Long range order in random-site dipole systems in a random field (KTaO3: Li, Na, Nb)

The destruction of long range order by static random field in spatially disordered systems with magnetic or electric dipole moments possessing uniaxial or cubic anisotropy is discussed in terms of a random field theory. It is shown that, in disordered random-site systems, the critical concentration for the appearance of long-range order is determined from the balance between the width of the static random field distribution and the interaction energy of the particles separated by a mean distance. The appropriate phase diagram is calculated for Ising spins (two-orientational dipoles) with an interaction potential of the form I(r) ∼ exp(-r/rc) and a Lorentz form for the static field distribution. Based on the assumption that critical concentrations for ferroelectric phase transitions in mixed crystals KTaO3: Li, Nb, Na are caused by the static random fields of extraneous impurities present, a width of the static polarization distribution of 50 mC/m2 is estimated and it is shown that quite different values of critical concentrations of Li, Nb and Na can be explained self-consistently.