LAUSR

Abstract Polycrystalline samples of Ca Ln 2 Ni 2 WO 9 ( Ln =La, Pr, Nd) have been synthesized and characterised by a combination of X-ray and neutron diffraction, electron microscopy and magnetometry. Each composition adopts a perovskite-like structure with a ~5.50, b ~5.56, c ~7.78 A, β ~90.1° in space group P 2 1 / n . Of the two crystallographically distinct six-coordinate sites, one is occupied entirely ( Ln =Pr) or predominantly ( Ln =La, Nd) by Ni 2+ and the other by Ni 2+ and W 6+ in a ratio of approximately 1:2. None of the compounds shows long-range magnetic order at 5 K. The magnetometry data show that the magnetic moments of the Ni 2+ cations form a spin glass below ~30 K in each case. The Pr 3+ moments in CaPr 2 Ni 2 WO 9 also freeze but the Nd 3+ moments in CaNd 2 Ni 2 WO 9 do not. This behaviour is contrasted with that observed in other ( A , A ') B 2 B 'O 9 perovskites.