Umesh Vasudeo Waghmare
@jncasr.ac.in
Professor, Theoretical Sciences Unit
Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR)
EDUCATION
PhD (Yale)
BTech (IIT Bombay)
RESEARCH INTERESTS
Computational Materials Science, Ferroelectrics, Shape Memory Alloys, Nanomaterials, Catalysis, Themoelectrics, Geometric Phases and Electronic Topology
Scopus Publications
Scholar Citations
Scholar h-index
Scholar i10-index
Scopus Publications
Prabir Dutta, Sushmita Chandra, Ivy Maria, Koyendrila Debnath, Divya Rawat, Ajay Soni, Umesh V. Waghmare, and Kanishka Biswas
Wiley
AbstractStructural mosaic of rare‐earth tri‐tellurides (RTe3) inlaid with non‐classical structural motifs like the 2D−polytelluride square nets has attracted immense attention owing to their enigmatic chemical bonding, unconventional structure, and harboring charge density wave (CDW) ground states. GdTe3, an archetypal RTe3, is a natural heterostructure of charged and van der Waals (vdW) layers, formed by intercalating vdW gap separated 2D square telluride nets [(Te2)−]n between the charged double corrugated slabs of n[GdTe]+. Here, he investigated the evolution of structural distortions along with the electrical and thermal transport properties of GdTe3 across its CDW transition through X‐ray pair distribution function analysis, thermal conductivity measurements, Raman spectroscopy and first principles theoretical calculations is investigated. The results reveal that the unusual structure of GdTe3 engenders a large anisotropic lattice thermal conductivity by concomitantly hampering the phonon propagation along parallel to the spark plasma sintering (SPS) pressing direction via chemical bonding hierarchy while facilitating phonon propagation along perpendicular to the SPS pressing direction through the metallic Te sheets and phason channel. The low lattice thermal conductivity is attributed to the strong vibrational anharmonicity caused by CDW‐induced concerted local lattice distortions of both Gd–Te slab and Te square net, and the robust electron–phonon coupling.
Sukanya Pal, Arijit Sinha, Luminita Harnagea, Prachi Telang, D. V. S. Muthu, U. V. Waghmare, and A. K. Sood
American Physical Society (APS)
Chaitali Sow, Meha Bhogra, Umesh V. Waghmare, and Giridhar U. Kulkarni
American Chemical Society (ACS)
Matthias Wuttig, Carl‐Friedrich Schön, Dasol Kim, Pavlo Golub, Carlo Gatti, Jean‐Yves Raty, Bart J. Kooi, Ángel Martín Pendás, Raagya Arora, and Umesh Waghmare
Wiley
AbstractA family of solids including crystalline phase change materials such as GeTe and Sb2Te3, topological insulators like Bi2Se3, and halide perovskites such as CsPbI3 possesses an unconventional property portfolio that seems incompatible with ionic, metallic, or covalent bonding. Instead, evidence is found for a bonding mechanism characterized by half‐filled p‐bands and a competition between electron localization and delocalization. Different bonding concepts have recently been suggested based on quantum chemical bonding descriptors which either define the bonds in these solids as electron‐deficient (metavalent) or electron‐rich (hypervalent). This disagreement raises concerns about the accuracy of quantum–chemical bonding descriptors is showed. Here independent of the approach chosen, electron‐deficient bonds govern the materials mentioned above is showed. A detailed analysis of bonding in electron‐rich XeF2 and electron‐deficient GeTe shows that in both cases p‐electrons govern bonding, while s‐electrons only play a minor role. Yet, the properties of the electron‐deficient crystals are very different from molecular crystals of electron‐rich XeF2 or electron‐deficient B2H6. The unique properties of phase change materials and related solids can be attributed to an extended system of half‐filled bonds, providing further arguments as to why a distinct nomenclature such as metavalent bonding is adequate and appropriate for these solids.
