Localization of oxygen interstitials in CeSrGa3O(7+δ) melilite.

Area of Science:

• Materials Science
• Solid State Chemistry
• Ceramics

Background:

• Melilite structures (La(1-x)Ce(x)SrGa3O(7+δ) and La(1.54-x)Ce(x)Sr0.46Ga3O(7.27+δ)) are investigated for their potential in electrochemical applications.
• Understanding the role of cerium (Ce) doping on thermal redox stability and electrical conductivity is crucial for optimizing material properties.

Purpose of the Study:

• To investigate the solubility of cerium in La(1-x)Ce(x)SrGa3O(7+δ) and La(1.54-x)Ce(x)Sr0.46Ga3O(7.27+δ) melilites.
• To determine the thermal redox stability of these melilites in air.
• To analyze the variation in conductivity associated with the oxidation of Ce(3+) to Ce(4+).

Main Methods:

• Synthesis of melilite solid solutions under varying atmospheric conditions (CO reducing and air).
• Electrical conductivity measurements at elevated temperatures (up to 600 °C).
• Neutron powder diffraction analysis to elucidate structural changes and oxygen incorporation.

Main Results:

• Complete substitution of La by Ce in La(1-x)Ce(x)SrGa3O(7+δ) is possible under reducing conditions, forming a solid solution stable in air up to ~600 °C for x ≥ 0.6.
• CeSrGa3O(7+δ) exhibits a conductivity of ~10(-4) S/cm at 600 °C in air, four orders of magnitude higher than LaSrGa3O7, due to enhanced electronic conduction from Ce(3+) 4f electrons.
• Oxidation of Ce(3+) to Ce(4+) slightly reduces conductivity, and extra oxygen does not significantly increase oxide ion conduction; structural distortion occurs due to interstitial oxygen.

Conclusions:

• Cerium doping significantly enhances the electronic conductivity of melilite materials.
• The accommodation and conduction mechanisms of oxygen interstitials in melilite structures are clarified, influenced by cationic size changes upon Ce oxidation.
• These findings offer insights into the design of advanced ceramic materials for electrochemical applications.