Synthesis of Hexagonal Nanophases in the La<sub>2</sub>O<sub>3</sub>–MO<sub>3</sub> (M = Mo, W) Systems
We report a study of nanophases in the La<sub>2</sub>O<sub>3</sub>–MO<sub>3</sub> (M = Mo, W) systems, which are known to contain a variety of good oxygen-ion and proton conductors. Mechanically activated La<sub>2</sub>O<sub>3</sub> + MO<...
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MDPI AG
2023-07-01
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| Online Access: | https://www.mdpi.com/1996-1073/16/15/5637 |
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| author | Egor Baldin Nikolay Lyskov Galina Vorobieva Igor Kolbanev Olga Karyagina Dmitry Stolbov Valentina Voronkova Anna Shlyakhtina |
| author_facet | Egor Baldin Nikolay Lyskov Galina Vorobieva Igor Kolbanev Olga Karyagina Dmitry Stolbov Valentina Voronkova Anna Shlyakhtina |
| author_sort | Egor Baldin |
| collection | DOAJ |
| description | We report a study of nanophases in the La<sub>2</sub>O<sub>3</sub>–MO<sub>3</sub> (M = Mo, W) systems, which are known to contain a variety of good oxygen-ion and proton conductors. Mechanically activated La<sub>2</sub>O<sub>3</sub> + MO<sub>3</sub> (M = Mo, W) mixtures and the final ceramics have been characterized by differential scanning calorimetry (DSC) and X-ray diffraction (XRD) with Rietveld refinement. The microstructure of the materials has been examined by scanning electron microscopy (SEM), and their conductivity in dry and wet air has been determined using impedance spectroscopy. In both systems, the formation of hexagonal La<sub>15</sub>M<sub>8.5</sub>O<sub>48</sub> (phase II, 5H polytype) (M = Mo, W) nanophases is observed for the composition 1:1, with exothermic peaks in the DSC curve in the range ~480–520 °C for La<sub>15</sub>Mo<sub>8.5</sub>O<sub>48</sub> and ~685–760 °C for La<sub>15</sub>W<sub>8.5</sub>O<sub>48</sub>, respectively. The crystallite size of the nanocrystalline tungstates is ~40 nm, and that of the nanocrystalline molybdates is ~50 nm. At higher temperatures (~630–690 and ~1000 °C), we observe irreversible reconstructive phase transitions of hexagonal La<sub>15</sub>Mo<sub>8.5</sub>O<sub>48</sub> to tetragonal γ-La<sub>2</sub>MoO<sub>6</sub> and of hexagonal La<sub>15</sub>W<sub>8.5</sub>O<sub>48</sub> to orthorhombic β-La<sub>2</sub>WO<sub>6</sub>. We compare the temperature dependences of conductivity for nanoparticulate and microcrystalline hexagonal phases and high-temperature phases differing in density. Above 600 °C, oxygen ion conduction prevails in the coarse-grained La<sub>18</sub>W<sub>10</sub>O<sub>57</sub> (phase I, 6H polytype) ceramic. Low-density La<sub>15</sub>W<sub>8.5</sub>O<sub>48</sub> and La<sub>15</sub>Mo<sub>8.5</sub>O<sub>48</sub> (phase II, 5H polytype) nanoceramics exhibit predominantly electron conduction with an activation energy of 1.36 and 1.35 eV, respectively, in dry air. |
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| language | English |
| last_indexed | 2024-03-11T00:28:32Z |
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| spelling | doaj.art-8a8e66c4cab846acb97644f36e5d1c442023-11-18T22:50:52ZengMDPI AGEnergies1996-10732023-07-011615563710.3390/en16155637Synthesis of Hexagonal Nanophases in the La<sub>2</sub>O<sub>3</sub>–MO<sub>3</sub> (M = Mo, W) SystemsEgor Baldin0Nikolay Lyskov1Galina Vorobieva2Igor Kolbanev3Olga Karyagina4Dmitry Stolbov5Valentina Voronkova6Anna Shlyakhtina7N.N. Semenov Federal Research Center for Chemical Physics, Russian Academy of Sciences, Moscow 119991, RussiaFederal Research Center of Problems of Chemical Physics and Medical Chemistry RAS, Chernogolovka, Moscow 142432, RussiaN.N. Semenov Federal Research Center for Chemical Physics, Russian Academy of Sciences, Moscow 119991, RussiaN.N. Semenov Federal Research Center for Chemical Physics, Russian Academy