Mopidevi Manikanta Kumar, C. Aparna, Amit Kumar Nayak, Umesh V. Waghmare, Debabrata Pradhan, and C. Retna Raj
American Chemical Society (ACS)
The transition metal phosphide (TMP)-based functional electrocatalysts are very promising for the development of electrochemical energy conversion and storage devices including rechargeable metal-air batteries and water electrolyzer. Tuning the electrocatalytic activity of TMPs is one of the vital steps to achieve the desired performance of these energy devices. Herein, we demonstrate the modulation of the bifunctional oxygen electrocatalytic activity of nitrogen-doped carbon-encapsulated CoP (CoP@NC) nanostructures by surface tailoring with ultralow amount (0.56 atomic %) of Ru nanoparticles (2.5 nm). The CoP at the core and the Ru nanoparticles on the shell have a facile charge transfer interaction with the encapsulating NC. The strong coupling of Ru with CoP@NC boosts the electrocatalytic performance toward oxygen reduction (ORR), oxygen evolution (OER), and hydrogen evolution (HER) reactions. The surface-tailored catalyst requires only 35 mV to deliver the benchmark current density of 10 mA·cm-2 for HER. A small potential gap of 620 mV between ORR and OER is achieved, making the catalyst highly suitable for the development of rechargeable zinc-air batteries (ZABs). The homemade ZAB delivers a specific capacity of 780 mA·hgZn-1 and peak power density of 175 mW·cm-2 with a very small voltaic efficiency loss (1.1%) after 300 cycles. The two-electrode water splitting cell (CoP@NC-Ru||CoP@NC-Ru) delivers remarkably low cell voltage of 1.47 V at the benchmark current density. Stable current density of 25 mA·cm-2 for 25 h without any significant change is achieved. Theoretical studies support the charge transfer interaction-induced enhanced electrocatalytic activity of the surface-tailored nanostructure.
Arpita Sen, Ayush K. Narsaria, Meghna A. Manae, Sharan Shetty, and Umesh V. Waghmare
Royal Society of Chemistry (RSC)
Using DFT, we demonstrate external electric field assisted CO2 capture on different MgO facets, leading to carbonate formation in strongly adsorbed cases, and in some cases even CO2 reduction to CO on polar MgO(111) facet.
Raagya Arora, Umesh Waghmare, and C. N. R. Rao
Wiley
AbstractAn unusual set of anomalous functional properties of rocksalt crystals of Group IV chalcogenides were recently linked to a kind of bonding termed as metavalent bonding (MVB) which involves violation of the 8‐N rule. Precise mechanisms of MVB and the relevance of lone pair of Group IV cations are still debated. With restrictions of low dimensionality on the possible atomic coordination, 2D materials provide a rich platform for exploration of MVB. Here, we present first‐principles theoretical analysis of the nature of bonding in five distinct 2D lattices of Group IV chalcogenides MX (M: Sn, Pb, Ge and X: S, Se, Te), in which the natural out‐of‐plane expression of the lone pair versus in‐plane bonding can be systematically explored. While their honeycomb lattices respecting the 8‐N rule are shown to exhibit covalent bonding, their square and orthorhombic structures exhibit MVB only in‐plane, with cationic lone pair activating the out‐of‐plane structural puckering that controls their relative stability. Anomalies in Born‐effective charges, dielectric constants, Grüneisen parameters occur only in their in‐plane behaviour, confirming MVB is confined strictly to 2D and originates from p‐p orbital interactions. Our work opens up directions for chemical design of MVB based 2D materials and their heterostructures.
