Structure and magnetic ordering of the “free-electron” rare-earth halides RE 2X 5 (RE-Ce, Pr; X=Br, I)

Abstract

Magnetic and neutron diffraction investigations performed on the rare-earth halides RE 2X 5=(RE 3+) 2(X -) 5(e - (RE=Pr, Ce; X=Br, I) showed that these compounds possess remarkably high magnetic ordering temperatures. Pr 2Br 5 orders antiferromagnetically at T N=50(1)K, corresponding to the Shubnikov space group P2 1/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 T N= 40(2)K, Ce 2Br 5 shows a similar antiferromagnetic ordering corresponding to k = 0. There is no evidence for a second phase transition at lower temperatures. Below T N= 37(1)K, Pr 2I 5 orders antiferromagnetically corresponding to k=[½,0,0] and the Shubnikov space group P a2 1. Structural investigations and X-ray absorption near-edge spectra showed that the compounds contain rare-earth ions RE 3+ as the “free-electron” formula (RE 3+) 2(X -) 5 (e -) suggests, although measurements of the average electrical conductivities of polycrystalline Pr 2X 5 (X = Br, I) and Ce 2Br 5 indicate semiconducting behaviour in the “powder average”. Presumably the delocalized electron causes via RKKY interactions the high ordering temperatures.