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We investigated single crystals of ${\mathrm{Ba}}_{3\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Nb}}_{5}{\mathrm{O}}_{15}$ with a tetragonal tungsten bronze (TTB) structure, which exhibits a metal-insulator transition with $x$. We found that there is a one-dimensional anisotropy with a higher dc conductivity along the $c$ axis in the metallic phase, but it disappears in the insulating phase. The optical conductivity spectra of the metallic samples show similar anisotropy to the dc conductivity, but the anisotropy persists even in the insulating phase. These experimental results, together with the result of magnetic susceptibility, indicate that the disorder caused by the Sr ions loosely bound to the surrounding oxygen ions plays an important role in the metal-insulator transition.

We studied single crystals of orthorhombic ${\mathrm{Sr}}_{2}{\mathrm{VO}}_{4}$ containing spin-singlet dimers of ${\mathrm{V}}^{4+}\phantom{\rule{0.28em}{0ex}}(3{d}^{1})$ and those with Ti or Nb substituted for V. We found that both Ti and Nb dopings induce the evolution of a Curie-Weiss component in the magnetic susceptibility, but the rate of evolution with doping is several times higher for Nb doping. We also found that Nb doping induces the conduction of $d$ electrons. These results can be explained by the hybridization of the Nb $4d$ states with the $3d$ states of the neighboring V ions, possibly causing rearrangement of the $d$ electron in the doubly degenerate $e$ states in the V ion.

${\mathrm{Ba}}_{0.9}{\mathrm{Sr}}_{0.1}{\mathrm{V}}_{13}{\mathrm{O}}_{18}$ 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.