A POWDER NEUTRON-DIFFRACTION STUDY OF STRUCTURE AND MAGNETISM IN NICR2S4

Powder neutron diffraction data for NiCr2S4 have been collected over t he temperature range 1.8 less than or equal to T less than or equal to 286 K. The Cr3S4 structure (space group I2/m) is adopted at all tempe ratures. Refinement of 286 K data (a = 5.8954(14) Angstrom, b = 3.4066 (8) Angstrom, c = 11.0589(22) Angstrom, beta = 91.362(4)degrees) demon strates that the distribution of nickel and chromium cations between s ites in a fully occupied metal layer and sites in an ordered vacancy l ayer is close to that expected for the normal structure type. Data col lected at 1.8 K (a = 5.8621(5) Angstrom, b = 3.4051(2) Angstrom, c = 1 1.0297(9) Angstrom, beta = 91.607(6)degrees) reveal the presence of lo ng-range magnetic order, involving a doubling of the unit cell in both the a and c directions, Cations in the fully occupied metal layer pos sess an ordered magnetic moment of 1.57(6) mu(B) which is directed par allel to the cation layers, Cations in the vacancy layer have a moment of 1.36(8) mu(B) which is directed toward the anion layers, The magne tic ordering temperature was determined as 180(5) K, These results are correlated with magnetic susceptibility and electrical conductivity d ata. (C) 1997 Academic Press.