dependence of the reaction state. Systems of this type are of potential interest for practical application; experiments with permanent magnets in contact with a spring loaded Cu, +,Cr2Se4 working electrode demonstrated that, in principle, it is possible to construct a magnetic switch operated isothermally by the chemical process under discussion. Conversely, the latter arrangement represents a magnetic sensor device for the reaction state of the chemical system.
The characteristic difference in reactivity direction between CuTi2S4[21 and CuCr2Se4 can be discussed qualitatively in terms of the relative stability in chalcogenospinel lattices of the formal oxidation states of the transition metal ions involved. Since copper has the oxidation state + 1 in chal~ogenides,[~] the compounds can be described in an ionic model as mixed valence phases Cu@(Ti3@Ti4@)(S2"), and C U @ ( C ~' @ C ~~@ ) ( S ~* " ) ~.
While CuTi2S4 can only be ox- idized to Ti2S4, CuCr2Se4 can only be reduced to Cu2Cr2Se4 under the conditions applied:
This can be explained reasonably well in terms of the lower stability of Ti3@ and the higher stability of Cr3@ in relation to the coexisting quadrivalent states in chalcogen ligand fields.
The magnetic ordering in copper chalcogenospinels CuCr2X4 has been interpreted in terms of Cr"'/Cr"' (superexchange) and Cr"'/Cr"' interaction (double ex-~hange).[~".~l We attribute the significant decrease in T, of C u l +$r2Se4 with increasing degree of conversion (y) to the higher strength of ferromagnetic coupling of Cr"'/Crlv as compared to Cr"'/Cr'''.[5c1 Extended studies of the related ferromagnetic chalcogenospinels CuCr2S4 and CuCr2Te4 revealed a behavior quite similar to that o f the selenide. Current work is related to the investigation of the influence of Lie intercalation in these phases because of the strong difference in bond ionicity Cu/X and Li/X; the possibility of Lie insertion in spinel type oxides and chalcogenides has been demonstrated in recent publication^.'^,^.^^