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 at T=6 K in a magnetic field of 4.6 T applied along the [101] rhombohedral direction, the two Sm sites were found to have different magnetic characters: SmI, along the compressed diagonal of the cube, carries a moment of 0.017(5) μ Β with a Sm3+ form factor, whereas SmII, along the other diagonals, carries a moment of 0.063(5), μ Β with a Sm2+ form factor. (authors)