Structure and magnetic ordering of the ''free-electron'' rare-earth halides RE2X5 (RE = Ce, Pr; X = Br, I)

Magnetic and neutron diffraction investigations performed on the rare-earth halides RE2X5=(RE3+)2(X-)5(e-) (RE=Pr, Ce; X=Br, I) showed that these compounds possess remarkably high magnetic ordering temperatures. Pr2Br5 orders antiferromagnetically at TN=50(1) K, corresponding to the Shubnikov space group P21/m', i.e. k=0. Below T=20 K there is a second magnetic phase transition causing an abrupt increase of the magnetic moments. At TN=40(2) K, Ce2Br5 shows a similar antiferromagnetic ordering corresponding to k=0. There is no evidence for a second phase transition at lower temperatures. Below TN=37(1) K, Pr2I5 orders antiferromagnetically corresponding to k=[1/2, 0, 0] and the Shubnikov space group Pa21. Structural investigations and X-ray absorption near-edge spectra showed that the compounds contain rare-earth ions RE3+ as the ''free-electron'' formula (RE3+)2(X-)5(e-) suggests, although measurements of the average electrical conductivities of polycrystalline Pr2X5 (X=Br, I) and Ce2Br5 indicate semiconducting behaviour in the ''powder average''. Presumably the delocalized electron causes via RKKY interactions the high ordering temperatures. (orig.)