Varadharajan Srinivasan - Academia.edu (original) (raw)

Papers by Varadharajan Srinivasan

Isolated Fe-sites on silica substrate have recently been reported for direct and non-oxidative co... more Isolated Fe-sites on silica substrate have recently been reported for direct and non-oxidative conversion of gaseous methane with high selectivity. The activated catalyst was proposed to be FeC2 cluster embedded in silica. Using a combination of density-functional theoretic methods and micro-kinetic modeling, we show that under the same reaction conditions (1223 K , 1 atm) FeC2 sites convert to FeC3 and the latter is instead responsible for the observed activity. We investigate the detailed mechanism of conversion of methane to methyl radical and hydrogen on FeC3@SiO2 under different conditions of methane partial pressure. We find that methyl radical evolution is the rate-determining step for the overall conversion. Our calculations also indicate that the conversion of embedded FeC3 to FeC4 competes with methyl radical evolution from the active catalyst. However, due to the higher stability of FeC3 sites, we anticipate that formation of higher carbides can be inhibited by controllin...

The pressure induced polymerization of molecular solids is an appealing route to obtain pure, cry... more The pressure induced polymerization of molecular solids is an appealing route to obtain pure, crystalline polymers without the need for radical initiators. Here, we report a detailed density functional theory (DFT) based study of the structural and chemical changes that occur in defect free solid acrylamide, a hydrogen bonded crystal, when it is subjected to hydrostatic pressures. Our calculations predict a polymerization pressure of 94 GPa, in contrast to experimental estimates of 17 GPa, while being able to reproduce the experimentally measured pressure dependent spectroscopic features. Interestingly, we find that the two-dimensional hydrogen bond network templates a topochemical polymerization by aligning the atoms through an anisotropic response at low pressures. This results not only in conventional C-C, but also unusual C-O polymeric linkages, as well as a new hydrogen bonded framework, with both NH... O and C-H...O bonds.

Physical Review B

We report a first-principles study of Bi-based 3d-5d ordered double perovskite oxides (A2BB O6) w... more We report a first-principles study of Bi-based 3d-5d ordered double perovskite oxides (A2BB O6) with a 3d atom (Fe) at the B-site and 5d atoms (Re,Ir) at the B-site while keeping highly polarizable ions (Bi 3+) at the Asite. We find that, under coherent heteroepitaxy, Bi2FeReO6 exhibits a strain-driven anti-ferromagnetic insulator to ferrimagnetic semi-metal transition, while Bi2FeIrO6 shows correlation driven ferromagnetic insulator to ferrimagnetic half-metal transition with calculated magnetic moments of 5 µB/f.u. and 3 µB/f.u., respectively. These properties along with the low band gaps in the insulating phases make the compounds appealing for spintronics applications. Furthermore, in Bi2FeIrO6, the conduction and valence states are localized on different transition metal sublattices implying more efficient electron-hole separation upon photoexcitation, a desirable feature for photovoltaic applications.

Physical Review Letters

Epitaxial strain has been shown to drive structural phase transitions along with novel functional... more Epitaxial strain has been shown to drive structural phase transitions along with novel functionalities in perovskite-based thin-films. Aliovalent doping at the A-site can drive an insulator-to-metal and magnetic transitions in perovskites along with a variety of interesting structural and electronic phenomena. Using firstprinciples calculations, we demonstrate here, how coupling epitaxial strain with A-site hole doping in a multiferroic double perovskite, Bi2FeCrO6, could lead to mitigation of issues related to anti-site defects and lowered magnetisation in thin-films of the material. We also show that epitaxial strain can be used to manipulate the hole states created by doping to induce half-metal to insulator, antipolar to polar, antiferromagnetic to ferromagnetic, orbital ordering and charge ordering transitions. We also predict the formation of a half-metallic polar phase with a large magnetic moment which could be of immense fundamental and technological significance.

