Displacive or Order-Disorder Phase Transition? The H-bond Dynamics in Multicaloric Ammonium Sulfate

3 0 Ju l 1 99 9 Crossover between displacive and order - disorder phase transition

The phase transition in a 3D array of classical anharmonic oscillators with harmonic nearest-neighbour coupling (discrete φ 4 model) is studied by Monte Carlo (MC) simulations and by analytical methods. The model allows to choose a single dimensionless parameter a determining completely the behaviour of the system. Changing a from 0 to +∞ allows to go continuously from the displacive to the ord...

Computer Simulation of the Order-Disorder Transition in Ammonium Chloride

The phase transition at Tc = 243 K in ammonium chloride is well established to be of the order-disorder type. The ammonium chloride has a CsCl-type cubic structure with hydrogen atoms pointing toward four of the eight chloride ions at the corners of the unit cell. This means that there are two possible orientations of ammonium tetrahedron; a and b. In the ordered phase below the Tc temperature ...

Monte Carlo Studies of the Orientational Order-Disorder Phase Transition in Solid Ammonium Chloride

Monte Carlo methods are used to study the phase transition in NH4Cl(s) from the orientationally ordered δ phase to the orientationally disordered γ phase. An effective pair potential is used to model the interaction between ions. Thermodynamic properties are computed in the canonical and isothermal-isobaric ensembles. Each ammonium ion is treated as a rigidly rotating body and the lattice is fi...

EFFECT OF THE NEXT-NEAREST NEIGHBOR INTERACTION ON THE ORDER-DISORDER PHASE TRANSITION

In this work, one and two-dimensional lattices are studied theoretically by a statistical mechanical approach. The nearest and next-nearest neighbor interactions are both taken into account, and the approximate thermodynamic properties of the lattices are calculated. The results of our calculations show that: (1) even though the next-nearest neighbor interaction may have an insignificant ef...

Pressure-induced decoupling of the order-disorder and displacive contributions to the phase transition in diguanidinium tetrachlorostannate