Neutron diffraction study of (K_xNa_1−x)NbO₃-based ceramics from low to high temperatures

A neutron diffraction study of (K_xNa_1−x)NbO₃-based ceramics has been carried out from 5 K to high temperatures well above the Curie temperature. The diffraction data were analysed using Rietveld refinement. For pure KNN samples, especially at the Na-rich side of the phase diagram, the low-temperature structure of simple rhombohedral symmetry changes to a highly complex monoclinic structure at a higher temperature. Chemical analysis on the samples showed good agreement of the expected and actual compositions. Trigonal, monoclinic, orthorhombic, tetragonal and cubic phase models as well as two-phase mixtures are observed depending on the temperature of measurement. Space groups R3c, P1m1, P11m, Amm2, P4mm, and their combinations are used to refine the trigonal, monoclinic, orthorhombic, tetragonal, cubic and mixed phases, respectively. For the (K_0.48Na_0.48Li_0.04)(Nb_0.86Ta_0.1Sb_0.04)O₃ sample at temperatures between 5 and 300 K, the monoclinic P11m space group gives the best refinement fit. For the (K_0.17Na_0.83)NbO₃ sample, a two-phase refinement using the trigonal R3c and monoclinic Pm space groups gave the best fit at 300 K, while at 5–150 K the trigonal R3c space group gives the best fit. The understanding of the structure of these lead-free ceramics will help in the optimization of their piezoelectric properties.