Lifetimes of positrons annihilating in NH4Cl and in nitrate glasses have been measured as a function of temperature. The observed temperature dependence of tau 2 and I2 indicates that the I2 is significantly affected by changes in molar volumes accompanying the phase transitions in these solids. I2 varies in the same direction as the molar volume thus underlining the role of free volume in the positron annihilation process in condensed media.
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