Gradient de champ électrique dans SnO2 et SnF4 interprétation des mesures d'éclatement quadrupolaire et évaluation de la polarisabilité anionique

A lattice sum calculation of the electric field gradient at the Sn 4+ site in SnO2 and SnF4 (self-consistent monopole-dipole model) is used to explain the Mössbauer quadrupole splittings ; agreement between calculated and experimental values is obtained with the following polarizabilities : α(O =) = 1.60 Å3 and α(F-) = 0.98 Å3.Les composantes monopolaires et dipolaires du gradient de champ électrique au niveau de l'ion Sn4+ ont été calculées pour les structures SnO2 et SnF4. La comparaison avec les résultats de spectroscopie Môssbauer (éclatement quadrupolaire) conduit aux valeurs suivantes de la polarisabilité : α(O=) = 1,60 Å3 et α(F -) = 0,98 Å3

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

Mössbauer study of (NH4)2FeCl5·H2O

Mössbauer spectra of the compound (NH4)2FeCl5·H2O have been studied as a function of temperature. Two phase transitions are observed in the temperature range between 7 K and 9 K. The transition at 9 K is structural and presents an unusually high thermal hysteresis. Around T=8 K the substance orders magnetically and different Fe3+ contributions are present.This work was supported by a grant from the Spanish-French Commitee for Scientific Cooperation. M.C. Moron is grateful for support given by the C.N.R.S. (France).Peer Reviewe

Mössbauer study of the α and β forms of (NH4)2FeF5

The Mössbauer spectra of the two crystallographic forms α- and β-(NH4)2FeF5 have been studied in the temperature range 4.2-300 K. The α-form presents typical one-dimensional behavior showing a transition to magnetic ordering at Tc = (7.5±0.5) K. The spectra of the β-form indicates the presence at low temperature of two different iron sites, while the room temperature crystal structure shows only one site for the iron ions. Differential scanning calorimetry measurements evidence the presence of a reversible structural phase transition at Ts = (168.0±1) K on heating and Ts = (187.0±3) K on cooling. This form also exhibits a magnetic ordering phase transition at Tc = (13±1) K.The work in Zaragoza has been supported by grant MAT88-0174, from the Comision Interministerial de Ciencia y Tecnologia. M.C.M. is grateful for the receipt of a NATO postdoctoral fellowship. The cooperation has been partially supported by Spanish-French Integrated Action 151/90.Peer Reviewe

Neutron Powder Diffraction Studies of Fluorite and Pyrochlore NdxZr1-xO2-x/2 Solid Solutions with 0.25<x<0.55

Neutron powder diffraction studies on stoichiometric and non-stoichiometric pyrochlore phases of the ZrO2- Nd2O3 system show remarkable differences in the occupation factor of the 8b site of space group Fd3m to which the pyrochlore phase belongs. Stoichiometric Nd2Zr2O7 can be described as a perfect pyrochlore phase in which the 8b site is either vacant or at least has a low occupation factor. The occupation factors of the 8b sites of non-stoichiometric NdxZr1-xO2-x/2 pyrochlores reveal that these contain a large degree of disorder in the anion sublattice. Best refinement results on non-stoichiometric materials are achieved when the overall structure of the non-stoichiometric pyrochlore phase is described as a hybrid phase consisting of a major volume fraction of the stoichiometric pyrochlore phase and a minor fraction of the defect fluorite phase