Anomalous Curie temperature behavior of A-site Gd-doped BaTiO3 ceramics: The influence of strain

The influence of A-site Gd3+ doping on the c/a ratio, cell volume and Curie temperature (T-c) of ferroelectric BaTiO3 according to the titanium vacancy mechanism, viz., Ba1-xGdxTi1-x/4O3 has been investigated and compared to that of La3+-doping. The c/a ratio and cell volume of Gd-doped samples are larger than the equivalent La-doped samples and T-c decreases at a rate of only similar to 8 degrees C/at. % for Gd3+ substitution compared to similar to 24 degrees C/at. % for La3+ substitution. These trends are opposite to that expected based on ionic-radii and tolerance factor arguments. This anomalous behavior is attributed to an ion-size mismatch (cation variance) effect on the A-site between the Ba2+ and Gd3+ ions that induces local strain in the lattice that suppresses the reduction in the c/a ratio and cell volume, and as a consequence in the rate of decrease in T-c. (C) 2011 American Institute of Physics. [doi:10.1063/1.3563710]