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The dependence on temperature from 15 to 40K of the average tunnel frequency f t in (NH 4 ) 2 PbCl 6 is explained as resulting from very short lifetime approximately=10 -12 s in the first excited torsional state of NH 4 + . A comment of f t in (NH 4 ) 2 SnCl 6 and NH 4 ClO 4 is also given.
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I. Svare (1979) studied this question.
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