Inelastic neutron scattering and lattice dynamics studies in complex solids

At Trombay, lattice dynamics studies employing coherent inelastic neutron scattering (INS) experiments have been carried out at the two research reactors, CIRUS and Dhruva. While the early work at CIRUS involved many elemental solids and ionic molecular solids, recent experiments at Dhruva have focussed on certain superconductors (cuprates and intermetallics), geophysically important minerals (Al₂SiO₅, ZrSiO₄, MnCO₃) and layered halides (BaFCl, ZnCl₂). In most of the studies, theoretical modelling of lattice dynamics has played a significant role in the interpretation and analysis of the results from experiments. This talk summarises the developments and current activities in the field of inelastic neutron scattering and lattice dynamics at Trombay.     

Inelastic neutron scattering study of lattice dynamics in α-ZnCl<Subscript>2</Subscript>

Inelastic neutron scattering experiments have been carried out to measure the phonon density of states in polycrystalline α-ZnCl₂ at Dhruva, Trombay. Lattice dynamical calculations, based on an interatomic potential model, are accomplished to study phonons associated with this otherwise extremely hygroscopic compound. Our calculated data are found to be well-compatible with the available measured ones.     

Inelastic neutron scattering and lattice dynamics of GaPO<Subscript>4</Subscript>

We report here measurements of phonon spectrum and lattice dynamical calculations for GaPO₄. The measurements in low-cristobalite phase of GaPO₄ are carried out using high-resolution medium-energy chopper spectrometer at ANL, USA in the energy transfer range 0–160 meV. Semiempirical interatomic potential in GaPO₄, previously determined using ab-initio calculations have been widely used in studying the phase transitions among various polymorphs. The calculated phonon spectrum using the available potential show fair agreement with the experimental data. However, the agreement between the two is improved by including the polarisability of the oxygen atoms in the framework of the shell model. The lattice dynamical models are also exploited for calculations of various thermodynamic properties of GaPO₄.     

Inelastic neutron scattering and lattice dynamics of ZrO2, Y2O3 and ThSiO4

Zirconia (ZrO₂), yttria (Y₂O₃) and thorite (ThSiO₄) are ceramic materials used for a wide range of industrial applications. The dynamical properties of these materials are of interest as they exhibit numerous interesting phase transitions at high temperature and pressure. Using a combination of inelastic neutron scattering and theoretical lattice dynamics we have studied the phonon spectra and thermodynamic properties of these compounds. The experimental data validate the theoretical model, while the model enables microscopic interpretations of the observed data. The calculated thermodynamic properties are in good agreement with the experimental data.      

Inelastic neutron scattering from and lattice dynamics of α-KNO3

Coherent inelastic neutron scattering techniques are employed to measure several branches of the phoon dispersion relation in KNO₃ in its orthorhombic (α-phase or phase II) form at room temperature. Group theoretical selection rules for external modes of the crystal have been used in the measurements along the three symmetry directions Σ(ξ00), Δ(0ξ0) and Λ(00ξ). Theoretical investigation of the lattice dynamics of the crystal is carried out on the basis of a rigid molecular-ion model using the external mode formalism. A two-body potential consisting of the Coulombic interaction and the Born-Mayer type short range interaction is assumed. The effective charges and radii of different atoms are determined by applying the stability criterion for the crystal. Dispersion curves are calculated, representation by representation, making use of group theoretical information. Comparison of theoretical results with experimental information on elastic constants, optical data and neutron results are made. Agreement between theoretical and the various experimental results may be considered very satisfactory.     

Lattice dynamics of strontium tungstate

We report here measurements of the phonon density of states and the lattice dynamics calculations of strontium tungstate (SrWO₄). At ambient conditions this compound crystallizes to a body-centred tetragonal unit cell (space group I4₁/a) called scheelite structure. We have developed transferable interatomic potentials to study the lattice dynamics of this class of compounds. The model parameters have been fitted with respect to the experimentally available Raman and infra-red frequencies and the equilibrium unit cell parameters. Inelastic neutron scattering measurements have been carried out in the triple-axis spectrometer at Dhruva reactor. The measured phonon density of states is in good agreement with the theoretical calculations, thus validating the interatomic potential developed.      

