Phase transitions and the role of vanadium t2g states in A V₁₃ O₁₈ (A=Sr,Ba )

We studied A V₁₃ O₁₈ (A=Sr,Ba), in which V ions form a face-centered-cubic lattice with periodically missing ions. It was found that there is a phase transition in both SrV₁₃ O₁₈ (Tc =270 K) and BaV₁₃ O₁₈ (Tc =200 K), where anomalies are observed in resistivity and magnetic susceptibility. Resistivity shows a contrasting behavior in the low-temperature phase between SrV₁₃ O₁₈ (a metallic behavior) and BaV₁₃ O₁₈ (an insulating behavior). By synchrotron x-ray powder-diffraction measurement, it was found that V trimerization occurs at the phase transition of SrV₁₃ O ₁₈. On the other hand, BaV₁₃ O₁₈ does not show a large change in the x-ray powder diffraction but superlattice peaks at (0 1 2 1 2) was observed in the electron diffraction. Possible orbital states of the t2g electrons in the V ions are discussed.