A polarized neutron investigation of charge-ordering in mixed-valence Sm4Bi3

Cubic Sm4Bi3 undergoes a first-order transition into a rhombohedral phase at about 260 K. This change has been ascribed to a charge-ordering transition from the thermal valence fluctuating state into a static mixture of di- and trivalent sites. In order to determine the positions of the Sm2+ and Sm3+ ions below the transition temperature, we have investigated a Sm4Bi3 single crystal by polarized neutron diffraction. A uniaxial pressure was applied along the [111] direction to avoid the formation of domains. The rhombohedral nuclear structure was first refined using unpolarized neutron measurements. From the flipping ratios measured with polarized neutrons at T = 6K in a magnetic field of 4.6 T applied along the [(1) over bar 01] rhombohedral direction, the two Sm sites were found to have different magnetic characters: Sm-I, along the compressed diagonal of the cube, carries a moment of 0.017(5)mu(B) with a Sm3+ form factor, whereas Sm-II, along the other diagonals, carries a moment of 0.063(5)mu(B) with a Sm2+ form factor. (C) 1999 Elsevier Science B.V. All rights reserved.