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 [1 1 1] 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 = 6 K in a magnetic field of 4.6 T applied along the [1̄ 0 1] 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)μ_B with a Sm³⁺ form factor, whereas Sm_II, along the other diagonals, carries a moment of 0.063(5)μ_B with a Sm²⁺ form factor.