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In the present work we report an anelastic spectroscopy study of pure FeCO 3 (siderite) and of carbon steel samples covered by siderite films. Uncovered steel samples have also been investigated for reference. The siderite shows two relaxation processes below room temperature, at about 220 K and 140 K, while the covered steel sample displays only the peak at lower temperature. The presence of such a relaxation can be used to monitor the deposition of siderite.

A detailed study of the relaxation process occurring in polycrystalline ZrV 2 H x (0 ≤ x ≤ 1) below room temperature has been carried out by means of acoustic spectroscopy. At low H concentrations the main effect is due to the well known martensitic phase transformation around 120 K. Moreover a H-related thermally activated peak is detected around 240 K, at a vibration frequency of about 3.5 kHz. The peak features and the relaxation parameters are discussed in terms of the possible kinds of H dynamic giving rise to the anelastic effect. For the highest values of x a clear effect due to H ordering is visible around 200 K. In addition, a new thermally activated process is detected around 140 K and the experimental data can be satisfactorily fitted with the superposition of two broad peaks with activation energies of 0.14 and 0.17 eV. These processes are due to the jumps of H atoms, likely among tetrahedral interstitial sites in the lattice of the Laves phase.

Acoustic spectroscopy and electrical resistivity measurements have been employed to investigate the two relaxation processes occurring at liquid He temperature in very dilute Y–H and Sc–H alloys, and ascribed to H tunnelling systems near single or paired O traps. At very low H concentration, the peak due to the O–H tunnelling system becomes extremely narrow and is describable in terms of incoherent tunnelling with phonon assisted transitions.

The elastic energy loss of InP has been measured between 1.9 and 450 K in the frequency range 1 to 16 kHz. In the InP samples displaying the semi-insulating state, obtained either by thermally treating at 950°C the undoped material or by doping with Fe, a well-developed thermally activated relaxation process appears at 300 K. The corresponding peak is described by a single-time Debye curve with an activation energy E=0.67 eV and a pre-exponential factor of the relaxation rate τ0−1=3×1014 s−1, indicating the presence of a species mobile at room temperature and constituted by atomic complexes. Instead, in the non-semi-insulating InP sample, the peak at 300 K is either absent or masked by the background. The possibility is discussed that the mechanism giving rise to the relaxation peak is due to hydrogen-related defects.

Venezia Tecnologie SpA (ENI Group), Via delle Industrie 39, I-30175 P. Marghera (VE), Italy~Received 2 December 1999!Elastic energy absorption measurements versus temperature on semiconducting, semi-insulating~SI!, andFe-doped InP are reported. A thermally activated relaxation process is found only in the SI state, which isidentified with the hopping of H atoms trapped at In vacancies. It is proposed that the presence of In vacanciesin InP prepared by the liquid encapsulated Czochralski method is due to the lowering of their energy by thesaturation of the P dangling bonds with H atoms dissolved from the capping liquid containing H

The motion regimes of hydrogen and deuterium in p -type Si:B and GaAs:Zn have been studied by anelastic spectroscopy (elastic energy loss and dynamic modulus) between 1 and 600 K in the frequency range (1–40) kHz. Interstitial H or D in Si:B are bound to the acceptor acting as a trapping centre, and display rather high jumping rates between the four equivalent bond centre sites around B; the respective relaxation peaks are nearly single Debye processes. The H(D) reorientation rate τ −1 ( T ) is satisfactorily interpreted by the classical Arrhenius law in terms of overbarrier hopping, within the temperature range of measurements. However, by joining the data from anelastic relaxation with those from infrared absorption, the range of the measured H(D) relaxation rates is widened to 11 orders of magnitude, and a slight deviation from the Arrhenius law to higher rates is observed below liquid nitrogen temperature. The analysis of the relaxation curves indicates that the models of incoherent tunnelling used to interpret the behaviour of hydrogen (deuterium) in metals hardly apply to the presently investigated systems. The influence of quantum tunnelling is even more evident in GaAs:Zn charged with H or D, which display relaxation processes at liquid helium temperatures characterised by transition rates of more than 10 orders of magnitude faster than all those previously reported in hydrogenated semiconductors. The mobility parameters obtained and the features of the relaxation spectra may suggest that the mobile species causing the effect performs coherent tunnelling within close sites, and the relaxation rates are not related to jumps, but to transitions among the quantized energy levels of the tunnel systems. Although the species causing the effect has not yet been univocally individualised, the data shows that the presence of both H(D) and Zn in the relaxing complex is required.

