At the Gates: The Tantalum-Rich Phase Hf₃Ta₂O₁₁ and its Commensurately Modulated Structure

Generic mixtures in the system (Zr,Hf)O₂-(Nb,Ta)₂O₅ are employed as tunable gate materials for field-effect transistors. Whereas production processes and target compositions are well-defined, resulting crystal structures are vastly unexplored. In this study, we summarize the sparse reported findings and present the new phase Hf₃Ta₂O₁₁ as synthesized via a sol-gel route. Its commensurately modulated structure represents the hitherto unknown, metal(V)-richest member of the family (Zr,Hf)_x(Nb,Ta)₂O_2x+5. Based on electron, neutron, and X-ray diffraction, the crystal structure is described within modern superspace [Hf_1.2Ta_0.8O_4.4, Z = 2, a = 4.7834(13), b = 5.1782(17), c = 5.064(3) Å, q = 1/5c∗, orthorhombic, superspace group Xmcm(00Î)s00] and supercell formalisms [Hf₃Ta₂O₁₁, Z = 4, a = 4.7834(13), b = 5.1782(17), c = 25.320(13) Å, orthorhombic, space group Pbnm]. Transmission electron microscopy shows the microscopic structure from film-like aggregates down to atomic resolution. Cation ordering within the different available coordination environments is possible, but no significant hint at it is found within the limits of standard diffraction techniques. Hf₃Ta₂O₁₁ is an unpredicted compound in the above-mentioned oxide systems, in which stability ranges have been disputably fuzzy and established only by syntheses via solid-state routes so far.