Debasmita Pariari, Paribesh Acharyya, Arijit Sinha, Ashutosh Mohanty, Shaili Sett, Navkiranjot Kaur Gill, Arindam Ghosh, Umesh V. Waghmare, Kanishka Biswas, and D. D. Sarma
American Chemical Society (ACS)
Meha Bhogra, Andrew L. Goodwin, Anthony K. Cheetham, and Umesh V. Waghmare
American Physical Society (APS)
Md Mokhlesur Rahman, Shazia Janwari, Minsu Choi, Umesh V. Waghmare, and Jaichan Lee
Elsevier BV
Animesh Bhui, Subarna Das, Raagya Arora, Usha Bhat, Prabir Dutta, Tanmoy Ghosh, Riddhimoy Pathak, Ranjan Datta, Umesh V. Waghmare, and Kanishka Biswas
American Chemical Society (ACS)
Defect engineering, achieved by precise tuning of the atomic disorder within crystalline solids, forms a cornerstone of structural chemistry. This nuanced approach holds the potential to significantly augment thermoelectric performance by synergistically manipulating the interplay between the charge carrier and lattice dynamics. Here, the current study presents a distinctive investigation wherein the introduction of Hg doping into AgSbTe2 serves to partially curtail structural disorder. This strategic maneuver mitigates potential fluctuations originating from pronounced charge and size disparities between Ag+ and Sb3+, positioned in octahedral sites within the rock salt structure. Hg doping significantly improves the phase stability of AgSbTe2 by restricting the congenital emergence of the Ag2Te minor secondary phase and promotes partial atomic ordering in the cation sublattice. Reduction in atomic disorder coalesced with a complementary modification of electronic structure by Hg doping results in increased carrier mobility. The formation of nanoscale superstructure with sizes (2-5 nm) of the order of phonon mean free path in AgSbTe2 is further promoted by reduced partial disorder, causes enhanced scattering of heat-carrying phonons, and results in a glass-like ultralow lattice thermal conductivity (∼0.32 W m-1 K-1 at 297 K). Cumulatively, the multifaceted influence of Hg doping, in conjunction with the consequential reduction in disorder, allows achieving a high thermoelectric figure-of-merit, zT, of ∼2.4 at ∼570 K. This result defies conventional paradigms that prioritize increased disorder for optimizing zT.
Paribesh Acharyya, Koushik Pal, Abdul Ahad, Debattam Sarkar, Kewal Singh Rana, Moinak Dutta, Ajay Soni, Umesh V. Waghmare, and Kanishka Biswas
Wiley
Shivani Grover, Keith T. Butler, Umesh V. Waghmare, and Ricardo Grau‐Crespo
Wiley
Bismuth ferrite, BiFeO 3 , is a multiferroic solid that is attracting increasing attention as a potential photocatalytic material, because the ferroelectric polarisation enhances the separation of photogenerated carriers. With the motivation of finding routes to engineer the band gap and the band alignment, while conserving or enhancing the ferroelectric properties, we have investigated the thermodynamic, electronic and ferroelectric properties of BiCo x Fe 1- x O 3 solid solutions, with 0 < 𝑥 < 0.13, using density functional theory. We show that the band gap can be reduced from 2.9 eV to 2.1 eV by cobalt substitution, while simultaneously increasing the spontaneous polarisation, which is associated with a notably larger Born effective charge of Co compared to Fe cations. We discuss the interaction between Co impurities, which is strongly attractive and would drive the aggregation of Co, as evidenced by Monte Carlo simulations. Phase separation into a Co-rich phase is therefore predicted to be thermodynamically preferred, and the homogeneous solid solution can only exist in metastable form, protected by slow cation diffusion kinetics. Finally, we discuss the band alignment of pure and Co-substituted BiFeO 3 with relevant redox potentials, in the context of its applicability in photocatalysis.
Lakshay Dheer, Meghna A. Manae, and Umesh V. Waghmare
American Chemical Society (ACS)
Narendra Kumar and Umesh V. Waghmare
Elsevier BV
Gurpreet Kaur, Ayushi Shukla, Arijit Sinha, Koyendrila Debnath, Kaliyamoorthy Justice Babu, Himanshu Bhatt, Umesh V. Waghmare, and Hirendra N. Ghosh
Royal Society of Chemistry (RSC)
Deploying advanced techniques-time-resolved transient absorption and terahertz spectroscopy, we reveal intriguing insights into the dynamics of diverse energy excitons and unconventional carrier transport in Cs2SnI6 nanocrystals.