Acoustic spectroscopy has been employed to investigate the nature of the processes occurring at liquid He temperature in pure polycrystalline Y and Sc for frequencies in the kHz range. The anelastic spectra indicate that hydrogen is delocalized in at least two different tunnel systems. Possible tunnel systems are discussed: the isolated or interacting H atom tunnelling between pairs of adjacent tetrahedral sites, the H atom tunnelling near single or paired O traps. It is concluded that, in the present dilute alloys, the observed relaxations have to be attributed to tunnel systems constituted by H trapped by O displaying transition rates governed by the multiphonon process.

CNR Area di Ricerca di Tor Vergata Istituto di Acustica O.M. Corbino, Via del Fosso del Cavaliere, I-00133 Roma

In the oxide superconductors with the 123 and 214 structure several anelastic relaxation processes are observed with activation energies below 0.2 eV, but there is no general agreement on their mechanisms. One of these processes, occurring in semiconducting YBCO and having an activation energy of 0.11 eV, has been observed by both the acoustic absorption and NQR measurements. This effect can be interpreted equally well in terms either of jumps of isolated O atoms in the nearly empty CuOx plane, or of polaron hopping. Also for the other processes some kind of polaron relaxation has been proposed, but no convincing evidence has been produced up to now of the involvement of polarons. After discussing what kind of polarons can give rise to the observed anelastic effects, new measurements are presented of the acoustic absorption peak with the lowest activation energy, 76 meV, in superconducting YBa2Cu3O6+x. It is shown that its intensity is maximum at the highest O stoichiometry, and steeply decreases below it, becoming undetectable already for x between 0.8 and 0.9. A connection is proposed with thepz holes which have been observed by X-ray absorption spectroscopy in the apical oxygens atx > 0.8. It is proposed that the anelastic relaxation is due to the reorientation of pairs of these holes.

The nature of the numerous thermally activated processes occurring in yttrium has been investigated by acoustic spectroscopy in polycrystalline samples. The measurements have been carried out between 1.1 and 600 K in the kHz range, varying the concentration of interstitial hydrogen and oxygen in annealed and deformed samples. Four processes have been observed, besides the main dissipation peak around room temperature due to the formation or dissolution of H pairs and that at liquid He temperature attributed to H tunneling. The processes below 300 K have been interpreted in terms of the motion of interstitial hydrogen trapped by oxygen or dragged by dislocations, while the large relaxation detected around 450 K has been attributed to the hopping of oxygen in a solid solution. {copyright} {ital 1997} {ital The American Physical Society}

The dynamics of H and D in p-type Si:B and GaAs:Zn in n-type Si:P and GaAs:Si, have been studied by acoustic spectroscopy measurements in the temperature range 1 K-550 K and at frequencies between 1 and 30 kHz. By combining the data from anelastic relaxation and from the dichroism decay, the relaxation rates of the H–B pair are obtained over 11 decades and there are clear indications of a deviation at low temperature from the classical dependence. However, no conclusions can be drawn at present on the mechanism governing the H(D) transitions. The complete absence of relaxation effects in Si:P may confirm that H occupies backbonding sites in this system. In deuterated GaAs:Zn, a new peak is observed at 20 K. This peak is much broader than a single-time Debye peak, and is due to a species performing unexpectedly high transition rates: more than 15–20 orders of magnitude higher than in all the other semiconductors measured so far. This species has been identified as the D–Zn complex. The analysis of data demonstrates that the nature of the relaxation is strongly quantistic. Measurements on GaAs:Si–D suggest that the energy barrier separating the four D antibonding sites around Si may be rather high.

