Thermal Diffusivity of Ordered Double Perovskite A₂FeMoO₆ ( A = Ca, Sr and Ba)

Thermal diffusivity has been investigated in ordered double perovskite Sr₂FeMoO₆ by means of transient surface grating technique in the temperature range of 300--450K. The thermal diffusivity shows an appreciable decrease from 39 mm²/s at 300 K to 37 mm²/s at 360 K in the ferromagnetic phase, and then steeply drops to 10 mm²/s with further increasing temperature above the critical temperature T_c～380 K. Such an abrupt decrease of the thermal diffusivity has been ascribed to the structural phase transition at T_c. We further investigate the lattice effect on the thermal diffusivity in A₂FeMoO₆ (A= Ca, Sr and Ba) by substitution of Ca²⁺ or Ba²⁺ ions for Sr²⁺ ions at 300 K. We find that the thermal diffusivity increases from 35 mm²/s for A= Ca to 41 mm²/s for A= Ba. Considering the change of the Fe--O--Mo bond angle from 152.4°C for A= Ca to 180°C for A= Ba, the increased thermal diffusivity for Ba compound has been ascribed to the enhanced hybridization between transition-metal d and oxygen p states due to the larger Fe--O--Mo bond angle and hence the wider one-electron bandwidth