[en] In search for the microscopic nature of the tunneling states (TS), which dominate the ''anomalous'' low-temperature properties of amorphous solids, neutron irradiated quartz turned out to be very interesting as a model for the glassy state: by varying the neutron dose, a gradual transition from a crystalline to an amorphous material can be obtained which allows a comparison between the induced structural damage and the ''glassy'' properties at low temperature. Recently we developed a microscopic model for the TS in neutron irradiated quartz: chains of coupled SiO₄ tetrahedra which transform from one (α₁) to the other configuration (α₂) in α-quartz. On the other hand studies of the TS after heat treatment showed that annealing could induce changes of the typical ''glassy'' distribution of the relaxation times of the TS. Here we present new data and a further analysis of annealing studies in neutron irradiated quartz. The influence of increasing the neutron dose is studied for different frequencies. It is found that when the dose approaches the threshold dose for transformation, after heat treatment, to the amorphous phase, annealing does not influence the distribution of the relaxation times of the TS like it does for lower doses. (orig.)

[en] Neutron scattering measurements were performed on the x=0, 0.5, and 0.7 concentrations of the insulating spinel Co₂Ru_1-xMn_xO₄. The experiments show that the x=0.7 system orders as a long-range ferrimagnet, the x=0.5 system has a ground state that is a mix of long-range ferrimagnetic order and short-range order, and the x=0 system only possesses short-range order

[en] Ultrasonic attenuation measurements are carried out for a quartz specimen irradiated with 4.7 x 10¹⁹ n/cm² and annealed at 700^oC during 1 hour. This dose is interesting because it is situated in the ''threshold region''. The measurements are carried out as a function of temperature (0.3-20 K) for different frequencies (80-300 MHz). The data show a behaviour that is qualitatively similar to previous observations for a much lower dose, and indicate a tendency to return to the crystalline state. (Author)

[en] This paper deals with the study of the tunneling states (TS) in neutron-irradiated quartz. Ultrasonic attenuation measurements are carried out for a dose of 1.2 x 10¹⁹ n/cm², before and after heat treatment at 700 C and 840 C. The data give evidence for significant changes in the distributions of the tunneling parameters upon heat treatment. these modifications provide interesting information about the nature of the tunneling states and will be discussed in view of the possible relation between the TS and the twin domains; the influence of heat treatment of the distribution of the tunneling states can be related to changes in the twin formation upon annealing

[en] Ultrasonic measurements in neutron-irradiated quartz are carried out for an intermediate dose of 2.6x10¹⁹ n/cm². The variation of the velocity of sound has been measured and previous attenuation measurements are extended to temperatures below 1.2 K. The TS-parameters anti P and γ₁ are calculated from numerical fittings to the tunneling model. The obtained values continue the tendency of previous measurements for lower neutron doses, where a linear increase of anti P with the dose was found. This behaviour, however, is not followed by a higher dose, situated near the ''threshold regime''. (orig.)

[en] Several investigations have recently been undertaken in order to achieve a more complete understanding of the radiation-damage mechanisms in A₂B₂O₇ pyrochlore-structure compounds. The present work represents the first systematic study of the irradiation-induced amorphization of a pyrochlore with A- and B-site cation valences of +2 and +5, respectively. Relatively large single crystals of Cd₂Nb₂O₇ were grown for these experiments. In situ ion-irradiation experiments were carried out in a transmission electron microscope in conjunction with ex situ Rutherford backscattering measurements of ion-irradiated Cd₂Nb₂O₇ single crystals. Cd₂Nb₂O₇ can be amorphized in situ by Ne or Xe ions at temperatures up to 480 and 620 K, respectively. At room temperature, the amorphization fluence was 36 times higher for 280 keV Ne⁺ than for 1200 keV Xe²⁺, corresponding to a displacement dose that was higher by a factor of 3. Disordering of Cd and Nb over the available cation sites occurs at intermediate ion doses prior to amorphization. The temperature dependence of the amorphization dose is modeled, and the results are compared to those of a previous model. The bulk-sample Rutherford backscattering spectroscopy (RBS) results were generally consistent with the in situ TEM measurements. Effects of crystallographic orientation and ion charge state had relatively little effect on the damage accumulation in bulk crystals. The RBS data are consistent with a defect-accumulation, cascade-overlap model of amorphization of Cd₂Nb₂O₇, as are the in situ TEM observations

[en] Mean-square atomic displacement in lanthanum triiron cobalt dodecaantimonide, determined as a function of temperature using single-crystal neutron diffraction, show that the La atom exhibits an anomalously large displacement at room temperature, U_eq = 0.0196 (9) A², because it is too small to fill the atomic cage formed by the corner-linked octahedral framework of M₄Sb₁₂, M = Fe, Co. Site-occupancy refinements show 25% vacancies on the La site and an actual Fe:Co ratio of 2.17:1. Analysis of the temperature dependence of the atomic displacements identifies a significant temperature-independent component for the La atom ascribed to static disorder, which amounts to 19% of the room-temperature value. The large-amplitude rattling of the La atom can be effectively linked to the dramatic decrease of the lattice contribution to the thermal conductivity, which is a key factor for improving the thermoelectric behavior of these materials. This structure-property relationship offers a new paradigm for the exploration of thermoelectric materials. (orig.)

[en] Ultrasonic velocity change and attenuation measurements are performed in neutron-irradiated z-cut quartz, for three different doses. From numerical fits of the data using the tunneling model, the typical parameters of the tunneling states (TS) are determined and compared with those found in comparable x-cut samples (irradiated under the same conditions and with similar doses). This study reveals a remarkable anisotropy, which has to be attributed to the tunneling mechanism itself. In search of the microscopic origin of the tunneling states, these results give direct experimental evidence for a microscopic picture of the TS as a rotation of coupled SiO₄ tetrahedra. At the same time, they provide important evidence for the missing link to obtain a common interpretation of all three low-frequency excitations in vitreous silica

[en] Ultrasonic attenuation measurements in neutron-irradiated quartz show distinct changes upon heat treatment in the behavior at the so-called plateau region. Since this plateau, observed in glasses and defective crystals such as neutron-irradiated quartz, is attributed to the specific distribution of the relaxation rates of the tunneling states (TS), the observations show that heat treatment can induce significant changes in this distribution, which provides an interesting tool to get more information about the nature of the tunneling states. The analysis in the framework of the tunneling model gives evidence that the TS with long relaxation times vanish upon heat treatment, which can be interpreted in terms of the disappearance of tunneling states with large mass

[en] Short communication