The elastic energy loss of YBa{sub 2}Cu{sub 3}O{sub 6+{ital x}} is measured below room temperature with {ital x} close to the maximum stoichiometry, where three low-activation-energy peaks are present. It is found that the intensity of the process with the lowest activation energy, {ital E}{approximately}0.076 eV decreases very steeply when {ital x} is lowered just below its maximum value; it becomes undetectable when the sample is still in the {ital T}{sub {ital c}}=90 K plateau and all the other peaks are nearly unaffected. The possible mechanisms for this process are discussed, mainly in terms of hopping of polarons or off-center atoms. The characteristics that these defects should possess in order to produce anelastic relaxation are specified. The most natural way to interpret the peak is the assumption of the formation and reorientation of bipolarons among orbitals which are occupied by holes only at the highest O stoichiometries; according to Tolentino {ital et} {ital al}. [Physica C {bold 192}, 115 (1992)], such holes could reside in the {ital p}{sub {ital z}} orbitals of the apical O atoms. {copyright} {ital 1996 The American Physical Society.}

The mobility of hydrogen and its isotopes in metals has been the object of investigation for several years, whereas the diffusion studies of H in doped semiconductors started more recently. Although the H diffusion coefficient in metals may be several orders of magnitudes higher than in semiconductors, the dynamics of H in metals and semiconductors presents many common features, like precipitation trapping by heavier impurities and, as indicated by recent results, quantum tunnelling at low temperature. In metals two regimes of the H mobility are observed: hopping with deviations from a classical Arrhenius motion, and a much faster tunnelling within few close sites. In the latter regime the H dynamics does not consist of jumps but of transitions between the quantized energy levels of the tunnel systems. The types of interactions assisting the H transitions and the geometry of the tunnel systems are an open problem: although the two-level tunnel system (TLS) has been widely used to explain neutron diffusion, specific heat, and acoustic spectroscopy results in interstitial solutions (NbO x H y ), recently this model has appeared not to be valid in substitutional solutions (Nbzr x Hy, NbTi x H y ) where the tunnel systems have a higher symmetry. The four-level systems seem to be more appropriate, although the corresponding model has not been developed as much as the TLS yet. In boron doped silicon, the relaxation rates τ -1 (T) of H around B obtained from anelastic relaxation were joined with those from infrared absorption: the remarkably wide range obtained (11 decades) clearly shows a deviation of τ -1 (T) from the classical dependence at low temperature. However, no conclusions can be drawn at present on the mechanism governing the H(D) dynamics. Most recently, the investigation of the dynamics of H(D) in GaAs doped with Zn revealed a dissipation peak at 20 K in the kHz range. This relaxation has the highest rate found so far for H in a semiconductor: more than 15 orders of magnitude higher than in all the other semiconductors measured so far. The analysis of the dissipation curves indicates that the nature of the H reorientation is strongly quantistic.

The dynamics of deuterium in GaAs doped with Zn has been investigated by anelastic relaxation. The most likely configuration is D trapped by substitutional Zn, although D trapped at a Ga vacancy cannot be excluded. The relaxation of D occurs at about 20 K in the kHz range and has the highest rate found up to now for a hydrogen isotope in a semiconductor. The shape of the curves of the elastic energy loss vs temperature indicates that the nature of the D re-orientation is strongly quantistic.

The electric properties of the oxide superconductors of the RBa2Cu3O6+x family are determined by the stoichiometry and ordering of oxygen in their CuOX planes. These O atoms are also very mobile even at room temperature and below, as demonstrated by the change of the electric properties of O deficient samples with aging at room temperature. Detailed information on the atomic mobility can be obtained by anelastic relaxation experiments (acoustic absorption), and here is made a short review of the main results obtained by this technique on the mobility of O in RBa2Cu3O6+X. Three elastic energy loss peaks as a function of temperature are attributed to jumps of O in different environments. The O atoms near a O vacancy in the OI domains (x=1) can jump with an activation energy of 1.1eV, those in the OII domains (x=0.5) or in disordered chains cause a separate relaxation process with a similar activation energy (-1.2eV). The isolated O atoms are much more mobile, and they are supposed to be the cause of a relaxation process with an activation energy of only 0.1eV in the highly O deficient samples. The apparent contrast between such a high mobility and the slow time constants for reaching equilibrium in O deficient samples is discussed.

