Enthalpy increments of Ba2Te3O8(s) and Ba3Te2O9(s) compounds

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Abstract

The enthalpy increments of the compounds Ba2Te3O8(s) and Ba3Te2O9(s) were measured in the temperature range 298–965 K, using a high temperature Calvet calorimeter. The values were least square fitted in the following temperature dependent polynomial equations:ΔH298.15KT(J/mol)Ba2Te3O8(s)=77,233+234.87T+0.08108T2(298.15862.8K),ΔH298.15KT(J/mol)Ba2Te3O8(s)=21,533209.39T+0.46007T2(862.8965K),ΔH298.15KT(J/mol)Ba3Te2O9(s)=116,177+338.96T+0.00741T2+4,310,572T.

The temperature differentials of the above equations gave the heat capacity equations for the respective compounds, which were compared with Neumann–Kopp's estimated heat capacities. As heat capacity of TeO3 was not available in literature, it was estimated from that of TeO2. Using the Gibbs energy of formation data of Ba3Te2O9(s), reported in literature, ΔfH298.15K°, and S298.15K° of the compound were calculated as −2258 kJ/mol and 822 J/(mol K), respectively.

Introduction

The chemistry of tellurium in nuclear reactor is of great importance as tellurium is one of the highly corrosive fission products. In its free form, it can easily migrate to the fuel surface and attack the clad. In many fuel pin failures, tellurium is found to be the main culprit. To be able to predict the availability of elemental tellurium that can cause the damage, we need to know the thermodynamic stability of all products of tellurium with other fission products or fuel elements, which can probably be formed in the reactor. Barium and strontium are the alkaline-earth fission products that form stable M–Te–O type compounds in oxide fuels. Thermodynamic data of many of these compounds have been reported in literature [1], [2], [3], [4], [5], [6], [7]. However, enthalpy increment values of only few of these compounds are reported. Chattopadhyaya and Juneja [8] have constructed a tentative phase diagram for the Ba–Te–O system on the basis of its constituent binaries. The enthalpy increment data are of great importance as the heat capacity equation calculated from this enables the calculation of thermodynamic parameter of the compound at various temperatures. Therefore, enthalpy increments of Ba2Te3O8(s) and Ba3Te2O9(s) were measured in the temperature range 298–965 K, using high temperature Calvet calorimeter.

Section snippets

Experimental

The compounds Ba2Te3O8(s) and Ba3Te2O9(s) were prepared by first preheating BaCO3(s) (E. Merk, Germany, mass fraction purity = 0.99999) and TeO2(s) (prepared from telluric acid, AR, BDH, UK, mass fraction purity = 0.99999). The stoichiometric amounts of these two compounds were then carefully mixed in agate mortar. The mixture for Ba3Te2O9 was heated in a platinum boat, at 900 K for 5 h and 1050 K for 24 h, in pure oxygen atmosphere with intermittent grindings. The pale yellow coloured compound thus

Results and discussion

A plot of the enthalpy increment values of Ba2Te3O8(s) versus temperature showed a change in slope at 863 K. It could not be attributed to a second-order, order–disorder transition as the heat capacity values at high temperatures, calculated from the slope of enthalpy increment data, did not come back in the range of heat capacity values observed just below the transition. Therefore, this change can be either due to a first-order phase transition, accompanied with small enthalpy of transition,

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