Structural, elemental, and electrical conduction properties of corundum-type Mg2Co2Nb2O9.

M g 2 C o 2 N b 2 O 9 is a new member of the corundum-type A 4 B 2 O 9 family. Its crystal structure is trigonal with space group P 3 ¯ c 1. Mg and Co ions share the A-site with fifty-fifty occupancy. In M g 2 C o 2 N b 2 O 9 , Co exhibits a 2 + oxidation state, whereas Nb shows a reduction in the charge state from 5 + to 4 + due to the presence of vacancy-trapped electrons in N b 2 O 9 dimmer. The conductivity spectra analysis in view of various transport models states that overlapping large polaron tunneling is the predominant transport process (polaron radius 10.65 Å) in the low-temperature range (303-473 K), while for the high-temperature range (573-673 K), the conduction is intrinsic, and the carriers are primarily small polarons. The proximity between optical bandgap (∼1.60 eV) and conduction activation energy (1.64 eV) is due to the formation of intermediate Co 3 d energy bands near the Fermi level and their participation in high-temperature intrinsic conduction. [ABSTRACT FROM AUTHOR]