Substitution Effect on Metal-Insulator Transition of K2V8O16

The effect of the substitution of various ions on the metal-insulator (MI) transition at 170K in K2V8O16 has been investigated. Both Rb and Ti form complete solid solution systems: K2-xRbxV8O16 and K2V8-yTiyO16, respectively. The substitution of Rb for K or of Ti for V splits the transition into two transitions: the high-temperature transition is a first-order MI transition from a tetragonal structure to a tetragonal structure, and the low-temperature transition is a second-order transition to a monoclinic structure. In K2-xRbxV8O16, the former terminates to an MI transition at around 220 K in Rb2V8O16, while the latter disappears at x > 0.6. In K2V8-yTiyO16, both transitions disappear at y > 0.5. The substitution of Cr for V also results in a similar splitting of the transition and the rapid disappearance of both transitions. The substitution of Na or Ba for K suppresses the MI transition without any splitting of the transition, although the solubility of both ions is limited. These substitution effects reveal that the MI transition of K2V8O16 consists of two parts: a first-order MI transition and a parasitic second-order structural transition; the substitution of some ions causes a clear splitting of these transitions, probably due to the difference between the chemical pressure effects on the two transitions. The first-order MI transition is very sensitive to charge randomness, suggesting the charge ordering nature of the MI transition, while the second-order structural transition is very sensitive to both charge and structural randomnesses.