Glasses in the ternary system Cr 2 O 3-PbO-P 2 O 5 were prepared by direct melting of the mixture with stoichiometric proportions of the reagents Cr 2 O 3 , PbO and (NH 4) 2 HPO 4 at 1080˚C. The glasses obtained are transparent in colour and have a non-hygroscopic appearance. The study of the dissolution rate was carried out on ternary glasses xCr 2 O 3-(45-x)PbO-55P 2 O 5 with (1 ≤ x ≤ 4; mol%), immersed in distilled water at 90˚C for 24 days, indicating a maximum of chemical durability when the level of chromium oxide passed through 2 mol%. Both, IR spectra and X-ray diffraction have indicated the predominance of metaphosphate or cyclic metaphosphate groups with some traces of isolated orthophosphate groups when the Cr 2 O 3 content is equal to x = 2. Analysis of the density values also, has showed a maximum density for x = 2 mol%. The covalent radius values of oxygen have indicated that the minimum value rcal (O 2−) is observed for x = 2 mol% and therefore a relatively high reinforcement of the metal-oxygen-phosphorus (Cr-O-P) bonds. SEM Micrographs have exhibited two phases, a vitreous phase and a crystalline phase. The radical change in the structure from ultraphosphate Q 3 groups to ring metaphosphate Q 2 and orthophosphate groups Q 0 seems to be the cause of the formation of crystallites. Beyond 2 mol% of Cr 2 O 3 , the structure of the glass changed relatively and the orthophosphate phases increased to the detriment of the metaphosphate phases. We observed a decrease in chemical durability. However, it was confirmed that the dissolution rate (D R) of the S 2 analysed compound is comparable to the values of borosilicate glasses which are used as alternative materials for the immobilisation of nuclear waste substances.

Various characterisation techniques were used to study the composition of the glass series 55-P 2 O 5-2Cr 2 O 3-(43-x) Na 2 O-xPbO (with 8 ≤ x ≤ 38; mole %) in terms of chemical durability, IR spectroscopy and scanning electron microscopy (SEM). The change in the dissolution rate as a function of time when the studied glasses were kept submerged in distilled water at 90˚C for 20 days showed an improvement in the chemical durability when Na 2 O content was substituted to PbO content. IR spectroscopy revealed a structural change from ultraphosphate groups to pyrophosphate, orthophosphate and probably ring metaphosphate groups. SEM revealed the existence of two phases: a vitreous phase and a crystalline phase. The presence of Cr 2 O 3 , even in small amounts, seems to play an important role in the formation of crystallites in the glass network. The improved chemical durability is attributed to the replacement of the easily hydrated Na-O-P and P-O-P bonds by covalent and resistant Pb-O-P bands. Both the increase in PbO content and in the Pb + Cr/P ratio causes an increase in the number of covalent Pb-O-P and Cr-O-P bonds, making the glass structure more rigid. The increase of the covalent Pb-O-P bands leads to a clear evolution of the structure and chemical resistance, caused by grain-boundary resistance as a result of glass crystallisation. The IR spectra indicate that the increase in PbO content favours the formation of isolated 3 4 * Corresponding author. N. Beloued et al. 150 limit, the equilibrium between the glass bath and these crystallites is no longer maintained; we notice, once, a decrease in the chemical durability.

Various characterization techniques were used to study the composition of the glass series xAl 2 O 3 -(40 -x)CaO-10Na 2 O-50P 2 O 5 (with 0 ≤ x ≤ 10) in terms of chemical durability, X-ray diffraction, IR spectroscopy and scanning electron microscopy (SEM). The improved chemical durability was attributed to the replacement of easily hydrated P-O-P bonds by covalent and resistant Ca-O-P and Al-O-P bonds. However, the change in the dissolution rate (D R ) versus time showed a marked decrease in chemical durability with increasing the Al 2 O 3 content to the detriment of the CaO content. The X-ray diffraction analysis of glasses annealed at 550˚C and 660˚C for 48 hours indicated the presence of pyrophosphate phases and predominant metaphosphates or cyclic metaphosphate phases when the Al 2 O 3 content was ≤7.5 mol%. Nevertheless, both, X-ray diffraction and IR spectroscopy confirmed the structural tendency change from metaphosphate (Q 2 ) and pyrophosphate structural units (Q 1 ). Toward short isolated orthophosphate units (Q 0 ) when the Al 2 O 3 content above 7.5 mol%. SEM micrographs illustrated that the number of crystallites increased in the glass network when the Al 2 O 3 content increased at the expense of the CaO content. An increase in the Al 2 O 3 content to 10 mol% led to the formation of a larger number of crystallites of different sizes, dominated by small crystallite sizes assigned to short isolated orthophosphate groups. This phenomenon led to a decrease in chemical durability and seems to be a favorable factor for the formation of the apatite layers which enclose the glass, in a SBF solution test, able of regenerating bone tissue in biomedical application.