Effect of titanium additives on the growth of tellurium dioxide crystals in a sol–gel process A. Huriet, S. Daniele * , L.G. Hubert-Pfalzgraf Universite ´ Claude Bernard Lyon 1, IRC-UPR 5401, 2 avenue A. Einstein, 69626 Villeurbanne, Cedex, France Received 20 January 2005; accepted 13 February 2005 Available online 1 April 2005 Abstract Hydrolysis of tellurium isopropoxide, in the presence of tetraalkylammonium bromide solution, led to oriented a-TeO 2 crystals. Anisotropic growth was enhanced by adding trace amounts of titanium isopropoxides which allowed to get higher aspect ratio a-TeO 2 crystals. Higher amounts of titanium isopropoxides afforded mixtures of a-TeO 2 rods and h-TeO 2 square crystals or TiTe 3 O 8 crystalline materials after annealing at 480 -C. Those materials have been characterised by FT-IR, XRD, XPS and SEM-EDX techniques. D 2005 Elsevier B.V. All rights reserved. Keywords: Tellurium dioxide; Alkoxide; Sol–gel; Aspect ratio; TiTe 3 O 8 1. Introduction Tellurium oxide materials (TeO 2 , TeO x ) have been widely investigated for optical applications such as optical memory storage due to their reversible phase changes induced by thermal treatment (e.g. by low power laser diodes) [1]. In particular, paratellurite a-TeO 2 is of great importance as transparent acousto-optic and piezoelectric material [2]. These materials are generally performed by physical processes such as vacuum evaporation [3], laser ablation [2] or RF magnetron sputtering [4]. Sol–gel techniques are limited to few studies due to the high reactivity of the tellurium alkoxides and chlorides and the difficulties to control their hydrolysis reactions [5–9]. Only, Pierre and al have reported the production of crystalline a-TeO 2 powders at room temperature from tellurium alkoxide hydrolysed in excess water [5]. Gen- erally, air-stable tellurium (VI) alkoxide or less-reactive tellurium (IV) isopropoxides modified by organic additives such as acetic acid, 1,3-propanediol, 2-methyl-2,4-pentane- diol, acetylacetone or ethyl acetyoacetate were employed in sol–gel process [6–9]. In these works, a-TeO 2 was obtained only at high temperature (around 360 -C) via subsequent crystallisations as metallic tellurium, g-TeO 2 and/or h-TeO 2 (tellurite). The presence of HNO 3 was found to promote a direct transformation from the amorphous form to the a-TeO 2 at around 340 -C [6,7]. However, to our knowledge, no effect of inorganic additives on the growth and the crystallization of tellurium dioxide crystals in sol–gel processing has been reported. We recently published the synthesis of nanocrystalline particles of titania at low temperature (100 -C) by a sol–gel process using titanium alkoxide and bromide ammonium salts as catalysts [10]. Herein, we report the effect of titanium additives in aqueous sol–gel preparations of crystalline and oriented TeO 2 crystals. 2. Experimental Ti(OPr i ) 4 was purchased from Aldrich and Te(OPr i ) 4 was prepared as previously reported [11]. They were distilled under vaccum and used under inert atmosphere using Schlenk tubes and vacuum line techniques with anhydrous solvents. The as prepared powders issued from the sol–gel process were analysed by FT-IR 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.02.082 * Corresponding author. Tel.: +33 4 72 44 53 60; fax: +33 4 72 44 53 99. E-mail address: daniele@catalyse.univ-lyon1.fr (S. Daniele). Materials Letters 59 (2005) 2379 – 2382 www.elsevier.com/locate/matlet