1. Ascarelli, G., and Baxter, J.E., 1972, Solid State Commun., 10:315.
2. Baba, T., and Masumi, T., 1978, Solid State Commun., 27:1113.
3. Bassani, F., Knox, R.S., and Fowler, W.B., 1965, Phys. Rev. 137:A1217.
4. Behnke, G., and Büttner, H., 1978, phys. stat. sol., (b) 90:53.
5. Brown, F.C., 1973, in: “Treaties on Solid State Chemistry”, Vol. 4, B. Hannay, ed., Plenum Press, New York, p. 333.