The formation, phase relations, crystal chemistry and physical properties were investigated for the solid solution deriving from binary clathrate with a solubility limit of 8 Zn atoms per formula unit at 800 °C ( is a vacancy). Single-crystal x-ray data throughout the homogeneity region confirm the clathrate type I structure with cubic primitive space group type . Temperature-dependent x-ray spectra as well as heat capacity define a low-lying, almost localized, phonon branch, whereas neutron spectroscopy indicates a phonon mode with significant correlations. The transport properties are strongly determined by the Ge/Zn ratio in the framework of the structure. Increasing Zn content drives the system towards a metal-to-insulator transition; for example, shows metallic behaviour at low temperatures, whilst at high temperatures semiconducting features become obvious. A model based on a gap of the electronic density of states slightly above the Fermi energy was able to explain the temperature dependences of the transport properties. The thermal conductivity exhibits a pronounced low-temperature maximum, dominated by the lattice contribution, while at higher temperatures the electronic part gains weight. Zn-rich compositions reveal attractive Seebeck coefficients approaching −180 µV K−1 at 700 K.

A very small capacitive sensor for measuring thermal expansion and magnetostriction of small and irregular shaped samples has been developed. A capacitive method with tilted plates is used. The tilted plate capacitance formula is used for the calculation of the capacitor gap, the calibration is performed by measuring the signal of a standard material. The active length of the sample can be less than 1 mm. The absolute resolution is about 1 Å. All mechanical connections of the dilatometer are carried out by tiny CuBe springs, enabling the small force on the sample to be adjusted ͑50-500 mN͒ and no additional sample fixing is necessary. The cell has been tested in the temperature range 0.3-200 K and in static magnetic fields up to 15 T. The zero signal of the dilatometer has been determined by measuring a silver sample. The correct operation and reproducibility has been verified by measuring the thermal expansion of Cu. The thermal expansion and magnetostriction of a DyCu 2 single crystal has been determined. The advantage of this method compared to specific heat measurements is that a large temperature range can be covered with one equipment. This high static and dynamic range of sample length, temperature, and magnetic field suggests a number of possible applications, like the investigation of crystal field effects on the magnetoelastic properties of single crystals or structural phase transitions.

Specific heat, resistivity, magnetic susceptibility, linear thermal expansion (LTE), and highresolution synchrotron X-ray powder diffraction investigations of single crystals Fe1+yTe (0.06 ≤ y ≤ 0.15) reveal a splitting of a single, first-order transition for y ≤ 0.11 into two transitions for y ≥ 0.13. Most strikingly, all measurements on identical samples Fe1.13Te consistently indicate that, upon cooling, the magnetic transition at TN precedes the first-order structural transition at a lower temperature Ts. The structural transition in turn coincides with a change in the character of the magnetic structure. The LTE measurements along the crystallographic c-axis displays a small distortion close to TN due to a lattice striction as a consequence of magnetic ordering, and a much larger change at Ts. The lattice symmetry changes, however, only below Ts as indicated by powder X-ray diffraction. This behavior is in stark contrast to the sequence in which the phase transitions occur in Fe pnictides.

We report on a new high resolution apparatus for measuring magnetostriction suitable for use at cryogenic temperatures in pulsed high magnetic fields which we have developed at the Hochfeld-Magnetlabor Dresden. Optical fibre strain gauges based on Fibre Bragg Gratings are used to measure the strain in small (∼ 1 mm) samples. We describe the implementation of a fast measurement system capable of resolving strains in the order of 10 −7 with a full bandwidth of 47 kHz, and demonstrate its use on single crystal samples of GdSb and GdSi.