Priyanka Jain, Gayatri Kumari, Meha Bhogra, Premakumar Yanda, Boby Joseph, Umesh V. Waghmare, and Chandrabhas Narayana
American Chemical Society (ACS)
The zeolitic imidazolate framework, ZIF-4, exhibits soft porosity and is known to show pore volume changes with temperatures, pressures, and guest adsorption. However, the mechanism and adsorption behavior of ZIF-4 are not completely understood. In this work, we report an open to narrow pore transition in ZIF-4 around T ∼ 253 K upon lowering the temperature under vacuum (10-6 Torr) conditions, facilitated by C-H···π interactions. In the gaseous environment of N2 and CO2 around the framework, characteristic Raman peaks of adsorbed gases were observed under ambient conditions of 293 K and 1 atm. A guest-induced transition at ∼153 K resulting in the opening of new adsorption sites was inferred from the Raman spectral changes in the C-H stretching modes and low-frequency modes (<200 cm-1). In contrast to a single vibrational mode generally reported for entrapped N2, we show three Raman modes of adsorbed N2 in ZIF-4. The adsorption is facilitated by dispersive and quadrupolar interactions. From our temperature-dependent Raman results and theoretical analysis based on the density functional tight-binding approach, we conclude that the C-Hs are the preferred adsorption sites on ZIF-4 in the following order: C4-H, C5-H > C2-H > center of the Im ring (interacting with C-H centers) > center of the cavity. We also show that with an increasing concentration of N2 adsorbed at low temperatures, the ZIF-4 structure undergoes shear distortion of the window formed by 4-imidazole rings and consequent volumetric expansion. Our results have immediate implications in the field of porous materials and could be vital in identifying subtle structural transformations that may favor or hinder guest adsorption.
V. Rajaji, Raagya Arora, B. Joseph, Subhajit Roychowdhury, Umesh V. Waghmare, Kanishka Biswas, and Chandrabhas Narayana
American Physical Society (APS)
Arpita Paul and Umesh V. Waghmare
American Physical Society (APS)
Ivy Maria, Raagya Arora, Moinak Dutta, Subhajit Roychowdhury, Umesh V. Waghmare, and Kanishka Biswas
American Chemical Society (ACS)
Metavalent bonding has attracted immense interest owing to its capacity to impart a distinct property portfolio to materials for advanced functionality. Coupling metavalent bonding to lone pair expression can be an innovative way to propagate lattice anharmonicity from lone pair-induced local symmetry-breaking via the soft p-bonding electrons to achieve long-range phonon dampening in crystalline solids. Motivated by the shared chemical design pool for topological quantum materials and thermoelectrics, we based our studies on a three-dimensional (3D) topological insulator TlBiSe2 that held prospects for 6s2 dual-cation lone pair expression and metavalent bonding to investigate if the proposed hypothesis can deliver a novel thermoelectric material. Herein, we trace the inherent phononic origin of low thermal conductivity in n-type TlBiSe2 to dual 6s2 lone pair-induced intrinsic lattice shearing that strongly suppresses the lattice thermal conductivity to a low value of 1.1-0.4 Wm-1 K-1 between 300 and 715 K. Through synchrotron X-ray pair distribution function and first-principles studies, we have established that TlBiSe2 exists not in a monomorphous R-3m structure but as a distribution of distorted configurations. Via a cooperative movement of the constituent atoms akin to a transverse shearing mode facilitated by metavalent bonding in TlBiSe2, the structure shuttles between various energetically accessible low-symmetry structures. The orbital interactions and ensuing multicentric bonding visualized through Wannier functions augment the long-range transmission of atomic displacement effects in TlBiSe2. With additional point-defect scattering, a κlatt of 0.3 Wm-1 K-1 was achieved in TlBiSeS with a maximum n-type thermoelectric figure of merit (zT) of ∼0.8 at 715 K.
Raagya Arora, Umesh V. Waghmare, and C. N. R. Rao
Wiley
A distinct type of metavalent bonding (MVB) was 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, we demonstrate emergence of MVB in rocksalt chalcogenides as a consequence of weakly broken symmetry of parent metallic simple-cubic crystals of Group V metalloids. High electronic degeneracy at the nested Fermi surface of the 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 respectively. MVB involves bonding and antibonding pairwise interactions alternating along linear chains of at least five atoms, which facilitate long range electron transfer in response to polar fields and cause unusual properties. Our precise picture of MVB predicts anomalous second order Raman scattering as an addition to set of their unusual finger-printing properties, and will guide in design of new metavalent materials with improved thermoelectric, ferroelectric and nontrivial electronic topological properties. This article is protected by copyright. All rights reserved.
