1. Ceramic Technology Division, Regional Research Laboratory, Trivandrum 695 019, India, Department of Electronics, Cochin University of Science and Technology, Cochin 682 022, India, Departamento de Física, ICEX, UFMG, C.P. 702, Belo Horizonte-MG 30123-970, Brazil, and Departamento de Engenharia Metalúrgica e de Materiais, UFMG, Rua Espírito Santo 35, Sala 206, Belo Horizonte-MG 30160-030, Brazil