Tuning of metal-metal bonding and magnetism via the electron count in GaxV4-yCryS8

Mixed crystals of the series Ga-x(V4-yCry)S-8 with x = 1 and 1.33 and y = 0-4 were synthesized and their crystal structures, magnetic and electrical properties were investigated. GaV4S8 crystallizes in the GaMo4S8 structure (F (4) over bar 3m) containing V-4 clusters and gallium atoms ordered in (1)/(16) of the tetrahedral holes of a face-centered cubic (fcc) sulfur packing. This structure is maintained up to y approximate to 2 with (V4-yCry) mixed clusters. Higher chromium contents (y >= 2) lead to filling of a second tetrahedral site with Ga atoms (x > 1) corresponding to a deficient spinel structure with partial disorder on the gallium sites (F (4) over bar 3m). Single-crystal structure data prove the lengthening of the metal-metal bonds and the disbanding of the (V4-yCry) clusters around y approximate to 2. Experimental magnetic moments agree with a cluster molecular orbital (MO) model up to y approximate to 2 and with separated V3+/Cr3+ ions for y > 2. Structural and magnetic data both verify the transition from the GaMo4S8 structure to the spinel type. The destabilization of the clusters in the series GaxV4-yCryS8 and the nonexistence of GaCr4S8 is due to unfavorable spin pairing energy in the cluster MO at y > 2, but is also a result of the stability of trivalent chromium in sulfides.