Effect of alkaline-earth and transition metals on the electrical transport of double perovskites

Abstract

The transport properties of A2BMoO6 (A=Ca, Sr, and Ba; B=Cr, Mn, and Fe) crystals are investigated from studies of x-ray diffraction, thermoelectric power (TEP) S, and resistivity ρ measurements over a wide temperature range (20–900 K). The resistivity of A2FeMoO6 (A=Ca, Sr, and Ba) crystals shows a metallic character and its analysis suggests that the electron-electron scattering is one of the contributions governing the conduction mechanism of these compounds. On the other hand, Ca2MnMoO6 shows an insulator-metal transition similar to that reported in rare earth nickelate [Phys. Rev. B 45, 8209 (1992)]. The conductivity σ(T) data of Sr2CrMoO6 can be best fitted in terms of simultaneous contributions to σ with an activated-type process and variable range hopping (VRH) mechanism. In the case of Sr2MnMoO6, ρ(T) behavior follows an activated-type hopping for T>400 K whereas for T<85 K it can be described satisfactorily by VRH mechanism. For all the compounds, TEP exhibits negative values (n type) except for Ca2FeMoO6 which shows a crossover from positive (p type) to negative values (n type) around T=147 K. From the metallic diffusion part of TEP, Fermi energy EF is estimated to be 0.86 eV, 1.2 eV, and 0.8 eV for Sr2FeMoO6, Ca2FeMoO6, and Ca2MnMoO6, respectively.