Structural rationalization of the microhardness trends of rare-earth aluminosilicate glasses: Interplay between the RE3 + field-strength and the aluminum coordinations

Many studies have sought to rationalize property/structure relationships in rare-earth (RE) bearing aluminosilicate oxide or oxynitride glasses. In a set of 48 RE2O3–Al2O3–SiO2 (RE = La, Y, Lu, Sc) glasses of widely spanning compositions and RE3 + cation field-strengths (CFS), we observe strong correlations between the microhardness, glass compactness, and the average coordination number of Al. We argue that the well-known microhardness elevation for increasing RE3 + CFS stems not primarily from the RE3 + ions themselves, as hitherto believed, but merely from a structure-strengthening by AlO5/AlO6 polyhedra that cross-link different glass–network segments. The high-coordination Al populations grow together with the RE3 + CFS.