There is continued interest in Niobate-based oxides for a variety of energy-related applications, from mixed conducting electrode materials for Li⁺/Na⁺ batteries to high energy density dielectrics for pulse power electronic applications. Here we report on the influence of A-site vacancies on the structural stability and electrical functionality of (i) NaNbO₃ perovskite and (ii) Sr₂NaNb₅O₁₅ tetragonal tungsten bronze.

(i) Partial replacement of Na⁺ by Bi³⁺ with the creation of A-site vacancies in NaNbO₃ produces a solid solution based on Na_1-3xBi_xNbO₃ which induces intrinsic clustering of lattice distortions that leads to displacive order–disorder behaviour with increasing x [1]. This remarkable solid solution initially exhibits interesting polar dielectric phenomena such as anti-ferroelectricity (x = 0.00) and relaxor ferroelectricity (x = 0.10) before undergoing an order-disorder transition that promotes co-existence of weakly coupled relaxor ferroelectricity and Na⁺ ion conduction (x = 0.20).  We now compare and contrast this range of functionality with that based on a range of A-site Rare Earth (RE³⁺ = La-Nd) and Alkaline Earth (AE²⁺ = Ca,Sr) dopants based on the mechanisms, Na_1-3xRE_xNbO₃ and Na_1-2xAE_xNbO₃, respectively. This allows a ‘holistic’ view of the electrical properties of A-site deficient NaNbO₃ to be presented.

(ii) Sr₂NaNb₅O₁₅ (SNN) has been reported as a novel high-temperature dielectric material [2]; however, there are questions about its thermodynamic stability and the influence of A-site vacancies based on the existence and thermal heat treatment of the solid solution Sr_2+xNa_1-2xNb₅O₁₅ with 0 ≤ x ≤ 0.20. Here we present results on the thermal stability and dielectric properties of a solid solution based on x = 0.20 and designed as Sr_2.2Na_0.6+yNb_5-yTi_yO₁₅ with 0 ≤ y ≤ 0.20 to systematically vary the A-site vacancy concentration from ~ 6.66 to 0%. The results show the thermodynamic stability is linked to the concentration of A-site vacancies and that the lower permittivity maximum at ~ -70 ^oC for x = 0.20 is invariant with the level of A-site vacancies; however, the magnitude and temperature of the ferro- to para-electric transition at ~ 230 ^oC for x = 0.20 show substantial variations. These results are discussed in a wider context on how to control the thermodynamic stability and temperature coefficient of capacitance in SNN-based dielectrics.