Since the -OH species in the oxide materials have great influence on the electrochemical performances , it is necessary to further explore the existence of -OH species in the three hydrated samples. FT-IR spectra shown in Fig. display the clear evolution of the multiple board peaks in the TVP glass samples upon the humidity treatment. The peak located at ~3450 cm -1 originated from the stretching vibration modes of the fundamental -OH in both OH -and H2O molecules while that located at ~1630 cm -1 is ascribed to the molecular H2O bending mode . The intensity of both peaks gradually increases with increasing the relative humidity, suggesting that more active -OH groups were generated in the samples. Fig. shows the DSC curves of the studied four samples. The endothermic peak at around 400 K become more intense from 65RH to 85RH samples. Combining with the TGA results in Fig. , this endothermic peak could be associated with the release of water absorbed during hydration. In addition, DSC curves of TVP, 45RH and 65RH demonstrate pronounced glass transition peaks while no glass transition peak could be observed in the DSC upscan of 85RH. The glass transition temperature (Tg) decreases from 543 K (for 'dry' TVP) to 512 K (for 65RH) with the increasing humidity as clearly displayed in the inset of Fig. . It is important to mention that no glass transition peak appears in 85RH sample, instead, an endothermic peak occurs at about 485 K. This peak might arise from the loss of covalently bounded H2O in the H4V3P3O16.5•xH2O crystals, which originate from the crystallization of almost all the glass matrix as confirmed by the XRD pattern (Fig. ). This is also why no glass transition peak appears in the DSC curve of 85RH sample. With the further increased scanning temperature, no obvious thermal events occur in TVP glass sample, but a very tiny endothermic peak appears at 787 K for 45RH sample, which might be due to the melting of H4V3P3O16.5•xH2O crystals generated during the hydration treatment, as confirmed in Fig. . As a universal rule, the broad exothermic peak at around 700 K in each DSC curve of 65RH and 85RH samples, respectively, is caused by crystallization, while the subsequent endothermic peak is due to the melting of crystals. However, the real origin of the two peaks is contrary to the conventional wisdom, which is discussed in the following section.