FT–IR and Raman spectra of Gd phosphate crystals and glasses

Abstract IR and Raman spectra of x Gd 2 O 3 (1− x )P 2 O 5 glasses ( x =0.22–0.31) are reported with emphasis on glassy Gd(PO 3 ) 3 and the crystalline forms emerging upon its devitrification. Increasing the concentration of Gd 2 O 3 from 0.22 to 0.26 mole fractions leads to an abrupt lowering of the highest observed P–O stretching frequency of 25 cm −1 . The spectral shift is related to changes in both the coordination of the lanthanide ion and the increase in metal to phosphorus ratio. The comparison of vibrational spectra of vitreous gadolinium metaphosphate ( x =0.25) with those of its crystalline analogs reveals a decrease in POP and PO 2 bond angles in glass. The changes observed are consistent with an elongation of P–O − terminal and P–O–P bridging bonds in the PO 4 tetrahedra. The redistribution of band frequencies is explained with the more covalent interaction of Gd 3+ with polyphosphate anions in glasses. The spectra of (Gd 2 O 3 ) 0.27 (P 2 O 5 ) 0.73 and (Gd 2 O 3 ) 0.31 (P 2 O 5 ) 0.69 glasses contain the characteristic bands of the metaphosphate structure along with features due to vibrations of PO 3 end groups. Their presence is interpreted in terms of the progressive fragmentation of the polyphosphate chains at these glass compositions.