Anomalies in elastic properties and features of electronic transitions in lanthanum and praseodymium at pressures up to 84 kbar

The propagation velocities of longitudinal and transverse elastic waves in polycrystalline praseodymium have been measured by a pulsed ultrasound method at pressures up to 84 kbar at room temperature. The density, elastic constants, and Debye temperature are calculated as functions of pressure. The anomalies found in the pressure dependence of the sound velocities in praseodymium, as in lanthanum, which was studied previously, reflect an additional anharmonicity of longitudinal and transverse acoustic phonons at two phase transitions which occur in the intervals 12–50 and 66–80 kbar. The experimental results on lanthanum and praseodymium are discussed on the basis of a pseudopotential method and in the Friedel model with 6s-5d electronic transitions. Estimates show that the observed anomalies in the elastic characteristics of both of these metals are caused by an accelerated 6s-5d “flow” of electrons. Similar anomalies should be observed in caesium II near the first-order isomorphic electronic 6s-5d transition at 42.2 kbar.