Lattice Phonon Spectra Research Articles

Complexities in the structural evolution with pressure of water-ammonia mixtures.

The structural evolution with pressure of icy mixtures of simple molecules is a poorly explored field despite the fundamental role they play in setting the properties of the crustal icy layer of the outer planets and of their satellites. Water and ammonia are the two major components of these mixtures, and the crystal properties of the two pure systems and of their compounds have been studied at high pressures in a certain detail. On the contrary, the study of their heterogeneous crystalline mixtures whose properties, due to the strong N-H⋯O and O-H⋯N hydrogen bonds, can be substantially altered with respect to the individual species has so far been overlooked. In this work, we performed a comparative Raman study with a high spatial resolution of the lattice phonon spectrum of both pure ammonia and water-ammonia mixtures in a pressure range of great interest for modeling the properties of icy planets' interiors. Lattice phonon spectra represent the spectroscopic signature of the molecular crystals' structure. The activation of a phonon mode in plastic NH3-III attests to a progressive reduction in the orientational disorder, which corresponds to a site symmetry reduction. This spectroscopic hallmark allowed us to solve the pressure evolution of H2O-NH3-AHH (ammonia hemihydrate) solid mixtures, which present a remarkably different behavior from the pure crystals likely to be ascribed to the role of the strong H-bonds between water and ammonia molecules characterizing the crystallites' surface.

Topological phonon analysis of the two-dimensional buckled honeycomb lattice: An application to real materials

By means of group theory, topological quantum chemistry, first-principles, and Monte Carlo calculations, we analyze the topology of the 2D buckled honeycomb lattice phonon spectra. Taking the pure crystal structure as an input, we show that eleven distinct phases are possible, five of which necessarily have nontrivial topology according to topological quantum chemistry. Another four of them are also identified as topological using Wilson loops in an analytical model that includes all the symmetry allowed force constants up to third-nearest neighbors, making a total of nine topological phases. We then compute the ab initio phonon spectra for the two-dimensional crystals of Si, Ge, P, As, and Sb in this structure and construct its phase diagram. Despite the large proportion of topological phases found in the analytical model, all of the crystals lie in a trivial phase. By analyzing the force constants space using Monte Carlo calculations, we elucidate why topological phonon phases are physically difficult to realize in real materials with this crystal structure.

Bulk and Surface-Stabilized Structures of Paracetamol Revisited by Raman Confocal Microscopy.

We revisit the polymorphism of paracetamol by means of a micro-Raman technique, which has proved to be a powerful tool for structure recognition. Distinct lattice phonon spectra clearly identified the pure phases. Confocality enabled us to detect phase mixing between form II and either I or III on a micrometric scale in the same crystallite. Following the most recent findings on surface-mediated structures, we also investigated spin-coated films grown on glass, gold, and polystyrene substrates, confirming the selectivity of these surfaces for the metastable form III, which shows an unprecedented stability over a time span of several months. A mechanism of its transformation to phase II, via a partially ordered intermediate state, is suggested by polarized Raman measurements.

Micro Raman Investigation of the Photodimerization Reaction of 9-Cyanoanthracene in the Solid State

The classic photodimerization reaction of 9-cyanoanthracene in the solid state has been studied by means of Raman microscopy, analyzing the time evolution both of the molecular and of the lattice phonon spectra from the reactant to the product. This has allowed us to verify that the formation of the dimer crystal structure always takes place with some delay with respect to the onset of the chemical reaction, as already observed in another crystal to crystal photoreaction.1 This evidence points to a phenomenon of reconstructive phase transition.2 The kinetic analysis of the spectroscopic data confirms the previous hypothesis of an autocatalytic mechanism for the process.

Lattice phonons of solid phases (α,β,δ,ε) of carbon monoxide by optical studies

The phase diagram of solid carbon monoxide was investigated in the pressure range 0–10GPa and temperature range 30–300K by infrared and Raman spectroscopy. The tentative phase diagram known from the literature was expanded and specified in detail. The δ-phase region is divided into two subphases—δrot and δloc—similar to solid nitrogen. The pressure-temperature behavior of the elementary and combined excitations was also followed up. The vibron overtone region was carefully investigated by FTIR spectroscopy as a function of temperature at different pressures; the fundamental region was investigated by Raman spectroscopy. The features of the IR-active phonon sideband to the vibron overtone were investigated in detail in the entire pressure-temperature region. The lattice-phonon spectra were studied by Raman spectroscopy as a function of pressure (at lowest temperature) and by IR spectroscopy as a function of temperature at saturated vapor pressure.

