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

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Abstract
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.