arXiv subjects

S. L. Chaplot

Publications and source records attributed to S. L. Chaplot.

54 records · Page 3Linked to original sources

arXiv2010

Investigation of phonon dynamics of perovskite multiferroic manganites: RMnO3 (R=Tb, Dy, Ho)

We have used a shell model to study the phonon dynamics of multiferroic manganites RMnO3 (R= Tb, Dy, Ho). The calculated phonon dynamical properties, crystal structure, Raman frequencies and specific heat are found to be in good agreement with the available experimental data. Besides, the phonon density of states, elastic constants and phonon dispersion curves along high symmetry directions (sigma, delta and lambda) have also been computed. A zone-center imaginary Au mode is revealed in these phonon dispersion curves, which indicates the occurrence of metastability of the perovskite phase. The Gibbs free energy values of orthorhombic phase, when compared with those of hexagonal phase, indicate the possibility of coexistence of these two phases of these multiferroic manganites under ambient conditions.

cond-mat.mtrl-sci↗

arXiv2010

Origin of Large Dielectric Constant with Large Remnant Polarization and Evidence of Magnetoelectric Coupling in Multiferroic La modified BiFeO3-PbTiO3 Solid Solution

The presence of superlattice reflections and detailed analyses of the powder neutron and x-ray diffraction data reveal that La rich (BF$_{0.50}$-LF$_{0.50}$)$_{0.50}$-(PT)$_{0.50}$ (BF-LF-PT) has ferroelectric rhombohedral crystal structure with space group \textit{$R3c$} at ambient conditions. The temperature dependence of lattice parameters, tilt angle, calculated polarization $(P_{s})$, volume, and integrated intensity of superlattice and magnetic reflections show an anomaly around 170 K. Impedance spectroscopy, dielectric and ac conductivity measurements were performed in temperature range $473K \leq T \leq 573K$ to probe the origin of large remnant polarization and frequency dependent broad transitions with large dielectric constant near $T_c^{FE}$. Results of impedance spectroscopy measurements clearly show contributions of both grain and grain boundaries throughout the frequency range ($10^{3}$ Hz$\leq f\leq 10^{7} $ Hz). It could be concluded that the grain boundaries are more resistive and capacitive as compared to the grains, resulting in inhomogeneities in the sample causing broad frequency dependent dielectric anomalies. Enhancement in dielectric constant and remnant polarization values are possibly due to space charge polarization caused by piling of charges at the interface of grains and grain boundaries. The imaginary parts of dielectric constant ($ε^{\prime\prime}$) Vs frequency data were fitted using Maxwell-Wagner model at $T_c^{FE}(\sim 523$K) and model fits very well with the data up to $10^{5}$ Hz. Magnetodielectric measurements prove that the sample starts exhibiting magnetoelectric coupling at $\sim 170$ K, which is also validated by neutron diffraction data.

cond-mat.mtrl-sci↗

arXiv2009

Pressure dependence of phonon modes across the tetragonal to collapsed tetragonal phase transition in CaFe2As2

The pressure dependence of a large number of phonon modes in CaFe2As2 with energies covering the full range of the phonon spectrum has been studied using inelastic x-ray and neutron scattering. The observed phonon frequency changes are in general rather small despite the sizable changes of the lattice parameters at the phase transition. This indicates that the bonding properties are not profoundly altered by the phase transition. The transverse acoustic phonons propagating along the c-direction are an exception because they stiffen very significantly in response to the large contraction of the c-axis. The lattice parameters are found to change significantly as a function of pressure before, during and after the first-order phase transition. However, the frequencies change nearly uniformly with the change in the lattice parameters due to pressure, with no regard specifically to the first-order phase transition. Density functional theory describes the frequencies in both the zero pressure and in the collapsed phase in a satisfactory way if based on the respective crystal structures.

