Exploratory synthesis: crystal chemistry and physical properties of binary, ternary and quaternary alkaline-, rare-earth and group 12 metal tetrelides

Suen, Nian-Tzu
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University of Delaware
The relationship between crystal structures, compositions and physical properties, together with the reasons determining the formation of one phase among others, still remains as an open question in the realm of solid-state chemistry. The crystal structures and bonding patterns within compounds hold the key to further understanding the physical properties of the compounds in question. Therefore this inspires us to embark on a series of exploratory synthesis of binary, ternary and even quarternary intermetallic compounds containing lanthanide f -block metals (Rare-Earth metals; denote RE hereafter), alkaline-earth metals (Mg, Ca, Sr; denote AE hereafter) or group 12 (Zn) and group 14 elements (Ge, Sn and Pb; group 14 elements are also known as tetrel elements, which we denote Tt hereafter). The compositions and crystal structures of those phases were determined by using different kinds of techniques, such as single-crystal/ powder X-ray diffractions and elemental analysis. The physical properties including magnetic susceptibilities and electrical resistivities of the compounds were reported and discussed herein. In this work, the results can be separated into four chapters including: 1) The binary RE -Ge system: RE 5Ge3 (RE = La and Ce; Pu 5 Rh3 structure type). It was confirmed that RE 5Ge3 could adopt two different structure types depending on the synthesis temperature. It crystalized with Pu 5 Rh3 structure type of low temperature (<600 °C), while it transformed to Mn5 Si3 structure type at higher temperature. 2) The ternary RE-AE -Ge system. This chapter contains three sub chapters and reports several series in RE -Mg-Ge system and RE -Ca-Ge system. In RE -Mg-Ge system, the RE 2 MgGe2 (RE = Y, Nd, Sm, Gd-Tm, Lu), Ce5 Mg8 Ge8 , CeMg2- x Gex , RE4 Mg5 Ge 6 (RE = Ce and Pr) and RE4 Mg 7 Ge6 (RE = Ce-Nd and Sm) they are all made of [MgGe4 ] tetrahedrons and/or [Ge2 ] dimers to form a polyanionic framework with rare-earth metals filled in the open spaces. In addition, the RE2 MgGe2 and RE4 Mg7 Ge6 contained uncommon square-planar coordinated Mg atoms in the structures. In RE -Ca-Ge system, we had found the RE5-x Ca x Ge3 (RE = Y, Ce-Nd, Sm, Gd-Tm and Lu) series, which adopted Mn5 Si3 structure type with rare-earth metals having been substituted by Ca atoms. The amount of Ca substitution in the structure was limited and could be explained by the Zintl-Klemm concept. The magnetic properties of substituted phases RE5-x Cax Ge 3 (ferromagnetism) were very different from the parent phases RE5Ge3 (antiferromagnetism), which were reported and discussed as well. 3) The ternary RE -Zn-Tt system. In this chapter, we report two series of RE 2 Zn5Tt (RE = La-Nd; Tt = Sn and Pb) and RE7 Zn21 Tt2 (RE = La-Nd; Tt = Ge, Sn and Pb). The structures of these two series were related to each other. The Tt atoms connected to three Zn atoms and had triangular coordination. The Zn atoms bonded to each other and form Zn7 cluster in RE2 Zn5Tt and Zn17 clusters in RE7 Zn21Tt 2 . 4) The quaternary Eu-AE -Pb-Ge system. Two phases, Eu1-x Ca x PbGe2 and Sr1-x Eu x PbGe2 , were reported in this chapter. Their structures could be viewed as linear-intergrowth of CsCl- and AlB2 -like structures with different arrangements, where it seemed that the size of cations would have some degree of influence on the structure preference. There were 1-D [Ge2 ] zigzag chains in Sr1-x Eu x PbGe2 while very rarely seen [Ge4 ] tetramers existed in Eu1-x Cax PbGe 2 . In addition, the interatomic distance between Pb and Ge atoms suggested possible-bonding interactions of Pb-Ge in Eu1- x Cax PbGe2 , which was the first example reported in intermetallic phases to the best of our knowledge.