Confocal Raman Spectroscopy of α‐Sexithiophene: From Bulk Crystals to Field‐Effect Transistors

AbstractWe report confocal micro‐Raman spectra of the organic semiconductor α‐sexithiophene (T6) on bulk crystals and on thin films grown on technologically relevant substrates and devices. We show that the two polymorphs, which are clearly identified by their lattice phonon spectra, may coexist as physical impurities of one inside the other in the same crystallite. Spatial distribution of the two phases is monitored by Raman phonon mapping of crystals grown upon different conditions. Raman microscopy has then been extended to T6 thin films grown on silicon oxide wafers. We identify the crystal phase in thin films whose thickness is just 18 nm. The most intense total‐symmetric Raman vibration is still detectable for a two‐monolayer thick film. Comparative analysis between micro‐Raman and AFM of T6 thin films grown on field effect transistors shows that electrode‐channel steps favour the nucleation and growth of T6 molecules on the substrate, at least below 50 nm.

Structural and magnetic instabilities of two-dimensional quantum spin systems

We present a study of the magnetic order and the structural stability of two-dimensional quantum spin systems in the presence of spin-lattice couplings. For a square lattice it is demonstrated that the plaquette deformation yields the strongest gain in magnetic energy. The analysis of the dimer-dimer response function further shows that lattice distortions may generally coexist with magnetic long-range order, in contrast to the one-dimensional case. Similarly, the coupling to Einstein phonons is found to reduce, but not to eliminate, the staggered magnetic moment. In addition, we consider the renormalization of the square lattice phonon spectrum due to spin-phonon coupling in the adiabatic approximation. Toward low temperatures, significant softening mainly of zone boundary phonons is found, especially around the $(\ensuremath{\pi},0)$ point of the Brillouin zone. This result is compatible with the tendency to plaquette formation in the static limit. We also point out the importance of a ``magnetic pressure'' on the lattice due to spin-phonon coupling. At low temperatures, this results in a tendency toward shear instabilities of the lattice.

Mechanism for singular behavior in vibrational spectra of topologically disordered systems: Short-range attractions

At low-enough fluid densities, we have found some naive singular behavior, like the van Hove singularities in the phonon spectra of lattices, appearing in the instantaneous normal mode spectra of the Lennard-Jones (LJ) 2n-n fluids, which serve as a prototype of topologically disordered systems. The singular behavior cannot be predicted by the mean-field theory, but interpreted by the perturbed binary modes of some special pairs, called the mutual nearest neighbor pairs, at separations corresponding to the extreme binary frequencies, which are solely determined by the attractive part of the LJ 2n-n pair potential. By reducing the range of attraction in the pair potential under the conditions of the same particle diameter and well depth, the tendency for the appearance of the singular behavior shifts to higher fluid densities. From this study, we conclude that pair potential with a short-range attraction can be a mechanism to produce a counterpart of the van Hove singularity in the vibrational spectra of disordered systems without a reference lattice.

Phonon spectra for crystal lattices of V, Cr, Mn, Fe, Co, Ni, and Cu

Lattice phonon spectra for 3 d elements with a cubic structure (V, Cr, Mn, Fe, Co, Ni, and Cu) are calculated approximately using the Mie‐Gruneisen potentials. Calculations were performed based on the Born‐Karman‐De Launay‐Blackman (BKDB) method [1‐4]. The De Launay force constants α ij and β ij , as well as the Born‐Karman coupling constants A ij , were determined using the Mie‐Gruneisen potentials (1) The De Launay force constants β ij correspond to the first derivative of the binding energy U ( r ) j in the crystal lattice of element j . The derivative is represented as a function of the nearest neighbor interatomic distance r 1 normalized by radius r ij of the i th coordination sphere: (2)

The properties of one-dimensional quasiperiodic lattice's phonon spectrum

A numerical method with renormalization group transformation is used to study the scaling properties of phonon spectrum and its relevant state of one-dimensional quasiperiodic lattice which is constructed by reduced map. We find that the phonon spectrum at finite gaps' edges in the binary chain Fibonacci model is a Cantor-like set spectrum. The spectrum is singularly continuous and the state is a critical state.

Raman scattering and infrared absorption spectra of Co2+ions in CsMgBr3and CsCdBr3

Polarised Raman scattering spectra of the disordered crystals CsMg1-xCoxBr3and CsCd1-xCoxBr3at 15K are reported for the concentration range 0<or=x<or=0.4. The Co2+ion electronic spectra show features that can be assigned to single Co2+ions and Co2+ion pairs. Crystal-field analyses of the single-ion spectra are compared with previous studies of Co2+ions in CsMgCl3and the crystal-field parameters are discussed in terms of a superposition model. The Co2+pair spectrum for CsMg1-xCoxBr3is analysed using a two-parameter exchange Hamiltonian. The lattice phonon spectra for CsMg1-xCoxBr3exhibit normal one-mode behaviour for the concentration range studied, but for CsCd1-xCoxBr3some two-mode behaviour is evident. Additional phonon spectra results are reported for CsCdCl3, RbMgBr3, RbMgCl3and CsMnCl3.

