Baryum, geçiş metali, silikon ve germanyumdan oluşan I-tipte kafes bileşikleri ve TaNiSi bileşiğine katkılar

Throughout recent years, clathrate compounds attracted significant attention due totheir unique transport properties. These compounds are reported to have potentialsuperconducting, thermoelectric, electro-optic, and super-hard materials applications. Amongthese applications high thermoelectric efficiency has been the most challenging property to beobtained. Since the thermoelectric efficiency of clathrates is still low, towards for anapplication the compounds and their exact compositions have to be investigated andoptimized. Here, we report on investigations of type I clathrate compounds with thecomposition Ba8(Tx(Ge,Si)yGz)(Ge,Si)40 (T = Ni, Pd, Pt; G = defect; x+y+z = 6). The crystalstructure and homogeneity phase range of Ni containing Ge clathrates were determined andphysical properties were discussed in context with the Si clathrates and clathrates with othertransition metals. Single crystals with different compositions were obtained, their structuresand site occupancies were determined from single crystal X-ray analysis. Investigations ofelectrical-transport properties of compounds revealed a semiconductor-metal transition and abad metallic behavior for Ge and Si clathrates, respectively. The case for Ge was justified alsoby theoretical calculations. No superconductivity was observed for temperatures down to 2 K.Magnetic susceptibility measurements showed a large diamagnetic response for allcompounds. The low thermal conductivity observed for these compounds can be explainedvia strong scattering of phonons introduced by the dynamic rattling motion of the guest atomsinside the 24-atom polyhedra, deduced from the relatively high atomic displacementparameters obtained from low temperature single crystal X-ray analysis. TaNiSi compoundhas been found as a by product material during the preparation of Si clathrate compounds. Inthis work, a new low temperature route for the synthesis of TaNiSi was developed whichenables the access to single phase materials and well-shaped single crystals. Crystal structureof the compound has been determined using single crystal X-ray diffraction method. Thephysical properties have also been studied by resistivity, specific heat capacity and magneticsusceptibility measurements. The investigated compound behaves as Pauli paramagnet andshows bad metallic conductivity. Calculated Debye temperature (434 K) indicates relativelyhigh stiffness of the compound. DTA analysis shows that it melts at 1852 ºC which enables itas a candidate for high temperature processing. Additionally, a method of single-crystallinecoating on metal wires was successfully developed.