We studied the physical properties of Ba 1−x Sr x V 13 O 18 single crystals, which exhibit three different phases caused by the orbital and charge degrees of freedom in V t 2g states. We found from the magnetic susceptibility and the NMR measurement that the low-temperature phase of this series of compounds is characterized by the spin-singlet state. We also found from the electrical resistivity and optical conductivity spectra that these compounds become heavily anisotropic in the low-temperature phase with V trimerization, indicating that the orbital ordering of V t 2g states causes the anisotropy of the electronic structure of this series of compounds.

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Ba 0.9 Sr 0.1 V 13 O 18 exhibits two ordered phases caused by the charge/orbital ordering of the d electrons on the V ions. We have succeeded in observing photoinduced phase transitions over three phases in this compound: a transition from the low-temperature phase to the high-temperature phase within 0.3 ps, followed by a transition to the intermediate-temperature phase within 2 ps. The relationship between the symmetry at each of the three phases plays a key role in the dynamics of the photoinduced phase transitions in such a compound.