Effect of explicit cationic size and valence constraints on the phase stability of 1:2 B-site-ordered perovskite ruthenates.

The related parameters of cation size and valence that control the crystallization of Sr(3)CaRu(2)O(9) into a 1:2 B-site-ordered perovskite structure were explored by cationic substitution at the strontium and calcium sites and by the application of high pressure. At ambient pressures, Sr(3)MRu(2)O(9) stoichiometries yield multiphasic mixtures for M = Ni(2+), Mg(2+), and Y(3+), whereas pseudocubic perovskites result for M = Cu(2+) and Zn(2+). For A-site substitutions, an ordered perovskite structure results for Sr(3-x)Ca(x)CaRu(2)O(9), with 0

Site and oxidation-state specificity yielding dimensional control in perovskite ruthenates.

Sr(3)CaRu(2)O(9), a new 2:1 B-site ordered perovskite ruthenate, was synthesized and its structure determined based on powder X-ray, neutron and electron diffraction data. It is composed of one layer of CaO(6) alternating with two layers of RuO(6) perpendicular to the [111] axis of the cubic perovskite structure. The ordering leads to a [-Ru-Ru-Ca-] repeat unit along each of the pseudocubic directions. Sr(3)CaRu(2)O(9) is the first example of this structure-type to include a majority metal with d electrons (Ru(V), d(3)). Three-dimensional Sr(3)CaRu(2)O(9) can be transformed to the layered Ruddlesden-Popper phase Sr(1.5)Ca(0.5)RuO(4) (i.e., Sr(3)CaRu(2)O(8)) by reduction at 1200 degrees C in flowing argon. The original structure can be restored by oxidation of Sr(1.5)Ca(0.5)RuO(4) at 1000 degrees C in flowing oxygen. This remarkable transformation highlights the structural versatility afforded by the combination of ruthenium and calcium.