This is the post‐print (i.e. final draft post‐refereeing) of the publication. The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.ceramint.2016.08.066 Effect of lanthanide doping on structural, microstructural and functional properties of K 0.5 Na 0.5 NbO 3 lead-free piezoceramics X.Vendrell 1* , J.E. García 2 , E. Cerdeiras 1 , D.A. Ochoa 2 , F. Rubio-Marcos 3 , J. F. Fernández 3 , L.Mestres 1 1 Departament de Química Inorgànica i Orgànica, Secció Química Inorgànica, Universitat de Barcelona, 08028 Barcelona, Spain. 2 Department of Physics, Universitat Politècnica de Catalunya, 08034 Barcelona, Spain. 3 Electroceramic Department, Instituto de Cerámica y Vidrio, CSIC, 28049 Madrid, Spain. Abstract Lanthanides-doped K 0.5 Na 0.5 NbO 3 (KNN) lead-free piezoelectric ceramics are prepared by conventional solid-state reaction. The effects of lanthanum concentration and the lanthanide type on the structure, microstructure and ferro-piezoelectric properties are evaluated. Ln 3+ doping has a slight effect on the structure, but greatly inhibits the grain growth. Moreover, a decrease in the Curie temperature and an increase in the dielectric constant values are observed when doping. Large amounts of lanthanum induce a diffuse phase transition and an increase of the dielectric losses. The piezoelectric properties are greatly improved when doping with small amounts of dopants. As the ionic radii of the lanthanide is reduced, the piezoelectric properties of the ceramics are increased, the Eu 3+ -doped ceramics show an increase of 29% of the piezoelectric coefficient d 33 with respect to pure KNN. The results show that low concentrations of lanthanides improve significantly the functional properties of KNN lead-free piezoceramics. Doping with lanthanides should be taken into consideration in some other compositions based on KNN. Keywords: Lead-free piezoceramics, Donor doping, Piezoelectric properties, (K,Na)NbO 3 *Dr. Xavier Vendrell xavier.vendrell@ub.edu Telf: +34 934 021 270 Fax: +34 934 907 725