We measured the elastic energy loss of two single crystals of NbZr 0.0045 H x by exciting extensional and torsional vibrations along the [100] and [111] directions. The acoustic absorption ascribable to tunnelling of H trapped by substitutional Zr consists of at least two components. The smaller, lower temperature component appears only under [100] extensional and [111] torsional vibrations. The major component has a similar but less marked dependence on the vibration type. It is shown that such a dependence of the relaxation strength on the applied stress is exactly what is expected from a four-site tunnelling system near the substitutional impurity.

Acoustic emission activity was detected in niobium during hydrogen precipitation from solid solution. The observed emission is attributed to the nucleation and propagation of microcracks generated by the misfitting growing precipitate particles. Analysis of the acoustic signals reveals that the distribution of their amplitudes follows the Gutenberg—Ritcher law of earthquakes N(A′ >A) ≈ A−c, c = 0.9. The observed power law indicates that during the fracture processes caused by the precipitation, the NbH system goes into a self-organized critical state, similar to what occurs in the earth's crust during earthquakes.

We carried out an investigation on the symmetry of the tunnel system of H trapped by Zr in b.c.c. Nb. We measured the elastic energy dissipation of two NbZr 0.0045 H x single crystals excited with extensional and torsional vibrations along their [100] and [111] directions at frequencies around 40 kHz. Below 40 K at least two processes are observed under 〈110〉 shear ( c 11 − c 12 ); under other types of stress the intensity of the main contribution is reduced by about a factor of 5, while a smaller peak around 2 K is completely suppressed. Tunnelling over two or four sites in the perturbed region close to the substitutional impurity is considered; the absence or strong reduction of relaxation under 〈100〉 shear can be explained by the symmetry of a four-site tunnel system.

The evolution of two anelastic relaxation processes in RBa2Cu3O6+x (R = Y, Eu) above room temperature has been investigated and discussed over the whole range of the O stoichiometry. The processes seem to be associated with the O jumps in the O-I and O-II domains, and could provide the tool for evaluating the fractions of the 90 and 60 K superconducting phases.

The existing measurements of elastic energy dissipation and modulus on RBa2Cu3O6+x have been reviewed and the possible mechanisms have been discussed taking into account the whole range of temperatures and O stoichiometries. Most of the processes have been correlated with the dynamics of O in the CuOx planes: those above room temperature to jumps of O bound in chains and ordered domains, and those at low temperature with hops between off-centre positions within the chains. Free oxygen in the diluted tetragonal phase has a mobility which is anomalously high for an oxide. Phase transformations are observed both above and below room temperature, and have been discussed in connection with the relaxation processes.

We report on three phase transformations occurring between 120 and 170 K in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$ for xg0.2. It is proposed that one or two of them are due to the ordering of the O atoms among the off-center positions in the Cu-O zigzag chains. Since an electric dipole is associated to ${\mathrm{O}}^{2\mathrm{\ensuremath{-}}}$ ions in such off-center positions, ferroelectric (oxygens on the same side of the chain) and antiferroelectric (oxygens on opposite sides in zigzag fashion) domains should also form. The proposed interpretation correlates well with the overall phenomenology, including an anelastic relaxation process previously interpreted in terms of the nearly uncorrelated jumps of such oxygens. The phase transition around 170 K may correspond to a ferroelectric displacive transition appearing in the dielectric function.

