Shock determination of the Grüneisen parameter for lanthanum and the6s-5dtransition

Abstract

Shock-wave overtake measurements have been used to determine sound velocities in lanthanum metal from 0.73 to 2.3 Mbar along the Hugoniot curve. Since these pressures lie in the liquid region, the results can be used to obtain the corresponding pressure and volume dependences of the adiabatic bulk modulus, $B_S$, and of the Grüneisen parameter, γ. A calculation by McMahan, Skriver, and Johansson [Phys. Rev. B 23, 5016 (1981)] of the volume and temperature dependence of the 6s-5d transition in lanthanum predicts that γ should be anomalously large and have a maximum value in the present pressure range. Our results are in qualitative agreement with this prediction, with γ decreasing with increasing pressure from a maximum value of 1.30 (twice the P=0 value) at V/$V_0$=0.55 (0.73 Mbar, our lowest pressure) to 0.62 at V/$V_0$=0.443 (2.32 Mbar). Technical problems prevented the actual observation of a maximum in γ at lower pressures.