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A series of silicate glasses containing alkaline-earth element(AE),with AE=Mg,Ca,Sr,Ba,were prepared in order to investigate the compositional effects on the physical properties of the glasses.An anomalous phenomenon was observed on the glass transition temperature(Tg)and Vickers hardness of Mg-containing glass in contrast to Ca-,Sr-,Ba-containing glasses.The molecular origin of these macroscopic features can be explained by the free volume parameters measured from positron annihilation lifetime spectroscopy(PALS).A remarkably direct correspondence between the physical properties and fractional free volume trend is established with alkaline-earth element variation in the glasses.The effect of these structural changes on the glass transition temperature and Vickers hardness has been studied.