Susceptibility scaling functions for landau critical and tricritical points

In the framework of Landau theory the equations for susceptibility scaling function for critical and tricritical points are derived above and below Tc, respectively. They are used to calculate the nonzerofield critical invariants Q for the critical (Q = 2) and tricritical (Q = 4/3) points. Q is equal to the ratio χ(τmax, E = 0)/χ(τmax, E), where τmax(E) is the reduced temperature of maximum of nonzerofield susceptibility χ(τ, E) for a given value of electric field E. It is concluded that behaviour of TGS and TGSe ferroelectrics is not in agreement with either the Landau critical or tricritical scaling prediction and that the experimental nonlinearity in the Landau critical and tricritical equations for susceptibility scaling functions is a measure of invariant Q (4/3 < Q < 2). It is suggested that the nonlinearity method (determination of Q) is more sensitive for detection of deviations from classical behaviour than the exponent ratio method (determination of Δ/γ) and both methods should be used simultaneously.