Functional Materials Letters Vol. 3, No. 1 (2010) 15–18 © World Scientific Publishing Company DOI: 10.1142/S1793604710000865 ALKALINE NIOBATE-BASED PIEZOCERAMICS: CRYSTAL STRUCTURE, SYNTHESIS, SINTERING AND MICROSTRUCTURE MARIJA KOSEC ∗ , BARBARA MALI ˇ C, ANDREJA BEN ˇ CAN † , TADEJ ROJAC and JENNY TELLIER Jožef Stefan Institute Jamova cesta 39 Ljubljana, 1000, Slovenia ∗ marija.kosec@ijs.si † andreja.bencan@ijs.si Received 16 October 2009; Revised 12 December 2009 In this review, the crystal structure and the synthesis of the sodium potassium niobate (K 0.5 Na 0.5 NbO 3 ) as a promising candidate for lead-free piezoelectrics are addressed. The sintering and the microstructure as prerequisites for obtaining ceramics with reliable and sufficiently high piezoelectric properties for selected applications are discussed. Keywords: (K,Na)NbO 3 ; structure; synthesis; sintering. 1. Introduction The solid solution of ferroelectric KNbO 3 and antiferroelec- tric NaNbO 3 with the composition (K 0.5 Na 0.5 )NbO 3 (KNN) is one of the promising and most studied lead-free piezoceramics materials. 1 KNN has a high Curie temperature (420 ◦ C), a low density (4.51 g/cm 3 ), a dielectric permittivity of a few 100s and a piezo d 33 coefficient between 80 and 110 pC/N. 2, 3 In alkaline niobate-based ceramics, the enchancement of piezo- electric properties is due to a shift of polymorphic phase tran- sition towards lower temperatures. 4 Solid solutions of KNN and about 5 to 6mol% LiTaO 3 or LiNbO 3 , on the boundary between orthorhombic and tetragonal phases at room temper- ature, have a piezo d 33 coefficient of about 200 pC/N. 5, 6 Fur- ther modification with LiSbO 3 increases the value of the d 33 coefficient to approximately 300pC/N whereas the texturing of (K,Na,Li)(Nb,Ta,Sb)O 3 ceramics resulted in the enhance- ment of d 33 to almost 400 pC/N. 7 An updated overview of lead-free materials can be found in Ref. 8 and especially on alkaline based materials in Ref. 9. Problems related to KNN mainly address the solid state synthesis, sintering, microstructure and properties therefore the basic knowledge data on structure, phase relations, solid state reactions, defect chemistry, sintering mechanism, etc is needed. In this review, we address the crystal structure, † Corresponding Author. synthesis and sintering as the key parameters to control the microstructure and consequently the functional properties of KNN. Possible methods of controlling grain growth and template-assisted grain growth resulting in alkaline niobate single crystals are discussed. 1.1. Crystal structure and synthesis of K 0.5 Na 0.5 NbO 3 The KNN composition is, according to the phase diagram, isotructural with KNbO 3 . 1 For the refinement of the KNN cell parameters, different structures and symmetries have been used; the authors report the perovskite unit cell with the orthorhombic symmetry, the monoclinic symmetry and also the triclinic symmetry for particles smaller than 130 nm. 10, 11 According to our recently published X-ray diffraction analysis of K x Na 1-x NbO 3 (0.4 ≤ x ≤ 0.6) system, the unit cell parameters and symmetry of all compositions exhibit a monoclinic unit cell with a small monoclinic distortion at room temperature. 12 The monoclinic symmetry for KNN given by Tellier et al. 12 (a = 4.0046 Å, b = 3.9446 Å, c = 4.0020 Å and β = 90.3327 ◦ ) was additionally confirmed by transmis- sion electron microscopy (TEM) analysis of the KNN single crystal. 13 As shown in Fig. 1, the unit cell parameters (a , b 15