Unit Cell Indexing of Luminescent Tantalum Zinc Oxide
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Philip D. Rack and Michael D. Potter Advanced Vision Technolgies, Inc., West Henrietta, New York 14586
Thomas N. Blanton Eastman Kodak Company, Rochester, New York 14650 (Received 8 September 1999; accepted 8 March 2000)
Tantalum zinc oxide (TZO) with the chemical formula Ta2Zn3O8 can be generated from the reaction of 3 mol ZnO and 1 mol Ta2O5 at elevated temperatures. This phase has been shown to exhibit blue cathodoluminescence at low electron beam voltages. It has also been realized in thin film form on silicon substrate, making TZO an important material for monolithic field emission display devices. The structure type of TZO has been investigated using powder x-ray diffraction techniques. The unit cell of this phase has been determined and found to be monoclinic. These results allow for indexing of the powder pattern first reported by Kasper in 1967 and correct for a discrepancy in the single-crystal structure lattice constants reported by Waburg and Muller-Buschbaum in 1984. I. INTRODUCTION
Bulk Ta2Zn3O8 (TZO) has been synthesized from ZnO and Ta2O5 powders using conventional ceramic processing techniques.1 It has also been prepared in thin-film form by reacting a thin-film stack composite of Ta over ZnO at a temperature range of 900–1200 °C.2 TZO has been shown to luminesce at a peak wavelength of ∼385 nm, under photo and electron beam excitation.1–3 It is believed that the luminescence mechanism is the result of a metal-to-ligand radiative transition, where tantalum is the metal and oxygen is the ligand. The excited state is the 5d0 state of the tantalum metal whereas the ground state is the 2p6 state associated with the oxygen ligand.3 Doping TZO with a suitable concentration of manganese (Mn) has been shown to excite green and red luminescence3–4 giving a full color capability for emissive displays. To better understand the mechanism of luminescence of intrinsic and doped TZO, it is essential to have knowledge of the structure of this material. Kasper 5 reported a crystalline Ta2Zn3O8 phase formed by 700 °C processing of ZnO and Ta2O5 powders. However, he was unable to obtain a single crystal and the powder x-ray diffraction (XRD) pattern he obtained was not indexed. His diffraction pattern is referenced in the Powder Diffraction File, PDF 20-1237, with d-spacings and intensities listed.6 Waburg and Muller-Buschbaum7 reported a Ta 2 Zn 3 O 8 phase, which was generated by hightemperature (1800 °C) solid-state reaction of ZnO and Ta2O5 using a CO2 laser. Their work did result in producing single crystals and the crystal structure was rea)
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J. Mater. Res., Vol. 15, No. 6, Jun 2000 Downloaded: 17 Mar 2015
ported as being monoclinic (a ⳱ 9.499 Å, b ⳱ 8.411 Å, c ⳱ 8.881 Å,  ⳱ 116.08°, Space Group C2/c). Though Waburg and Muller-Buschbaum did not report a powder diffraction pattern, the International Center for Diffraction Data recently calculated powder XRD patterns based on the 40,000+ crystal structures
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