Structure refinement of polycrystalline orthorhombic calcium titanate substituted by lanthanum: Ca1 − x La x TiO3 (x = 0.1–0.4)

Perovskite-type oxynitrides AB(O,N)3 are potential candidates for photoelectrode materials in solar water splitting. A drawback of these materials is their low sintering tendency resulting in low electrical conductivities. Typically, they are prepared by ammonia treatment of insulating, wide band gap oxides. In this study, we propose an approach starting from small band gap oxides Eu1−xCaxTiO3−δ and then widen the band gaps in a controlled way by ammonolysis and partial Ca2+ substitution. Both together induced a distortion of the octahedral network and dilution of the Eu4f and N2p levels in the valence band. The effect is the stronger the more Ca2+ is present. Within the series of samples, Eu0.4Ca0.6Ti(O,N)3 had the most suitable optical band gap (EG ≈ 2.2 eV) for water oxidation. However, its higher Eu content compared to Eu0.1Ca0.9Ti(O,N)3 slowed down the charge carrier dynamics due to enhanced trapping and recombination as expressed by large accumulation (τon) and decay (τoff) times of the photovoltage of up to 109 s and 486 s, respectively. In contrast, the highly Ca2+-substituted samples (x ≥ 0.7) were more prone to formation of TiN and oxygen vacancies also leading to Ti3+ donor levels below the conduction band. Therefore, a precise control of the ammonolysis temperature is essential, since even small amounts of TiN can suppress the photovoltage generation by fast recombination processes. Water oxidation tests on Eu0.4Ca0.6Ti(O,N)3 revealed a formation of 7.5 μmol O2 from 50 mg powder together with significant photocorrosion of the bare material. Combining crystal structure, chemical composition, and optical and electronical band gap data, a first simplified model of the electronical band structure of Eu1−xCaxTi(O,N)3 could be proposed.

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Band Gap Adjustment in Perovskite-type Eu_1−xCa_xTiO₃via Ammonolysis

Widenmeyer

Kohler

Samoli

et al. 2020

Zeitschrift Für Physikalische Chemie

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Ceramic Mineral Waste-Forms for Nuclear Waste Immobilization

2019

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Crystalline ceramics are intensively investigated as effective materials in various nuclear energy applications, such as inert matrix and accident tolerant fuels and nuclear waste immobilization. This paper presents an analysis of the current status of work in this field of material sciences. We have considered inorganic materials characterized by different structures, including simple oxides with fluorite structure, complex oxides (pyrochlore, murataite, zirconolite, perovskite, hollandite, garnet, crichtonite, freudenbergite, and P-pollucite), simple silicates (zircon/thorite/coffinite, titanite (sphen), britholite), framework silicates (zeolite, pollucite, nepheline /leucite, sodalite, cancrinite, micas structures), phosphates (monazite, xenotime, apatite, kosnarite (NZP), langbeinite, thorium phosphate diphosphate, struvite, meta-ankoleite), and aluminates with a magnetoplumbite structure. These materials can contain in their composition various cations in different combinations and ratios: Li–Cs, Tl, Ag, Be–Ba, Pb, Mn, Co, Ni, Cu, Cd, B, Al, Fe, Ga, Sc, Cr, V, Sb, Nb, Ta, La, Ce, rare-earth elements (REEs), Si, Ti, Zr, Hf, Sn, Bi, Nb, Th, U, Np, Pu, Am and Cm. They can be prepared in the form of powders, including nano-powders, as well as in form of monolith (bulk) ceramics. To produce ceramics, cold pressing and sintering (frittage), hot pressing, hot isostatic pressing and spark plasma sintering (SPS) can be used. The SPS method is now considered as one of most promising in applications with actual radioactive substances, enabling a densification of up to 98–99.9% to be achieved in a few minutes. Characteristics of the structures obtained (e.g., syngony, unit cell parameters, drawings) are described based upon an analysis of 462 publications.

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Structure refinement of polycrystalline orthorhombic calcium titanate substituted by lanthanum: Ca1 − x La x TiO3 (x = 0.1–0.4)

Cited by 2 publications

References 23 publications

Band Gap Adjustment in Perovskite-type Eu_1−xCa_xTiO₃via Ammonolysis

Band Gap Adjustment in Perovskite-type Eu_1−xCa_xTiO₃via Ammonolysis

Ceramic Mineral Waste-Forms for Nuclear Waste Immobilization

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Structure refinement of polycrystalline orthorhombic calcium titanate substituted by lanthanum: Ca1 − x La x TiO3 (x = 0.1–0.4)

Cited by 2 publications

References 23 publications

Band Gap Adjustment in Perovskite-type Eu1−xCaxTiO3via Ammonolysis

Band Gap Adjustment in Perovskite-type Eu1−xCaxTiO3via Ammonolysis

Ceramic Mineral Waste-Forms for Nuclear Waste Immobilization

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Product

Resources

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Band Gap Adjustment in Perovskite-type Eu_1−xCa_xTiO₃via Ammonolysis

Band Gap Adjustment in Perovskite-type Eu_1−xCa_xTiO₃via Ammonolysis