Descriptions of outer d electrons in thiospinels

Abstract

Abstract The constraints [1] of symmetry imposed by the spinel structure and[2] of internal consistency within the periodic table are used to construct phenomenological energy diagrams for the transition-metal thiospinels. It is found that band narrowing via intra-atomic exchange is sufficient to localize the d electrons to a cation only if there are at least three unpaired spins at the cation. A-site Co2+ ions may have collective d electrons, but exhibit a spontaneous atomic moment in Co [Cr2]S4. The compounds A[Cr2]S4, where A = Zn, Cd, Mn and Fe, contain localized d electrons, although the antiparalled-spin electron per Fe2+ ion is just localized. It is argued that the compounds Cu[B2]S4, B = V, Cr, Rh and 1 2(Rh+Cr) , have one hole per molecule in primarily d-like, collective orbitais that are primarily Cu-S in character. Where the B cations have no spontaneous atomic moment (B ≠ Cr), the compounds are superconducting at lowest temperatures. Cu[Ti2]S4 and Cu[Co2]S4, though metallic and without spontaneous magnetism, exhibit no superconducting transition above 0.05°K. Cu[Ti2]S4 and probably Cu[Co2]S4 are distinguished by a lack of (or less than one per molecule) holes in the d-like, collective Cu-S states. Cu2+ + Cr3+ is argued to be more stable than Cu+ + Cr4+ in the chalcogenide spinels, as it is known to be in Cu[Cr2]O4. The low Curie constant of CuCrRhSe4, as compared with CuCrTiS4, is attributed to a temperature-dependent molecular field, which is primarily due to mobile charge carriers in the selenide, rather than to Cr4+ excited states at high temperatures. The decrease in optical energy gap below Tc in CdCr2Se4, in contrast to its increase in CdCr2S4, is attributed to high-spin and low-spin Cr2+ ions of approximately the same energy.