Electrical behavior of Eu at very high pressures and low temperatures

Normally Eu has a valence of two while its Periodic-Table neighbors Sm and Gd have valences of three. There is theoretical reason to believe that some densification of Eu by pressure would shift the balance of the $4f$ and $5d$ electrons to promote it to a valence of three and a different crystal structure. Some theoretical treatments suggest that the high-pressure form of Eu might be superconducting at sufficiently low temperatures. In our experiments, the electrical resistance of Eu specimens was observed to pressures above 400 kbar and temperature down to 2.3 K. In this range, no superconductivity was observed. Based upon reproducible discontinuities of dR/dT and of dR/dP, and the reported observations of others, a tentative P,T phase diagram is proposed. As the pressure is varied there is one phase below about 125-150 kbar and a different phase above. As the temperature is increased in the low-pressure phase there is a magnetic ordering transformation in the 90-75-K region, and in the high-pressure phase there is a change at about 150 K, the physical nature of which has not been established.