VALENCE AND STRUCTURAL PHASE TRANSITIONS IN THE SERIES Ba₂PrRu_1-xIr_xO₆

Brendan J. Kennedy,^a Leqing Li,^a Christopher J. Howard^b and Brett A. Hunter^b

^aThe School of Chemistry, the University of Sydney, Sydney, NSW 2006, Australia; ^bANSTO, PMB1 Menai NSW 2234, Australia; (b.kennedy@chem.usyd.edu.au)

There is considerable interest in the magnetic and electronic properties of the 4d and 5d oxides. The double perovskites Ba₂PrRuO₆ and Ba₂PrIrO₆ both exhibit rocksalt ordering of the B-type cations (Pr and Ru(Ir)). Previous studies have demonstrated that in the Ru compound the oxidation states are Pr(III) and Ru(V) whereas in the Ir compound they are Pr(IV) and Ir(IV). The series of oxides Ba₂PrRu_1-xIr_xO₆ might therefore be expected to show interesting properties and structures.

High-resolution powder synchrotron diffraction studies of 15 members in the series Ba₂PrRu_1-xIr_xO₆ have been undertaken at the ANBF. These studies have confirmed that the structure of Ba₂PrRuO₆ is best described in the monoclinic space group P2₁/n, however we find that Ba₂PrIrO₆ is in fact tetragonal and can be adequately described in P4/mnc. The transition between these two structural types is related to the relative size of the B-cations involved and occurs near x = 0.3. That is the Pr exists in the +3 oxidation state in the Ru rich compounds and this stabilizes a a⁺b^‑b^- tilt patterns so that the structure is monoclinic. In the Ir rich compounds the Pr is present in the +4 oxidation state and the reduction in size of the Pr cation alters the tilt system is a⁰a⁰c⁺. Interestingly although a continuous P4/mnc to P2₁/n transition is allowed, in the present compounds this is observed to be first order and there is a two-phase region between x = 0.4 and 0.5.

The temperature dependence of the P2₁/n to P4/mnc transition in two members of this series have been also investigated using both synchrotron and powder neutron diffraction methods. These measurements confirm the first order nature of the monoclinic to tetragonal transition. In both cases the volume of the tetragonal structure is noticeably smaller than the monoclinic structure as a consequence of the change in the Pr valency.