*2.2.1 Synthesis of Ba*2*Tb(Bi,Sb)O*<sup>6</sup> *samples*

The citrate pyrolysis technique as mentioned above was applied to synthesis of Ba2Tb(Bi,Sb)O6 compounds. After mixing stoichiometric quantities of high purity Ba (NO3)2, Tb4O7, Bi2O3, and Sb, the resultant mixture was dissolved in a nitric acid solution at 70–80°C. Furthermore, adding citric acid to the solution, the neutralizing process was in progress by adding aqueous ammonia to it while the pH value of the solution reached � 6*:*9. The transparent solution was dried under stirring on the halogen lamp hot plate, and the self-ignition process occurred in the 0.5 L beaker, resulting in the formation of the porous products. Finally, the precursors were ground into fine powders and they were annealed in air at 900–1000°C for 48-96 h, in order to synthesize the Ba2Tb(Bi,Sb)O6 double perovskite phase (samples #T1 and #T2).

For scanning electron microscope (SEM) measurements, Ba2TbBiO6 polycrystalline film on Ag substrate was fabricated from the single-phase powders by an electrophoretic deposition technique. The SEM image revealed the surface morphology and shape of the Ba2TbBiO6 powder sample.

### *2.2.2 Physical properties of Ba*2*Tb(Bi,Sb)O*<sup>6</sup> *samples*

We measured optical spectra by a diffuse reflectance method using a spectrophotometer (Hitachi U-3500) with the reference material of BaSO4. The energy band gaps for the powder samples were estimated from the reflectance data on the basis of conventional Kubelka-Munk function [18, 28]. The *dc* magnetization measurement was performed over a wide range of temperatures under the magnetic field cooling process of 0.1 T using a SQUID magnetometer.

The effective magnetic moment (*μ*eff ) estimated from the the magnetic susceptibility data using the Curie–Weiss law gives rise to the average valence of the Tb ion at the B site, which is related to the ratio of the Tb3<sup>þ</sup> and Tb<sup>4</sup><sup>þ</sup> ions.
