Polymorphie von SrTa2O6

Polymorphism of SrTa₂O₆ Orthorhombic SrTa₂O₆ is a new low temperature modification related to orthorhombic CaTa₂O₆. SrTa₂O₆(orh.) was obtained when the wellknown modification SrTa₂O₆(TTB) which is related to the tetragonal tungsten bronzes was heated in the presence of a transporting agent (chlorine) or a mineralizer (melt of B₂O₃) at temperatures below 1150°C. It could be prepared by the reaction of a 1:1 mixture of Sr(NO₃)₂ or SrCO₃ with Ta₂O₅ in a sealed quartz glass tube as well. SrTa₂O₆(orh.) also occurred as an intermediate phase of the reaction of the corresponding 1:2 mixture at temperatures below 900°C (e. g. 840°C). Indexing of Guinier powder patterns led to the following unit cell: a = 11.006 Å, b = 7.638 Å, c = 5.622 Å. At temperatures above 1220°C SrTa₂O₆(orh.) changes (in air) to SrTa₂O₆(TTB). A reversal of this transition could not be achieved without the presence of a mineralizer or a transporting agent. Ca_xSr_1?xTa₂O₆ solid solutions of the low temperature form could not definitely be established. However, at 1300°C solid TTB solutions of Ca_xSr_1?xTa₂O₆ were formed. For x > 0.05 the TTB unit cells are orthorhombically distorted. For x ≥ 0.85 the x-ray powder patterns of the solid solutions looked like the one of CaTa₂O₆(orh.) and no TTB-structure was observed at 1300°C.