The quadrupole splitting of (NH4)(2)FeF5 . H2O was measured in the 79-
340 K temperature range. A strong dependence of temperature was observ
ed. This effect was explained in terms of thermally activated reorient
ations of the NH4+ ion. Our data had resulted in an activation energy
of (750+/-30) K. A comparison with results previously reported of(NH4)
(2)FeCl5H2O is presented. Copyright (C) 1996 Elsevier Science Ltd.