The adiabatic elastic constantsc11andc44and the lsqb;100rsqb; longitudinal attenuation have been measured in single crystal ND4Cl, with emphasis on the region of the orderndash;disorder transition. Unlike NH4Cl, the transition is almost continuous at 1 atm. Not only the firsthyphen;order instability but also the variation ofc11in the disordered phase is very sensitive to deuteration. In contrast, the variation ofc11in the ordered phase is relatively insensitive to deuteration and can be well represented by a Pippard relation. The overall pattern of changes in velocity and attenuation on deuteration is very similar to those observed in NH4Cl on going from 1 atm to 1500 bar. The dynamical behavior corresponds to the conventional theory of critical slowing down, which is consistent with the prescence of a multicritical point in ND4Cl.
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