Sound velocity in liquid and glassy selenium

The speed of longitudinal sound waves at 7 and 22 MHz has been measured in liquid, supercooled, and amorphous selenium, including the region around the glass transition temperature, T_g, near 35 °C. In amorphous selenium the speed of shear waves at 7 MHz was also measured. The experiments were performed with high purity Se (99.9999%) hermetically sealed in an evacuated silica ampoule. Four temperature regions with strongly different relaxation times can be distinguished between room temperature and the melting point: (1) a glassy state below T_g, which is stable on the time scale of the experiments, (2) a glassy state above T_g, which is metastable on the time scale of the experiments, (3) a region where homogeneous crystal nucleation occurs, and (4) a supercooled liquid, which is stable on the time scale of the experiments. Each region is marked by a change in the slope of the temperature dependence of the sound velocity. Near the glass transition temperature the velocities of longitudinal and transverse sound exhibit hysteresis with a step-like drop on heating and a more continuous rise on cooling. The step-like anomaly in sound velocity may be a general property of the glass transition.     

Structure of liquids and glasses in the Ge–Se binary system

A summary is given of the partial structure factors that have been measured for liquids and glasses in the Ge–Se binary system by using diffraction methods. Information is presented on the pair correlation functions describing the atomic species and use is made of the Bhatia–Thornton formalism to separate those pair correlation functions describing the system topology from those that describe the chemical ordering. The information made available by the measured Bhatia–Thornton partial structure factors on the thermodynamic properties of Ge–Se mixtures is briefly summarized. The properties of the network structures formed in liquid and glassy Ge–Se compounds are investigated as a function of composition, temperature and pressure. For GeSe₂ at ambient pressure it is shown that the so-called first sharp diffraction peak, which appears in the measured diffraction patterns at a small scattering wavevector value of ≈1 Å⁻¹ and which is associated primarily with Ge–Ge correlations, does account for discernable features of the observed intermediate range order in both the liquid and glassy phase. The first sharp diffraction peak in the measured Ge–Ge partial structure factor for both phases is described by comparable parameters suggesting communality in the underlying intermediate range order which survives the glass transition. Homopolar bonds are found to be a feature in the structure of liquid GeSe and both liquid and glassy GeSe₂ with Ge–Ge and Se–Se distances in the ranges 2.33(3)–2.42(2) and 2.30(2)–2.34(2) Å, respectively.     

Acoustic properties of aspirin in its various phases and transformation stages studied by Brillouin scattering

The acoustic behaviors of aspirin were studied in its crystal, liquid, glass states as well as transformation stages between them. The Brillouin doublet of the longitudinal acoustic (LA) mode propagating to the [100] direction of aspirin crystals showed a splitting in a temperature range of 393-413 K below the melting temperature, which indicated a softening and distortion of the layered aspirin crystal in its premelting stage. One LA mode was observed in the liquid and glassy phases of aspirin, consistent with Brillouin selection rule for isotropic materials. The acoustic damping in the glassy aspirin was systematically higher than that in the crystal aspirin, which was in contrast to ibuprofen materials. These suggested different activity of remnant local dynamics that could couple to the acoustic waves, persistent in the glassy state of these two representative pharmaceuticals. In the supercooled liquid phase, the Brillouin spectrum displayed a new spectral feature characterized by a high-frequency cutoff and a distributed low-frequency contribution other than the Brillouin peak of the amorphous aspirin. This could be ascribed to the formation of crystalline grains in the glassy matrix due to cold crystallization.     

On the phase diagram of polymorphic ethanol: Thermodynamic and structural studies

It is well known that ethanol exhibits a very interesting polymorphism presenting different solid phases: a fully-ordered (monoclinic) crystal, a (bcc) plastic crystal, which by quenching becomes an orientationally-disordered crystal with glassy properties (hence sometimes named ‘glassy crystal’), and the ordinary amorphous glass. We have carried out calorimetric, X-ray diffraction, and Brillouin-scattering experiments above liquid-nitrogen temperatures and have found several new features that shed more light on the rich and interesting phase diagram of ethanol. Firstly, we have identified up to four different varieties of the monoclinic crystalline phase depending on the thermal history. We also present new specific-heat data of these glassy and crystalline phases below the glass transition temperature up to the melting temperature. Furthermore, we have unexpectedly found that the amorphous phase can also be obtained by the unusual route of a very slow cooling of the liquid in some particular experimental set-ups, evidencing the heterogeneous character of the crystallization kinetics of these molecular glass-formers.     

Acoustic behaviour of amorphous selenium

The attenuation of longitudinal sound waves in amorphous selenium has been measured at frequencies of 15 and 25 MHz and in the range of temperature between 10 and 310 K. It is shown that absorption exhibits a minimum around Tτ150 K, while a pronounced maximum takes place around 30 K. Such a behaviour is briefly discussed in the frame of acoustic properties of amorphous material below the glass temperature. It is shown that the experimental results can be described, almost qualitatively, in terms of a recently proposed theoretical model involving a stochastic resonance that seems to be characteristic of the disordered systems.     

