The effect of the substitution of various ions on the metal-insulator (MI) transition at 170 K in K 2 V 8 O 16 has been investigated. Both Rb and Ti form complete solid solution systems: K 2Àx Rb x V 8 O 16 and K 2 V 8Ày Ti y O 16 , 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 K 2Àx Rb x V 8 O 16 , the former terminates to an MI transition at around 220 K in Rb 2 V 8 O 16 , while the latter disappears at x > 0:6. In K 2 V 8Ày Ti y O 16 , 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 K 2 V 8 O 16 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.
Abstract. Rb-substitution effect on the metal-insulator transition of hollandite vanadate, K 2 V 8 O 16 has been investigated. The metal-insulator transition is accompanied by a charge ordering and a structural change from a tetragonal to a monoclinic structure with an increase of volume in the low-temperature monoclinic insulator phase. Rb-substitution for K atoms continuously raises the transition temperature up to 240 K for Rb 2 V 8 O 16 from 160 K for K 2 V 8 O 16 and does not make the transition broad in contrast to Ti-substitution. This significant rise of transition temperature can be understood as a negative chemical pressure effect caused by Rb-substitution.
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