Two new compounds, Sr 3 NiRhO 6 and Sr 3 CuRhO 6 , 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 X-ray Rietveld re5nements and magnetic measurements. The commensurate structure of Sr 3 NiRhO 6 is isostructural with the rhombohedral K 4 CdCl 6 structure (space group R3 c; Z ؍ 6; a ؍ 9.5951(1) A > , c ؍ 11.0621(2) A > ) while the structure of Sr 3 CuRhO 6 forms in a monoclinic distortion of the K 4 CdCl 6 structure (space group C 2/c; Z ؍ 4; a ؍ 9.2226(2) A > , b ؍ 9.6882(2) A > , c ؍ 6.6926(2) A > , ؍ 92.440(2)3). The structures of Sr 3 NiRhO 6 and Sr 3 CuRhO 6 contain chains of alternating face-sharing NiO 6 /CuO 6 trigonal prisms and RhO 6 octahedra. The magnetic susceptibility of Sr 3 NiRhO 6 shows an abrupt drop at 30 K, suggesting antiferromagnetic correlations between the transition metal containing chains. Sr 3 CuRhO 6 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 speci5c synthesis conditions, di4erent incommensurate structures can be obtained. The 5tted lattice parameters with space group R3m (cell 1) and P31c (cell 2) are a ؍ 9.6201(6) A > , c 1 ؍ 2.6732(2) A > , c 2 ؍ 3.9803(8) A > for Sr 3 NiRhO 6؉ and a ؍ 9.624(2) A > , c 1 ؍ 2.6981(5) A > , c 2 ؍ 3.9539(5) A > for Sr 3 CuRhO 6؉ . The magnetic susceptibilities of the incommensurate compounds are quite di4erent from their commensurate analogues, as all signs of long-range magnetic order disappear.
2002 Elsevier Science (USA)