Anupam Bhattacharya, Vishal Bhardwaj, Meha Bhogra, B. K. Mani, Umesh V. Waghmare, and Ratnamala Chatterjee
American Physical Society (APS)
Srishti Pal, Pallavi Malavi, Arijit Sinha, Anzar Ali, Piyush Sakrikar, Boby Joseph, Umesh V. Waghmare, Yogesh Singh, D. V. S. Muthu, S. Karmakar,et al.
American Physical Society (APS)
Pallavi Malavi, Arpita Paul, Achintya Bera, D. V. S. Muthu, Kunjalata Majhi, P S Anil Kumar, Umesh V. Waghmare, A. K. Sood, and S. Karmakar
American Physical Society (APS)
Quasi-two-dimensional layered BiSe, a natural super-lattice with Bi 2 Se 3 -Bi 2 -Bi 2 Se 3 units, has recently been predicted to be a dual topological insulator, simultaneously weak topological insulator as well as topological crystalline insulator. Here using structural, transport, spectroscopic measurements and density functional theory calculations, we show that BiSe exhibits rich phase diagram with the emergence of superconductivity with Tc ~8K under pressure. Sequential structural transitions into SnSe-type energetically tangled orthorhombic and CsCl-type cubic structures having distinct superconducting properties are identified at 8 GPa and 13 GPa respectively. Our observation of weak-antilocalization in magneto-conductivity suggests that spin-orbit coupling (SOC) plays a significant role in retaining non-trivial band topology in the trigonal phase with possible realization of 2D topological superconductivity. Theoretical analysis reveals that SOC significantly enhances superconducting Tc of the high-pressure cubic phase through an increase in electron-phonon coupling strength. Simultaneous emergence of Dirac-like surface states suggests cubic BiSe as a suitable candidate for the 3D-topological superconductor. enhances the linewidth. First-principles calculations on the (001) surface of high-pressure cubic phase of BiSe predict Dirac like linear crossing of the surface-related bands. The observed superconductivity in the high-pressure phases of BiSe and the predicted Dirac like surface states in the cubic phase make the high-pressure BiSe phase a remarkable candidate for a 3D-topological superconductor. AP is thankful to the National Supercomputing Mission, JNCASR for providing computational resources. Author Contributions: S.K., P.M. and A.K.S designed research; P.M., S.K., A.B., D.V.S.M., K.M., P.S.A.K. performed research (experiments); A.P. and U.W. performed research (first-principles calculations); P.M., S.K., and A.K.S. wrote the paper with inputs from all the authors.
Paribesh Acharyya, Tanmoy Ghosh, Koushik Pal, Kewal Singh Rana, Moinak Dutta, Diptikanta Swain, Martin Etter, Ajay Soni, Umesh V. Waghmare, and Kanishka Biswas
Springer Science and Business Media LLC
AbstractAs the periodic atomic arrangement of a crystal is made to a disorder or glassy-amorphous system by destroying the long-range order, lattice thermal conductivity, κL, decreases, and its fundamental characteristics changes. The realization of ultralow and unusual glass-like κL in a crystalline material is challenging but crucial to many applications like thermoelectrics and thermal barrier coatings. Herein, we demonstrate an ultralow (~0.20 W/m·K at room temperature) and glass-like temperature dependence (2–400 K) of κL in a single crystal of layered halide perovskite, Cs3Bi2I6Cl3. Acoustic phonons with low cut-off frequency (20 cm−1) are responsible for the low sound velocity in Cs3Bi2I6Cl3 and make the structure elastically soft. While a strong anharmonicity originates from the low energy and localized rattling-like vibration of Cs atoms, synchrotron X-ray pair-distribution function evidence a local structural distortion in the Bi-halide octahedra and Cl vacancy. The hierarchical chemical bonding and soft vibrations from selective sublattice leading to low κL is intriguing from lattice dynamical perspective as well as have potential applications.