Phase transitions and electrical transport in the mixed-valence V2+/V3+ oxide BaV10O15

BaV10O15 can be regarded as a Ba-doped V2O3 in which the Ba2+ ions substitute in the O2− 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, Ts, 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 V2+/V3+ charge ordering in either the Cmca or Pbca forms. Electrical conductivity data show semiconducting behavior above Ts which can be fitted to a small polaron hopping model for the most reduced samples (Ts=135K). The same sample shows a sharp but not discontinuous decrease in conductivity below Ts, consistent with carrier removal due to bond formation. More oxidized materials with Ts=105K show a more subtle anomaly. Evidence for correlated (Efros-Shklovskii) variable-range hopping at low temperatures is seen in the Ts=105K sample from analysis using a Hill-Zabrodski (logdE vs. logT) plot. Thermopower data on the Ts=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 Ts=135K phase transition. These results are not consistent with a simple one carrier model for small polaron hopping assuming that the V2+ 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.