Effect of Cooling Rate on Dopant Spatial Localization and Phase Transformation in Cu‐Doped Y‐Stabilized ZrO2 Nanopowders

International audienceThe effect of calcination temperature (TC = 500–1000 °C) and cooling rate on the dopant distribution in Cu‐doped Y‐stabilized ZrO2 nanopowders is studied. The powders are produced by co‐precipitation technique and investigated by attenuated total reflection, UV‐vis diffuse reflectance, electron paramagnetic resonance, and transmission electron microscopy methods. The cooling rate is found to affect the amount of Cu substances on grain surface, the powders subjected to fast cooling (quenching) showed higher amount of Cu‐related complexes on the grains’ surface than their counterparts cooled with furnace after calcination. It is observed that Cu impurities diffuse inside ZrO2 grains from Cu‐related surface substances when TC  800 °C, outward migration of Cu dopants takes place. Simultaneously, the intensity of 275‐nm absorption band decreases, the monoclinic ZrO2 phase forms and its contribution rises with TC. It is proposed that monoclinic phase formation is caused by the replacement of Cu atoms from lattice sites to interstitials leading to an appearance of the channels for Y out‐diffusion via cation vacancies and destabilization of ZrO2 tetragonal phase

Effect of Cooling Rate on Dopant Spatial Localization and Phase Transformation in Cu‐Doped Y‐Stabilized ZrO2 Nanopowders

International audienceThe effect of calcination temperature (TC = 500–1000 °C) and cooling rate on the dopant distribution in Cu‐doped Y‐stabilized ZrO2 nanopowders is studied. The powders are produced by co‐precipitation technique and investigated by attenuated total reflection, UV‐vis diffuse reflectance, electron paramagnetic resonance, and transmission electron microscopy methods. The cooling rate is found to affect the amount of Cu substances on grain surface, the powders subjected to fast cooling (quenching) showed higher amount of Cu‐related complexes on the grains’ surface than their counterparts cooled with furnace after calcination. It is observed that Cu impurities diffuse inside ZrO2 grains from Cu‐related surface substances when TC  800 °C, outward migration of Cu dopants takes place. Simultaneously, the intensity of 275‐nm absorption band decreases, the monoclinic ZrO2 phase forms and its contribution rises with TC. It is proposed that monoclinic phase formation is caused by the replacement of Cu atoms from lattice sites to interstitials leading to an appearance of the channels for Y out‐diffusion via cation vacancies and destabilization of ZrO2 tetragonal phase

Dielectric resonator in rectangular ??(102) cavity for electron paramagnetic resonance study of thin films

The improved compared to a rectangular TE(102 )cavity geometry of a dielectric resonator (DR) suitable for studying thin films and coatings has been calculated and experimentally verified. It is shown that electron paramagnetic resonance (EPR) signal of SiOx films can be enhanced by using as the DR of two rectangular parallelepipeds fabricated from BaTi4O9 + 8.5% ZnO ceramics (epsilon=36) with dimension of 5.64x5.5 x 5.9 mm(3 )and a gap of up to 0.5 mm between them. Located inside a standard rectangular metal TE(102 )cavity of the X-band EPR spectrometer, the DR increases the filling factor by 5-12 times depending on the size of a sample studied. The experimental use of the DR allows to increase the EPR signal of the 950 nm SiOx thin film by a factor of approximately 4.Funding Agencies|National Academy of Sciences of Ukraine [0118U002317, 0118U002317s]; Swedish Foundation for Strategic Research (SSF) [UKR22-0040]</p

Peculiarities of Thermally Activated Migration of Subvalent Impurities in Cu-Doped Y-Stabilized ZrO2 Nanopowders Produced From Zr Oxychlorides

International audienc

Optical and structural properties of Mn 4+ activated (ZnxMg1‐x)2TiO4 red phosphors

