Phase equilibria and metastability in the high-entropy A6B2O17 oxide family with A = Zr, Hf and B = Nb, Ta

The present work details experimental phase stabilization studies for the disordered, multi-cation A6B2O17 (A = Zr, Hf; B = Nb, Ta) system. We leverage both high-temperature in situ and ex situ X-ray diffraction to assess phase equilibrium and metastability in A6B2O17 ceramics produced via reactive sintering of stoichiometric as-received powders. We observe that the A6B2O17 phase can be stabilized for any stoichiometric combination of Group 4B and 5B transition metal cations (Zr, Nb, Hf, Ta), including ternary and quinary systems. The observed minimum stabilization temperatures for these phases are generally in agreement with prior calculations for each disordered A6B2O17 ternary permutation, offering further support for the inferred cation-disordered structure and suggesting that chemical disorder in this system is thermodynamically preferable. We also note that the quinary (Zr3Hf3)(NbTa)O17 phase exhibits enhanced solubility of refractory cations which is characteristic of other high-entropy oxides. Furthermore, A6B2O17 phases experience kinetic metastability, with the orthorhombic structure remaining stable following anneals at intermediate temperatures.