Ferroelectric phase transitions in KNO 3 embedded into porous glasses R. Poprawski, E. Rysiakiewicz-Pasek * , A. Sieradzki, A. Ci _ zman, J. Polan ´ska Institute of Physics, Wroclaw University of Technology, W. Wyspian ´ skiego 27, 50-370 Wroclaw, Poland Available online 1 October 2007 Abstract This paper presents the results of dielectric and dilatometric measurements of porous glasses filled with KNO 3 from water solution and from the melt. It was shown that ferroelectric phase transition is observed during the cooling process only in KNO 3 nanocrystals introduced into the porous matrix. The phase diagram of transition temperatures versus average values of pore dimensions was obtained. The phenomenological model describing the phase transition and size effects in KNO 3 crystals is proposed. Ó 2007 Elsevier B.V. All rights reserved. PACS: 77.80.Bh; 77.80.e; 77.55.+f; 78.55.Mb Keywords: Ferroelectric; Glasses; Porosity; Nanocrystals 1. Introduction Nano-sized ferroelectrics have attracted great interest in the last decade because they have important practical applications and are physical objects exhibiting physical properties different from their bulk counterparts. Some of the most important electronic applications of nano-sized ferroelectrics include nonvolatile memories, pyroelectric sensors, optical waveguides, optical memories, sound receivers and transmitters. These applications utilize the unique dielectric, piezoelectric, pyroelectric and electro- optic properties of low dimensional ferroelectric materials. At critical dimensions the corresponding crystals actually lose ferroelectric properties [1]. For the bulk KNO 3 crystals, ferroelectric phase is observed only when cooling at atmospheric pressure in the temperature range from 383 K to 398 K [2,3]. Lower dimensions of the KNO 3 lead to distinct and unusual anomalies of physical properties compared to the bulk material. It is also known that thin films of KNO 3 exhibit ferro- electric properties at room temperature [4–6]. Basic physi- cal properties of nano-sized KNO 3 strongly depend on the method of its fabrication, thickness of the ferroelectric layer and type of the electrodes [4], thus making interpreta- tion of experimental results strongly linked to the employed technology. One of the simple ways to produce nano-sized ferroelectrics is introducing them into the por- ous glass matrices [7,8]. The aim of this work is to study the size effects for KNO 3 introduced into porous glasses by different methods, in par- ticular the influence of size effects on phase transitions. We report the results of dielectric measurements, dilatometric studies and atomic force microscope imaging for porous glasses with two different mean sizes of pores filled with KNO 3 from water solutions and from the melted phase. It is worth to notice that the results of dielectric measurements are strongly influenced by the water absorbed inside the pores and interaction between the electrodes and investi- gated samples. The dilatometric parameters are mainly influ- enced by the thermal deformation of the ferroelectrics particles inside the pores. Thus, the dilatometric measure- ments can be used for the control of phase transition temper- ature shifts in porous glasses with ferroelectric particles and phase transition character changes related to size effects. 0022-3093/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.01.086 * Corresponding author. Tel.: +48 71 320 36 14; fax: +48 71 328 36 96. E-mail address: ewa.rysiakiewicz-pasek@pwr.wroc.pl (E. Rysiakie- wicz-Pasek). www.elsevier.com/locate/jnoncrysol Available online at www.sciencedirect.com Journal of Non-Crystalline Solids 353 (2007) 4457–4461