cond-mat.supr-con↗

arXiv2009

Dynamic Disorder in Negative Thermal Expansion Compound Zn(CN)2

Dynamical disorder in negative thermal expansion compound Zn(CN)2 is investigated by quasielastic neutron scattering technique in the temperature range 170-320 K. Significant quasielastic broadening is observed above the phase transition temperature of about 250 K, however no broadening is observed at 220 K and below. Data at high temperatures are analyzed assuming the CN reorientation. Characteristic time associated with the CN orientation is estimated as 16 ps and 11 ps at 270 and 320 K respectively.

cond-mat.mtrl-sci↗

arXiv2009

Anomalous Phonons in CaFe2As2

Extensive inelastic neutron scattering measurements of phonons on a single crystal of CaFe2As2 allowed us to establish a fairly complete picture of phonon dispersions in the main symmetry directions. The phonon spectra were also calculated by density functional theory (DFT) in the local density approximation (LDA). There are serious discrepancies between calculations done for the optimized structure and experiment, because the optimised structure is not the ambient pressure structure but is very close to the collapsed structure reached at p = 3.5 kbar. However, if the experimental crystal structure is used the calculation gives correct frequencies of most phonons. The most important new result is that linewidths/frequencies of certain modes are larger/softer than predicted by DFT-LDA. We also observed strong temperature dependence of some phonons near the structural phase transition near 173K. This behavior may indicate anomalously strong electron phonon coupling and/or anharmonicity, which may be important to the mechanism of superconductivity.

cond-mat.supr-con↗

arXiv2009

Effects of magnetic doping and temperature dependence on phonon dynamics in CaFe\_{1-x}Co\_{x}AsF compounds (x = 0, 0.06, 0.12)

We report detailed measurements of composition as well as temperature dependence of the phonon density-of-states in a new series of FeAs compounds with composition CaFe1\_{1-x}Co\_{x}AsF (x = 0, 0.06, 0.12). The composition as well as temperature dependence of phonon spectra for CaFe\_{1-x}Co\_{x}AsF (x = 0, 0.06, 0.12) compounds have been measured using time of flight IN4C and IN6 spectrometers at ILL, France. The comparison of phonon spectra at 300 K in these compounds shows that acoustic phonon modes up to 12 meV harden in the doped compounds in comparison to the parent CaFeAsF. While intermediate energy phonon modes from 15 meV to 25 meV are also found to shift towards high energies only in the 12 % Co doped CaFeAsF compound. The experimental results for CaFe\_{1-x}Co\_{x}AsF (x = 0, 0.06, 0.12) are quite different from our previous phonon studies on parent and superconducting MFe2As2 (M=Ba, Ca, Sr) where low-energy acoustic phonon modes do not react with doping, while the phonon spectra in the intermediate range from 15 to 25 K are found to soften in these compounds. We argue that stronger spin phonon interaction play an important role for the emergence of superconductivity in these compounds. The lattice dynamics of CaFe\_{1-x}Co\_{x}AsF (x = 0, 0.06, 0.12) compounds is also investigated using the ab-initio as well as shell model phonon calculations. We show that the nature of the interaction between the Ca and the Fe-As layers in CaFeAsF compounds is quite different compared with our previous studies on CaFe2As2.

cond-mat.supr-con↗

arXiv2009

Lattice Dynamics and High Pressure Phase Stability of Zircon Structured Natural Silicates

