Acoustic Properties of Magnetite as a Function of Temperature and Cation Vacancies

The acoustic loss and resonant frequency of polycrystalline magnetite has been measured from 4.2° to 780°K for various cation vacancy levels between 0.001 and 0.01 vacancies per formula unit. Measurements were made using a magnetic drive technique at a frequency of approximately 1 Mc/sec. The samples show, in general, acoustic loss peaks near 40°, 100°, and 690°K. The 40°K loss peak, which has been previously attributed to electron exchange, is seen to decrease in amplitude with decreasing vacancy concentration and is absent in the sample with 0.001 vacancies. This observation is opposite to that expected from a simple interpretation of the electron exchange model. The 100°K loss peak, which is attributed to ordering of Fe2+ and Fe3+ ions, moves from 100° to 115°K as the number of vacancies changes from 0.01 to 0.001 per formula unit. This observation is consistent with resistivity measurements of Verwey and Haayman. The 690°K loss peak decreases in amplitude and shifts to 660°K as cation vacancies go fro...