MAGNETIC-PROPERTIES OF (NH4)(2)FEF5-CENTER-DOT-H2O - INFLUENCE OF A STRUCTURAL PHASE-TRANSFORMATION AND RELEVANCE OF AMBIENT-TEMPERATURE STRUCTURE DETERMINATIONS TO THE INTERPRETATION OF LOW-TEMPERATURE MAGNETIC-BEHAVIOR

Low temperature magnetic properties of (NH4)(2)FeF5 . H2O have been in vestigated via iron-57 Mossbauer spectroscopy and ac susceptibility me asurements. The high temperature ac susceptibility data can be fitted to a Curie-Weiss law with C = 4.22 +/- 0.05 emu K mol(-1) and Theta = -3.9 +/- 0.5 K while the fit of the low temperature data to a Heisenbe rg linear-chain model yields g = 1.97 +/- 0.02 and an intrachain const ant J/k(B) = -0.40 +/- 0.02 K. At lower temperatures (NH4)(2)FeF5 . H2 O exhibits a crossover to three dimensional magnetic ordering with T-c = 2.2 +/- 0.05 K and 1.61 +/- 0.05 K from Mossbauer spectroscopy and ac susceptibility, respectively. Differential scanning calorimetry mea surements suggest a first-order structural phase transition centered a t T-c = 139 +/- 1 K on heating and T-c = 125 +/- 1 K on cooling for (N H4)(2)FeF5 . H2O. No such transformation is suggested by scanning calo rimetry studies of the corresponding K+, Rb+ and Cs+ analogues. The li miting internal hyperfine field, H-n(0 K), is 45 T, indicating some 25 % zero point spin reduction consistent with significant 1-d magnetic b ehavior, All the experiments reported here have been performed followi ng varied and careful thermal treatments. A particularly interesting r esult is the observation of a persistent rapidly relaxing fraction tha t the Mossbauer spectra of (NH4)(2)FeF5 . H2O clearly exhibit below T- c but which is not seen in previous studies of the K+, Rh+, and Cs+ co mpounds. A probable explanation for this is the loss of magnetic equiv alence of the Fe3+ sites as a result of the structural phase transitio n. This behavior further calls into question the still common practice of interpretation of low temperature magnetic phenomena largely on th e basis of ambient temperature structure determinations.