Role of entropy in the stability of cobalt titanates K.T. Jacob * , G. Rajitha Department of Materials Engineering, Indian Institute of Science, Bangalore 560 012, India article info Article history: Received 15 October 2009 Received in revised form 13 February 2010 Accepted 26 February 2010 Available online 6 March 2010 Keywords: Co 2 TiO 4 CoTiO 3 CoTi 2 O 5 Gibbs free energy Enthalpy Entropy Thermodynamic properties Phase diagram abstract The standard molar Gibbs free energy of formation of Co 2 TiO 4 , CoTiO 3 , and CoTi 2 O 5 as a function of tem- perature over an extended range (900 to 1675) K was measured using solid-state electrochemical cells incorporating yttria-stabilized zirconia as the electrolyte, with CoO as reference electrode and appropri- ate working electrodes. For the formation of the three compounds from their component oxides CoO with rock-salt and TiO 2 with rutile structure, the Gibbs free energy changes are given by: D f G  ðoxÞ ðCo 2 TiO 4 Þ 104=ðJ  mol 1 Þ¼18865  4:108 ðT =KÞ D f G  ðoxÞ ðCoTiO 3 Þ 56=ðJ  mol 1 Þ¼19627 þ 2:542 ðT =KÞ D f G  ðoxÞ ðCoTi 2 O 5 Þ 52=ðJ  mol 1 Þ¼6223  6:933 ðT=KÞ Accurate values for enthalpy and entropy of formation were derived. The compounds Co 2 TiO 4 with spi- nel structure and CoTi 2 O 5 with pseudo-brookite structure were found to be entropy stabilized. The rela- tively high entropy of these compounds arises from the mixing of cations on specific crystallographic sites. The stoichiometry of CoTiO 3 was confirmed by inert gas fusion analysis for oxygen. Because of par- tial oxidation of cobalt in air, the composition corresponding to the compound Co 2 TiO 4 falls inside a two- phase field containing the spinel solid solution Co 2 TiO 4 –Co 3 O 4 and CoTiO 3 . The spinel solid solution becomes progressively enriched in Co 3 O 4 with decreasing temperature. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Cobalt titanate Co 2 TiO 4 finds application as a pigment [1] and CoTiO 3 as a gas sensor for ethanol [2] and high-k ultra-thin gate dielectric for MOSFET and DRAM applications [3]. It has smaller equivalent oxide thickness than the conventional SiO 2 gate oxide and minimal reaction with Si substrate. The gate dielectric is usu- ally prepared in situ by oxidation of a thin film of Co–Ti amorphous alloy deposited on nitrided Si surface. Information on the nonstoi- chiometry of CoTiO 3 and phase relations in the system Co–Ti–O is important for optimizing the performance of the dielectric. Accu- rate thermodynamic data are useful for computing phase diagrams of complex systems containing cobalt titanates. Taylor and Schmalzried [4] first measured the standard Gibbs free energy of formation of CoTiO 3 and Co 2 TiO 4 in the temperature range (1023 to 1473) K using solid-state electrochemical cells incorporating calcia-stabilized zirconia as the solid electrolyte. Gol- ubenko et al. [5] studied the reduction of CoTiO 3 with CO gas and concluded that the compound is nonstoichiometric with respect to oxygen and can be represented as CoTiO 3x with 0 6 x 6 0.15. On further reduction, the compound CoTiO 2.85 decomposes to metallic Co and TiO 2 in the temperature range (1223 to 1423) K. Gibbs free energy of formation of CoTiO 3 was evaluated form the re- sults. Brezny and Muan [6] determined phase relations in the sys- tem CoO–TiO 2 . They found two congruently melting compounds Co 2 TiO 4 (T m = 1835 K) and CoTiO 3 (T m = 1736 K) and an incongru- ently melting compound CoTi 2 O 5 (T decomp = 1755 K). Below T = 1413 K, CoTi 2 O 5 decomposes to a mixture of CoTiO 3 and TiO 2 [6]. The standard Gibbs free energy of formation of the three crys- talline phases from their oxide components at T = (1373 and 1573) K were determined by studying equilibria between pairs of oxide phases, metallic cobalt and a CO + CO 2 gas phase of known composition [6]. By analysis of ternary phase diagrams and tie-lines Navrotsky and Muan [7] obtained values for the Gibbs free energy of formation of CoTiO 3 and Co 2 TiO 4 at T = 1323 K. Popov and Levit- skii [8] used the solid-state EMF method with Fe + Fe 0.95 O as the ref- erence electrode to determine the Gibbs free energy of formation of CoTiO 3 and Co 2 TiO 4 in the temperature range (1160 to 1420) K. Using electrochemical reduction, they confirmed that CoTiO 3 is nonstoichiometric as suggested by Golubenko et al. [5]. Other investigators [4,6,7] considered CoTiO 3 as a stoichiometric. In a more recent study by Yankin et al. [9] the stability of oxide phases in the Co–Ti–O system was examined at reduced static pressures of oxygen in a vacuum chamber at T = (1073, 1173, and 1273) K. The oxygen partial pressure in the gas phase was determined by a solid-state oxygen probe based on yttria-stabilized zirconia 0021-9614/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2010.02.016 * Corresponding author. Tel.: +91 80 22932494; fax: +91 80 23600472. E-mail addresses: katob@materials.iisc.ernet.in, ktjacob@hotmail.com (K.T. Jacob). J. Chem. Thermodynamics 42 (2010) 879–885 Contents lists available at ScienceDirect J. Chem. Thermodynamics journal homepage: www.elsevier.com/locate/jct