International audienceWe report on optical and structural investigations of Mn4+ activated (ZnxMg1-x)2TiO4 red phosphors by X-ray diffraction, photoluminescence (PL) and electron paramagnetic resonance methods. The phosphors of Mg2TiO4, Zn2TiO4 and solid solutions (ZnxMg1-x)2TiO4 (x=0.25, 0.50, 0.75) with manganese content of 0.1 mol. % were synthesized at temperatures in the range of 800-1200 °C via solid state reaction. Formation of solid solutions of inverse spinel structure demonstrates the features similar to both the Mg2TiO4 and Zn2TiO4, i.e. decomposition of the (Zn,Mg)2TiO4 on the (Zn,Mg)TiO3 and MgO and separation of the secondary ZnO phase, respectively. These processes depend on the composition of solid solution and sintering temperature, and for some regimes the single phase phosphor can be obtained. The largest intensity of Mn4+ red PL is found in the Mn-doped (Zn0.25Mg0.75)2TiO4 phosphor sintered at 1100°C and it is 2 times larger than those in similar Mn4+ activated Mg2TiO4 phosphor. The differences in the PL intensity of the phosphors of solid solutions are explained by different PL thermal quenching as well as by competing processes of Mn incorporation in +2 and +4 charge states in zinc-magnesium titanate crystal lattice

Room-Temperature Electron Paramagnetic Resonance Study of a Copper-Related Defect in Cu 2 ZnSnS 4 Colloidal Nanocrystals

We report on electron paramagnetic resonance (EPR) spectroscopy of Cu-Zn-Sn-S nanocrystals (NCs) synthesized by a colloidal chemistry method. The chemical composition of the NCs is characterised by a strong deficit of tin and slight zinc enrichment with respect to the "ideal" stoichiometry of kesterite Cu2ZnSnS4. Nevertheless, X-ray photoemission spectroscopy (XPS) shows that the net cation/anion ratio is very close to 1:1 and the valences states of the constituents correspond to those of Cu2ZnSnS4, in particular Cu+. The kesterite crystal structure of the NCs is confirmed by X-ray diffraction and Raman spectroscopy. Observing the EPR features for such CZTS NCs characteristic of individual Cu2+ embedded in the crystal lattice can be directly related with cationic disorder and non-stoichiometry. In particular, as a result of Cu-Zn antisites, Sn deficiency, and likely multivalence, a very small fraction of copper ions may occur in the Cu2+ state. The concentration of Cu2+ sites of ~1017-1018 cm-3 assessed from the EPR spectra is below the sensitivity of XPS, therefore the latter is dominated by the prevailing Cu+ species. The temperature dependence of the I-V curves reveal a level at about 0.2 eV above the valence band that can be assigned to the CuZn antisite (with copper presumably in Cu2+ state) and a 14 meV level above valence band that can correspond to VCu. The new insights reported here into the relation between the structure and electronic states of CZTS NCs are important for advancing their applications

Data_Sheet_1_Peculiarities of Thermally Activated Migration of Subvalent Impurities in Cu-Doped Y-Stabilized ZrO2 Nanopowders Produced From Zr Oxychlorides.docx

<p>The influence of chlorine on spatial distribution of subvalent dopants and oxygen vacancies' content in Cu-doped Y-stabilized ZrO₂ nanopowders was studied as a function of calcination temperature (500–1,000°C) and Cu content (1 and 8 mol%). The powders were prepared by co-precipitation technique from a mixture of zirconium oxychloride, yttrium and copper nitrates. The powders were studied by X-ray diffraction, Auger spectroscopy, attenuated total and diffuse reflectance, as well as by electron paramagnetic resonance methods. The increase of calcination temperature stimulates structure transformation, variation of oxygen vacancies content in the grains, and amount of dispersed CuO at their surface. All these changes depend on Cu content being controlled by surface-volume copper redistribution: in-diffusion of copper (below 800°C) and its out-diffusion (at 800–1,000°C). The transformation of surface entities (Cu₂Cl(OH)₃ and Cu-OH complexes) followed by the formation of CuO was observed. At low Cu content (1 mol%), the amount of oxygen vacancies and CuO varies significantly and non-monotonically with T_c rise, while for higher Cu content (8 mol%), they change slightly. Chlorine was found to be present in the grain bulk with the content nearly equal to Cu content. It acts as compensator for Cu charge that prevents the appearance of oxygen vacancies. The Cu-Cl interaction hampers Cu out-diffusion from the grains, phase transformation and formation of CuO at their surface. The latter reduces the catalytic activity of the powders in the CO PROX reaction. Among all the powders, the highest CO conversion (about 80%) was found to be demonstrated by the powders calcined at 600°C contained 1 mol% of CuO.</p