Thermodynamic stability of the tetragonal tungsten bronze "Sr₂NaNb₅O₁₅"

There has been uncertainty for several decades over the crystal chemistry, composition and thermodynamic stability of the nominally stoichiometric tetragonal tungsten bronze (TTB), Sr₂NaNb₅O₁₅ (SNN). Recent interest in SNN-based ceramics sintered at ≥1300 °C has come about because of their high permittivity over a wide temperature range making them attractive for 'next generation', high temperature class II multi-layer ceramic capacitors. Here we use a solid-state reaction method between NaNbO₃ and SrNb₂O₆ powders in a 1 : 2 mol ratio to verify this SNN-based TTB is not thermodynamically stable below ∼1200 °C. Instead, a mixture of A-site deficient Sr-doped NaNbO₃ perovskite (i.e. Na_1-2x Sr _x NbO₃ with x ∼ 0.20) and SrNb₂O₆ is obtained. We confirm the thermal instability of an A-site deficient SNN by demonstrating its complete decomposition to the same biphasic mixture at 900 °C by in situ powder X-ray diffraction. The local bonding arrangements and co-ordination number (CN) of the A-site cations created by the NbO₆ octahedral arrangements play a crucial role in the thermodynamics of this system. For Sr-doped NaNbO₃, there is a random arrangement of Na⁺, Sr²⁺ ions and vacancies on the A sites with CN < 12 in a perovskite lattice of corner sharing NbO₆ octahedra. For SrNb₂O₆, the Sr²⁺ ions are in a corrugated arrangement along channels with CN = 8 based on a lattice of corner sharing Nb₂O₁₀ dimers. This phase assemblage is energetically more favourable below ∼1200 °C than Sr²⁺ and Na⁺ ions on A1 (CN = 12) and larger A2 (CN = 15) sites within a TTB lattice based on corner sharing NbO₆ octahedra. The thermodynamic instability of the TTB-SNN below ∼1200 °C has similarities to that for the metastable TTB-PbNb₂O₆ and has implications for the dielectric applications of SNN ceramics.