Pippard's relations applied to certain lambda transitions in dielectric and magnetic materials

When making thermodynamically based comparisons concerning behaviour in the vicinity of Curie points or Neel points, Pippard's relations are in principle more pertinent than Ehrenfest's. In a discussion supplementing the analyses of Sawatzky and Bloom (1962-64) and Skalyo, Cohen, Friedberg and Griffiths (1967), various magnetic Pippard's relations are considered, and it is re-emphasized that a relation derived by Fisher (1960-62) in statistical mechanical terms (that near an antiferromagnetic Neel point the rise in specific heat closely follows the temperature coefficient of the magnetic susceptibility) amounts very nearly to a relation of the purely thermodynamic type obtained by Pippard for lambda transitions generally. Relevant observations are analyzed in such terms for a number of representative cases, and comparisons made with the dependence of lambda temperatures on hydrostatic pressure and magnetic field. Correspondingly, for lambda transitions associated with a very rapid change of dielectric properties with temperature, the lambda temperature corresponds to a peak in the derivative of electric susceptibility with respect to temperature, and not to a maximum in the susceptibility itself.