• SHOBHANA NARASIMHAN

      Articles written in Bulletin of Materials Science

    • Exchange–correlation errors at harmonic and anharmonic orders: the case of bulk Cu

      Shobhana Narasimhan Stefano De Gironcoli

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      As an aid towards improving the treatment of exchange and correlation effects in electronic structure calculations, it is desirable to have a clear picture of the errors introduced by currently popular approximate exchange–correlation functionals. We have performed ab initio density functional theory and density functional perturbation theory calculations to investigate the thermal properties of bulk Cu, using both the local density approximation (LDA) and the generalized gradient approximation (GGA). Thermal effects are treated within the quasiharmonic approximation. We find that the LDA and GGA errors for anharmonic quantities are an order of magnitude smaller than for harmonic quantities; we argue that this might be a general feature. We also obtain much closer agreement with experiment than earlier, more approximate calculations.

    • Double stripe reconstruction of the Pt(111) surface

      Raghani Pushpa Shobhana Narasimhan

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      We have studied the reconstruction of the Pt(111) surface theoretically, using a 2D generalization of the Frenkel–Kontorova model. The parameters in the model are obtained by performing ab initio density functional theory calculations. The Pt(111) surface does not reconstruct under normal conditions but experiments have shown that there are two ways to induce the reconstruction: by increasing the temperature, or by depositing adatoms on the surface. The basic motif of this reconstruction is a `double stripe’ with an increased surface density and alternating hcp and fcc domains, arranged to form a honeycomb pattern with a very large repeat distance of 100–300 Å. In this paper, we have studied the `double stripe’ reconstruction of the Pt(111) surface. In agreement with experiment, we find that it is favourable for the surface to reconstruct in the presence of adatoms, but not otherwise.

    • Effect of coordination on bond properties: A first principles study

      Jaita Paul Shobhana Narasimhan

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      We have used density functional theory to obtain the binding curves for a variety of hypothetical periodic structures of Al, Si, Pb, Sn and Au. Upon examining the resulting database of results for equilibrium bond lengths and radial force constants (within a nearest-neighbour model), we find that both decrease smoothly as coordination is reduced. The effect of dimensionality appears to be small. We find that the force constants at equilibrium vary as the inverse eighth power of the equilibrium bond length. We also find evidence that the force constants are sensitive only to the bond length, and not to the coordination number. We believe these results will be useful in formulating interatomic potentials, e.g., for nanosystems.

    • Enhanced hydrogen evolution reactivity on Mo$_2$C–Mo$_2$N composites

      DEBDIPTO ACHARYA KASINATH OJHA NISHA MAMMEN PREETI DAGAR SOURAV MONDAL ASHOK K GANGULI SHOBHANA NARASIMHAN

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      We have studied the electrolysis of water, by performing a combined experimental and theoretical study of the hydrogen evolution reaction (HER) capability of Mo$_2$C–Mo$_2$N composites. Experimentally, we have synthesized nanowires with varying Mo$_2$C:Mo$_2$N ratios. We have found that the composites show good HER activity in an acidic medium, that is superior to that of either pristine Mo$_2$C or Mo$_2$N. These experimental results are supported by ab initio density functional theory calculations. Interestingly, we find that it is vital to incorporate van der Waals corrections toaccurately reproduce the experimentally observed structural transition from an orthorhombic to tetragonal phase as $x$, the N concentration in Mo$_2$C$_{1-x}$N$_x$, is increased. By computing Gibbs free energy for H adsorption on Mo$_2$C$_{1-x}$N$_x$ surfaces, our calculations confirm the experimental finding that mixed systems have superior HER activity to pristine systems, withN-rich systems being most active.

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      Posted on October 12, 2020

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