Quadrupolar freezing in potassium bromide cyanide ((KBr)1-x(KCN)x) mixed crystals

Abstract

We have used constant-pressure molecular dynamics to investigate the process of quadrupolar freezing in (KBr),_x(KCN)x mixed crystals. In agreement with experiment, a system with x = 0.25 was seen to remain cubic at all temperatures and form an orientational glass. The ground state is analyzed in terms of absolute and relative ordering of the CN" molecular anions. We find that neighboring quadrupoles exhibit a definite preference for ferroelastic ordering, though a broad distribution of relative orientations is also present. These results are in sharp contrast to those in a system with x = 0.73, which is unequivocally found to freeze into an orientationally ordered (crystal) state at low temperature.

Is the ground-state structure of (KCl)0.25(KCN)0.75a noncubic orientational glass?

“…(This explains the relatively high value of x c for this system.) In fact, as also observed numerically 8,19 as well as experimentally 14,20 in other systems, this is a direct manifestation of the competition between transition-rotation coupling, which tends to induce a structural transition, and strainrotation coupling, which opposes it. 10 Random strains are therefore particularly effective in this material, as it undergoes an extremely complicated transition from the high-temperature cubic phase to a low-temperature structure readily identified to be monoclinic (with one of the shears zero and the other two equal).…”

“…2,3 The nature of this state is not fully understood. 7,8 In the pure cyanides, the (first order) transition is primarily driven by the coupling of the translational to the rotational degrees of freedom of the aspherical CN ~ molecules. 9 As replacement of the CN ~ by halide ions proceeds, the presence of random strains in the system increases.…”

“…A preliminary analysis of orientational correlations in terms of Ed wards-Anderson-type order parameters 8,10 indicates that, on the average, the CN ~ molecular axes freeze into random {ill} directions of the parent cubic phase. To get better insight into the distribution of these orientations, we examined correlations between them as functions of both temperature and distance; we define the quantity P n (cosO), which gives the relative probability of finding a pair of rtth-nearest-neighbor CN ~ molecular ions such that 0 is the angle between them.…”

“…To get better insight into the distribution of these orientations, we examined correlations between them as functions of both temperature and distance; we define the quantity P n (cosO), which gives the relative probability of finding a pair of rtth-nearest-neighbor CN ~ molecular ions such that 0 is the angle between them. 8 The freezing process is thus monitored in terms of first-nearest-neighbor correlations P\(cos6)} At high temperature, where the molecules behave as quasiindependent rotors, there is no preferred relative orientation and the distribution is flat. As cooling proceeds, a tendency for neighboring molecules to align with their axes parallel or antiparallel develops.…”