Enhanced ordered structure and relaxor behaviour of 0.98Pb(Mg _1/3Nb_2/3)O₃-0.02La(Mg_2/3Nb _1/3)O₃ single crystals

High quality Pb(Mg_1/3Nb_2/3)O₃ (PMN) and La-doped PMN (PLMN) single crystals were grown via a top-seeded solution growth method. At a doping level as low as 2at.% of La, the 1:1 B-site cation (chemical) order was significantly enhanced as revealed by the presence of superlattice peaks in the x-ray diffraction pattern and comparatively large (∼100nm) chemically ordered regions (CORs) in the transmission electron microscopy dark field images. The average chemical order parameter was calculated (from the intensity of superlattice peaks) to be S≈0.44. Besides, x-ray diffraction revealed the presence of a low-symmetry (presumably rhombohedral) phase. The effect of La doping on the dielectric relaxation was not substantial. Though the magnitude of the dielectric constant peak in PLMN was reduced by half as compared to pure PMN, the peak temperature and diffuseness as well as the shape of the dielectric relaxation spectra remained almost unchanged. The analysis of the temperature dependence of the characteristic time for the main relaxation process in terms of the Vögel-Fulcher law revealed the freezing of dipole dynamics at temperature T_f = 192K, which was only slightly lower than the T_f = 213K in PMN. To reconcile the structural and dielectric data we suggest that the rhombohedral phase forms inside CORs while the chemically disordered matrix remains cubic and is populated by dynamic polar nanoregions that give rise to the characteristic dielectric response.