The influence of instrumental resolution on high frequency incoherent neutron scattering

The instrumental energy resolution Δω limits the analysis and interpretation of experimental data to times t≲(Δω)−1. Investigating high frequency molecular modes with a resolution Δω much bigger than the characteristic frequency of the lattice phonon spectrum the observed intensity has a dependence on momentum transfer Q and temperature T which is described by a Debye–Waller factor which contains only the amplitudes of the high frequency molecular modes. With improved resolution details of the signal coming from the molecular mode itself and from the lattice phonon, sidebands can be studied. This paper generalizes the recent analysis of Griffin and Jobic, which ignored instrumental resolution.

Lattice Phonon Spectra of Aluminum Oxide Anodic Films

Techniques have been developed for obtaining reproducible lattice phonon spectra of barrier aluminum oxide anodic films. Changes in the lattice phonon spectrum as a result of 15-keV proton bombardment are interpreted in terms of the formation of Al-OH bonds. Polarization spectra of stripped and unstripped aluminum oxide films confirm the findings of Vedder and Vermilyea, particularly with respect to the origin of the 960 cm−1 band. The close similarity in the observed and calculated band shapes (using the theory of Berreman) leave little doubt that the 960 cm−1 absorption is due to a longitudinal optic mode of the aluminum oxide lattice.

Level shifting effect

A level shifting effect in relation to the coupling of degenerate electronic states with vibrations in crystals is discussed. In this effect a degenerate level interacting with the lattice-phonon spectrum is shifted without the removal of degeneracy. This phenomenon is compared with Jahn-Teller effect in which the degeneracy is statically or dynamically removed without a shift. While the Jahn-Teller effect is essentially a tunnel effect with the system either tunneling over a hill (dynamic) or staying in one well (static) so that the degenerate state is instable, the present level-shifting effect is entirely stable.

Photoluminescence in SrTiO3

At 77 K photoluminescence from nominally pure, 0.05% Sm-doped and 0.005% Cr-doped SrTi${\mathrm{O}}_{3}$ gives identical spectra in the infrared (I.R.) with an intensity ratio of 1:20:400, respectively. In undoped and Sm-doped specimens, the intensity of emission is vanishingly weak at 4 K, in contrast to Cr-doped SrTi${\mathrm{O}}_{3}$, in which it is practically unchanged relative to that at 77 K. At 77 K, the spectrum consists of a narrow line at 1.5625 eV of half-width 0.5\ifmmode\times\else\texttimes\fi{}${10}^{3}$ eV, which we identify as the zero-phonon line, and 31 satellite lines, which are shown to be vibronics due to lattice phonons from the center and edge of the Brillouin zone. At 4 K the zero-phonon line is a doublet. Comparison with the reported lattice phonon spectrum is satisfactory. In addition, the data suggest the energies of several zone boundary phonons which have not been reported. In spite of identical emission spectra, undoped, Sm-doped, and Cr-doped specimens have different excitation spectra. This suggests energy transfer from Sm or Cr to a tightly bound center, isolated from the electronic energy bands, such as typically arise from rare-earth ions in ionic crystals. It is suggested that the center is an intrinsic defect or an exciton. In undoped SrTi${\mathrm{O}}_{3}$ an emission band in the visible---2.175 eV (77 K), 2.475 eV (4 K)---and its excitation spectrum are reported. The temperature dependence of both are anomalous. In contrast to the infrared emission, it is broad (half-width 0.63 eV) and structureless. Application of a dc electric field \ensuremath{\sim}100 V/cm, at 4 K, induces oscillations in time of the visible emission, a property not shared by the I.R. emission.

Heat Capacity and Structure of Vitreous Silica and Diamond‐Like Lattices (I)

AbstractThe phonon spectra of lattices with crystalline and glassy structures possessing skeletons of the purely covalent or ionic‐covalent type are discussed on the basis of the “one‐dimensional‐three‐dimensional model”. The upper frequency band, from ω′ to ωmax (ω′ ≪ ωmax), corresponds to a one‐dimensional frequency distribution g1(ω) in which elastic waves may propagate only in certain principal directions. In the lower band from 0 to ω′, the frequency distribution g3(ω) is three‐dimensional, as in the usual Debye‐Born‐von Karman theory. For the one‐dimensional distribution g1(ω) the exact solution is used and taken from papers by Pirenne and Renson, Deltour and Kartheuser for monoatomic and diatomic chains. The phonon spectrum for vitreous silica is discussed in terms of the present model, since its structure resembles that of β‐cristobalite. It is shown that the “chain model” applies to the thermal capacity of vitreous silica up to 900 °K.

Phonons in crystalline solids and dilute Bose gases

Phonons in a crystalline solid and a dilute Bose gas are treated from a uniform point of view. In each of the cases, the potential is expanded about ionic (or atomic) ‘equilibrium’ positions, with the important difference that in the gaseous state these positions are allowed to move. This motion leads to an additional term in the kinetic energy part of the gas Hamiltonian, resulting in a dispersion relation for long wavelength density fluctuations identical to that obtained by Bogoliubov. The usual lattice phonon spectrum is also reproduced. The limitations of the viewpoint when applied to liquids and Fermi gases are discussed.