Phonon density of states of tetracyanoethylene from coherent inelastic neutron scattering at Dhruva reactor

Inelastic neutron scattering experiments to determine phonon density of states of coherent scattering samples of polycrystalline complex solids are generally intensity-limited and therefore are feasible only at high flux facilities. Phonon density of states of the monoclinic phase of tetracyanoethylene at 300 K, obtained using the medium resolution triple axis spectrometer at the new Indian medium flux reactor Dhruva are reported here. The raw data is converted to the “neutron weighted” phonon density of states by applying suitable corrections. Comparison made with results from a theoretical calculation based on a semirigid molecule model of lattice dynamics is fair. Results from Dhruva are also consistent with that obtained (to be published) at the high flux pulsed neutron source (ISIS) of the Rutherford Appleton Laboratory in United Kingdom.     

A first-principle study of the structural and lattice dynamical properties of CaX (X=S,Se, and Te)

We have studied structural, elastic, thermodynamic (Debye temperature and melting temperature), and lattice dynamical (phonon dispersion curves, heat capacity, and entropy) properties of CaX via ab initio calculations within the local density approximations. The results are compared with the available experimental and other theoretical data, and the agreement is, generally, quite good. We also predict the temperature and/or pressure-dependent behaviors of some mechanical, lattice dynamical, and thermodynamic properties for the same compounds.     

Intrinsic bulk vortex dynamics revealed by time resolved small angle neutron scattering

Measurements of intrinsic vortex lattice (VL) dynamics in superconductors as for instance VL melting or Bragg glass transitions are typically performed by e.g. macroscopic transport or surface sensitive measurement techniques. Therefore, usually thin superconducting films are used for microscopic measurements of VL dynamics. A direct consequence of using thin films is the strong influence of surface effects and defects, sample quality and geometry. We succeeded to combine time resolved stroboscopic small angle neutron scattering (SANS) with an advanced, time varying magnetic field setup allowing to extend the time window for slow dynamical processes to the range of 10 ms up to several minutes. The new results demonstrate that it is possible to observe directly the intrinsic dynamics of the VL in a bulk niobium single crystal on a microscopic scale without limitations due to surface effects. Field and temperature dependent relaxation times of the VL from 100 to 500 ms could be observed for the first time, allowing to directly measure the VL of the tilt modulus. This new experimental technique provides the possibility to study also the dynamical magnetic properties of various strongly correlated electron systems.     

Polarized neutron scattering studies of the kagomé lattice antiferromagnet

We report polarized neutron scattering studies of spin-wave excitations and spin fluctuations in the lattice antiferromagnet KFe₃(OH)₆(SO₄)₂ (jarosite). Inelastic polarized neutron scattering measurements at 10 K on a single crystal sample reveal two spin gaps, associated with in-plane and out-of-plane excitations. The polarization analysis of quasi-elastic scattering at 67 K shows in-plane spin fluctuations with XY symmetry, consistent with the disappearance of the in-plane gap above the Néel temperature . Our results suggest that jarosite is a promising candidate for studying the 2D XY universality class in magnetic systems.     

Negative thermal expansion in framework compounds

We have studied negative thermal expansion (NTE) compounds with chemical compositions of NX₂O₈ and NX₂O₇ (N=Zr, Hf and X=W, Mo, V) and M₂O (M=Cu, Ag) using the techniques of inelastic neutron scattering and lattice dynamics. There is a large variation in the negative thermal expansion coefficients of these compounds. The inelastic neutron scattering experiments have been carried out using polycrystalline and single crystal samples at ambient pressure as well as at high pressures. Experimental data are useful to confirm the predictions made from our lattice dynamical calculations as well as to check the quality of the interatomic potentials developed by us. We have been able to successfully model the NTE behaviour of these compounds. Our studies show that unusual phonon softening of low energy modes is able to account for NTE in these compounds.      

Hydration of the chloride ion in concentrated aqueous solutions using neutron scattering and molecular dynamics

Neutron scattering experiments were performed on 6 m LiCl solutions in order to obtain the solvation structure around the chloride ion. Molecular dynamics simulations on systems mirroring the concentrated electrolyte conditions of the experiment were carried out with a variety of chloride force-fields. In each case the simulations were run with both full ionic charges and employing the electronic continuum correction (implemented through charge scaling) to account effectively for electronic polarisation. The experimental data were then used to assess the successes and shortcomings of the investigated force-fields. We found that due to the very good signal-to-noise ratio in the experimental data, they provide a very narrow window for the position of the first hydration shell of the chloride ion. This allowed us to establish the importance of effectively accounting for electronic polarisation, as well as adjusting the ionic size, for obtaining a force-field which compares quantitatively to the experimental data. The present results emphasise the utility of performing neutron diffraction with isotopic substitution as a powerful tool in gaining insight and examining the validity of force-fields in concentrated electrolyte solutions.     

