Electronic transition in solid Nb at high pressure and temperature

The electrical resistivity of high purity solid Nb has been measured at fixed pressures up to 5 GPa in a large volume press and temperatures up to ∼1900 K. The expected resistivity decrease with pressure and increase with temperature were found. A transition was observed in the temperature dependence of resistivity at high temperature. The transition is discussed in terms of the effects of pressure and temperature on the electronic band structure of Nb causing a resistivity behavior characteristic of a change from the "minus group" to the "plus group." Extrapolation of the pressure dependence of the transition temperature suggests that Nb would show plus group behavior at room T at an estimated pressure of ∼27 ± 7 GPa. The electronic thermal conductivity was calculated using the Wiedemann-Franz law and was in very good agreement with 1 atm data. We show that the temperature dependence of the calculated electronic thermal conductivity increases with a steep slope from room temperature up to the electronic transition temperature for all fixed pressures. Above the transition temperature, the T-dependence of electronic thermal conductivity remained constant at 2 GPa and exhibited an increasingly negative slope at higher pressures. The isothermal pressure-dependence of electronic thermal conductivity is positive.