Structural role of La2O3 in La2O3-CaO-Na2O-SiO2 glasses

Glasses of composition (1+z-x)CaO.(1+z)Na2O, x/3La2O3.2SiO(2) and (1+z) CaO.(1+z -x).Na2O. x/3La(2)O(3).2SiO(2) (z=0, 0.25; 0less than or equal toxless than or equal to1.25) were prepared and submitted to differential thermal analysis (DTA) to measure the glass transformation temperatures and to FTIR spectroscopy. Substitution of La2O3 for Na2O greatly affects the glass transformation temperature, T-g, while for CaO has much less effect. When plotting T-g values as a function of composition linear trends were in all cases obtained. In the course of substitution the FTIR stretching band of SiO4 tetrahedra slightly broadens. The results agree well with the hypothesis, based on well known criteria reported in the literature, of the network modifying role for La2O3. Taking into account that when La I ions are hosted in the silicate structure, coordination shells with three nonbridging oxygens must be built tip, the FTIR results can be attributed to the need of a broader distribution of the nonbridging oxygens among the tetrahedra.