Change of phonon Raman spectra with v trimerization in BaV ₁₀O ₁₅

We investigated Raman scattering of BaV ₁₀O ₁₅, which exhibits a structural phase transition with V trimerization at T _c=123K possibly dominated by the orbital ordering of V t _2g states. It was found that phonon Raman spectra drastically change at T _c. According to the analysis of the phonon spectra based on the bond-polarizability model, the force constant of the V-V bond increases by 75% and the peak width is reduced by half below T _c. This result can be explained by the existence of disorder originating from orbital degrees of freedom above T _c, which is suppressed by the long-range orbital ordering below T _c.