Spinodal Decomposition in the TiO₂–VO₂ System

Abstract

Spinodal decomposition is a ubiquitous phenomenon leading to phase separation\nfrom a uniform solution. We show that a spinodal decomposition occurs in a\nunique combination of two rutile compounds of TiO2 and VO2, which are\nchemically and physically distinguished from each other: TiO2 is a wide-gap\ninsulator with photo catalytic activities and VO2 is assumed to be a strongly\ncorrelated electron system which exhibits a dramatic metal-insulator transition\nat 342 K. The spinodal decomposition takes place below 830 K at a critical\ncomposition of 34 mol% Ti, generates a unidirectional composition modulation\nalong the c axis with a wavelength of approximately 6 nm, and finally results\nin the formation of self-assembled lamella structures made up of Ti-rich and\nV-rich layers stacked alternately with 30-50 nm wavelengths. A metal-insulator\ntransition is not observed in quenched solid solutions with intermediate\ncompositions but emerges in the thin V-rich layers as the result of phase\nseparation. Interestingly, the metal-insulator transition remains as sharp as\nin pure VO2 even in such thin layers and takes place at significantly reduced\ntemperatures of 310-340 K, which is probably due to a large misfit strain\ninduced by lattice matching at the coherent interface.\n