Physical Review Materials

Perovskite metal-organic frameworks (MOFs) have recently emerged as potential candidates for mult... more Perovskite metal-organic frameworks (MOFs) have recently emerged as potential candidates for multiferroicity. However, the compounds synthesized so far possess only weak ferromagnetism and low polarization. Additionally, the very low magnetic transition temperatures (Tc) also pose a challenge to the application of the materials.We have computationally designed a mixed metal perovskite MOF-[C(NH2)3][(Cu0.5Mn0.5)(HCOO)3]that is predicted to have magnetization two orders of magnitude larger than its parent ([C(NH2)3][Cu(HCOO)3]), a significantly larger polarization (9.9 µC/cm 2), and an enhanced Tc of up to 56 K, unprecedented in perovskite MOFs. A detailed study of the magnetic interactions revealed a novel mechanism leading to the large moments as well as the increase in the Tc. Mixing a non-Jahn-Teller ion (Mn 2+) into a Jahn-Teller host (Cu 2+) leads to competing lattice distortions which are directly responsible for the enhanced polarization. The MOF is thermodynamically stable as evidenced by the computed enthalpy of formation, and can likely be synthesized. Our work represents a first step towards rational design of multiferroic perovskite MOFs through the largely unexlpored mixed metal approach.

We are currently performing full-fledged Car-Parrinello molecular dynamics simulations (CP) for P... more We are currently performing full-fledged Car-Parrinello molecular dynamics simulations (CP) for PbTiO_3, both in bulk and in thin-layer forms. We use various technical features, including the newly developed Wannier-function approach, which among other things even allows dealing with finite electric fields. The bulk simulations are performed with the standard variable-cell CP code and no field. They provide genuine first-principle insight

Physical Review B, 2016

We have investigated the effect of epitaxial strain on the magnetic properties and B-site cation ... more We have investigated the effect of epitaxial strain on the magnetic properties and B-site cation ordering in multiferroic Bi2FeCrO6 (001) thin films using a density-functional theory approach. We find that in thin films with rock-salt ordering of Fe and Cr the ground state is characterised by C-type anti-ferromagnetic (AFM) order. This is in contrast to the bulk form of the material which was predicted to be a ferrimagnet with Gtype AFM order. Furthermore, the cation ordered thin-films undergo a transition with epitaxial strain from C to A-type AFM order. Other magnetic orders appear as thermally accessible excited states. We also find that B-site cation disordered structures are more stable in coherent epitaxial strains thereby explaining the lowered magnetic moments observed in these samples at room temperature. Strain varies both the sign as well as strength of the Fe-Cr superexchange coupling resulting in a very interesting phase diagram for Bi2FeCrO6 thin films.

Aps Meeting Abstracts, 2008

Nanoscale cantilevers have shown great promise as ultrasensitive, low-power chemical sensors base... more Nanoscale cantilevers have shown great promise as ultrasensitive, low-power chemical sensors based on the surface stresses induced by interactions between the target species and the receptor coating layer. However, the basic mechanism of these induced stresses is yet to be fully understood, and it is therefore of great fundamental and practical interest to elucidate their electronic and structural origins via

ABSTRACT CdSe, a potential material for hybrid solar cells, has a well known reconstruction at th... more ABSTRACT CdSe, a potential material for hybrid solar cells, has a well known reconstruction at the surface which removes the surface states. Using Density Functional Theory (DFT) we explore what happens to the now-removed surface states when CdSe is interfaced with two different metals, Al and Au. We compare and contrast this with the interfaces of a pristine unreconstructed CdSe surface with the two metals.

We perform ab-initio path integral simulations on protonated and deuterated KDP at different temp... more We perform ab-initio path integral simulations on protonated and deuterated KDP at different temperatures and lattice constants in order to probe the origin of the isotope effect of the ferroelectric phase transition in this material. By taking into account the quantum nature of the proton/deuteron our simulations are capable of distinguishing the direct effects of a pure mass change versus the indirect structural effect in the hydrogen bonding geometry upon deuteration. In reality, the direct and indirect effects amplify each other in a self-consistent manner, leading to the huge isotope effect on the transition temperature. With our calculation we can selectively investigate the manisfestation of the two phenomena. We characterize the ferro and paraelectric phases with the help of a recent modification of the path integral implementation in the CPMD package which enables us to compute momentum distributions of the proton/deuteron both above and below the transition temperature in ...

