Thermochimica Acta 450 (2006) 105–109 Heat capacity, enthalpy and entropy of strontium bismuth niobate and strontium bismuth tantalate J. Leitner a,∗ , M. Hampl a , K. R ˚ uˇ ziˇ cka b , D. Sedmidubsk ´ y c , P. Svoboda d , J. Vejpravov´ a d a Department of Solid State Engineering, Institute of Chemical Technology, Technick´ a 5, 166 28 Prague 6, Czech Republic b Department of Physical Chemistry, Institute of Chemical Technology, Technick´ a 5, 166 28 Prague 6, Czech Republic c Department of Inorganic Chemistry, Institute of Chemical Technology, Technick´ a 5, 166 28 Prague 6, Czech Republic d Department of Electronic Structures, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 120 00 Prague 2, Czech Republic Available online 19 September 2006 Abstract The heat capacities and enthalpy increments of strontium bismuth niobate SrBi 2 Nb 2 O 9 (SBN) and strontium bismuth tantalate SrBi 2 Ta 2 O 9 (SBT) were measured by the relaxation method (2–150 K), Calvet-type heat-conduction calorimetry (305–570 K) and drop calorimetry (773–1373 K). The temperature dependences of non-transition heat capacities in the form C pm = 324.47 + 0.06371T - 5.0755 × 10 6 /T 2 JK -1 mol -1 (298–1400 K) and C pm = 320.22 + 0.06451T - 4.7001 × 10 6 /T 2 JK -1 mol -1 (298–1400K) were derived for SBN and SBT, respectively, by the least-squares method from the experimental data. Furthermore, the standard molar entropies at 298.15 K S ◦ m (SBN) = 327.15 ± 0.80 and S ◦ m (SBT) = 339.23 ± 0.72 J K -1 mol -1 were evaluated from the low-temperature heat capacity measurements. © 2006 Elsevier B.V. All rights reserved. Keywords: SrBi 2 Nb 2 O 9 ; SrBi 2 Ta 2 O 9 ; Strontium bismuth niobate; Strontium bismuth tantalate; Heat capacity; Enthalpy increment; Entropy 1. Introduction Ceramic materials with ferroelectric properties have found many applications in electronics and optics. In the semicon- ductor industry, much attention is paid to materials used in non-volatile ferroelectric random access memories (FeRAM). At the present time, PZT (PbZr 1-x Ti x O 3 ) ceramics are gener- ally used for FeRAM fabrication. Bismuth layered perovskites SrBi 2 Ta 2 O 9 (SBT) and SrBi 2 Nb 2 O 9 (SBN) are now being exten- sively studied because of their lower fatigue, better compati- bility with CMOS (complementary metal-oxide-semiconductor) technology and lead-free composition [1–3]. Both ferroelectric compounds are orthorhombic (space group A2 1 am) at room tem- perature and transform to tetragonal paraelectric phases (space group I4/mmm) above the Curie temperature T C . Various val- ues of the ferroelectric transition temperature T C derived from dielectric constant measurements with sintered powder sam- ples are given in literature: T C = 543–608 K for SBT [4–10] and 687–723 K for SBN [4,5,8,11,12]. ∗ Corresponding author. Fax: +420 220 444 330. E-mail address: jindrich.leitner@vscht.cz (J. Leitner). SBT, SBN, and their solid solutions have been prepared as powder materials as well as thin films by a number of methods. Metalorganic chemical vapor deposition (MOCVD) has been investigated as the most practical preparation method to realize a high-density FeRAM because of the good step coverage and the uniform composition and film thickness over a large area. For better understanding of MOCVD processes, thermodynamic calculations of equilibrium compositions of relevant systems are often performed. For such a calculation, thermodynamic data for all considered substances are necessary. Morimoto et al. [5] measured the heat capacities of SBT and SBN in the temperature range 500–770 K by the thermal radi- ation calorimetry. Their measurements were performed with powder samples of approximately 5 g. While the temperature dependence of the heat capacity of SBT does not show any anomaly in this temperature region, a distinct peak with maxi- mum at 678 K was observed for SBN. Temperature dependence of the heat capacity of SBT in the form of polycrystalline sam- ple, single crystal as well as thin film were also determined by Onodera et al. [13–15]. The measurements were performed in the temperature range 80–800 K. A slight hump in C p was observed around 610 K for a polycrystalline sample, while a rather clear anomaly was detected at 613 K for the single crystal 0040-6031/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tca.2006.09.006