Crystal structure and compositional analysis of epitaxial (K 0.56 Na 0.44 )NbO 3 films prepared by hydrothermal method
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Hiro Einishi Department of Innovative and Engineered Materials, Tokyo Institute of Technology, Yokohama 226-8502, Kanagawa, Japan
Takao Shimizu and Hiroshi Funakuboa) Department of Innovative and Engineered Materials, Tokyo Institute of Technology, Yokohama 226-8502, Kanagawa, Japan; and Materials Research Center for Element Strategy (Tokyo Tech MCES), Yokohama 226-8502, Kanagawa, Japan
Minoru Kurosawa Department of Information Processing, Tokyo Institute of Technology, Yokohama 226-8502, Kanagawa, Japan
Hiroshi Uchida Department of Materials and Life Sciences, Sophia University, Chiyoda, 102-8554, Tokyo, Japan
Nobuhiro Kumada Center for Crystal Science and Technology, University of Yamanashi, Koufu, 400-8510, Yamanashi, Japan
Takanori Kiguchi and Toyohiko J. Konno Institute for Material Research, Tohoku University, Sendai 980-8577, Miyagi, Japan (Received 7 December 2015; accepted 27 January 2016)
(KxNa1x)NbO3 films were deposited on Nb-doped (100)SrTiO3 substrates at 240 °C for times between 1 and 6 h by a hydrothermal method. Over this time series, the measured (K 1 Na)/Nb ratio of the films was found to remain constant, but the bulk K/(K 1 Na) ratio, x, decreased from an initial value of 0.75–0.56. It was determined that film growth initially proceeded through crystallization of the K-rich phase (K0.75Na0.25)NbO3. For film growth times greater than 3 h, a second perovskite phase with a smaller unit cell volume was detected, with an estimated composition of (K0.36Na0.64)NbO3. As such, the measured bulk composition value x 5 0.56 was determined to be the result of a combination of these two phases, as opposed to originating from a single phase. Cross-sectional transmission electron microscopy analyses of films prepared for 6 h revealed that they consist of two layers in the direction normal to the substrate; this bilayer-type structure, only observed for hydrothermal growth of this material, is considered to arise from the large solubility mismatch between the Nb precursor and KOH and NaOH in the growth solution.
I. INTRODUCTION
Active devices based on piezoelectric films have been widely investigated for microelectromechanical system (MEMS) applications.1–4 Pb(ZrxTi1x)O3 films have been widely used in this field due to their excellent piezoelectric properties, especially the morphotropic phase boundary composition at x 5 0.5.5–7 However, alternative Pb-free piezoelectric materials have been highly sought after as a replacement for this material due to the toxicity of Pb.8 Pb-free piezoelectric materials based on the alkaline niobate structure have attracted a lot of attention in Contributing Editor: Amit Goyal a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.56
research,9 particularly (KxNa1x)NbO3, a solid solution between KNbO3 and NaNbO3. This material is considered to be one of the most promising candidates because (KxNa1x)NbO3 also possesses a phase boundary for compositions around x 5 0.5, and may exhibit a maximum piezoelectric response comparable
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