Higher-order critical phenomena in ND4Cl

The behavior of the length $L$ and the piezoelectric constant $d_{14}$ of N${\mathrm{D}}_4$Cl single crystals in the vicinity of the order-disorder transition has been investigated at 1 atm. In the absence of complications associated with changes in the domain structure, the value of $d_{14}$ is directly proportional to the order parameter $\Psi$. A laser interferometric technique, capable of detecting piezoelectrically induced vibrations as small as 4×${10}^{-12\mathrm{}}$ cm, made it possible to follow the variation in $\Psi$ very close to the transition. The temperature variation of $L$ and $d_{14}$ in the ordered phase determines the critical exponents ${\alpha }^{\prime }$ and $\beta$. Two N${\mathrm{D}}_4$Cl samples cut from the same 99.2% deuterated crystal gave the least-squares values ${\alpha }^{\prime }=0.69\mathrm{}\pm 0.06$ and $\beta =0.126\mathrm{}\pm 0.006$ with $T_c=249.690\mathrm{}\pm 0.030$ K for sample $A$ and $T_c=249.725\mathrm{}\pm 0.025$ K for sample $B$.