A new class of entropy stabilized oxides: Commensurately modulated A₆B₂O₁₇ (A = Zr, Hf; B = Nb, Ta) structures

High entropy oxides (HEOs) have experienced a meteoric rise in the past decade. Here, we report a new class of entropy stabilized A₆B₂O₁₇ (A = Zr, Hf; B = Nb, Ta) oxides whose enthalpies of formation have been determined by high temperature oxide melt solution calorimetry. All four compounds have endothermic enthalpies of formation from binary oxides (AO₂ and B₂O₅), indicating that these modulated phases are entropy stabilized. The unfavorable energetics of formation are counterbalanced by large configurational entropy arising from cation disorder. The calculated configurational entropy per formula unit for A₆B₂O₁₇ is 4.50R J/mol⋅K, which is ~2.8 times higher than the maximum configurational entropy value of 1.61R for an equimolar five cation containing “high entropy oxide”. Based on X-ray and neutron diffraction data, measured enthalpies of formation and calculated configurational entropies, one can infer that all four compounds have a high degree of disorder.