Higher-order critical phenomena in ND4Cl

Abstract

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 $\ensuremath{\Psi}$. A laser interferometric technique, capable of detecting piezoelectrically induced vibrations as small as 4\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}12}$ cm, made it possible to follow the variation in $\ensuremath{\Psi}$ very close to the transition. The temperature variation of $L$ and ${d}_{14}$ in the ordered phase determines the critical exponents ${\ensuremath{\alpha}}^{\ensuremath{'}}$ and $\ensuremath{\beta}$. Two N${\mathrm{D}}_{4}$Cl samples cut from the same 99.2% deuterated crystal gave the least-squares values ${\ensuremath{\alpha}}^{\ensuremath{'}}=0.69\ifmmode\pm\else\textpm\fi{}0.06$ and $\ensuremath{\beta}=0.126\ifmmode\pm\else\textpm\fi{}0.006$ with ${T}_{c}=249.690\ifmmode\pm\else\textpm\fi{}0.030$ K for sample $A$ and ${T}_{c}=249.725\ifmmode\pm\else\textpm\fi{}0.025$ K for sample $B$.