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

IR and Raman spectra of xGd2O3(1−x)P2O5 glasses (x=0.22–0.31) are reported with emphasis on glassy Gd(PO3)3 and the crystalline forms emerging upon its devitrification. Increasing the concentration of Gd2O3 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 PO2 bond angles in glass. The changes observed are consistent with an elongation of P–O− terminal and P–O–P bridging bonds in the PO4 tetrahedra. The redistribution of band frequencies is explained with the more covalent interaction of Gd3+ with polyphosphate anions in glasses. The spectra of (Gd2O3)0.27(P2O5)0.73 and (Gd2O3)0.31(P2O5)0.69 glasses contain the characteristic bands of the metaphosphate structure along with features due to vibrations of PO3 end groups. Their presence is interpreted in terms of the progressive fragmentation of the polyphosphate chains at these glass compositions.