We report a lattice dynamics study of relative stability of various phases of natural silicates MSiO4 (M=Zr, Hf, Th, U) as a function of pressure (P) and temperature (T), which is important in the context of their use in nuclear waste storage media. Extending our previous work on ZrSiO4, the Gibbs free energy has been calculated using a transferable interatomic potential in various phases over a range of P and T. Due to an interesting interplay between the vibrational entropy and atomic packing, the zircon (body centered tetragonal, I41/amd), scheelite (body centered tetragonal, I41/a) and huttonite (monoclinic, P21/n) phases occur at different P and T. It is shown that for ThSiO4 at high P, the huttonite and scheelite phases are favored at high and low T respectively. However, for both USiO4 and HfSiO4 the huttonite phase is dynamically unstable and the scheelite phase is stable as the high pressure phase. In fact, the calculations reveal that the stability of the huttonite phase is determined by the ionic size of the M-atom; this phase is unstable for the silicate with the smaller Hf and U ions and stable with the larger Th ion. The calculated phase diagrams are in fair agreement with the reported experimental observations. The calculated structures, phonon spectra, and various thermodynamic properties also fairly well reproduce the available experimental data. The low-energy phonons in the huttonite phase that contribute to its large vibrational entropy are found to involve librational motion of the silicate tetrahedral units.

cond-mat.mtrl-sci↗

arXiv2009

Phonon Softening and Pressure-Induced Phase Transitions in Quartz Structured Compound, FePO4

Nuclear resonant inelastic x-ray scattering on quartz structured 57FePO4 as a function of pressure, up to 8 GPa reveals hardening of the low-energy phonons under applied pressures up to 1.5 GPa, followed by a large softening at 1.8 GPa upon approaching the phase transition pressure of ~2 GPa. The pressure-induced phase transitions in quartz-structured compounds have been predicted to be related to a soft phonon mode at the Brillouin-zone boundary (1/3, 1/3, 0) and to the break-down of the Born-stability criteria. Our results provide the first experimental evidence of this predicted phonon softening.

cond-mat.mtrl-sci↗

arXiv2009

Phonon spectra in CaFe2As2 and Ca0.6Na0.4Fe2As2: Measurement of the pressure and temperature dependence and comparison with ab-initio and shell model calculations

We report the pressure and temperature dependence of the phonon density-of-states in superconducting Ca0.6Na0.4Fe2As2 (Tc=21 K) and the parent compound CaFe2As2, using inelastic neutron scattering. We observe no significant change in the phonon spectrum for Ca0.6Na0.4Fe2As2 at 295 K up to pressures of 5 kbar. The phonon spectrum for CaFe2As2 shows softening of the low-energy modes by about 1 meV when decreasing the temperature from 300 K to 180 K. There is no appreciable change in the phonon density of states across the structural and anti-ferromagnetic phase transition at 172 K. These results, combined with our earlier temperature dependent phonon density of states measurements for Ca0.6Na0.4Fe2As2, indicate that the softening of low-energy phonon modes in these compounds may be due to the interaction of phonons with electron or short-range spin fluctuations in the normal state of the superconducting compound as well as in the parent compound. The phonon spectra are analyzed with ab-initio and empirical model calculations giving partial densities of states and dispersion relations.

cond-mat.supr-con↗

arXiv2008

Phonon dynamics in Sr0.6K0.4Fe2As2 and Ca0.6Na0.4Fe2As2

We report inelastic neutron scattering measurements of the phonon density-of-states in superconducting Sr0.6K0.4Fe2As2 (Tc=30 K) and Ca0.6Na0.4Fe2As2 (Tc=18 K). Compared with the parent compound BaFe2As2 doping affects mainly the lower and intermediate frequency part of the vibrations. Mass effects and lattice contraction cannot explain these changes. A lattice dynamical model has been used to identify the character of the various phonon bands. Softening of phonon modes below 10 meV has been observed in both samples on cooling from 300 K to 140 K. In the Ca doped compound the softening amounts to about 1 meV while for the Sr doped compound the softening is about 0.5 meV. There is no appreciable change in the phonon density of states when crossing Tc.

