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

Abstract

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 κ_T 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 ${\kappa }^L\mathrm{=\; (\kappa }{{\displaystyle {lim}^{\sim }}}^L{\mathrm{+\; B\; \phi }}^{\mathrm{-y}}{\text{) / (1 + 2 pB φ}}^{\mathrm{-y}})$ , where ${\mathrm{\phi \; \equiv \; |\hspace{0.17em}\; p-p}}_c{\mathrm{\hspace{0.17em}\; |\; /\hspace{0.17em}\; p}}_c$ and p_c is the transition pressure for the given isotherm. The quantity κ^L has been assigned the constant value ${\text{1.57 × 10}}^{\text{−6}}\mathrm{bar}\text{−1}$ 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 ${\alpha }^L\mathrm{=\; \alpha }{{\displaystyle {lim}^{̃}}}^L{\mathrm{+\; A\epsilon }}^{\mathrm{-x}}$ , where ${\mathrm{\epsilon \; \equiv \; |\hspace{0.17em}\; T-T}}_c{\mathrm{\hspace{0.17em}\; |/T}}_c$ and T_c is the transition temperature for the given isobar. The quantity $\alpha {{\displaystyle {lim}^{̃}}}^L$ has been assigned the constant value ${\text{5.0 × 10}}^{\text{−5}}\mathrm{deg}\text{−1}$ 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.