Relaxation of anisotropic alkali metaphosphate glass

•Anisotropic metaphosphate glass has highly oriented chain structures.•The anisotropic glass shows a minimum in storage modulus around Tg.•The unusual behavior is caused by deformation and shrinkage in the chain direction.•The deformation is due to weakening of bond strength between the chains below Tg.•The shrinkage is due to a disordering of the oriented chains above Tg. The anisotropic glass of 12.5Li2O-12.5Na2O-12.5K2O-12.5Cs2O-50P2O5 (mol%) shows significant shrinkage during relaxation to an isotropic structure. To clarify the relaxation mechanism, the change in storage modulus (Eˊ) and structure for isotropic and anisotropic glasses was investigated in temperatures ranging from room temperature to above the glass transition temperature (Tg.). The Eˊ for the isotropic glass gradually decreased with increasing temperature. On the other hand, the anisotropic glass showed a marked decrease in the Eˊ even below Tg, then increased, reaching the minimum around Tg, and finally reached the value of the isotropic glass. Raman spectra for the anisotropic glass with increasing temperature showed that the bond strength between alkali cations and non-bridging oxygen, of some local structures in oriented -P-O-P- chains, rapidly weakened. These changes were accompanied by a decrease in O−-P-O− bond angle below Tg. Additionally, the number of these local structures increased with increasing temperature. This structural change led to deformation in the oriented chain and loss of directionality. The change became significant above Tg, and the highly oriented structure could not be maintained. This unique relaxation in the direction of the oriented chain produced the noticeable shrinkage of the anisotropic glass.