Lanthanide-doped glasses under high pressure

Abstract

According to Zanotto and Mauro [1],“glass is a nonequilibrium, noncrystalline con-densed state of matter that exhibits a glass transition. The structure of glasses islike that of their parent supercooled liquids. Their fate, in the limit of infinite time,is to crystallize.”Glasses are usually formed by quenching of a molten mixture insuch a way that the atoms do not have time to reach the equilibrium spatial config-uration, forming an amorphous solid with short-range order. The glass transitiontemperature,Tg, consists in the temperature range between the rigid solid-statestructure and a more flexible, viscous structure. Good candidates to make glassesmust have high viscosity in the molten state, where the atomic rearrangement isconstrained. Materials that are not able to form glasses are, sometimes, used as net-work modifiers. In this case, they induce the formation of dangling bonds in theprimary three-dimensional glass network, usually decreasing density. The physicalproperties of glasses can be tuned by adjusting their chemical composition.Silica(SiO2), the major constituent of sand, has been the raw material of glassesfor thousands of years. It is formed by SiO4tetrahedra sharing four oxygen atoms(bridging oxygen atoms) in a random spatial configuration. To overcome the highmelting point of silica (1,723 °C), alkali metal oxides such as Na2O, K2O, and Li2Oare used as network modifiers. For instance, the addition of 33 mol% of Na2Ode-creases the melting temperature to 875 °C [2, 3]. The presence of alkali metal posi-tive ions as modifiers induces the formation of nonbridging oxygen (NBO) atoms,so the SiO4tetrahedra would share less than four oxygen atoms, affecting the localstructure and properties of the glass [4–8].Germania(GeO2) is also a good glass former, like SiO2,consistingofGeO4tetrahe-dra sharing four oxygen atoms in a random spatial configuration. It was first synthe-sized by Dennis and Laubengayer in 1926 [9]. The incorporation of glass modifiersinduces an atypical behavior, known asgermanate anomaly: thermophysical proper-ties present maxima or minima depending on the amount of the modifier. In theatomic level, the anomaly is related to modifications in the coordination number ofGe atoms or to the formation of small rings of GeO4[10–13].