Origin of Rayleigh scattering and anomaly of elastic properties in vitreous and molten GeO2

Vitreous (v) and molten (m) GeO₂ were studied by Rayleigh and Mandel’shtam–Brillouin scattering spectroscopy and high-temperature acoustics. Original measurement apparatus and procedure were used that included Bayseian deconvolution of light scattering spectra of vGeO₂ and a specially designed high-temperature (up to 1500 °C) acoustic interferometer to measure temperature and frequency dependence of ultrasonic (US) velocity and attenuation in mGeO₂. Landau–Placzek ratios for vGeO₂ were found optically (from the light scattering spectrum) and acoustically (through the Schroeder’s formalism). Dispersion of optical and other physical parameters of vGeO₂ found by many authors is explained by the existence of small amount of GeO in the samples. It means that properties of vGeO₂ are under the influence of redox synthesis conditions controlling the GeO₂ ↔ GeO and coordination [GeO₄] ↔ [GeO₆] equilibrium in vGeO₂. Measurements of temperature dependencies of longitudinal ultrasonic velocities in mGeO₂ and in the PbO–GeO₂ glass melts as a function of PbO concentration shows existence of ‘water-like anomaly’ in mGeO₂ and in liquid germanates with the rich content of GeO₂ where equilibrium sound velocity increases with the temperature.     

High frequency dynamics of an orientationally disordered molecular crystal

The dynamic structure factor of the polycrystalline plastic crystal 1-cyanoadamantane has been measured in the THz frequency region by inelastic X-ray scattering as a function of the exchanged momentum Q in the range 1–10 nm⁻¹ and as a function of the temperature in the two solid disordered phases: plastic crystal and glassy crystal. We find that the dispersion of the acoustic excitations is crystal-like in the two phases and that the attenuation Γ(Q) shows a negligible T dependence similarly to structural glasses and a Q^α behavior with 1.15 < α < 1.50 at variance with structural glasses characterized by a Q² dependence.     

Microscopic collective dynamics in liquid tellurium

The spectra of collective excitations of liquid tellurium have been studied by means of inelastic neutron scattering. Here we report on the dynamics of liquid Te as measured at two different temperatures, just above melting (T_m = 723 K) and at ∼1000 K. Estimates for the velocity of propagating excitations for both temperatures have been obtained from the experimental data and a contrasting behavior is found with respect to anomalies shown by the adiabatic sound velocity measured by ultrasound methods.     

Acoustic damping in Li2O–2B2O3 glass observed by inelastic X-ray and optical Brillouin scattering

The dynamic structure factor of lithium-diborate glass has been measured at several values of the momentum transfer Q using high resolution inelastic X-ray scattering. Much attention has been devoted to the low-Q-range, below the observed Ioffe–Regel crossover q_IR ≃ 2.1 nm⁻¹. We find that below q_IR, the linewidth of longitudinal acoustic waves increases with a high power of either Q, or of the frequency Ω, up to the crossover frequency Ω_IR ≃ 9 meV that nearly coincides with the center of the boson peak. This new finding strongly supports the view that resonance and hybridization of acoustic waves with a distribution of rather local low frequency modes forming the boson peak is responsible for the end of acoustic branches in strong covalent glasses. Further, we present high resolution Brillouin light-scattering data obtained at much lower frequencies on the same sample. These clearly rule out a simple Ω²-dependence of the acoustic damping over the entire frequency range.     

Composition dependence of the glass transition temperatures of PdNiP and PtNiP glasses

Thermal properties of glassy PdNiP and PtNiP alloys have been measured as a function of the concentration of transition metals. The glass transiion temperature, T_g, of these alloys glasses exhibits a negative linear deviation with transition metal content - which is in contrast to the increasing T_g of binary glassy alloys with increasing metalloids.It is suggested that the suppression of the glass transition temperature of these glassy alloys may be attributed to the excess configurational entropy of disorder associated with a mixture of hard spheres differing in radius. In contrast, the increasing T_g of binary glassy alloys with the metalloid content may be associated with the short-range order resulting from strong interactions between metal and metalloid atoms.     

Dynamical properties of solutions of LiNO3 in (1,3)-diaminopropane in the liquid and glassy states

Quasielastic neutron scattering measurements have been made for concentrated solutions of LiNO₃ in (1,3)-diaminopropane in the liquid and glassy states. The results indicate that a harmonic lattice vibration-like motion of the atoms in solid glasses changes around the glass transition temperature to an intermediate one prior to a translational diffusive motion at higher temperatures. This picture is consistent with the previous results obtaiend from NMR relaxation mearsuremts.     

Influence of pressure on fast dynamics in polyisobutylene

Influence of pressure on fast dynamics and elastic properties in polyisobutylene is studied using Raman, Brillouin and neutron scattering spectroscopy. Analysis of the results shows that the boson peak frequency increases with pressure stronger than the longitudinal sound velocity measured by Brillouin scattering. Moreover, the boson peak intensity decreases under pressure stronger in Raman scattering than in neutron scattering suggesting a decrease in the light-to-vibrations coupling coefficient C(ν). The strong decrease of the microscopic peak intensity under pressure in Raman spectra supports this suggestion. We argue that variations in C(ν) might be related to amplitude of structural fluctuations. We speculate that change in disorder and/or overall density under pressure is the main cause for the observed variations.     

