Sol-gel approach to low-temperature synthesis of single-phase metastable La_(2)Ga_(3)O_(7.5) melilite with enhanced grain-boundary oxide ionic conductivity via a kinetically favorable mechanism  

作　　者：Yuan ZHANG Longfei ZHAO Zhupeng YE Yanwei ZENG 

机构地区：[1]College of Materials Science and Engineering,Nanjing Tech University,Nanjing 211816,China

出　　处：《Journal of Advanced Ceramics》2022年第10期1613-1625,共13页先进陶瓷（英文）

基　　金：supported by the Priority Academic Program Development(PAPD)of Jiangsu Higher Education Institutions,China.

摘　　要：Starting with the stoichiometric and highly homogeneous gel-precursor,single-phase metastable melilite La_(2)Ga_(3)O_(7.5),as the end-member of solid solution La_(1+x)Sr_(1−x)Ga_(3)O_(7+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 La_(2)Ga_(3)O_(7.5) shows an orthorhombic symmetry with crystal cell parameters a=11.4690(1)A,b=11.2825(4)A,and c=10.3735(4)A,while has more Raman active modes than LaSrGa_(3)O_(7) with a tetragonal structure,which was also synthesized under the same conditions for comparison,but tends to slowly decompose into perovskite LaGaO_(3) and Ga2O_(3) 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,E_(a)=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.

关 键 词：MELILITE La_(2)Ga_(3)O_(7.5) low temperature synthesis gel-precursor meta-stability kinetic mechanism 

分 类 号：TB383.3[一般工业技术—材料科学与工程] TQ174.758.1[化学工程—陶瓷工业]