Aluminum speciation, vibrational entropy and short-range order in calcium aluminosilicate glasses

The heat capacity and vibrational entropy of a calcium aluminate and three peraluminous calcium aluminosilicate glasses have been determined from 2 to 300 K by heat-pulse relaxation calorimetry. Together with previous adiabatic data for six other glasses in the system CaO-Al2O3-SiO2, these results have been used to determine partial molar heat capacities and entropies for five species namely, SiO2, CaO and three different sorts of Al2O3 in which Al is 4-, 5- and 6-fold coordinated by oxygen. Given the determining role of oxygen coordination on low-temperature heat capacity, the composition independent entropies found for SiO2 and CaO indicate that short-range order around Si and Ca is not sensitive to aluminum speciation up to the highest fraction of 25% observed for VAl by NMR spectroscopy. Because of the higher room-temperature vibrational entropy of IVAl2O3 (72.8 J/mol K) compared to VAl2O3 (48.5 J/mol K), temperature-induced changes from IVAl to VAl give rise to a small negative contribution of the order of 1 J/mol K to the partial molar configurational heat capacity of Al2O3 in melts. Near 0 K, pure SiO2 glass distinguishes itself by the importance of the calorimetric boson peak. On a g atom basis, the maximum of this peak varies with the composition of calcium aluminosilicate glasses by a factor of about 2. It does not show smooth variations, however, either as a function of SiO2 content, at constant CaO/Al2O3 ratio, or as a function of Al2O3 content, at constant SiO2 content.