Heat capacity of NH4Cl and ND4Cl single crystals at high pressure

The constant-pressure heat capacity of N${\mathrm{H}}_4$Cl and N${\mathrm{D}}_4$Cl has been investigated in the vicinity of the order-disorder transition at 1 atm and at high pressures. The ac calorimetry technique has been used and has been shown to work well for investigating solid samples under hydrostatic gas pressures up to ∼3 kbar. The "multicritical" pressure (above which the transition is continuous and below which a small first-order instability occurs) is 1500 bar for N${\mathrm{H}}_4$Cl and is very close to 1 atm for N${\mathrm{D}}_4$Cl. A correspondence was found between the $C_p$ variation for N${\mathrm{D}}_4$Cl at 1 atm and N${\mathrm{H}}_4$Cl at 1500 bar and also between that for N${\mathrm{D}}_4$Cl at 1500 bar and N${\mathrm{H}}_4$Cl at 3134 bar. At the multicritical pressure, the behavior of the heat capacity in the ordered phase can be well represented by a power law, and the critical exponent ${\alpha }^{\prime }=0.50\mathrm{}\pm 0.07$ for N${\mathrm{D}}_4$Cl and 0.57±0.07 for N${\mathrm{H}}_4$Cl. At higher pressures, the peak $C_p$ values are significantly but reversibly decreased and the data show systematic deviations from a single power-law fit.