Metal-insulator transition in AV$_{10}$O$_{15}$ (A=Ba,Sr)

In AV$_{10}$O$_{15}$ (A=Ba, Sr), the V ions take a mixed- valence state, V$^{2.8+}$ ($3d^{2.2}$), and form a modified triangular lattice, in which V triangles are periodically missing from a normal triangular lattice. It is known that BaV$_ {10}$O$_{15}$ undergoes a structural phase transition at around 120 K. We have succeeded in growing large single crystals of AV$_{10}$O$_{15}$ (A=Ba, Sr) by a floating-zone method. We found a large jump of electrical resistivity by $\sim 10^{3}$ times at the structural transition temperature (123 K) of BaV$_ {10}$O$_{15}$, which can be regarded as a metal-insulator transition presumably dominated by a charge/orbital ordering of V. We also found an antiferromagnetic ordering at 43 K in the same compound. On the other hand, SrV$_{10}$O$_{15}$ did not show any structural anomaly down to the lowest temperature, and a spin-glass behavior was observed. These results indicate a strong correlation between the structural anomaly (charge/orbital ordering) and the magnetism in this series of compounds. We also measured the optical reflectivity of BaV$_ {10}$O$_{15}$ and found the opening of a charge gap in the optical conductivity spectrum at low temperatures.