Protonic defects and water incorporation in Si and Ge-based apatite ionic conductors

Oxygen Defects and Novel Transport Mechanisms in Apatite Ionic Conductors: Combined 17O NMR and Modeling Studies

“…14, 17, 18 Indeed, the actual location of the interstitial site in the apatite systems has attracted significant controversy 5. 13–20…”

“…The oxide‐ion excess (y) has been reported to be either between the oxygen sites at the center of the channels, or associated with the framework 5. 7, 13–20 One reason for these literature discrepancies is the large local distortions around the interstitial site, making determinations from diffraction studies difficult, as these focus on the long‐range average structure.…”

Oxygen Defects and Novel Transport Mechanisms in Apatite Ionic Conductors: Combined 17O NMR and Modeling Studies

“…Despite the intense research on these systems, there is still a need to develop new electrolytes with improved properties to allow operation of SOFCs at intermediate temperatures (500-700 8C). In this respect there has been interest in a range of new structure classes showing high oxide-ion conduction, including apatite-type silicates/germanates (La 9.33+x (Si/ Ge) 6 O 26+3x/2 ) [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] and melilite-type La 1+x Sr 1Àx Ga 3 O 7+x/2 . [22] An unusual feature of these systems is that rather than oxide-ion vacancies being the conducting defects (as for conventional fluorite and perovskite oxides), the conduction is mediated by oxide ions located at interstitial sites.…”

“…[7-9, 14, 17, 18] Indeed, the actual location of the interstitial site in the apatite systems has attracted significant controversy. [5,[13][14][15][16][17][18][19][20] These apatite materials have the general formula, A 10Àx (Si/Ge) 6 O 26+y (A = rare earth/alkaline earth), and their structures may be considered as composed of an A 4Àx (Si/ GeO 4 ) 6 framework, with the remaining A 6 O 2 units occupying the channels within this framework. The oxide-ion excess (y) has been reported to be either between the oxygen sites at the center of the channels, or associated with the framework.…”

“…The oxide-ion excess (y) has been reported to be either between the oxygen sites at the center of the channels, or associated with the framework. [5,7,[13][14][15][16][17][18][19][20] One reason for these literature discrepancies is the large local distortions around the interstitial site, making determinations from diffraction studies difficult, as these focus on the long-range average structure.…”