文摘
A new Sr3(Si1−xAlxO4)O1−x/2□x/2 (0.0 ≤ x ≤ 0.2) oxysilicate series has been prepared, existing as a single phase for x ≤ 0.11. The evolution of the cell parameters was smooth along the series. The crystal structure of Sr3(Si0.9Al0.1O4)O0.95□0.05 was determined from neutron powder diffraction data at room temperature. This compound crystallizes in the P4/ncc space group, Z = 4, with a = 6.9427(1) Å, c = 10.7992(1) Å, and V = 520.53(1) Å3. The Rietveld R values were RWP = 2.54% and RF = 1.33%. The Si4+/Al3+ aliovalent substitution is accompanied by oxygen vacancies for charge compensation. The crystal structure is described, as is a possible pathway for the “loosely bound” oxide anions that justifies the observed oxygen conductivity. Impedance data showed that the overall conductivity increased by more than 2 orders of magnitude from Sr3(SiO4)O to Sr3(Si0.89Al0.11O4)O0.945□0.055. Further electrochemical characterization under various atmospheres (dry air, wet air, dry 5% H2 in Ar, and wet 5% H2 in Ar) and impedance data as a function of the oxygen partial pressure indicated that there was a significant proton contribution to the overall conductivity. The oxide ion transference numbers, determined using a modified electromotive force method, ranged from 0.98 to 0.95 under strong reducing conditions. A large p-type electronic contribution was also observed for Sr3(Si0.89Al0.11O4)O0.945□0.055 under oxidizing conditions, decreasing the oxide ion transference number to ~0.01 at 1173 K.