Raman study NH4Cl near the tricritical point

The Raman spectrum of NH₄Cl has been studied as a function of temperature and pressure close to the multicritical point at which the λ-transition changes from first to second order. Variations in the intensities of appropriate bands have been used to calculate the critical exponents for the order parameter. By varying the temperature at fixed pressures of zero, 3.0 k bar and 1.5 k bar, exponent values of β = 0.154 ± 0.012, β = 0.33 ± 0.06 and β = 0.26 ± 0.06 have been obtained for the first-order, second-order and multicritical regions respectively.Our first-order exponent value is close to that observed by previous workers, our second-order value is close to that predicted by the three dimensional Ising model, and our multicritical value is close to that predicted by mean field theory.