Pressure-induced tuning of phase transition and role of disorder in electrical transport properties of $\beta \text{−}{\mathrm{Sr}}_x{\mathrm{V}}_6{\mathrm{O}}_{15}$

We report the resistivity and thermoelectric power of $\beta \text{−}{\mathrm{Sr}}_x{\mathrm{V}}_6{\mathrm{O}}_{15}$, for various stoichiometries, $0.6⩽x⩽1$, and under pressures up to $1.7\mathrm{GPa}$. The pristine system $(x=1)$ exhibits a semiconductor-insulator transition at $155\mathrm{K}$, which is evidenced in both resistivity and thermopower and is probably induced by charge ordering. We observe a pronounced change in the nature of the phase transition under pressure and we attribute it to the tuning of the nearest neighbor Coulomb interaction $V$. At ambient pressure, as the system moves away from stoichiometry to $x<1$, disorder is introduced into the strontium sublattice and the phase transition is immediately suppressed. The temperature dependence of the thermoelectric power gradually weakens as the system moves away from $x=1$, indicating the importance of disorder. While for $x<1$ compound thermoelectric power shows evidence of a localized contribution to the conduction, which may involve polaronic effects, the activation energies speak against small polarons in the pristine $x=1$ compound. We explain our results in a model of conduction through localized states in the off-stoichiometric systems and of thermally activated conduction in the pristine system.

Figure 1

(Color online) The crystal structure of the stoichiometric $\beta \text{−}\mathrm{Sr}{\mathrm{V}}_6{\mathrm{O}}_{15}$ compound. The ${\mathrm{V}}_2{\mathrm{O}}_5$ skeleton consists of chains formed by $\mathrm{V}{\mathrm{O}}_6$ and $\mathrm{V}{\mathrm{O}}_5$ polyhedra, running along the $b$ axis. The three crystallographically different vanadium sites, V1, V2, and V3, are indicated by green, red, and blue (online). The gray balls represent the intercalated strontium atoms.

Figure 2

(Color online) Resistivity (top) and thermoelectric power (bottom) under ambient pressure. The top inset shows the fit of the resistivity data, in the temperature range above the phase transition, to the activated model (left axis) and to the small polaron model (right axis). The bottom inset shows the high-temperature $(T>155\mathrm{K})$ $1∕T$ dependence of the thermopower.

Figure 3

(Color online) Temperature dependence of the resistivity (upper panel), the logarithmic derivative of resistivity (middle panel), and the thermoelectric power (lower panel) is shown under various pressures. The logarithmic derivative of resistivity clearly indicates an evolution from second to first order phase transition. The top inset displays the hysteresis in the resistivity for $p=1.22\mathrm{GPa}$. The arrows indicate the direction of increasing pressure.

Figure 4

(Color online) The temperature of the phase transition, as determined from the resistivity (closed circles), and the thermopower (open circles) shows remarkable sensitivity to the applied pressure. In the high-pressure phase, we determine the $T_c$ from the thermopower as the temperature of the local minimum. The error bars are smaller than the size of symbols, unless indicated otherwise. The dashed line is a guide for the eyes.

Figure 5

(Color online) The energy gaps extracted from the resistivity of $\mathrm{Sr}{\mathrm{V}}_6{\mathrm{O}}_{15}$ under different pressures, fitted to an activated behavior. Whereas the difference between the high- and low-temperature gaps is large at low pressures, above $0.6\mathrm{GPa}$, the two values of the gap are very close.

Figure 6

The pressure dependence of the high-temperature energy gaps extracted from the resistivity $\left({\Delta }_{\rho }\right)$, fitted to an activated behavior, and thermoelectric power $\left({\Delta }_S\right)$, fitted to Eq. (2). The ${\Delta }_S$ is smaller than ${\Delta }_{\rho }$ because the presence of both electrons and holes leads to their partial compensation in the thermopower.

Figure 7

(Color online) The ambient-pressure resistivity and thermoelectric power for various stoichiometries. The inset shows the dependence of the high-temperature phase activation energy ${\Delta }_{\rho }$ on stoichiometry.

Figure 8

(Color online) Electrical resistivity under pressure for the $x=0.6$ compound. The inset shows activation energies obtained from a fit to an activated behavior.

Figure 9

(Color online) Thermoelectric power for the $x=0.6$ compound, under various pressures. While the weak temperature dependence of the thermopower is not much influenced by pressure, its absolute value decreases significantly.