Mössbauer effect studies of alkali borate glasses

Abstract

Mössbauer studies of a large number of glass samples prepared with alkali oxides in the region of glass formation are reported. Some representative samples are studied in the temperature range 85–500 K. The following glass systems are studied. (a) (x) Me₂0 : (94 - x) B₂0₃ : 6 Fe₂O₃ : Me = Li, Ns, K, (b)25 Na₂0 (x) Al₂0₃ : (72 - x) B₂O₃:3 Fe₂O₃, (c) 15 Al₂0₃ : (x)Na₂0 : (82 - x) B₂0₃ 3 Fe₂03. (d) 25 Na₂0 (x) Ca0 (69- x)B₂0₃ : 6 Fe₂O₃. Room-temperature isomer shift values decrease gradually with the addition of alkali oxide and the values fall sharply for alkali content higher than 22 mol%. We conclude that iron is in the ferric state and that the oxygen polyhedra of iron changes from octahedral to tetrahedral. The structural change results from the fact that the alkali introduces an extra non-bridging oxygen ion. The size of the alkali cation introduced into the glass has considerable influence on the isomer shift values of iron. In fact, the polarizing power decreases in the order LiNaK, hence the s-character of the FeO bond increases in the order LiNaK. Addition of Al2O3 has no effect on the isomer shift values of iron, showing that aluminium ions occupy network-forming positions. For some representative samples the second-order Doppler effect was studied as a function of temperature in the range 85–500 K. The thermal red shifts due to second-order Doppler effect are used to estimate the local specific heats of ferric ion in the glass system. The quadrupole splitting has weak temperature dependence, showing that Fe³⁺ is in a high-spin state.