Thermochimica Acta 513 (2011) 38–42 Contents lists available at ScienceDirect Thermochimica Acta journal homepage: www.elsevier.com/locate/tca Thermodynamic studies on Ba 2.875 UO 5.875 (s) Smruti Dash ∗ , Ziley Singh Product Development Division, Bhabha Atomic Research Centre, Mumbai 400085, India article info Article history: Received 23 March 2010 Received in revised form 26 October 2010 Accepted 2 November 2010 Available online 11 November 2010 Keywords: Barium uranates Gibbs energy of formation Enthalpy of formation Entropy Manometric technique abstract The Gibbs energy of formation of Ba 2.875 UO 5.875 (s) has been determined using manometric technique. The equilibrium CO 2 (g) pressures over the three phase mixture: {Ba 2.875 UO 5.875 (s) + Ba 3 UO 6 (s) + BaCO 3 (s)} have been measured using an absolute pressure transducer in the temperature range 1030–1220 K. The corresponding Gibbs energy expression as a function of temperature can be given as:  f G ◦ m (T )(Ba 2.875 UO 5.875 , s,T ) (kJ mol -1 ) ± 5 =-3153.1 + 0.5573 (T/K)  f H ◦ m (298.15 K) and S ◦ m (298.15 K) data have been calculated from the above Gibbs energy expres- sion using required values from the literature. The corresponding values are -3143.4 ± 2 kJ mol -1 and 270.8 ± 4JK -1 mol -1 , respectively. The  f H ◦ m (298.15 K) value has also been calculated from third law method as -3137.7 ± 4 kJ mol -1 . © 2010 Elsevier B.V. All rights reserved. 1. Introduction Barium is an important fission product in an irradiated nuclear fuel. Thus, thermochemical stabilities of compounds in the Ba- U-O system are of importance. The stability of a compound can be calculated from its Gibbs energy of formation values. Hence,  f G ◦ m (T ) of all barium uranates are required to compute the chem- ical form of barium in the reactor environment. Allpress [1] and Touzelin and Pialoux [2] investigated Ba-U-O system using X- ray diffraction technique. Thermodynamic functions of barium uranates are reported in the literature [3–14]. However,  f G ◦ m (T ) data are limited [10,15,16]. Recently Banarjee et al. [15] have measured  f G ◦ m (T ) of BaU 2 O 7 (s) and Ba 2 U 3 O 11 (s) using e.m.f. technique. Yamawaki et al. [16] investigated the vaporization of BaUO 3 (s) by means of Knudsen effusion mass spectrometry in the temperature range of 1770–1920 K. Matsuda et al. [10] have cal- culated  f G ◦ m (BaUO 3 , s,T ) from their heat capacity measurements and literature data. The  f G ◦ m (T ) values for other uranates are not reported. In this study, the CO 2 (g) pressure measurements have been carried out to determine  f G ◦ m (Ba 2.875 UO 5.875 , s,T ) val- ues using a static manometric technique in the temperature range 1030–1220 K. ∗ Corresponding author. Tel.: +91 22 2559 0648; fax: +91 22 2550 5151. E-mail address: smruti@barc.gov.in (S. Dash). 2. Experimental 2.1. Material preparation Ba 2.875 UO 5.875 (s) was prepared by the solid state reaction of BaCO 3 (s) (0.997 mass fraction, B.D.H, UK) with U 3 O 8 (s) (0.9999 mass fraction purity, Nuclear Fuel Complex, Hyderabad, India) in the presence of air. BaCO 3 (s) and U 3 O 8 (s) were mixed thoroughly in the ratio of 8.625:1 and calcined at 1273 K for 300 h in an alu- mina boat with intermediate grindings. Ba 2.875 UO 5.875 (s) has been formed due to following reaction: BaCO 3 (s) + U 3 O 8 (s) + 1/2O 2 (g) = 3Ba 2.875 UO 5.875 (s) + CO 2 (g). (1) The resulting reddish-orange coloured compound was charac- terized as Ba 2.875 UO 5.875 (s) by X-ray diffraction (XRD). The XRD pattern was taken on a STOE X-ray diffractometer, Germany, using Ni filtered Cu K ( radiation. The X-ray diffraction (XRD) pattern shown in Fig. 1 agreed well with that reported in the literature [17,18]. Griffiths and Kemmler [17] found this compound besides Ba 3 UO 6 (s) in the BaO-UO 3 system. A three phase mixture: {Ba 2.875 UO 5.875 (s) + Ba 3 UO 6 (s) + BaCO 3 (s)} was prepared under dry condition and made into pellets under a pressure of 100 MPa. The pellets were dried and stored in a des- iccator for manometric measurements. 2.2. Equilibrium technique An all-metal ultra-high vacuum system similar to that employed earlier [19] was used to measure CO 2 (g) pressures. The assem- 0040-6031/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tca.2010.11.007