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Journal of Advanced Ceramics 2022, 11(10): 1613-1625 https://doi.org/10.1007/s40145-022-0635-x
Research Article | Open Access | Issue | Published: 24 September 2022

Sol–gel approach to low-temperature synthesis of single-phase metastable La2Ga3O7.5 melilite with enhanced grain-boundary oxide ionic conductivity via a kinetically favorable mechanism

Show Author's Information Yuan ZHANG,  Longfei ZHAO,  Zhupeng YE,  Yanwei ZENG( )
College of Materials Science and Engineering, Nanjing Tech University, Nanjing 211816, China
Keywords:
melilite, La2Ga3O7.5, low temperature synthesis, gel-precursor, meta-stability, kinetic mechanism
Cite this article:
ZHANG Y, ZHAO L, YE Z, et al. Sol–gel approach to low-temperature synthesis of single-phase metastable La2Ga3O7.5 melilite with enhanced grain-boundary oxide ionic conductivity via a kinetically favorable mechanism. Journal of Advanced Ceramics, 2022, 11(10): 1613-1625. https://doi.org/10.1007/s40145-022-0635-x
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Abstract Full text Electronic supplementary material About this article

Starting with the stoichiometric and highly homogeneous gel-precursor, single-phase metastable melilite La2Ga3O7.5, as the end-member of solid solution La1+xSr1−xGa3O7+x/2 (0 ≤ x ≤ 1), has been synthesized by solid-state reaction at 700 ℃ for 2 h via a kinetically favorable mechanism and characterized by X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), AC impedance spectroscopy, etc. It has been revealed that the as-synthesized melilite La2Ga3O7.5 shows an orthorhombic symmetry with crystal cell parameters a = 11.4690(1) Å, b = 11.2825(4) Å, and c = 10.3735(4) Å, while has more Raman active modes than LaSrGa3O7 with a tetragonal structure, which was also synthesized under the same conditions for comparison, but tends to slowly decompose into perovskite LaGaO3 and Ga2O3 when annealed at 700 ℃ for over 20 h driven by its meta-stability. Moreover, the metastable La2Ga3O7.5 shows a higher XPS binding energy for the excess oxide ions in the crystal structure than those at normal lattice sites. Its intrinsic grain oxide ion conductivity can reach as high as 0.04 and 0.51 mS·cm–1 at 550 and 700 ℃, respectively, characterized by a simple Arrhenius relationship ln(σT)–1/T with invariable activation energy, Ea = 1.22 eV, over the temperature range from 300 to 700 ℃, along with an apparent grain boundary conductivity that is about double that from the grains thanks to the clean grain boundaries. This paper provides a new strategic approach to the synthesis of complex oxides that may be of high performance but are difficultly achieved by the conventional ceramic method at high temperatures.


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About this article

Sol–gel approach to low-temperature synthesis of single-phase metastable La2Ga3O7.5 melilite with enhanced grain-boundary oxide ionic conductivity via a kinetically favorable mechanism

Show Author's information Yuan ZHANG, Longfei ZHAO, Zhupeng YE, Yanwei ZENG( )
College of Materials Science and Engineering, Nanjing Tech University, Nanjing 211816, China

Abstract

Starting with the stoichiometric and highly homogeneous gel-precursor, single-phase metastable melilite La2Ga3O7.5, as the end-member of solid solution La1+xSr1−xGa3O7+x/2 (0 ≤ x ≤ 1), has been synthesized by solid-state reaction at 700 ℃ for 2 h via a kinetically favorable mechanism and characterized by X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), AC impedance spectroscopy, etc. It has been revealed that the as-synthesized melilite La2Ga3O7.5 shows an orthorhombic symmetry with crystal cell parameters a = 11.4690(1) Å, b = 11.2825(4) Å, and c = 10.3735(4) Å, while has more Raman active modes than LaSrGa3O7 with a tetragonal structure, which was also synthesized under the same conditions for comparison, but tends to slowly decompose into perovskite LaGaO3 and Ga2O3 when annealed at 700 ℃ for over 20 h driven by its meta-stability. Moreover, the metastable La2Ga3O7.5 shows a higher XPS binding energy for the excess oxide ions in the crystal structure than those at normal lattice sites. Its intrinsic grain oxide ion conductivity can reach as high as 0.04 and 0.51 mS·cm–1 at 550 and 700 ℃, respectively, characterized by a simple Arrhenius relationship ln(σT)–1/T with invariable activation energy, Ea = 1.22 eV, over the temperature range from 300 to 700 ℃, along with an apparent grain boundary conductivity that is about double that from the grains thanks to the clean grain boundaries. This paper provides a new strategic approach to the synthesis of complex oxides that may be of high performance but are difficultly achieved by the conventional ceramic method at high temperatures.

Keywords: melilite, La2Ga3O7.5, low temperature synthesis, gel-precursor, meta-stability, kinetic mechanism

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Publication history

Received: 04 April 2022
Revised: 12 July 2022
Accepted: 17 July 2022
Published: 24 September 2022
Issue date: October 2022

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© The Author(s) 2022.

Acknowledgements

This work was supported by the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions, China. We would like to thank Mengyan Chen and Weiwei Wang from Shiyanjia Lab (www.shiyanjia.com) for the XPS and HRTEM analysis.

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