Sr3NiRhO6 and Sr3CuRhO6––Two New One-Dimensional Oxides. Magnetic Behavior as a Function of Structure: Commensurate vs Incommensurate

Two new compounds, Sr3NiRhO6 and Sr3CuRhO6, belonging to a family of 2H-perovskite related materials, were synthesized in both a commensurate and an incommensurate form. The oxides were structurally characterized by powder Xray Rietveld refinements and magnetic measurements. The commensurate structure of Sr3NiRhO6 is isostructural with the rhombohedral K4CdCl6 structure (space group R c; Z=6; a=9.5951(1) ?, c=11.0621(2) ?) while the structure of Sr3CuRhO6 forms in a monoclinic distortion of the K4CdCl6 structure (space group C 2/c; Z=4; a=9.2226(2) ?, b=9.6882 (2) ?, c=6.6926(2) ?, beta=92.440(2)°). The structures of Sr3NiRhO6 and Sr3CuRhO6 contain chains of alternating facesharing NiO6/CuO6 trigonal prisms and RhO6 octahedra. The magnetic susceptibility of Sr3NiRhO6 shows an abrupt drop at 30 K, suggesting antiferromagnetic correlations between the transition metal containing chains. Sr3CuRhO6 displays ferromagnetic-type ordering below 10 K. When the samples are heated for prolonged periods of time, a transition to an incommensurate structure takes place. Depending on the specific synthesis conditions, different incommensurate structures can be obtained. The fitted lattice parameters with space group R3m (cell 1) and P31c (cell 2) are a=9.6201(6) ?, c1= 2.6732(2) ?, c2=3.9803(8) ? for Sr3NiRhO6+delta and a=9.624(2) ?, c1=2.6981(5) ?, c2= 3.9539(5) ? for Sr3CuRhO6 +delta. The magnetic susceptibilities of the incommensurate compounds are quite different from their commensurate analogues, as all signs of long-range magnetic order disappear.