X-RAY-DIFFRACTION STUDY ON THE MIXED ALKALI-ALKALINE EARTH EFFECT IN BA, NA METAPHOSPHATE GLASSES

In ternary Ba, Na phosphate glasses, the mixed oxide effect was studie d along the join of the metaphosphate members BaP2O6 and NaPO3 by mean s of X-ray diffraction. The oxygen polyhedra of the modifier cations N a+ and Ba2+ are deformed if compared with those in the end members. Th is behaviour is expected according to the competition for the oxygens by the Ba2+ and Na+ cations. The preference of dissimilar pairs of cat ions is found to be a small effect. The number of such Na-Ba neighbour s at distances of about 0.40 nm does not deviate much from that expect ed for a random occupation of cationic sites. A discernible feature in the scattering intensities of the single glasses, the prepeak or shou lder in front of the first largest maximum, is not detected in the sca ttering of the mixed glasses. The origin of this prepeak was previousl y deduced to be due to the mutual ordering of the phosphate chains (an d/or rings) and the cation-centered oxygen polyhedra. However, at this time no explanation for the absence of the prepeak in the X-ray scatt ering functions of the mixed glasses can be proposed. (C) 1998 Elsevie r Science B.V.