A polarized neutron investigation of charge-ordering in mixed-valence Sm 4Bi 3

Abstract

Cubic Sm 4Bi 3 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 Sm 2+ and Sm 3+ ions below the transition temperature, we have investigated a Sm 4Bi 3 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 Sm 3+ form factor, whereas Sm II, along the other diagonals, carries a moment of 0.063(5) μ B with a Sm 2+ form factor.