Phase Diagrams with Asymptotic Transition Lines

While linear and curved phase transition lines in P(T) or H(T) diagrams can be described by adding linear and quadratic terms in a joint expansion of the Landau coefficients a, b in terms of P–P_t and T–T_t where (P_t, T_t) is a critical point, a new class of phase diagram is obtained by allowing for bilinear, ϵ(P–P_t)(T–T_t), and higher product terms. In this contribution we relate these new expansion terms to some hitherto unexplained experimental P–T and H–T phase diagrams.