Ternary and quaternary phases in the alkali-earth - rhenium - oxygen system
In this work crystalline phases in the system of alkali-earth elements, rhenium and oxygen have been prepared and characterised with respect to their crystal structures and magnetic properties. The results are summarised in schemes of phase stability for Ca-Re-O, Sr-Re-O and Ba-Re-O. Hereby new comp...
| Summary: | In this work crystalline phases in the system of alkali-earth elements, rhenium and oxygen have been prepared and characterised with respect to their crystal structures and magnetic properties. The results are summarised in schemes of phase stability for Ca-Re-O, Sr-Re-O and Ba-Re-O. Hereby new compounds could be established like Sr3ReO6, M11Re4O24 (M = Sr, Ba) and Sr7Re4O19 and their crystal structures were solved and refined. In the Ca-Re-O system two new compounds were synthesised and their cell parameters were defined. Two more substances in this system, Ca11Re4O24 and Ca5Re3O15, were synthesised in a pure phase polycrystalline form for the first time. Furthermore, the crystal structures of the already reported compounds could be solved, Ca11Re4O24, or had to be reconsidered, Ca5Re3O15. These results are discussed in the light of the conditions of synthesis (related to the phase stability, especially with respect to the partial oxygen pressure) and the relationships between the different crystal structures. Investigations in the Mg-Re-O system were carried out additionally. Despite of the different synthetic techniques applied no new compounds with rhenium in the low oxidation states could be prepared in this system. Furthermore, the crystal structures of the already reported M2MgReO6 (M = Ca, Sr, Ba) compounds were determined, and the cell parameters of the Ca- and Sr- containing compounds were redefined. Magnetic properties of all new compounds were investigated. In all cases an unusual temperature dependence of magnetisation was observed. The general feature of magnetic properties of all oxides studied is a weak ferromagnetism. At the same time the ordering temperature depends unambiguously on contents of the Re+6 ions in the structures. This dependence is so strong that in the case of Sr11Re4O24, where only half of the rhenium atoms have the oxidation state of +6, the ordering temperature is 12(1) K, since in the case of Sr7Re4O19, where all rhenium atoms are in +6 oxidation state, hysteresis loops were observed at 300 K and the estimated ordering temperature is about 380 K. |
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