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Submitted on: 04 December 1999.
Revised on: 21 March 2000.
Accepted on: 02 May 2000.
___________________Journal of Non-Crystalline Solids
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Submitted on: 25 December 1999.
Revised on: 11 April 2000.
Accepted on: 20 May 2000.
___________________Journal of Non-Crystalline Solids
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Submitted on: 05 June 1998.
Revised on: 03 September 1998.
Accepted on: 15 September 1998.
___________________Journal of Non-Crystalline Solids
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