Summary of the Scientific Activity

Summary of the Scientific Activity

The research activity of Rosario Cantelli was oriented to fundamental studies of the condensed matter, conceiving new experiments, and interpreting the obtained results, through the measurements of: i) anelastic spectroscopy (elastic energy absorption and elastic modulus as a function of temperature and frequency); ii) acoustic emission; iii) calorimetry, like thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), differential thermal analysis (DTA); iv) neutron scattering, XRD, EXAFS.

He is the author of more than 250 papers published in international scientific Journals, and of nearly the same number of Conference contributions. His papers have been cited more than 2500 times.

The main systems studied are schematically summarized in the following.

Metal-hydrogen systems.

Study of the interstitial dynamics of hydrogen and of its motion regimes as a function of temperature, from 900 K to 2 K; precipitation of metallic hydrides;

– 1968. He discovered the Gorsky effect due to the long-range diffusion of interstitial hydrogen, as theoretically predicted by Gorsky in 1935;

– he determined the metal-hydrogen phase diagrammes at low H concentrations, where the other techniques are insensitive;

– 1979. He observed for the first time the acoustic emission generated by hydride precipitation in Nb, Ta, V. It was shown that the stress waves emitted during the progressive lattice destruction caused by precipitation display the same features of the self-organized-criticality (SOC) which governs, through the Guthenberg-Richter law, the earthquakes and market movements.

– 1981. The anelastic spectroscopy experiments in Nb, Ta, V, and their alloys were extended down to and below the liquid helium temperatures. The measurements revealed new relaxation processes markedly deviating from the classical law, which were interpreted as caused by the quantum tunnelling of hydrogen around interstitial or substitutional trapping centres: the two-level systems (previously adopted to explain the dynamical processes in glasses), and the four-level systems.

Solid-state storage of hydrogen as an alternative ecological energy carrier. New absorbers.

He devoted remarkable attention to the study of the H-accumulation in novel artificially nanoassembled absorbers. Those materials are of remarkable interest for the applications, and his main aim was always that of attaining a better understanding of the microscopic mechanisms which govern the hydrogen storage and release.

He studied the complex hydrides, which present promising storage features, like the alanates, borohydrides, amides, imides, alanes. In those materials, hydrogen is part of the formula unit of the compound and is not an interstitially added entity, as it occurs in metals and alloys.

In the studies of alanates (NaAlH4), he proposed a H vacancy-assisted model of the atomistic mechanism occurring during the decomposition accompanying the H release, which was subsequently adopted in many theoretical studies.

High Temperature Superconductors.

He studied the superconducting oxides of type YBa2Cu3O6+x since their discovery in 1986 by the Nobel Prizes Bednorz and Müller, and observed many phenomena for the first time: phase transformations around 220, 120, and 500 K, thermally activated processes with activation energies between 0.16 and 0.19 eV in the conducting material, jumps of oxygen atoms in the Cu-O chains in the orthorhombic I (full chains) and orthorhombic II (alternately full and empty chains) phases.

In the semiconducting state (x~0) of Y-Ba-Cu-O, a new single-time relaxation effect was discovered at low temperature, characterized by a very low activation energy (0.11 eV), which was identified as the Snoek peak due to the O long-range diffusion, i.e. to the O(1)-O(5) jumps of isolated O atoms in the Co-Ox plane. For its importance, this effect was subsequently cited hundreds of times.

The complete anelastic spectrum of oxides of type La2-xSrxCuO4+d as a function of O- and Sr-doping was also measured between 1.3 and 900 K. In the insulating stoichiometric material (d~0) new and extremely intense peaks were first observed, due to pseudodiffusive lattice modes identified in the motion of apical O (in the octahedra) in a local potential with many minima. In this configuration, two different types of motion were identified: i) a collective dynamics, which demonstrated that the tilt of the CuO6 octahedra previously observed by other techniques is not static but controlled by a relatively slow dynamics (~ MHz); ii) a local motion with a much faster dynamics dominated by tunnelling.

Semiconductor-hydrogen systems.

Anelastic spectroscopy measurements in Si:B charged with H demonstrated that hydrogen jumps with a rather fast dynamics around the four bond centre sites created by B, which acts as a trapping centre. Indeed, the thermally activated relaxation peak measured at about 130 K at a few kHz, gave an activation energy for the reorientation process of 0.22 eV. The relaxations rates t-1 obtained were plotted together with the low frequency rates from infrared absorption measurements. The obtained plot of t-1(T-1) spanned of more than 10 orders of magnitudes and revealed a deviation at low temperature from the classical motion described by the Arrhenius law. This observation constituted the first irrefutable evidence for the quantum motion of hydrogen in semiconductors.

In GaAs:Zn a thermally activated relaxation effect due to the Zn-H(D) complex was first found at the liquid helium temperatures, whose rate is tens of orders of magnitude faster than those measured for the reorientation of H or D in similar complexes (see Si:B-H). This result gave, again, evidence of the quantum behaviour of H at low temperatures in semiconductors, through the formation of tunnelling states, with H in off-centre occupation with respect to the axis of the Ga-Zn bond.

The semiconducting-semiinsulating transition taking place in InP after empirical thermal treatments, whose mechanism was never rigorously understood before, was explained by the anelastic spectra in terms of H vacancies formation around In.