Ionic conductivity of lanthanoid silicates, Ln10(SiO4)6O3(Ln = La, Nd, Sm, Gd, Dy, Y, Ho, Er and Yb)

Abstract

Ionic conductivities have been investigated for lanthanoid silicates of the Lnlo(Si04)603 solid solution series and related compounds. The activation energy and conductivity at 500 "C were estimated to be 69 kJ mol-' and 1.8 x S cm-' for Lalo(Si04),03 and 71 kJ mol-' and 1.5 x ~O -~ Scm-' for Ndlo(Si04),03. The a and c lattice constants of the hexagonal phase decreased with decreasing radius of the Ln3+ ion for Lnl0(SiO4),O3 (Ln = La, Nd, Sm, Gd and Dy). The activation energy increased and the conductivity decreased when Ln3+ ions with smaller ionic radii were used. The sole carrier in these materials is the 0'ion.

Oxygen-17 nuclear magnetic resonance measurements on apatite-type lanthanum silicate (La9.33(SiO4)6O2)

“…Apatite-type Ln silicates, Ln9.33+2x/3(SiO4)6O2+x, (Ln: rare earth elements) and related materials are promising oxide-ion conductors and are used as electrolytes in high-temperature solid oxide fuel cells because of their high conductivity, which is comparable or superior to that of yttria-doped zirconia [1][2][3][4][5][6][7]. The high conductivity was first observed by one of the authors [1,2].…”

“…The high conductivity was first observed by one of the authors [1,2]. Although the conduction mechanism has been extensively investigated [8][9][10][11][12], it has not been fully understood.…”

“…Because the O4 site is located in the largest channel along the c-axis, the oxide ion at the O4 site seems to be the main migration specie [1]. In the study of single crystal Nd-Si-O apatite, the conductivity along the c-axis was found to be greater than that perpendicular to the c-axis; however, the difference between them decreases with increasing temperature, suggesting that the oxide ions migrate along the 001 direction.…”

“…Among the apatite-type materials, we chose lanthanum silicate because of its high conductivity [1,7]. O-17 NMR has been applied to investigate the structure around oxide ions [15] or defects in oxide-ion conductors [16][17][18][19].…”

Structural studies of apatite-type oxide ion conductors doped with cobalt

“…Studies have shown that conduction in these systems is wholly anionic 1,3 , with high oxygen transference numbers (>0.9) across a wide range of oxygen partial pressures.…”