ISSN:
1573-4803
Source:
Springer Online Journal Archives 1860-2000
Topics:
Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes:
Abstract Rare-earth molybdates of the type R2(MoO4)3 with R=La, Ce, Pr, Nd, Sm and Eu were prepared and characterized, and the electrical conductivity, σ and Seebeck coefficient, S in the temperature range 450–1200 K were measured. These molybdates are concluded to be insulating solids with a band gap which increases slowly going down the series from 2.30 eV for La molybdate to 3.20 eV for Eu molybdate. The plots of log σ and S versus T −1 show, in general, three linear regions with two break temperatures T 1 and T 2 occurring due to a change in the conduction mechanism. At higher temperatures the intrinsic conduction in these solids occurs via a band mechanism. The O2− ∶2p and Mo6+ ∶4d orbitals form the valence and conduction bands, respectively. These bands are the main support of conduction in La, Sm and Eu molybdates; however, for Ce, Pr and Nd molybdates 4f n levels fall within the band gap and become very effective in electrical conduction. The main charge-carrying entities seem to be electrons in Ce, Pr and Nd molybdates and holes in La, Sm and Eu molybdates. On the basis of mobility calculations of charge carriers it is concluded that the charge carriers in these bands become polarons which are, in fact, the charge carrying entities. At lower temperatures electrical conduction is mainly extrinsic. Cerium molybdate shows a semiconductor-semimetal transition around 940 K.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00353184
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