First-principles study of the electronic and magnetic properties of Ti-substituted K2Cr8O16

Abstract

The hollandite K2Cr8O16 is a paramagnetic metal (PM) at the high temperature (HT) tetragonal I4/m structure. This material encounters a first step transition from PM to ferromagnetic metal at Tc = 180 K and finally a metal–insulator transition (MIT) at 95 K. It is well known that the formation of tetramers, accompanied by dimerization in the Cr-Cr double chains is responsible for the MIT in K2Cr8O16. Nevertheless, the substitution effect has a marked impact on the MIT of other hollandite compounds. For example, Rb substitution for K or Ti substitution for V in K2V8O16 significantly increases the transition temperature. There is no available report on the electronic and magnetic properties of Ti-substituted K2Cr8O16. In this spirit, Ti-substitution effect on the electronic and magnetic properties of K2Cr8O16 are extensively investigated for its HT structure. It is revealed in this study that this compound preserves its ferromagnetism at 25% (the corresponding compound is K2Cr6Ti2O16), 50% (the corresponding compound is K2Cr4Ti4O16) and 75% (the corresponding compound is K2Cr6Ti4O16) Ti substitution for Cr. We observed that K2Cr8O16 undergoes MIT for 75% Ti substitution. The cooperative effect of Coulomb correlation and Cr3+-Ti4+ and Ti4+-Ti4+ charge ordering is responsible for the observed MIT in K2Cr2Ti6O16. The ferromagnetism in all the Ti-substituted compounds arises from Cr-O coupling. The strength of ferromagnetism decreases with Ti concentration due to the weakening in the Cr-O coupling. More interestingly, Tc reduces significantly by 45 K, 76 K and 81 K respectively for 25%, 50% and 75% Ti-substitution.