of Sciences, Moscow 119991, RussiaEmanuel Institute of Biochemical Physics RAS, Russian Academy of Sciences, Moscow 119334, RussiaDepartment of Chemistry, Lomonosov Moscow State University, Moscow 119991, RussiaDepartment of Physics, Lomonosov Moscow State University, Moscow 119991, RussiaN.N. Semenov Federal Research Center for Chemical Physics, Russian Academy of Sciences, Moscow 119991, RussiaWe report a study of nanophases in the La<sub>2</sub>O<sub>3</sub>–MO<sub>3</sub> (M = Mo, W) systems, which are known to contain a variety of good oxygen-ion and proton conductors. Mechanically activated La<sub>2</sub>O<sub>3</sub> + MO<sub>3</sub> (M = Mo, W) mixtures and the final ceramics have been characterized by differential scanning calorimetry (DSC) and X-ray diffraction (XRD) with Rietveld refinement. The microstructure of the materials has been examined by scanning electron microscopy (SEM), and their conductivity in dry and wet air has been determined using impedance spectroscopy. In both systems, the formation of hexagonal La<sub>15</sub>M<sub>8.5</sub>O<sub>48</sub> (phase II, 5H polytype) (M = Mo, W) nanophases is observed for the composition 1:1, with exothermic peaks in the DSC curve in the range ~480–520 °C for La<sub>15</sub>Mo<sub>8.5</sub>O<sub>48</sub> and ~685–760 °C for La<sub>15</sub>W<sub>8.5</sub>O<sub>48</sub>, respectively. The crystallite size of the nanocrystalline tungstates is ~40 nm, and that of the nanocrystalline molybdates is ~50 nm. At higher temperatures (~630–690 and ~1000 °C), we observe irreversible reconstructive phase transitions of hexagonal La<sub>15</sub>Mo<sub>8.5</sub>O<sub>48</sub> to tetragonal γ-La<sub>2</sub>MoO<sub>6</sub> and of hexagonal La<sub>15</sub>W<sub>8.5</sub>O<sub>48</sub> to orthorhombic β-La<sub>2</sub>WO<sub>6</sub>. We compare the temperature dependences of conductivity for nanoparticulate and microcrystalline hexagonal phases and high-temperature phases differing in density. Above 600 °C, oxygen ion conduction prevails in the coarse-grained La<sub>18</sub>W<sub>10</sub>O<sub>57</sub> (phase I, 6H polytype) ceramic. Low-density La<sub>15</sub>W<sub>8.5</sub>O<sub>48</sub> and La<sub>15</sub>Mo<sub>8.5</sub>O<sub>48</sub> (phase II, 5H polytype) nanoceramics exhibit predominantly electron conduction with an activation energy of 1.36 and 1.35 eV, respectively, in dry air.https://www.mdpi.com/1996-1073/16/15/5637mechanochemical synthesisnanomaterialslanthanum tungstatelanthanum molybdatepolytypismoxygen ion conductivity |
| spellingShingle | Egor Baldin Nikolay Lyskov Galina Vorobieva Igor Kolbanev Olga Karyagina Dmitry Stolbov Valentina Voronkova Anna Shlyakhtina Synthesis of Hexagonal Nanophases in the La<sub>2</sub>O<sub>3</sub>–MO<sub>3</sub> (M = Mo, W) Systems Energies mechanochemical synthesis nanomaterials lanthanum tungstate lanthanum molybdate polytypism oxygen ion conductivity |
| title | Synthesis of Hexagonal Nanophases in the La<sub>2</sub>O<sub>3</sub>–MO<sub>3</sub> (M = Mo, W) Systems |
| title_full | Synthesis of Hexagonal Nanophases in the La<sub>2</sub>O<sub>3</sub>–MO<sub>3</sub> (M = Mo, W) Systems |
| title_fullStr | Synthesis of Hexagonal Nanophases in the La<sub>2</sub>O<sub>3</sub>–MO<sub>3</sub> (M = Mo, W) Systems |
| title_full_unstemmed | Synthesis of Hexagonal Nanophases in the La<sub>2</sub>O<sub>3</sub>–MO<sub>3</sub> (M = Mo, W) Systems |
| title_short | Synthesis of Hexagonal Nanophases in the La<sub>2</sub>O<sub>3</sub>–MO<sub>3</sub> (M = Mo, W) Systems |
| title_sort | synthesis of hexagonal nanophases in the la sub 2 sub o sub 3 sub mo sub 3 sub m mo w systems |
| topic | mechanochemical synthesis nanomaterials lanthanum tungstate lanthanum molybdate polytypism oxygen ion conductivity |
| url | https://www.mdpi.com/1996-1073/16/15/5637 |
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