Solid State Ionics 127 (2000) 125–132 www.elsevier.com / locate / ssi Electrical properties of ferroelectric addition compound K SeO .Te(OH) 2 4 6 * M. Dammak, H. Khemakhem, N. Zouari, A.W. Kolsi, T. Mhiri ´ Laboratoire de l’ Etat Solide ( LES), Faculte des Sciences, Sfax, 3038 Tunisia Received 12 November 1998; received in revised form 5 March 1999; accepted 20 July 1999 Abstract Single crystals of K SeO .Te(OH) , (KTSe) were obtained by slow evaporation at 300 K. The differential scanning 2 4 6 calorimetry diagram showed three anomalies at 433, 480 and 495 K, respectively. The dielectric constant evolution as a function of frequency and temperature revealed the first two anomalies. The peak at 433 K was attributed to the ferroelectric–paraelectric phase transition and the second one to the superionic phase transition. X-ray powder diffraction data confirmed that the third anomaly at 495 K characterizes the decomposition before melting of the material.  2000 Elsevier Science B.V. All rights reserved. Keywords: Ferroelectric phase; Paraelectric phase; Phase transition; Superionic conduction 1. Introduction troscopy, dielectric relaxation spectroscopy, calorimetric and X-ray diffraction measurements. Selenate, sulfate and phosphate tellurate are addi- tion compounds between telluric acid and various cations of selenates, sulfates and phosphates, respec- 2. Experimental tively [1–4]. Some of these compounds of general formula M AO .Te(OH) (where M is a monovalent The crystals of KTSe have been prepared by the 2 4 6 cation and A 5 S, Se, P . . . ) exhibit structural phase reaction with stoichiometric quantities of H TeO 6 6 transition and interesting physical properties such as and K SeO (1 M) in aqueous solution: 2 4 ferroelectricity and superionic conductivity [5–7]. 6 H TeO 1 K SeO → K SeO .Te(OH) In the course of a systematic investigation of these 6 6 2 4 2 4 ' compounds, we have synthesized K SeO .Te(OH) 2 4 6 which crystallizes in the monoclinic cell Cc with the Single crystals have been grown from water ˚ unit cell dimensions a 5 11.552(2) A; b 5 6.432(1) solution by slow evaporation at room temperature ˚ ˚ A; c 5 13.919(2) A and b 5 105.92(1)8 [2]. during approximately 10 days. Several recrystalliza- We report here some results of impedance spec- tions were necessary to obtain single crystals suitable for the study of the physical properties and X-ray *Corresponding author. analysis. The crystals had a parallelepipedic form 0167-2738 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738(99)00256-8