cond-mat.supr-con↗

arXiv2008

Inelastic neutron scattering and lattice dynamical calculations in BaFe2As2

We report here first extensive measurements of the temperature dependence of phonon density of states of BaFe2As2, the parent compound of the newly discovered FeAs-based superconductors, using inelastic neutron scattering. The experiments were carried out on the thermal time-of-flight neutron spectrometer IN4 at the ILL on a polycrystalline sample. There is no appreciable change in the spectra between T = 10 K and 200 K, although the sample undergoes a magnetic as well as a tetragonal-to-orthorhombic structural phase transition at 140 K. This indicates a rather harmonic phonon system. Shell model lattice dynamical calculations based on interatomic potentials are carried out to characterize the phonon data. The calculations predict a shift of the Ba-phonons to higher energies at 4 GPa. The average energy of the phonons of the Ba-sublattice is also predicted to increase on partial substitution of Ba by K to Ba0.6K0.4. The calculations show good agreement with the experimental phonon spectra, and also with the specific heat data from the literature.

cond-mat.supr-con↗

arXiv2008

Lattice dynamical calculations of infinite layer iron oxides SrFeO2 and CaFeO2

We report extensive lattice dynamical calculations of the newly discovered infinite-layer iron oxides SrFeO2 and CaFeO2. For SrFeO2, the parameters of the interatomic potential have been determined to reproduce the zone-centre phonon frequencies reported using ab-initio calculations. Further we have extended the potential model for calculations of CaFeO2. The potential parameters are found to be transferable between the two compounds, and are used to calculate the phonon spectra in the whole Brillouin zone and several thermodynamic properties for these compounds. The calculations show fair agreement with the available experimental data of structure, thermal expansion, and mean-squared amplitudes of the atoms.

cond-mat.mtrl-sci↗

arXiv2007

Fast ion diffusion, superionic conductivity and phase transitions of the nuclear materials UO2 and Li2O

Lattice dynamics and molecular dynamics studies of the oxides UO2 and Li2O in their normal as well as superionic phase are reported. Lattice dynamics calculations have been carried out using a shell model in the quasiharmonic approximation. The calculated elastic constants, phonon frequencies and specific heat are in good agreement with reported experimental data, which help validate the interatomic potentials required for undertaking molecular dynamics simulations. The calculated free energies reveal high pressure fluorite to cottunite phase transitions at 70 GPa for UO2 and anti-fluorite to anti-cotunnite phase transformation at 25 GPa for Li2O, in agreement with reported experiments. Molecular dynamics studies shed important insights into the mechanisms of diffusion and superionic behavior at high temperatures. The calculated superionic transition temperature of Li2O is 1000 K, while that of UO2 is 2300 K.

cond-mat.mtrl-sci↗

arXiv2007

Competing antiferroelectric and ferroelectric interactions in NaNbO3 : Neutron diffraction and theoretical studies

Neutron diffraction studies using powder samples have been used to understand the complex sequence of low temperature phase transitions of NaNbO3 in the temperature range from 12 K-350 K. Detailed Rietveld analysis of the diffraction data reveal that the antiferroelectric to ferroelectric phase transition occurs on cooling around 73 K while the reverse ferroelectric to antiferroelectric transition occurs on heating at 245 K. However, the former transformation is not complete till down to 12 K and there is unambiguous evidence for the presence of the ferroelectric R3c phase coexisting with an antiferroelectic phase (Pbcm) over a wide range of temperatures. The coexisting phases and reported anomalous smearing of the dielectric response akin to dipole glasses and relaxors observed in the same temperature range are consistent with competing ferroelectric and antiferroelectric interactions in NaNbO3. We have carried out theoretical lattice dynamical calculations which reveal that the free energies of the antiferroelectric Pbcm and ferroelectric R3c phases are nearly identical over a wide range of temperature. The small energy difference between the two phases is of interest as it explains the observed coexistence of these phases over a wide range of temperature. The computed double well depths and energy barriers from paraelectric Pm m to antiferroelectric Pbcm and ferroelectric R3c phases in NaNbO3 are also quite similar, although the ferroelectric R3c phase has a slightly lower energy.

cond-mat.mtrl-sci↗

arXiv2006

Ab initio studies of phonon softening and high pressure phase transitions of alpha-quartz SiO2

