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. Na1-2x 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.