Enhanced ordered structure and relaxor behaviour of0.98Pb(Mg1/3Nb2/3)O3–0.02La(Mg2/3Nb1/3)O3 single crystals

Abstract

High quality Pb(Mg1/3Nb2/3)O3 (PMN) and La-doped PMN (PLMN) single crystals were grown via a top-seededsolution growth method. At a doping level as low as 2 at.% of La, the 1:1 B-sitecation (chemical) order was significantly enhanced as revealed by the presenceof superlattice peaks in the x-ray diffraction pattern and comparatively large(∼100 nm) 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) tobe S≈0.44. Besides, x-ray diffraction revealed the presence of a low-symmetry (presumablyrhombohedral) phase. The effect of La doping on the dielectric relaxation was notsubstantial. Though the magnitude of the dielectric constant peak in PLMN was reducedby half as compared to pure PMN, the peak temperature and diffuseness as well as theshape of the dielectric relaxation spectra remained almost unchanged. The analysis of thetemperature dependence of the characteristic time for the main relaxation process in termsof the Vögel–Fulcher law revealed the freezing of dipole dynamics at temperatureTf = 192 K, which was onlyslightly lower than the Tf = 213 K in PMN. To reconcile the structural and dielectric data we suggest that therhombohedral phase forms inside CORs while the chemically disordered matrix remainscubic and is populated by dynamic polar nanoregions that give rise to the characteristicdielectric response.