Infrared phonon anomalies and orbital ordering in single-crystalline MnV ₂O ₄ spinel

We used infrared ellipsometry to study phonons in a spinel MnV ₂O ₄ single crystal. At 300 K, the imaginary part of the complex dielectric function spectra show four phonon peaks. Below the cubic-tetragonal structural transition temperature at 58 K, small peaks appear around the cubic modes and the number of phonons becomes more than 12. The group analysis predicted 12 infrared modes for the I4 ₁/a symmetry. The observed modes support the I4 ₁/a or lower crystal symmetry, which reflects the antiferro ordering of V3 ⁺ t _2g orbitals.