Magnetic properties of (NH4)2FeF5·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)2FeF5·H2O have been investigated via iron-57 Mössbauer spectroscopy and ac susceptibility measurements. 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 = -3.9 ± 0.5 K while the fit of the low temperature data to a Heisenberg linear-chain model yields g = 1.97 ± 0.02 and an intrachain constant J/kB = -0.40 ± 0.02 K. At lower temperatures (NH4)2FeF5·H2O exhibits a crossover to three dimensional magnetic ordering with Tc = 2.2 ± 0.05 K and 1.61 ± 0.05 K from Mössbauer spectroscopy and ac susceptibility, respectively. Differential scanning calorimetry measurements suggest a first-order structural phase transition centered at Ts = 139 ± 1 K on heating and Ts = 125 ± 1 K on cooling for (NH4)2FeF5·H2O. No such transformation is suggested by scanning calorimetry studies of the corresponding K+, Rb+ and Cs+ analogues. The limiting internal hyperfine field, Hn(0 K), is 45 T, indicating some 25% zero point spin reduction consistent with significant 1-d magnetic behavior. All the experiments reported here have been performed following varied and careful thermal treatments. A particularly interesting result is the observation of a persistent rapidly relaxing fraction that the Mössbauer spectra of (NH4)2FeF5·H2O clearly exhibit below Tc but which is not seen in previous studies of the K+, Rb+, and Cs+ compounds. A probable explanation for this is the loss of magnetic equivalence of the Fe3+ sites as a result of the structural phase transition. This behavior further calls into question the still common practice of interpretation of low temperature magnetic phenomena largely on the basis of ambient temperature structure determinations.We thank the North Atlantic Treaty Organization for travel monies to W.M.R. and Y.C. under a Nato Collaborative Research Grant, No. 920936; Grants MAT94-43/ MAT92-896/MAT91-681 from CICYT (Spain) are acknowledged.Peer Reviewe