Proprietes des molybdates et tungstates vitreux et fondus

There is a tendency to form infinite chains in which common tetrahedral and octahedral coordination (the latter for the alkali ions in groups [RO6]) is interlinked with [MO5] and even (with M = Mo) [MoO5]4- groups. The [MoO5]4- anion is in this case in the shape of a tetragonal-pyramidal configuration. With M = W there is a lower tendency to appear in coordinations above [WO4], preferring the coordinations [WoO4] and perhaps [WoO6]. Glass formation in Mo and W oxy salts require an extremely rapid quenching techniques, e.g., by application of the principle of a glass drop in a Pt-PtRh thermocouple loop and a rate of cooling of 10-1 to 102 degree/sec on a microscope stage. \"Splash\"-cooling devices can be used to produce layers of less than 1m thickness on a \"collector\" plane, when the sample drop is molten in a thermocouple loop and then \"explosively\" splashed onto the substrate. The rate of cooling may then reach 106 degrees/sec. The ir absorption spectra of such molybdate and tungstate glasses show important bands at 250 to 650 cm-1 for the bonds Mo-O and W-O. Evaluation of the vibrational bands confirms the possibility for formation of variable bands [MOn], with n > 4, and the existence even of chainlikeconfigurations. The viscosity of such molybdate and tungstate glasses are extremely low at 950 Deg, with a monotonic isotherm as a function of concn. in MO3. The same is valid for calcd. mol. vols. which show linear curves for W, more curved ones for Mo salts, about in the same dimensions. With a stepwise removal of R2O from the longer chains, typical changes in the mol. vol. data indicate structural effects of widening in the structure. Specifically, for Li+ in its tetrahedral coordination, the glass formation is distinctly reduced, in comparison with the conditions in the vitrification of Na and K molybdates and tungstates. Measurements of the surface-tension parameters, by using the ring balance method at 950 Deg, show decreasing surface tension for glasses with increasin alkali ionic radii, smaller for the molybdates than for the tungstates. Evidently, the surface layer of the molten glass is enriched in MO3, the alkali ions tending to move away from the surface. Calcn. of the Eotvos free mol. surface enthalpy isotherms shows a surprising coincidence of those for the molybdates and tungstates with Na+ and Li+ ions, whereas that for K+ is lower by seyeral percent. [on SciFinder (R)