Order-Disorder Phenomena. VII. Critical Variations in the Length of NH4Cl Single Crystals at High Pressures

A high-precision capacitance method has been used to investigate variations in the length L of an NH4Cl single crystal in the vicinity of its order—disorder transition line. At low pressures, there is a small first-order discontinuity ΔL superimposed on the lambdalike variation in L. At 255.95°K and 1491.8 bar, L varies continuously but κT and α appear to diverge. At higher pressures, the variation in L at the transition becomes progressively more gradual as the pressure increases. The linear isothermal compressibility in the immediate vicinity of the transition is well represented along an isotherm by κ L = (κ lim ∼L + B φ −y) / (1 + 2 pB φ −y), where φ ≡ | p−p c | / p c and pc is the transition pressure for the given isotherm. The quantity κL has been assigned the constant value 1.57 × 10−6bar−1 for all isotherms; B and y (which are assumed to be independent of pressure) vary with the temperature. The linear isobaric coefficient of thermal expansion is fairly well represented along an isobar by α L = α lim ̃L + Aε−x, where ε ≡ | T−T c |/T c and Tc is the transition temperature for the given isobar. The quantity α lim ̃L has been assigned the constant value 5.0 × 10−5deg−1 for all isobars; A and x (which are assumed to be independent of temperature) vary with the pressure. The exponents x and y, which range from 0.4 to 0.9, are much larger than those associated with an ideal Ising lattice.