Quasi-elastic neutron scattering study of dynamics in condensed matter

Quasi-elastic neutron scattering (QENS) technique, known to study stochastic motions has been successfully used to elucidate the molecular motions and physical properties related to them, in a variety of systems. QENS is a unique technique that provides information on the time-scale of the motion as well as the geometry of the motions. In this paper, results of some of the systems studied using the facility available at Dhruva, Trombay and other mega-facilities are discussed. Emphasis is given on the results obtained from three different systems studied using QENS, namely, (1) alkyl chain motions in monolayer protected metal clusters, (2) molecular motions of propane in Na-Y zeolitic systems and (3) the study of reorientational motions of liquid crystal innO.m series in different mesophases.     

Inelastic neutron scattering and lattice dynamics of NaNbO<Subscript>3</Subscript> and Sr<Subscript>0.70</Subscript>Ca<Subscript>0.30</Subscript>TiO<Subscript>3</Subscript>

NaNbO₃ and (Sr,Ca)TiO₃ exhibit an unusual complex sequence of temperature- and pressure-driven structural phase transitions. We have carried out lattice dynamical studies to understand the phonon modes responsible for these phase transitions. Inelastic neutron scattering measurements using powder samples were carried out at the Dhruva reactor, which provide the phonon density of states. Lattice dynamical models have been developed for SrTiO₃ and CaTiO₃ which have been fruitfully employed to study the phonon spectra and vibrational properties of the solid solution (Sr,Ca)TiO₃.      

Wn(n=3—27)原子团簇结构的第一性原理计算

使用密度泛函理论下的第一性原理方法,对W_n原子团簇(n=3—27)的结构特性进行了理论计算. 得到了W_n团簇(n=3—7)的最低能量结构和(n=8—27)的局域能量极小的典型结构. 使用凝胶模型,提出的电子组态1s²1p⁶1d¹⁰2s²1f¹⁴2p⁶3s     

Wn(n=3—27)原子团簇结构的第一性原理计算

使用密度泛函理论下的第一性原理方法,对W_n原子团簇(n=3—27)的结构特性进行了理论计算. 得到了W_n团簇(n=3—7)的最低能量结构和(n=8—27)的局域能量极小的典型结构. 使用凝胶模型,提出的电子组态1s²1p⁶1d¹⁰2s²1f¹⁴2p⁶3s^{关键词：W团簇结构性质稳定性从头计算}     

Inelastic neutron scattering an ab-initio calculation of negative thermal expansion in Ag2O

The compound Ag₂O undergoes large and isotropic negative thermal expansion over 0–500 K. We report temperature dependent inelastic neutron scattering measurements and ab-initio calculations of the phonon spectrum. The temperature dependence of the experimental phonon spectrum shows strong anharmonic nature of phonon modes of energy around 2.4 meV. The ab-initio calculations reveal that the maximum negative Grüneisen parameter, which is a measure of the relevant anharmonicity, occurs for the transverse phonon modes that involve bending motions of the Ag₄O tetrahedra. The thermal expansion is evaluated from the ab-initio calculation of the pressure dependence of the phonon modes, and found in good agreement with available experimental data.     

Hydration and translocation of an excess proton in water clusters: Anab initio molecular dynamics study

The hydration structure and translocation of an excess proton in hydrogen bonded water clusters of two different sizes are investigated by means of finite temperature quantum simulations. The simulations are performed by employing the method of Car-Parrinello molecular dynamics where the forces on the nuclei are obtained directly from ‘on the fly’ quantum electronic structure calculations. Since no predefined interaction potentials are used in this scheme, it is ideally suited to study proton translocation processes which proceed through breaking and formation of chemical bonds. The coordination number of the hydrated proton and the index of oxygen to which the excess proton is attached are calculated along the simulation trajectories for both the clusters.