Heat capacities and thermodynamic properties of antimony substituted lanthanum orthoniobates

Abstract

The results of heat capacity measurements for the lanthanum orthoniobate substituted with 10, 20 and 30 mol% of antimony (LaNb_0.9Sb_0.1O₄, LaNb_0.8Sb_0.2O₄ and LaNb_0.7Sb_0.3O₄) are presented and discussed. Temperature dependence of low temperature heat capacity was analyzed within the Debye and Einstein models. The Debye temperature decreased, whereas the Einstein temperature increased with antimony content. The decrease of the Debye temperature with increasing antimony content was correlated with decreasing scheelite–fergusonite transition temperature. The increase of the Einstein temperature of LaSb_xNb_1−xO₄ with increasing antimony content may indicate increasing frequency of optical vibrations of Nb(Sb)–O₄⁻² polyhedra relative to La³⁺ cations. Using the heat capacity data, standard entropies of the phases were calculated and combined with previously measured enthalpies of formation to obtain Gibbs energies of formation. Standard thermodynamic properties were tabulated.

Introduction

Understanding the thermodynamic properties of ceramic materials operating in a wide temperature range is crucial in terms of applications and from the geochemical point of view. In this work we focus on lanthanum orthoniobates, that is, on compounds based on LaNbO₄ stoichiometry. Properties of these materials may be modified by different substitutions. For example, temperature of the structural phase transition from the monoclinic to the tetragonal phase depends on the type and amount of substituting atoms. The phase transition temperature of stoichiometric LaNbO₄ is about 770 K [1] but it may be either decreased or increased if Nb is partially substituted by other elements. For instance, substituting niobium by an element with higher ionic radius, such as Ta, leads to increase of the phase transition temperature [2], [3]. However, particularly interesting is the possibility of decreasing the transition temperature below room temperature which may be achieved through substitution with antimony or vanadium [4], [5], [6], [7], [8]. In our previous work we analyzed the influence of antimony substitution on the spontaneous strain and Landau order parameter [7]. It was shown that the introduction of antimony atoms into the niobium sublattice does not change the nature of the phase transition which is second order. Further investigation by high temperature oxide melt solution calorimetry has allowed us to analyze the energetic stability of materials containing different concentrations of substituent. The investigation showed that the enthalpy of structural phase transition for these compounds is relatively low [7]. In the present work, heat capacity measurements at 2–870 K for lanthanum orthoniobate substituted by 10, 20 and 30 mol% of antimony are presented and discussed. Standard entropies and Gibbs energies of formation are calculated.