We report new effects on the elastic energy dissipation and modulus of ReBa 2 Cu 3 O 6+ x (Re = Eu, Y) occuring around 500 K for x ≤ 0.3. The observed phenomenology is discussed in relation with the kinetics of the O ordering and with the phase diagram of ReBa 2 Cu 3 O 6+ x . The achievement of equilibrium is thought to pass through metastable states, each associated to the activation energy characteristic for dissociation of the particular species, like O pairs, isolated chain fragments or ordered domains.

The anelastic spectra of a single crystal of Nb 0.9955 Zr 0.0045 containing controlled amounts of O and D have been measured to clarify the nature of a narrow peak due to H tunnelling, which appears only at the lowest impurity contents in the dilute Nb substitutional alloys. It is shown that the process is due to Zr–D pairs without the involvement of unwanted O impurities. The saturation of the peak intensity can be understood in terms of the increasing interactions among the H atoms and the consequent broadening of the distribution of the energy asymmetries of the tunnel systems.

We report elastic-energy-dissipation measurements in YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} from 50 to 300 K at frequencies between 0.48 and 6.3 kHz. It is shown unambiguously that the two peaks around {Tc} are thermally activated with activation energies of 0.16 and 0.19 eV. When oxygen is reduced to about 6.5 atoms per formula unit, the higher-temperature process nearly disappears, while the other one increases. The peaks are interpreted in terms of jumps of the O atoms in the Cu-O basal planes, and two mechanisms are proposed and discussed: short jumps between the off-center positions in the chains and jumps between O(4) and O(5) positions of isolated atoms in hypothetical oxygen-depleted islands of the orthorhombic basal planes.

The anelastic spectrum of two intermetallic polycrystalline ZrV 2 H x samples (0 ≤ x ≤ 0.09) has been investigated from 1 to 600 K in the frequency range 2–50 kHz. Besides the well-known martensitic phase transformation around 120 K, two thermally activated peaks are detected in the H doped alloys around 230 and 410 K at a vibration frequency of about 3 kHz. The process at the lower temperature is due to the jumps of H atoms likely among tetrahedral interstitial sites in the cubic lattice of the Laves phase. The profile can be satisfactorily fitted with the superposition of two peaks, broader than single time Debye relaxations, with activation enthalpies of 0.33 and 0.41 eV. The peak above room temperature, which appears only on the sample containing impurity phases (primary solution phase of vanadium and zirconium), presents features which suggest a mechanism involving the interaction between point defects and dislocations.

The anelastic spectrum of the solid solution Sc–O has been investigated on a polycrystalline sample from 360 to 570 K for oxygen concentrations varying between 0.024 and 0.91 at.% O, as estimated by electrical resistivity and intentional doping. Two relaxation processes appear at 430 and 520 K for the vibration frequency of 3.5 kHz; both peaks are stable with thermal cycling and their intensities increase with the oxygen content. The process at lower temperature has been tentatively interpreted as due to the stress-induced hopping of oxygen atoms between the non equivalent tetrahedral and octahedral interstitial sites. A possible mechanism for the higher temperature process could be the dissolution/formation of interacting O–O pairs.

A detailed study of the relaxation processes occurring below room temperature has been carried out by charging polycrystalline Sc with different amount of hydrogen and deuterium. For all the concentrations, we have observed relaxation effects ascribed to tunnelling of H (D) between the two equivalent first neighbour tetrahedral sites in the c direction, which strongly depend on the H (D) content. In diluted Sc–H (up to few hundreds ppm H), due to the presence of interstitial O, the main anelastic peak is related to the tunnelling of H trapped by O. Increasing the H (D) concentration the traps are saturated and new peaks appear at lower temperature. A marked isotope effect is observed; the peaks shift in temperature and are at least four times smaller with H than with D, when measured at the same frequency. In deuterated sample the main peak has been analysed in terms of the two-level-systems (TLS). A good fit for the case of 1 at.% D could be obtained with a relaxation time τ ∝ T −7 and a small distribution of the asymmetries. This indicates that, quite surprisingly, the rate in this temperature range is dominated by multiple phonons processes.

