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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.

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.

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.

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 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.

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.