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 30mol% of antimony (LaNb0.9Sb0.1O4, LaNb0.8Sb0.2O4 and LaNb0.7Sb0.3O4) 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 LaSbxNb1-xO4 with increasing antimony content may indicate increasing frequency of optical vibrations of Nb(Sb)-O4 -2 polyhedra relative to La +3 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.

Thermal and Chemical Expansion in Proton Ceramic Electrolytes and Compatible Electrodes

“…At the time, it was considered somewhat of a breakthrough, as LaNbO 4 represented a new oxide system posing as an alternative electrolyte material to the traditional Y-doped BaZrO 3 or BaCeO 3 perovskites. Following that paper, significant attempts went into optimizing the performance of the oxide by investigating alternative doping strategies [63,64,200,201,205,[211][212][213][214], as well as different synthesis routes and conditions [202,[215][216][217][218][219]. However, most of this work ultimately resulted in minimal improvements compared to the original work by Haugsrud and Norby.…”

Antimony substituted lanthanum orthoniobate proton conductor – Structure and electronic properties

“…16 It has been shown that both vanadium and antimony substitutions influence phonon properties. 7,10,17 In our recent study, we reported that both A-site (with calcium) and B-site (with antimony) substitutions in lanthanum orthoniobate increase its water uptake. 18 Moreover the introduction of dopants elevated not only the water uptake but also the protonic conductivity of these compounds.…”

“…Therefore the phase transition occurs with a change in niobium coordination number (from 6 to 4) and no change in coordination of lanthanum. 4 The main strategy for shifting the phase transition temperature is the substitution of niobium by other penta-or tri-valent elements like antimony, [5][6][7][8] vanadium, [8][9][10] arsenic, 11 or tantalum. 8,[12][13][14] Differences in ionic radius and electronegativity compared to niobium result in either elevating (for Ta) or decreasing (for Sb, V, and As) the transition temperature.…”

Influence of Sb-substitution on ionic transport in lanthanum orthoniobates

“…We have reported effects of isovalent substitution in lanthanum niobate on structural and thermal properties of LaNb1-xSbxO4 29,31,32 . It was shown that the tetragonal Scheelite structure could be stabilised above room temperature with a constant TEC value of 8.1x10 -6 1/K 29 .…”

“…Out of this group lanthanum orthoniobate is considered to be among the best materials combining appreciable proton conductivity and chemical stability 22,[24][25][26][27] , reaching conductivity close to 10 -3 S/cm at 900 °C under wet conditions 22 . Doping and substitutions in lanthanum niobate, both alio-and isovalent, have a substantial influence on these materials properties and much effort to understand the influence of dopants on structural [24][25][26][28][29][30][31][32][33][34] and electrical [22][23][24][25][26][27][35][36][37][38][39][40][41][42][43] properties have been made. Lanthanum orthoniobate undergoes a structural phase transition at about 500 °C, which is accompanied by a decrease in the activation energy of the conductivity 37,39,43 and nearly a twofold change of the thermal expansion coefficient (TEC) [24][25][26]29 .…”