The formation of single‐crystal by a gel method and the electrical conductivity of these crystals are reported. In the high‐temperature region above 245°C, the conduction is intrinsic with an activation energy of 1.70 eV and is controlled by the motion of the interstitial positive ions. At lower temperatures, from 190 to 245°C, the conductivity is extrinsic with an activation energy of 0.63 eV and is controlled by the motion of the ions through ion vacant lattice points. The conductivity below 190°C is interpreted to be due to p‐type conduction and has an activation energy of 0.30 eV. In this region, the conduction is controlled by the motion of holes. It is clear that remarkable changes in the conductivity characteristics are caused by heat‐treatment. The conductivity of heat‐treated crystalline increases by more than one order of magnitude compared with an as‐grown sample in the hole conduction region where the activation energy is 0.30 eV. Acceptor levels created by diffused Na also show an increase in conductivity after heat‐treatment.
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Takashi Unagami (1999) studied this question.