Submitted on: 28 February 1998.
Revised on: 04 April 1998.
Accepted on: 09 April 1998.
SOLID STATE SCIENCES---ELSEVIER
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Submitted on: 17 September 1998.
Revised on: 10 October 1998.
Accepted on: 04 November 1998.
SOLID STATE SCIENCES
Submitted on: 14 July 2003.
Revised on: 05 August 2003.
Accepted on: 16 August 2003.
SOLID STATE SCIENCES---ELSEVIER