**Acknowledgements**

The authors would like to acknowledge the Third World Academy of Sciences (RG/PHYS/LA Nos. 99-050, 02-225 and 05-043), and the ICTP, Trieste-Italy, for financial support of the Latin-American Network of Ferroelectric Materials (NET-43). Thanks to CNPq and FAPEMIG agencies of Brazil. Dr Aimé Peláiz-Barranco acknowledges Le Conseil Régional de LanguedocRoussillon for her invitation to the University of Nîmes, France. Thanks to the Embassy of France in Havana, Cuba, for financial support for the scientific cooperation between the University of Nîmes and Havana University. Dr Aimé Peláiz-Barranco would like to thank Tongji University, Shanghai, China.
