Effects of substitution of Ti⁴⁺ by Nb⁵⁺ on the electrical properties of BaTi₂O₅ prepared by a floating zone method

Nb⁵⁺-substituted single-crystalline BaTi₂O₅ (Ba(Ti_1−xNb_x)₂O_5+z) was synthesised by a floating zone method, and the effect of Nb⁵⁺ substitution on the dielectric properties of Ba(Ti_1−xNb_x)₂O_5+z was investigated. The growth direction of Ba(Ti_1−xNb_x)₂O_5+z was parallel to the b axis. The a lattice constant and β angle increased from 1.6906 to 1.6918 nm and from 103.09° to 103.13° at x=0–0.7 at%, respectively. With increasing x, the Curie temperature of Ba(Ti_1−xNb_x)₂O_5+z decreased from 750 to 652 K. A complex impedance plane plot of BaTi₂O₅ at 598 K in the b direction showed a semicircle at high frequencies and a spike inclined at 45° to the real axis at low frequencies, indicating ionic conduction of bulk BaTi₂O₅. At x>0.002 (0.2 at%), the complex impedance plots of Ba(Ti_1−xNb_x)₂O_5+z showed no inclined spike, suggesting dominantly electronic conduction. The electrical conductivity of Ba(Ti_1−xNb_x)₂O_5+z increased with temperature with an activation energy of 1.4 to 1.7 eV and was minimum at x=0.001 (0.1 at%). The spontaneous polarisation of Ba(Ti_0.998Nb_0.002)₂O_5+z was 2.7×10⁻² C m⁻², which was 2.5 times higher than that of BaTi₂O₅.