Exploring the Growth of Charge Balanced Tetrelide Phases from Mg/Al Flux

Flux synthesis is a useful method for the exploration and development of new tetrelide Zintl phases. Reactions of heavy divalent metals (A = Ca, Sr, Ba, Eu, Yb) with tetrels (Tt = Si, Ge, Sn) in Mg/Al flux mixtures produce charge-balanced tetrelides with the general formula (A/Mg)2Tt; these compounds have hexagonal structures analogous to those of a large family of rare earth transition metal phosphides. Initial reactions of ytterbium, barium and silicon in Mg/Al melts produced three new phases: Ba2Yb0.9Mg11.1Si7, Ba5Yb2Mg17Si12, and Ba20Yb5Mg61Si43, isostructural to the ternary phosphides Zr2Fe12P7, Ho5Ni19P12, and Ho20Ni66P43 respectively. Later a fourth silicide (Ba6Yb1.84Mg18.16Si13, analogous to Zr6Ni20P13) was synthesized. All these phases exhibit Pauli paramagnetic behavior, indicative of divalent Yb2+ ions. From the density of states (DOS) calculations on Ba2Yb0.9Mg11.1Si7, Ba5Yb2Mg17Si12 and Ba6Yb1.84Mg18.16Si13, these compounds are expected to be poor metals/semimetallic. Due to the very complex nature of the Ba/Yb/Mg/Si system (there are four competing, structurally related compounds) quenching experiments and in situ neutron powder diffraction studies were carried out to determine the reaction parameters that favor certain products. Under slow cooling conditions, Ba5Yb2Mg17Si12 precipitates from the flux at 800 °C. A faster cooling rate of an identical reaction results in the formation of single crystals of Ba20Yb5Mg61Si43 in the flux at 640 °C. This indicates that the crystallization of products in this metal flux reaction does not involve precipitation and interconversion of different phases but instead depends on the rate of cooling across the supersaturated metastable zone in this system. The other two compounds (Ba2Yb0.9Mg11.1Si7 and Ba6Yb1.84Mg18.16Si13) were not seen in the in situ neutron diffraction experiments. These phases might form under rare and difficult to reproduce conditions (caused by temperature fluctuations that occur when opening the furnace door) that position the reaction at a specific point in the metastable zone. Further exploration of reactions of tetrels with divalent alkaline earth or rare earth metals in Mg/Al flux was carried out. All the (A/Mg)2Tt products are tetrelide analogs of hexagonal ternary phosphides (Zr2Fe12P7, Ho5Ni19P12, Zr6Ni20P13 or Ho20Ni66P43 structure types). The charge-balanced stoichiometries result in semimetallic behavior, and the complex structures and heavy element incorporation yield low thermal conductivity, confirmed by thermoelectric measurements on two representative compounds. Ba5Eu2.85Mg16.15Si12 and Ba5Yb2.26Mg16.74Si12 have room temperature thermal conductivities of 2 – 3 W/mK, and Seebeck coefficients of +160 uV/K and -75 uV/K respectively, with Ba5Eu2.85Mg16.15Si12 exhibiting magnetoresistance around 2 K. These materials are promising for thermoelectric applications at high temperatures. Incorporation of europium results in compounds exhibiting very weak magnetic transitions at low temperatures. Magnetic susceptibility measurements on Ba5Eu2.85Mg16.15Si12 and Eu4.64Yb2.32Mg18.04Si12 indicated that both phases show no ordering below 3 K and that their Eu ions are divalent.