Study of the microdynamics of liquid lithium and lithium-hydrogen melt by inelastic neutron scattering

The frequency spectra of vibrations of Li atoms at temperatures of 22, 227, 397, and 557°C and the lithium-hydrogen melt (98 at % ⁷Li, 2 at % H) at 557°C have been obtained from the experimental neutron inelastic scattering data. On the basis of the frequency spectra, the temperature dependences of the mean-square displacement of Li atoms, the mean-square amplitudes of atomic vibrations, and the velocity autocorrelation function of atoms have been calculated. The speed of sound in liquid lithium has been estimated within the Debye model. The frequency spectra of lithium-hydrogen melt and solid lithium hydride are compared. A generalized frequency spectrum of vibrations of hydrogen atoms in lithium-hydrogen melt is obtained.     

High precision measurements of the temperature dependence of the sound velocity in selected liquid metals

The temperature dependence of the sound velocity in liquid lead, tin, bismuth and antimony has been measured with high precision (errors of less than 0.35%) by the ultrasonic pulse transmission technique. The measurements were performed from the melting temperature to approximately 1000 °C. A smooth temperature dependence of the sound velocity was found in liquid lead and antimony. In liquid lead, a linear dependence with a negative temperature coefficient was observed whereas for liquid antimony the sound velocity displays a distinct maximum. The high precision of the measurements enabled uncovering localized features in the temperature dependence of the sound velocity in liquid tin and bismuth. The measurements provide some information on the temperature dependent structure of the molten state of these four elements.     

High frequency collective dynamics in liquid potassium

We investigated by inelastic X-ray scattering the dynamical properties of molten potassium in a wide range of momentum transfer, Q, from 1 nm⁻¹ up to the main peak of the structure factor Q ≈ 17 nm⁻¹. The observed increase of sound velocity in the low Q region (1 < Q < 3 nm⁻¹), has been described within a model characterized by two distinct relaxation processes for the collective dynamics. The obtained results are discussed and compared with those from previous neutron scattering experiments. In particular, we associate the speed-up of the sound velocity to the ‘instantaneous’ disorder of the liquid as opposed to the argument, supported by some neutron scattering studies, of a transition from a liquid to solid like response of the system.     

Low-temperature properties of glassy and crystalline states of n-butanol

We present low-temperature measurements of the specific heat and the thermal conductivity for the three solid phases of n-butanol, namely glass, crystal and “glacial” phases. Whereas crystal and glass ones are found to exhibit the expected thermal behavior for crystalline and non-crystalline solids, respectively (i.e. Debye theory for crystals, and universal low-temperature properties with a boson peak and a concomitant plateau in the thermal conductivity for glasses), the so-called “glacial phase” behaves as a very defective crystal, confirming earlier work that identifies it as a mixed phase of nanocrystallites and a disordered matrix. We have also measured longitudinal and transverse sound velocities at low temperatures for the glass phase. The elastic Debye coefficient and Debye temperature of the glass determined from these measurements are found to agree very well with the calorimetric ones obtained from a SPM analysis of the specific heat.     

The structure of phosphate glass evidenced by small angle X-ray scattering

Numerous phosphate glass systems with compositions covering the entire glass forming range have been examined by small angle X-ray scattering. The experiments were carried out in a widely extended interval of s values between 0.055 and 31 nm⁻¹. A small angle scattering effect was detected for all samples investigated with the exception of the aluminium metaphosphate glass. The small angle scattering recorded indicates the presence of two differently-sized heterogeneity regions of electron density. The small-scale heterogeneities are present in magnesium and zinc phosphate glasses only. Their size is about 1 nm diameter in the magnesium and about 2 nm in the zinc phosphate glasses. The species do not result from liquid–liquid phase separation. Examinations to extract information on the nature of these heterogeneity regions are described in detail. There is no fundamental knowledge on the large-scale heterogeneities established. The small angle scattering suggests that their occurrence is related to water contamination. Anomalous small angle X-ray scattering experiments of a series of zinc polyphosphate glasses and a strontium metaphosphate glass sample are performed to examine the distribution of the Zn or Sr atoms, respectively.     

Molar volume and compressibility of the liquid Sb---Se system

Molar volume and sound velocity measurements on the liquid Sb---Se system have been made up to 900°C to investigate thermodynamic properties. The molar volume, thermal expansion coefficient and adiabatic compressibility show the behaviour in the temperature and composition range characteristic of a non-metal-metal transition. The results are interpreted as being due to the existence of a covalent Sb₂Se₃ entity in the liquid. The two-liquid phase between Sb and 50 at.% Sb has been found to be localized in a much smaller temperature-composition range than previously assumed.