Increase in the superconducting transition temperature in Zr-Hf alloys due to $s-d$ electron transfer under pressure

The superconducting transition temperatures $T_c$ of Zr-Hf alloys are measured as a function of pressure up to 47.4 GPa. It is found that the value of $T_c$ experiences jumps due to the transition of the hexagonal $\omega$ phase to the $\beta$ phase: $T_c$ of the Zr$_{80}$Hf$_{20}$ alloy increases from 3.2 to 11 K at $P=35.0$ GPa, and a jump in $T_c$ from 3.4 to 10 K is observed for the Zr$_{67}$Hf$_{33}$ alloy at $P=40.9$ GPa. The isobars of the concentration dependence $T_c(c)$ of the bcc Zr-Hf alloys are qualitatively similar to the $T_c(c)$ curves for the bcc phases in the systems of V$b$–IV$b$ subgroup elements at $P=1$ atm. The data obtained indicate that, because of the $s-d$ electron transition, the IV$b$ subgroup metals become similar in their crystal structure and superconducting properties to the V$b$ subgroup metals.