization, suggesting that the range of reported polarizations may instead correspond to distinct switching paths in structural space. Modern measurements on well-characterized bulk samples are required to confirm this interpretation.

10.1103/physrevb.71.014113

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Citations (1,341)

First-principles free energies and Ginzburg-Landau theory of domains and ferroelectric phase transitions inBaTiO3

Physical Review B (2010)

Anil Kumar Umesh V. Waghmare

We present a method based on combination of (a) constrained polarization molecular dynamics and (b) thermodynamic integration to determine the free-energy landscape relevant to structural phase transitions and related phenomena in ferroelectric materials, bridging the gap between first-principles calculations and phenomenological Landau-type theories. We illustrate it using a first-principles effective Hamiltonian of ${\text{BaTiO}}_{3}$ to (a) uncover the quantitative features of the free-energy function that are responsible for its first-order ferroelectric transitions, (b) calculate the minimum free-energy pathways for the polarization switching and (c) evaluate temperature-dependent free energy of domain walls, and a minimum free-energy pathway to formation of ferroelectric domains. We use our method within a variational mean-field theory to connect with Landau theory and show through comparison with numerically exact simulations that (a) the state constrained to have vanishing order (away from the equilibrium) below the transition temperature is highly degenerate due to fluctuations that drive the phase transition first order, and (b) certain terms need to be added to the phenomenological Landau-Devonshire free-energy functions to capture the physics of spatial fluctuation in order parameter. Our method can be readily generalized to any classical microscopic Hamiltonian and ensembles characterized with a given constraint.

Hamiltonian (control theory)

Landau theory

Energy landscape

Phenomenological model

Ginzburg–Landau theory

10.1103/physrevb.82.054117

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Citations (37)

Activation of Lattice Oxygen of TiO₂ by Pd²⁺ Ion: Correlation of Low-Temperature CO and Hydrocarbon Oxidation with Structure of Ti_1–xPd_xO_2–x (x = 0.01–0.03)

Chemistry of Materials (2012)

Bhaskar Devu Mukri Gargi Dutta Umesh V. Waghmare M. S. Hegde

Lattice oxygen of TiO2 is activated by the substitution of Pd ion in its lattice. Ti1–xPdxO2–x (x = 0.01–0.03) have been synthesized by solution combustion method crystallizing in anatase TiO2 structure. Pd is in +2 oxidation state and Ti is in +4 oxidation state in the catalyst. Pd is more ionic in TiO2 lattice compared to Pd in PdO. Oxygen storage capacity defined by "amount of oxygen that is used reversibly to oxidize CO" is as high as 5100 μmol/g of Ti0.97Pd0.03O1.97. Oxygen is extracted by CO to CO2 in absence of feed oxygen even at room temperature which is more than 20 times compared to pure TiO2. Rate of CO oxidation is 2.75 μmol g–1 s–1 at 60 °C over Ti0.97Pd0.03O1.97 and C2H2 gets oxidized to CO2 and H2O at room temperature. Catalyst is not poisoned on long time operation of the reactor. Such high catalytic activity is due to activated lattice oxygen created by the substitution of Pd ion as seen from first-principles density functional theory (DFT) calculations with 96 atom supercells of Ti32O64, Ti31Pd1O63, Ti30Pd2O62, and Ti29Pd3O61. The compounds crystallize in anatase TiO2 structure with Pd2+ ion in nearly square planar geometry and TiO6 octahedra are distorted by the creation of weakly bound oxygens. Structural analysis of Ti31Pd1O63 which is close to 3% Pd ion substituted TiO2 shows that oxygens associated with both Ti and Pd ions in the lattice show bond valence sum of 1.87, a low value characteristic of weak oxygen in the lattice compared to oxygens with valence 2 and above in the same lattice. Exact positions of activated oxygens have been identified in the lattice from DFT calculations.

Lattice (music)

10.1021/cm301704u

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Citations (47)

Domain formation and dielectric response in PbTiO$_3$: A first-principles free energy landscape analysis

Bulletin of the American Physical Society (2013)

Anil Kumar Karin M. Rabe Umesh V. Waghmare

Dielectric response

Energy landscape

Source

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Theoretical Prediction of a Stable 2D Crystal of Vanadium Porphyrin: A Half-Metallic Ferromagnet

The Journal of Physical Chemistry C (2015)

Harish K. Singh Pawan Kumar Umesh V. Waghmare

We predict a 2D ferromagnetic half-metal based on vanadium porphyrin (V–PP) using first-principles density functional theoretical analysis. We establish the dynamical stability of its planar structure and magnetic ground state through determination of energetics and phonon dispersion. We find that the exchange interaction between spins of nearest neighbor V atoms is mediated by delocalized states of porphyrin and determine its strength from the relative energies of states with ferromagnetic and antiferromagnetic ordering. Using it in an Ising model, our estimate of its Curie temperature (Tc) is 197 K, which is higher than that of 2D manganese phthalocyanine (Mn–Pc) and 2D Cr–PP. With estimated work function of 4.9 eV, moderate in-plane stiffness, and a branch of very low energy flexural modes evident in its phonon dispersion, we find that 2D V–PP is quite suitable for use in flexible spintronic devices.

Density of states

Magnon

10.1021/acs.jpcc.5b09763

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Citations (25)

Strain coupling in perovskite structural transitions: A first principles approach

Ferroelectrics (1997)

Karin M. Rabe Umesh V. Waghmare

Abstract Effects of strain coupling on ferroelectric transitions in the perovskite oxides are investigated through analysis of first-principles effective Hamiltonians and the related “eight-site” model. When strain coupling is set to zero in these models, classical Monte Carlo simulations show a single second-order cubic-rhombohedral transition. Mean field analysis of the full models, including strain coupling, leads to a second-order transition directly to a phase with the same symmetry as the ground state. Only in classical Monte Carlo simulations of the full models are intermediate tetragonal and orthorhombic phases and first-order transitions observed. This suggests that nonanalytic strain-related fluctuation corrections are essential for producing the observed transition behavior in ferroelectric perovskites.

Tetragonal crystal system

Orthorhombic crystal system

Mean field theory

Strain (injury)

10.1080/00150199708016087

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Citations (12)

Destabilizing excitonic insulator phase by pressure tuning of exciton-phonon coupling

Physical Review Research (2020)

Sukanya Pal Shivani Grover Luminita Harnagea Prachi Telang Anupam Singh

The authors use high-pressure Raman spectroscopy along with a first-principles model analysis to study the role of exciton-phonon coupling in stabilizing the excitonic insulator phase of Ta${}_{2}$NiSe${}_{5}$ at room temperature.

10.1103/physrevresearch.2.043182

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Citations (14)