Clathrates Ba₈{Zn, Cd}ₓSi₄₆₋ₓ, x~7: synthesis, crystal structure and thermoelectric properties

Abstract

Novel ternary type-I clathrate compounds Ba8{Zn, Cd}xSi46−x , x ∼ 7 have been synthesized from the elements by melting and reacting in quartz ampoules. Structural investigations for both compounds, i.e. x-ray single-crystal data at 300, 200 and 100 K for Ba8Zn7Si39 and Rietveld data for Ba8Cd7Si39, confirm cubic primitive symmetry consistent with the space group type Pm¯3n (aBa8Zn7Si39 = 1.043 72(1) nm; aBa8Cd7Si39 = 1.058 66(3) nm). Whereas for Ba8Zn7Si39 site 16i is completely occupied by Si atoms, a random atom distribution with different Zn/Si ratio exists for the two sites, 6d (0.77Zn + 0.23Si) and 24k (0.91Si + 0.09Zn). No vacancies are encountered and all atom sites are fully occupied. This atom distribution is independent of temperature. Rietveld refinements for Ba8Cd7Si39 show that the 6d site is fully occupied by Cd atoms, leaving only the 24k site for a random occupation (0.96Si + 0.04Cd) consistent with the chemical formula Ba8Cd7Si39. The temperature-dependent x-ray spectra for Ba8Zn7Si39 define an Einstein mode, E,U33 = 80 K. Studies of transport properties show electrons as the majority charge carriers in the system. Although the Cd- and Zn-based samples are isoelectronic, a significantly different electronic transport points towards substantial differences in the electronic density of states in both cases.