Navida Nasir

Formation of clathrates Ba-M-Ge (M = Mn, Fe, Co)

Abstract In order to define the ability of magnetic elements M = Mn, Fe, Co to stabilise clathrate structures, alloys of the Ba – M – Ge system were investigated in the as-cast state and after annealing at 700°C and 800°C by means of X-ray powder diffraction, light optical and electron-probe microanalysis. Temperatures of phase transformations were derived from differential thermal analysis. Results are summarised in (i) isothermal sections at 700°C and 800°C, (ii) solidus and liquidus surfaces covering the region of existence for both clathrate phases in these systems. Invariant reactions during crystallisation are presented in form of Schultz – Scheil diagrams. In all three cases only limited solubility of the M element was found for clathrate IX (Ba6Ge25) i. e. the Ge-framework in the crystal structure of Ba6MxGe25 – x dissolves 0.6 atom of Mn, and about 1 atom of Fe and Co per unit cell. The maximum solubility of iron in clathrate type I (Ba8Ge46 – x) was found to be less than 0.5 Fe atom per unit cell, and clathrates with Mn and Co contain up to 1.0 and 2.5 atoms in the unit cell, respectively. Whilst Fe does not decrease the formation temperature of the clathrate phase, Mn and Co decrease it from 770°C (for binary Ba8Ge43) to 766°C and 749°C, respectively.

Evaluation of the thermoelectric potential of the type-I clathrate Ba8NiyZnxGe46−x−y

A detailed investigation of the high-temperature thermoelectric properties of the ternary clathrate of type-I Ba8ZnxGe46−x for the range 7.2 ≤ x ≤ 7.8 is presented. Electrical resistivity, Seebeck coefficient, thermal conductivity and Hall-effect measurements clearly demonstrate that the maximum ZT-value can be expected for the composition Ba8Zn7.7Ge38.3, which reaches ZT ~ 0.46 at a temperature of 730 K. Furthermore, the influence on the thermoelectric properties was studied for partial substitution of Zn by one Ni-atom (Ba8NiyZnxGe46−x−y with ynom = 1, and 6.5 ≤ x ≤ 7.2) or by three Ni-atoms per unit cell (Ba8NiyZnxGe46−x−y with ynom = 3 and x = 1 to 3). Whereas a small amount of Ni increases the figure of merit ZT to 0.65 at 800 K, a higher Ni-content reduces the thermoelectric performance. Hall-effect measurements reveal that this improvement is essentially due to an increased charge carrier mobility.