The high-temperature anelastic spectrum of the solid solution Sc–O has been investigated on a polycrystalline sample at oxygen concentrations between 0.024 and ∼0.9 at.% O, as estimated by residual resistivity and intentional O doping. Two thermally activated relaxation processes appear near 430 and 520 K for a vibration frequency of 3.5 kHz; both peaks are stable with thermal cycling and their intensities increase with the oxygen content, indicating that they are due to O jumps. The process at lower temperature has an intensity that strongly increases with increasing temperature, when measured at higher frequency (42 kHz), indicating that the relaxation occurs between states differing in energy by ∼0.3 eV. The peak is describable by a single relaxation time, and is interpreted as due to the stress-induced hopping of single oxygen atoms between the non-equivalent tetrahedral and octahedral interstitial sites. The process at high temperature is tentatively attributed to O pairs. An estimate of the specific resistivity of O atoms has been provided.

The anelastic relaxation due to the reorientation of the ZrH pair has been measured in two Nb single crystals at low H and Zr contents. The two crystals were oriented with the longer dimension along the [100] and [111] directions, and were excited in their extensional, torsional and flexural modes. The observed dissipation between 50 and 200 K consists of at least two peaks. One of them is not observed under [100] torsional and [111] extensional vibrations, and this is interpreted as due to the redistribution of H among four-site tunnel systems over the faces of the cube containing the Zr atom. The other peak is predominant and is always observed, implying the occupation of additional sites.

Recent progress and new results are discussed from anelastic spectroscopy studies of H complexes with substitutional dopants and vacancies in Si, GaAs and InP. Anelastic relaxation processes due to the reorientation of H around dopants are found in the case of acceptors (B in Si and Zn in GaAs) but not of donors (P in Si, and Si Ga in GaAs). It is argued that this is an indication of bond-center occupancy in the first case and anti-bonding in the second one, in accordance with theoretical predictions. The reorientation rate of H around B in Si can be followed over 11 orders of magnitude, including the data from infra-red spectroscopy, and a deviation from an Arrhenius law with effective barrier of 0.22 eV at low temperature indicates the involvement of tunneling in the reorientation process. The relaxation rate of H and D in GaAs:Zn is much faster than any other dopant-H complex studied so far, and the analysis of the process demonstrates a totally non-classical nature of the H motion. This also provides evidence of off-centre occupation of H from the Zn–As bond. In InP, a new relaxation process has been found, which is present only in the semi-insulating state. It is argued that the defect is an In vacancy with the P dangling bonds partially saturated by H, and that such vacancy-H complexes have a role in obtaining semi-insulating InP, which is of technological interest.

Evidence of a very high mobility of the isolated O atoms in the Cu(1)-O basal planes of tetragonal YBa2Cu3O6.2 is reported. The jumps of the isolated O atoms between the O(4) and O(5) positions are expected to give rise to a well detectable anelastic relaxation process, and a recently reported elastic energy dissipation peak in YBa2Cu3O6.2 (63 K, 1.1 kHz) was attributed to this mechanism, resulting in an O diffusion coefficient in the basal plane of 4 × 10 −4 exp (−0.11 eV kT ) cm2/sec. Measurements are produced here which show that all anelastic processes above room temperature in orthorhombic YBa2Cu3O7-x disappear in YBa2Cu3O6.2, confirming that the low temperature peak is indeed due to the diffusion of the free oxygen.

In order to clarify the nature of the numerous thermally activated processes occurring in yttrium, the anelastic relaxation spectrum of polycrystalline samples has been investigated between 1.1 and 600 K in the kHz range, by varying the concentration of interstitial hydrogen and oxygen and in presence of plastic deformation. Besides the main dissipation peak around 300 K due to the formation /dissolution of H pairs and the peak at liquid He temperature attributed to H tunnelling, four processes have been observed. The processes below 300 K can be interpreted by mechanisms involving hydrogen trapped by oxygen, and dislocations dragging interstitial H, whilst the large relaxation observed around 450 K has to be ascribed to oxygen motion in solid solution.

