Phase transitions and electrical transport in the mixed-valence V ²⁺/V³⁺ oxide BaV₁₀O₁₅

BaV₁₀O₁₅ can be regarded as a Ba-doped V ₂O₃ in which the Ba²⁺ ions substitute in the O^2- close-packed layers. The Ba²⁺ ions order within these layers and direct the occupation of the octahedral sites by V²⁺ and V³⁺ ions resulting in a structure with subtle differences from that of V₂O₃ which can be described in Cmca at room temperature. Magnetic susceptibility data show evidence for two phase transitions at 135 and 40K. The higher temperature transition at 135K is shown to be structural in origin to another orthorhombic form, Pbca. The structural transition temperature, T_s, decreases with decreasing V²⁺ content to a minimum value of 105K. Crystallographic and DSC data support a first-order transition driven by partial bond formation which results in a 7% reduction in the distance between two of the five crystallographically distinct V atoms. There is no conclusive crystallographic evidence for V ²⁺/V³⁺ charge ordering in either the Cmca or Pbca forms. Electrical conductivity data show semiconducting behavior above T_s which can be fitted to a small polaron hopping model for the most reduced samples (T_s=135K). The same sample shows a sharp but not discontinuous decrease in conductivity below T_s, consistent with carrier removal due to bond formation. More oxidized materials with T _s=105K show a more subtle anomaly. Evidence for correlated (Efros-Shklovskii) variable-range hopping at low temperatures is seen in the T_s=105K sample from analysis using a Hill-Zabrodski (logdE vs. logT) plot. Thermopower data on the T_s=135K material show an anomalously small value of S∼+1μV/K at room temperature which increases to >+200μV/K upon cooling to 90K. Plots of dS/dT show evidence for the T _s=135K phase transition. These results are not consistent with a simple one carrier model for small polaron hopping assuming that the V ²⁺ ions are the carriers, which would predict S∼-100μV/K, but seem to demand a two carrier model with n∼p at room temperature for which n-type carriers are trapped as a result of bond formation at the phase transition as temperature is lowered. The lower temperature phase transition near 40K is magnetic in origin and will be discussed in a subsequent publication.