MULTISPECTROSCOPIC STUDIES ON  ALUMINATE PHASES WITH UNUSUAL COORDINATIONS

Simion Simon

Babes-Bolyai University, Faculty of Physics, 3400 Cluj-Napoca, Romania

The development of new aluminate phases from amorphous xerogels and glasses doped with iron and gadolinium has been characterized by Magic Angle Spinning Nuclear Magnetic Resonance  (MAS NMR),  Electron Paramagnetic Resonance (EPR) and Infrared (IR) spectroscopies . The crystallization of amorphous precursors takes place between 760 and 860oC in the case of xerogels and around 1000oC for glasses. The spectroscopic results show that the local structure dramatically changes when in amorphous precursors are developed crystalline phases. The major changes are evidenced in the boron surrounding where from the majority threecoordinated borons in amorphous precursors almost only tetracoordinated species are detected in the crystalline samples. Aluminium occurs in amorphous samples as hexa-, penta- and tetra-coordinated by oxygens while in the crystalline ones it appears preponderantly pentacoordinated. While in the amorphous samples lanthanum is high coordinated, in the developed crystalline phases low coordinated lanthanum has been evidenced. These unusual coordinations of boron, aluminium and lanthanum in crystalline phases bring a special interest for these compounds. Different sites for the doped paramagnetic ions were identified in crystalline samples by EPR measurements.