Density functional perturbation theory calculations of alpha-quartz using extended norm conserving pseudopotentials have been used to study the elastic properties and phonon dispersion relations along various high symmetry directions as a function of bulk, uniaxial and non-hydrostatic pressure. The computed equation of state, elastic constants and phonon frequencies are found to be in good agreement with available experimental data. A zone boundary (1/3, 1/3, 0) K-point phonon mode becomes soft for pressures above P=32 GPa. Around the same pressure, studies of the Born stability criteria reveal that the structure is mechanically unstable. The phonon and elastic softening are related to the high pressure phase transitions and amorphization of quartz and these studies suggest that the mean transition pressure is lowered under non-hydrostatic conditions. Application of uniaxial pressure, results in a post-quartz crystalline monoclinic C2 structural transition in the vicinity of the K-point instability. This structure, intermediate between quartz and stishovite has two-thirds of the silicon atoms in octahedral coordination while the remaining silicon atoms remain tetrahedrally coordinated. This novel monoclinic C2 polymorph of silica, which is found to be metastable under ambient conditions, is possibly one of the several competing dense forms of silica containing octahedrally coordinated silicon. The possible role of high pressure ferroelastic phases in causing pressure induced amorphization in silica are discussed.

cond-mat.mtrl-sci↗

arXiv2005

Collective dynamics in crystalline polymorphs of ZnCl$_{2}$: potential modelling and inelastic neutron scattering study

We report a phonon density of states measurement of $α$-ZnCl$_{2}$ using the coherent inelastic neutron scattering technique and a lattice dynamical calculation in four crystalline phases of ZnCl$_{2}$ using a transferable interatomic potential. The model calculations agree reasonably well with the available experimental data on the structures, specific heat, Raman frequencies and their pressure variation in various crystalline phases. The calculated results have been able to provide a fair description of the vibrational as well as the thermodynamic properties of ZnCl$_{2}$ in all its four phases.

cond-mat.mtrl-sci↗

arXiv2005

Lattice vibrations in high-pressure phases of LiYF$_{4}$

Possible variations in the dynamical behaviour of LiYF$_{4}$ due to its several structural changes under pressure are examined by making use of the complementary techniques of quasi-harmonic lattice dynamics and molecular dynamics simulations. The phonon spectra in the entire Brillouin zone together with the respective Gibbs free energies are calculated for the three high-pressure polymorphs of LiYF$_{4}$ (that are stable at T = 0) with a view to better understand their relative stabilities as functions of pressure and temperature. The present work predicts anomalous thermal expansion at low temperatures in phases I and IIa while irreversibilty of phase II $\to$ phase III transition on subsequent pressure release. Molecular dynamics simulations provide qualitative impressions about a temperature-driven second-order transition and also of kinetic effects in the subsequent pressure-driven first-order phase transformation.

cond-mat.mtrl-sci↗

arXiv2004

Vibrational and thermodynamic properties of high-pressure phases in LiYF_4

Possible variations in the dynamical behaviour of LiYF$_{4}$ due to its structural changes following several pressure-induced phase transitions are examined by making use of the complementary techniques of quasi-harmonic lattice dynamics and molecular dynamics simulation. The phonon spectra in the entire Brillouin zone and the respective Gibbs free energies are calculated for the three high-pressure polymorphs of LiYF$_{4}$ that are stable at T = 0, with an aim to understand their relative stabilities as functions of pressure and temperature in terms of volume compression and vibrational entropy. Molecular dynamics simulations provide qualitative impressions about a temperature-driven second-order transformation and also of kinetic effects in the subsequent pressure-driven first-order phase transition. In addition, the calculations predict anomalous thermal expansion at low temperature in phases I and IIa while irreversibilty of phase II $\to$ phase III transition on subsequent pressure release.

cond-mat.mtrl-sci↗