Linking intermetallics and zintl compounds: An investigation of ternary trielides (Al, Ga, In) forming the NaZn13 structure type

A populous group of ternary trielide rich (Al, Ga, In) intermetallics formi ng the NaZn13 structure type has been synthesized from stoichiometric combi nations of the elements in an are melter. These ternary compounds have the general formula AM(x)T(13-x), where A = Ba, Sr, La, Eu, M = Cu and Ag, and T = Al, Ga, and In, with 5 less than or equal to x less than or equal to 6. 5, and have been structurally characterized by both powder and single-cryst al X-ray diffraction. Furthermore, magnetic susceptibility, electrical resi stivity, XPS, and EDS measurements are reported for some of the samples. Si ngle-crystal X-ray diffraction experiments on BaCu5Al8 (BaCu5.10(7)Al-7.90( 7), cubic, a = 12.205(4) Angstrom, Z = 8) and EuCu6.5Al6.5 (EuCu6.41(5)Al-6 .59(5), cubic, a = 11.928(1) Angstrom, Z = 8) indicate that the quasi-infin ite three-dimensional [CuxAl13-x] framework involves mostly Cu atoms center ing icosahedra, with its vertexes randomly occupied by the remaining Cu and Al atoms. On the other hand, when M = Ag, Al shows a greater tendency to o ccupy the center of the icosahedra. A systematic study of the compositional variation in BaCuxAl13-x demonstrates that the NaZn13 type phase exists wi thin a narrow range of x between five and six. To examine the role of the c ation A in stabilizing this structure, quaternary phases, e.g., BaSrAg12Al1 4, (BaSrAg12.0(1)Al-14.0(1) cubic, a = 12.689(1) Angstrom, Z = 4) and SrCeC u12Al14 (SrCeCu11.74(2)Al-14.26(2), cubic, a = 11.938(1) Angstrom, Z = 4), were prepared and characterized. Extended Huckel calculations on these tern ary aluminides demonstrate how the tuning of the system's stoichiometry max imizes the bonding within the atom-centered icosahedral framework. These ca lculations also address the substitution pattern of M and T within the [MxT 13-x] network. Tight-binding LMTO calculations have also been applied to ex amine the charge-density and electron localization functions (ELF) in this structure for different electron counts in order to address the nature of c hemical bonding in these phases. One important conclusion from the theoreti cal results is that the NaZn13 type phases show optimal stability for 40-42 valence electrons for the [MxT13-x] framework.