The authors acknowledge the collaboration of the Institut d'Estudis Catalans;
the Service of Electron Microscopy of the Universitat Polit6cnica de Catalunya and the Department of Inorganic Chemistry of our University. The work was financed by the Comisi6n Asesora de Investigaci6n Cientifica y T6cnica del Ministerio de Educaci6n y Ciencia under Contracts 3967/79 and 1106/81;