The Mixed Alkali Effect (MAE) is associated with the non-linear variation of physical properties with composition when two dissimilar alkali ions are present in the glass matrix. The MAE is manifested mainly by properties which depend directly or indirectly on ionic mobility such as electrical conductivity, ionic diffusion, internal friction, viscosity and glass transition temperature. Despite the plethora of studies devoted to mixed alkali glasses there are still aspects which are not well resolved or appear to depend on the particular mixed alkali system. These include the effect of alkali mixing on the short-range order of glass as well as on the nature of sites favored by dissimilar alkali ions. To address such questions we investigate structural properties of mixed alkali borate glasses by employing infrared spectroscopy and molecular dynamics simulations.
Mixed Li-Cs and Li-Na borate glasses of composition 0.3[(1-x)Li2O-xCs2O]-0.7B2O3 and 0.3[(1-x)Li2O-xNa2O]-0.7B2O3 were prepared for several values of x by the conventional melt quenching technique and simulated by molecular dynamics. The short-range order structure was found to consist of borate tetrahedra, BØ4-, and of neutral, BO3, and charged, BØ2O-, triangular borate units [Ø=bridging oxygen atom]. The fraction of BØ4- units in Li-Cs glasses was found to decrease from Li to Cs and to exhibit negative deviation from linearity, in agreement with earlier findings by infrared reflectance spectroscopy. However, no appreciable changes were detected for the network structure of glasses in the Li-Na system. Alkali ions in mixed glasses were found to occupy sites similar to those formed in single alkali borates. Nevertheless, the Li ion-coordination environments were found better defined and the Li-O interactions strengthened upon alkali mixing in both systems considered, these changes occurring at the expense of Cs-O and Na-O interactions. These trends of metal ion-oxygen bonding were traced to the formation of dissimilar cation pairs around non-bridging oxygen (NBO) atoms in mixed glasses, where the unlike-cation pairs were found to constitute the dominant cation configurations. A key implication of this structural characteristic is the drastic reduction in the population of NBO's coordinated to like-cation configurations, which becomes minimum at maximum mixing (x=0.5). This striking result points towards a structural interpretation of the MAE. Besides their effect on local bonding and ion transport, the unlike-cation configurations affect also the vibrational properties of metal ions in their sites as manifested in both simulations and far-infrared measurements of mixed alkali glasses.
A. Vegiri, C.-P.E. Varsamis and E.I. Kamitsos, "Molecular dynamics investigation of mixed-alkali borate glasses: Short range order structure and alkali ion environments", Phys. Rev. B 80, 184202 (2009)
Short Range Order Structure of Mixed Alkali Borate Glasses
Molar fractions of short-range order structural units in simulated borate glasses 0.3[(1-x)Li2O-xM2O]-0.7B2O3 (M=Cs, Na) as a function of alkali mixing, x, at T=300 K; (a) BØ4- units, X4, and (b) BØ2O- units, X2.
A Structural Interpretation of the MAE?
Percentage of NBO atoms that sense in their vicinity like-alkali ion configurations in simulated glasses 0.3[(1-x)Li2O-xM2O]-0.7B2O3 glasses (M=Cs, Na) as a function of alkali mixing, x, at T=300 K.
Simulated Vibrational Properties of Metal Ions in
Mixed Alkali Borate Glasses
Calculated power spectra at T=300K for Li ions (a) and for Cs ions (b) that are in the vicinity of non-bridging oxygen atoms (denoted Linb and Csnb) in simulated glasses 0.3[(1-x)Li2O-xCs2O]-0.7B2O3 for x=0 and x=1 (black lines) and x=0.5 (red lines).
Far Infrared Spectra of Mixed Alkali Borate Glasses
The experimental far infrared spectra of glasses in the mixed alkali system 0.33[(1-x)Li2O-xCs2O]-0.67B2O3 provide direct evidence for the influence of alkali mixing on the metal ion-oxygen interactions.