A powder neutron diffraction study of magnetic phase transitions and spin frustration in ilvaite, a mixed-valence iron silicate showing a semiconductor-insulator transition

Abstract Ilvaite, Ca(Fe 2+ , Fe 3+ )Fe 2+ Si 2 O 7 O(OH) with double octahedral chains of Fe 2+ (A) and Fe 3+ (A) ions with Fe 2+ (B) octahedra occurring above and below shows electron delocalization at the A sites and a semiconductor-insulator transition (∼370K) followed by a crystallographic transition (346 K). In addition, it shows two magnetic transitions at 123 and 50 K. The magnetic structure derived from powder neutron diffraction consists of A site ribbons composed of two single ferromagnetic (Fe 2+ −Fe 3+ −Fe 2+ −Fe 3+ …) chains, whose spins are aligned antiparallel to each other below 123 K; the Fe 2+ (B) spins are weakly coupled to their A site neighbors and are mostly ordered below 50 K. Both A and B site spins are nearly parallel and antiparallel to b . Below 123 K, the Fe 2+ (A) and Fe 3+ (A) moments increase rapidly with decreasing temperature, showing a sharp increase at 50 K indicating the second transition, whereas Fe 2+ (B) moment increases very slowly below 123 with a substantial increase at 50 K indicating considerable spin fluctuation between 123 and 50 K. The moments extrapolated to 0 K are 3.6, 4.6, and 3.6 μ B for Fe 2+ (A), Fe 3+ (A) and Fe 2+ (B) sites respectively. The difference in the thermal evolution of the magnetic moments at A and B sites is due to the frustration of Fe 2+ (B) spins, which occurs at the apex of a square pyramid, a corner common between two triangles with respect to two sets of Fe 2+ (A)-Fe 3+ (A) pairs with opposite spin directions.