Tetragonal tungsten bronze compounds: relaxor versus mixed ferroelectric-dipole glass behavior.

We demonstrate that recent experimental data (Castel et al 2009 J. Phys.: Condens. Matter 21 452201) on the tungsten bronze compound (TBC) Ba(2)Pr(x)Nd(1-x)FeNb(4)O(15) can be well explained in our model predicting a crossover from ferroelectric (x = 0) to orientational (dipole) glass (x = 1), rather then relaxor, behavior. We show that, since a 'classical' perovskite relaxor like Pb(Mn(1/3)Nb(2/3))O(3) is never a ferroelectric, the presence of ferroelectric hysteresis loops in the TBC shows that this substance actually transits from ferroelectric to orientational glass phase with x growth. To describe the above crossover theoretically, we use the simple replica-symmetric solution for the disordered Ising model.