A brief survey is made of the numerous anelastic relaxation processes with activation energies below 0.2 eV, which are found in the oxide superconductors with the 123 and 214 structure. Up to now there is no general agreement on their mechanisms, although some kind of electronic relaxation is often invoked. New measurements are presented of the process with the lowest activation energy, ∼80 meV, in superconducting YBa 2 Cu 3 0 6+x . It is shown that the intensity of this peak is maximum at the maximum oxygen stoichiometry, x = 1, and steeply decreases in the restricted range 0.9 0.8 by X-ray absorption spectroscopy.

The hopping and tunnelling of deuterium trapped by a Zr substitutional atom in Nb have been investigated by elastic energy loss measurements. A Nb 0.995 Zr 0.0045 D x single crystal was excited in its c′ and c 44 modes at frequencies between 6.3 and 130 kHz, from 1 to 300 K. The results are compared with previous experiments on the same sample doped with H and on Nb 1− y Ti y (H,D) x . The main peak around 150 K due to hopping of D around Zr is accompanied by secondary peaks at lower temperature, like in the alloys previously investigated. This indicates that D (as well as H) has two distinct types of occupation around the Zr atom. The most striking feature is the isotope dependence of a narrow peak at 2 K due to H (D) tunnelling. The intensity of the peak with D is more than one order of magnitude higher than that with H. An explanation of this fact is proposed, which only requires the reduction of the tunnelling matrix element when passing from H to D. Tunnelling within both two or four equivalent sites near the Zr atom is considered.

An overview is provided of the thermally activated processes and phase transformations observed in RBa2Cu3O6+x, in the framework of an interpretation as consistent as possible with them. New measurements are also reported of highly degassed YBa2Cu3O6+x, where it is shown that the only thermally activated peak left, with an activation energy of 0.11 eV, is reduced in intensity by extreme degassing. This observation supports the hypothesis that such a peak is due to the Snoek-type reorientation of isolated O atoms in the CuOx plane.

Elastic energy loss experiments were made in order to investigate the O mobility and reordering in the CuO x planes of YBa 2 Cu 3 O 6+x at the lowest values of x. The height of a dissipation peak with an activation energy of 0.11 eV, attributed to jumps of isolated oxygens, has been shown to decrease both after severe degassing treatments in vacuum and after several months of room temperature ageing. The apparent inconsistency between the high mobility of free O and the slow reordering kinetics is discussed. The repulsive Coulomb interaction between the isolated O ions is proposed to have a major influence on their aggregation probability, and consequently on the fraction of free O at equilibrium and on the time necessary for reaching it.

The statistical analysis of the acoustic signals, emitted by hydrogenated Nb samples during hydrogen precipitation from solid solution, reveals that the distribution of their amplitudes follows the Gutenberg-Richter power law, N(A'gA)\ensuremath{\sim}${\mathit{A}}^{\mathrm{\ensuremath{-}}\mathit{c}}$ with c=0.9 over more than 2 orders of magnitude. This behavior can be considered as evidence that fracture processes due to hydride precipitation in the Nb-H system determine a self-orignized critical state.

In the present study a comparison has been made between intubating condition obtainable after anesthesia induction with Thiopental or Propofol, using Vecuronium Bromide to achieve muscle relaxation. Data were collected about hemodynamic parameters, vocal cords position, coughing or bucking, and involuntary movements. Three-hundred patients, males and females, ASA classes I and II, not premedicated, were included in the study; they all had to undergo surgery requiring tracheal intubation. The patients were divided in six different groups, and in each of them intubation was performed at different times from injection of inducing agents (2-2, 30-3-4-5-6 minutes). Overall results show a lack of satisfying intubating conditions on the extreme of selected times (2 and 5-6 minutes), with no significant difference between Thiopental and Propofol, except for a minimal unlike behaviour in hemodynamics. Therefore, on the basis of our data, as far as intubating conditions are considered, we can conclude that there is no reason to prefer one of the two inducing agents.