Refractive-index and linear-birefringence measurements on KTai_ x Nb x 0 3 , with x = 0.008, 0.012, and 0.02, reveal smeared phase transitions into long-range order. In accordance with previous caloric, second-harmonic-generation, dielectric, and x-ray results, the crystal optical data can be explained on the basis of a "cooperative dipole glass" model. This involves local ordering within the finite Curie region of a diffuse phase transition followed by a strain-induced collective reorientation of frozen-in microdomains.PACS numbers: 64.70. Kb, 77.80.Bh, 78.20.Fm, 78.20.Jq The influence exerted by impurities on systems undergoing structural phase transitions (PT) has been discussed very thoroughly in the last few years. 1 Particular interest has been focused on soft defects, which do not a priori break the symmetry seen by the local ordering coordinate, t>ut which nevertheless enhance the actual PT temperature of the system. The way in which soft defects really initiate the PT by coupling of the local and the bulk soft modes has been the subject of quite a number of investigations. 2 " 4 Fundamental differences are expected if the critical region is attained such that the defects are already frozen-in or such that they are still mobile between their off-center potential wells. In the latter case, true long-range order (LRO) with somewhat modified critical behavior is likely to occur rather generally. 5 For the frozenin-type defects the very existence of a unique transition temperature T 0 and of a low-temperature phase with true LRO is still a matter of controversy. There is much evidence that LRO is suppressed by random symmetry-breaking defects in systems, where the order parameter has either only one (Ising-type symmetry 4 ) or an infinite number of components (continuous symmetry 6 ). The influence of random fields exerted by the defects in the case 1 < n < oo still lacks theoretical studies. 1 One of the most extensively investigated doped systems, suitable for studies of soft defects, is KTa0 3 containing small amounts of Nb 5+ (KTN). Whereas KTa0 3 is an incipient ferroelectric, which does not undergo any PT, KTa!_ x Nb x 0 3 (x > 0.008) seems to exhibit a cubic-to-rhombohedral PT at finite temperature. 7,8 It has been argued 9 that as a result of the short hopping time of the Nb 5+ ions the defect-centered clusters may reorient themselves near T 0 to give a phase with true LRO. This seems to be proven by measurements of the spontaneous polarization P s . l0 > n Moreover, electric-field-induced Raman scattering of the soft mode in KTN with x ^ 0.05 did not reveal any evidence for Nb 5+ being a symmetry-breaking defect. 12 Additional experimental results, which are compatible with LRO, are available from investigations of second-harmonic generation, 13 of 35 K NMR, 14 and of first-order Raman spectra. 12,15 Very recently, however, the occurrence of LRO in ferroelectric KTN has been questioned 16 on the basis of dielectric-relaxation measurements as a function of temperature and hydrostatic pressure. Th...
