Unusual elastic and mechanical behaviors of copper phosphate glasses with different copper valence states

Elastic and mechanical properties such as Young's modulus E, Poisson's ratio v, Debye temperature theta (D), Vickers hardness H-v, fracture toughness K-c, and fracture surface energies gamma (f) of yCuO(x). (100-y)P2O5 glasses (y = 45, 50, 55) with different copper valence states, i.e., R(Cu+) = Cu+/(Cu+ + Cu2+), at room temperature (humidity 64%) have been examined. The following features have been found: (1) the glass transition temperature (218-434 degreesC), H-v (2.7-4.4 GPa), E (50.6-78.2 GPa), and theta (D) (358-434 K) decrease largely with increasing R(Cu+); (2) the mean atomic volume, K-c (0.56-1.14 MPa.m(1/2)), and gamma (f) (1.9-11.2 J.m(-2)) tend to increase with increasing R(Cu+); (3) 50CuO(x). 50P(2)O(5) glasses with R(Cu+) = 0.42 and 0.55 have a high resistance against crack formation in Vickers indentation tests and no crack is observed in the 45CuO(x). 55P(2)O(5) glass with R(Cu+) = 0.57 under an applied load of about 98 N. The results demonstrate that elastic and mechanical properties of yCuO(x). (100-y)P2O5 glasses depend strongly on the copper valence state and the CuOx/P2O5 ratio. The unusal mechanical and elastic properties of copper phosphate glasses are well explained qualitatively by considering unique oxygen coordination and bonding states of Cu+ ions, i.e., lower coordination number and more covalent bonding compared with Cu2+ ions.