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

Abstract

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)2O2x+5. Based on electron, neutron, and X-ray diffraction, the crystal structure is described within modern superspace [Hf1.2Ta0.8O4.4, Z = 2, a = 4.7834(13), b = 5.1782(17), c = 5.064(3) Å, q = ⅕c*, orthorhombic, superspace group Xmcm(00γ)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 abovementioned oxide systems, in which stability ranges have been disputably fuzzy and established only by syntheses via solid-state routes so far. model. To settle this matter finally, neutron diffraction with an exceptionally high signal-to-noise ratio at 2θ < 25° would be needed to overcome diagnostic uncertainty. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Additional figures and tables: diffraction pattern with fit of an α-PbO2-type model, ellipsoid plots, stereoview of crystal structure, bond lengths, de-Wolff sections, and Bärnighausen tree (PDF). Accession Codes. CCDC 1861844-1861847 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures. Copyright Notice. The TOC graphic is a derivative work of "P-Channel MOSFET 20V 24Alow Vgs(th)" 39 by SparkFun Electronics, used under CC BY 2.0. 40

3D Electron Diffraction: The Nanocrystallography Revolution

“…Recently the dynamical refinement was generalized to the case of modulated structures and applied to deduce the structure of Hf 3 Ta 2 O 11 . 67…”

“…Lanza et al determined and refined the natural modulated structure of the mineral daliranite (PbHgAs 2 S 5 ). Recently the dynamical refinement was generalized to the case of modulated structures and applied to deduce the structure of Hf 3 Ta 2 O 11 …”

“…Since 2007, the three-dimensional ED method has spread with an exponential trend. Very soon, still in the hand of few specialists, it allowed the structure determination of landmark samples, mostly inorganic, that were considered impracticable for X-ray methods. − Later, the introduction of ultrafast data acquisition procedures and of more sensitive detectors allowed establishing robust experimental protocols also for organic and metalorganic materials. − Finally, three-dimensional ED found successful applications for macromolecules − and other structures of biologic interest. − The growing attention on three-dimensional ED is confirmed by the number and impact of related publications in the very last year, − and by the fact that this method has been considered one of the most important recent scientific breakthroughs …”

Commensurate Nb2Zr5O15: Accessible Within the Field Nb2ZrxO2x+5 After All

“…All positions are strongly underbonded for niobium(V), with Nb14 being the least disfavored. The metal(V) BVS are noticeably lower than in Hf 3 Ta 2 O 13 , which is probably caused by the higher relative number and thereby stronger predominance of metal(IV) ions. With a calculated overall ratio of r (Nb/Zr)=2 : 19, the BVS cannot account for a clear separation of cations and leaves a scenario with Nb11/Zr11, Zr12, Nb13/Zr13, and Nb14 as most probable candidate.…”

“…With Nb 2 Zr 5 O 15 now and Hf 3 Ta 2 O 11 before, we have synthesized two compounds off stoichiometry via a sol‐gel route that “locked in” to commensurately modulated structures M IV n M V 2 O 2 n +5 ( M : metals) with the next higher metal(IV) content. An excess of metal(V) precursors is unnecessary for synthesis.…”