Elena Tchernychova

Compositional and Structural Study of a (K 0.5 Na 0.5 )NbO 3 Single Crystal Prepared by Solid State Crystal Growth

In this work we investigated the chemical composition and structure of (K0.5Na0.5)NbO3 (KNN) single crystals grown by the solid state crystal growth method. The optical, scanning, and transmission electron microscopies were employed for the analysis of the chemical homogeneity and domain structure of the KNN crystal. No compositional inhomogeneities within experimental error were encountered in the KNN single crystals. The domain structure of the KNN single crystal, with a monoclinic unit cell, is composed of large 90 degrees domains of up to 100 microm width, which further consist of smaller 180 degrees domains with widths from 50 to 300 nm.

Successful application of spatial difference technique to electron energy‐loss spectroscopy studies of Mo/SrTiO3 interfaces

The electron energy‐loss near‐edge structure (ELNES) of Mo/SrTiO3 interfaces has been studied using high spatial resolution electron energy‐loss spectroscopy (EELS) in a dedicated scanning transmission electron microscope. Thin films of Mo with a thickness of 50 nm were grown on (001)‐orientated SrTiO3 surfaces by molecular beam epitaxy at 600 °C. High‐resolution transmission electron microscopy revealed that the interfaces were atomically abrupt with the (110)Mo plane parallel to the substrate surface. Ti‐L2,3 (∼460 eV), O‐K (∼530 eV), Sr‐L2,3 (∼1950 eV) and Mo‐L2,3 (∼2500 eV) absorption edges were acquired by using the Gatan Enfina parallel EELS system with a CCD detector. The interface‐specific components of the ELNES were extracted by employing the spatial difference method. The interfacial Ti‐L2,3 edge shifted to lower energy values and the splitting due to crystal field became less pronounced compared to bulk SrTiO3, which indicated that the Ti atoms at the interface were in a reduced oxidation state and that the symmetry of the TiO6 octahedra was disturbed. No interfacial Sr‐L2,3 edge was observed, which may demonstrate that Sr atoms do not participate in the interfacial bonding. An evident interface‐specific O‐K edge was found, which differs from that of the bulk in both position (0.8 ± 0.2 eV positive shift) and shape. In addition, a positive shift (0.9 ± 0.3 eV) occurred for the interfacial Mo‐L2,3, revealing an oxidized state of Mo at the interface. Our results indicated that at the interface SrTiO3 was terminated with TiO2. The validity of the spatial difference technique is discussed and examined by introducing subchannel drift intentionally.

Growth and Characterization of Single Crystals of Potassium Sodium Niobate by Solid State Crystal Growth

Due to the toxic nature of PbO in Pb(Zr,Ti)O3 (PZT), the most common type of piezoceramic, many studies on lead-free materials are being conducted worldwide (Roedel et al., 2009). One of the most studied groups of lead-free ferroelectric materials is based on a solid solution of potassium sodium niobate, K0.5Na0.5NbO3 (KNN) (Jaffe et al., 1971; Kosec et al., 2008). However, activity in attempts to find a lead-free replacement for PZT really accelerated with the discovery of textured (K,Na,Li)(Nb,Ta,Sb)O3 ceramics, with properties comparable to those of PZT (d33> 300 pC/N, relative permittivity > 1500, planar coupling coefficient kp> 60%)(Saito et al., 2004). One way to enhance the piezoelectric properties of KNN is to grow KNN-based single crystals along certain crystallographic directions, as has been demonstrated for the case of KNbO3 (Wada et al., 2004) and relaxor-based ferroelectric single crystals (Park & Shrout, 1997). The most frequently used methods for growing alkali niobate based single crystals are top seeded solution growth and the flux methods (Chen et al. , 2007; Davis et al., 2007; Kizaki et al., 2007; Lin et al., 2009). However, these methods are not yet fully commercialized due to high costs and poor reproducibility related to compositional inhomogeneity within the crystals. A possible way to improve the homogeneity of crystals with complex composition is to grow them by the low cost Solid State Single Crystal Growth (SSCG) method. The SSCG method is essentially a form of induced abnormal grain growth, a phenomenon which is very well known in the solid state sintering community. A significant breakthrough in the solid state synthesis of lead-free materials has been achieved in recent years (Kosec at al., 2010; Malic et al., 2008a). However, due to the strongly hygroscopic nature of alkaline carbonates usually used in solid state synthesis, different diffusion rates of involved ionic species during annealing, and the high sublimation rates of the alkaline species at high temperatures (Bomlai et al., 2007; Jenko et al., 2005; Kosec & Kolar, 1975; Malic et al., 2008b), it may be rather difficult to obtain compositionally homogeneous alkali niobate-based single crystals by SSCG.

APPROACHES FOR A RELIABLE COMPOSITIONAL ANALYSIS OF ALKALINE-BASED LEAD-FREE PEROVSKITE CERAMICS USING MICROANALYTICAL METHODS

The article describes the approaches for a reliable, quantitative compositional analysis of lead-free perovskite ceramics in powder and bulk forms that contain volatile alkaline compounds. The combination of scanning electron microscopy (SEM) and transmission electron microcopy (TEM) with electron-probe analytical techniques, such as energy-dispersive X-ray spectroscopy (EDS), wavelength-dispersive X-ray spectroscopy (WDS) and electron-energy-loss spectroscopy (EELS) makes it possible to determine the true chemical composition, from precursor powders to synthesized ceramics or single crystals. The microscale (SEM) and nanoscale (TEM) analytical methods also give an insight into the local variations of the chemical composition.