1. [1] V. K. Bokov, G. V. Novikov, Yu. G. Saksonov, V. A. Trukhtanov & S. L. Yushchuk, Sov. Phys.- Solid State, vol. 16, 2364, 1975.
2. [2] W. Kirsten, Phy. Stat. Sol. (a), vol. 18, 447, 1975.
3. [3] Deepika Kothari, Ph.D Thesis, Delhi University, India, 1988.
4. [4] D. R. Sagar, Chandra Prakash, S. N. Chatterjee and Pran Kishan, Proc. ICF5 (India), 441, 1989.
5. [5] Pran Kishan, D.R. Sagar, S.N. Chatterjee, L.K. Nagpaul, Nitendar Kumar and K.K. Laroia, Adv. in Ceramics, vol.15, 507, 1986.