ORDER-DISORDER TRANSITION OF ND4CL AND NH4CL

ND4Cl and NH4Cl single crystals, as model systems for orientational or der-disorder phase transitions, were investigated at atmospheric press ure with x-ray diffraction methods. NH4Cl undergoes a first-order tran sition whereas for ND4Cl the transition is very cll,se to a tricritica l point (TCP). The order parameter m(T), which describes a low-tempera ture phase with ferro-ordered NH4+ or ND4+ tetrahedra, is obtained by analyzing the temperature dependence of the lattice constant a. It is shown that the temperature dependence of a strongly modifies the criti cal exponent beta(TCP) Of the order parameter. In particular, the data yield beta(TCP)=0.16+/-0.01 in direct neighborhood to the TCP, in goo d agreement with published values. The departure from the expected mea n field value (beta(TCP)=0.25) is explained by rescaling the critical temperature due to the order-parameter-dependent lattice contraction. For ND4Cl we got quantitative results for the lattice contraction usin g a microscopic electrostatic model. Fluctuations related to local ant iferro (AF) ordering of the tetrahedra were observed, too. Previously, AF ordering and fluctuations were only observed in NH4Br and ND4Br. T he correlation length xi(AF) Of these AF fluctuations exhibits values of 30 Angstrom for NH4Cl and 10 Angstrom (2-3 lattice constants) for N D4Cl. [S0163-1829(98)01026-1].