Optics Express, 2015

In this article, we experimentally demonstrate for the first time that photobleaching (PB) can be... more In this article, we experimentally demonstrate for the first time that photobleaching (PB) can be induced in morphologically disordered a-Se thin film, an observation which is opposite of the previously well-known photodarkening (PD) effects in morphologically ordered films. Further, the optical response of the film shows many fold increase with increase in control beam intensity. To explain the observed extraordinary phenomenon, we have proposed a model based on the morphological disorder of a modified surface and its subsequent photo-annealing. Our results demonstrate an efficient and yet simple new method to engineer the optical response of photosensitive thin films. We envision that this process can open up many avenues in optical field-enhanced absorption-based technologies.

ABSTRACT We have recently shown ootnotetextPhys. Rev. Lett. 101, 185504 (2008) that adsorption-in... more ABSTRACT We have recently shown ootnotetextPhys. Rev. Lett. 101, 185504 (2008) that adsorption-induced stresses in alkanethiolate-covered Au(111) contribute significantly to the stress-response in nano-mechanical cantilever sensors. In particular, we proposed a local stress relief (LSR) mechanism whereby charge removal by the Au-S bond from the Au surface promotes a stress reducing rearrangement of surface Au atoms. Since LSR depends on the nature of the Au-S bond it was unclear how the contribution to the stress- response would depend on the alkanethiolate structure. We present a first-principles study of the chain-length dependence in surface stresses of alkanethiolate-covered Au(111). We find that the surface stress upon adsorption is anisotropic and tensile, increasing in magnitude with the chain-length. We analyze this trend in the context of the LSR mechanism and inter-adsorbate interactions.

The Journal of Physical Chemistry C, 2011

ACS Nano, 2011

We have used scanning tunneling microscopy, Auger electron spectroscopy, and density functional t... more We have used scanning tunneling microscopy, Auger electron spectroscopy, and density functional theory calculations to investigate thermal and photoinduced structural transitions in (fulvalene)tetracarbonyldiruthenium molecules (designed for light energy storage) on a Au(111) surface. We find that both the parent complex and the photoisomer exhibit striking thermally induced structural phase changes on Au(111), which we attribute to the loss of carbonyl ligands from the organometallic molecules. Density functional theory calculations support this conclusion. We observe that UV exposure leads to pronounced structural change only in the parent complex, indicative of a photoisomerization reaction.

Physical review letters, Jan 31, 2008

First-principles calculations were employed to elucidate the origin of adsorption-induced surface... more First-principles calculations were employed to elucidate the origin of adsorption-induced surface stresses in alkanethiolate self-assembled monolayers on an Au(111) surface. Our results suggest a mechanism that accounts for the huge relief of the tensile stress compared to the bare surface in terms of a local rearrangement of surface Au atoms accompanying charge removal from the surface towards the Au-S bond. A purely interadsorbate interaction model is shown to be inconsistent with the anisotropy and the magnitude of the calculated stress.

We have undertaken the first ever fully first-principles simulations of ferroelectric crystals at... more We have undertaken the first ever fully first-principles simulations of ferroelectric crystals at finite temperature with an aim to understand the nature of their phase transitions. In particular, we have studied the different aspects of phase transitions in two protypical ferroelectrics - PbTiO3 and KH2PO4. In PbTiO3, we have successfully reproduced the temperature-driven transition from a tetragonal to a cubic

We investigate the quantum-mechanical localization of protonated and deterated isotopes in the sy... more We investigate the quantum-mechanical localization of protonated and deterated isotopes in the symmetric low-barrier hydrogen-bonds of potassium dihydrogen phosphate (KDP) crystals in the paraelectric phase. The spatial density distributions of these hydrogen atoms are suspected to be responsible for the surprisingly large isotope effect observed for the ferroelectric phase transition in KDP. We employ ab initio path integral molecular dynamics