https://libjncir.jncasr.ac.in/xmlui/handle/10572/1548 2026-04-04T05:31:59Z https://libjncir.jncasr.ac.in/xmlui/handle/10572/2575 Competing Cation-Anion Interactions and Noncentrosymmetry in Metal Oxide-Fluorides: A First-Principles Theoretical Study Mishra, Abhishek Kumar; Marvel, Michael R.; Poeppelmeier, Kenneth R.; Waghmare, Umesh V. Anomalous Born dynamical charges in perovskite oxides, such as BaTiO3 and KNbO3, are known to be indicators of their tendency to turn polar through cation off-centering and measure of the interaction between d states of transition metal and p states of oxygen. Here, we use first-principles density functional theory based calculations to determine Born charges of noncentrosymmetric ICNaNbOF5 and centrosymmetric CsNaNbOF5 with a goal to assess the cation anion interactions relevant to the breaking of their centrosymmetry. We find that while noncentrosymmetry is favored by the primary Nb-O interaction, covalency in the competing interaction of Cs with anions suppresses it stabilizing the centrosymmetric structure and is reflected clearly in the deviation of Born effective charges (BECs) from their nominal ionic values. We identify specific features in the electronic structure that correlate with stability of the centrosymmetric structure and show that polarization of the noncentrosymmetric KNaNbOF5 estimated using the Berry phase method is rather weak similar to 0.21 mu C/cm(2), consistent with the finding that it originates from the competition between the primary and secondary electronic distortions. Restricted Access 2014-01-01T00:00:00Z https://libjncir.jncasr.ac.in/xmlui/handle/10572/2578 Emergence of Ferroelectricity at a Metal-Semiconductor Transition in a 1T Monolayer of MoS2 Shirodkar, Sharmila N.; Waghmare, Umesh V. Using a combination of Landau theoretical analysis and first-principles calculations, we establish a spontaneous symmetry breaking of the metallic state of the 1T monolayer of MoS2 that opens up a band gap and leads to an unexpected yet robust ferroelectricity with ordering of electric dipoles perpendicular to its plane. Central to the properties of this thinnest known ferroelectric is a strong coupling of conducting states with valley phonons that induce an effective electric field. The current in a semiconducting 1T-MoS2 channel can, thus, be controlled independently by changing its ferroelectric dipolar structure with a gate field, opening up a possibility of a class of nanoscale dipolectronic devices. Our analysis applies equally well to MoSe2, WS2, and WSe2, giving tunability in design of such devices based on two-dimensional chalcogenides. Restricted Access 2014-01-01T00:00:00Z https://libjncir.jncasr.ac.in/xmlui/handle/10572/2579 Evidence of scaling in the high pressure phonon dispersion relations of some elemental solids Srivastava, Divya; Waghmare, Umesh V.; Sarkar, Subir K. First principles searches are carried out for the existence of an asymptotic scaling law for the zero temperature phonon dispersion relation of several elemental crystalline solids in the high pressure regime. The solids studied are Cu, Ni, Pd, Au, Al, and Ir in the face-centered-cubic (fcc) geometry and Fe, Re, and Os in the hexagonal-close-packed (hcp) geometry. At higher pressures, the dependence of the scale of frequency on pressure can be fitted well by a power law. Elements with a given crystalline geometry have values of the scaling exponent very close to each other (0.32 for fcc and 0.27 for hcp -with a scatter below five percent of the average). (C) 2014 AIP Publishing LLC. Restricted Access 2014-01-01T00:00:00Z https://libjncir.jncasr.ac.in/xmlui/handle/10572/2576 Composition-dependent photoluminescence and electronic structure of 2-dimensional borocarbonitrides, BCXN (x=1, 5) Moses, Kota; Shirodkar, Sharmila N.; Waghmare, Umesh V.; Rao, C. N. R. Layered borocarbonitrides BCN and BC5N with a wide difference in composition have been prepared by the urea route. These 2D materials show a significant difference in the photoluminescence spectra, with BCN and BC5N showing maxima at 340 and 410 nm (3.61 and 3.0 eV), besides exhibiting different electrical resistivities. First-principles calculations show that BCN and BC5N are associated with different band gaps, the gap of the carbon-rich composition being lower. The change in the electronic structure and properties is related to the composition of BCXN i.e. the ordering of the graphene and BN domains. Restricted Access 2014-01-01T00:00:00Z