A Mossbauer study of (NH₄)₂FeCl₅.H₂O

Abstract

Mossbauer spectra of (NH4)2FeCl5.H2O have been measured over the temperature range 1.5-297 K for powder and various single-crystal absorbers. This antiferromagnet has shown complicated magnetic behaviour but, contrary to a previous suggestion, the spectra show that below the ordering temperature the Fe spins are aligned along the alpha axis. The antiferromagnetic spectra may be fitted with a small number of components coupled with relaxation broadening due to critical fluctuations. The data suggest two closely spaced ordering temperatures. Details of the hyperfine electrostatic interaction have been deduced from the paramagnetic spectra. No evidence was found for a reported structural phase change in the paramagnetic region.

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

“…Considering the persistent rapidly relaxing phase detected by Mössbauer spectroscopy experiments for (NH 4 ) 2 FeF 5 ·H 2 O below the ordering temperature, a highly probable explanation is the destruction of the magnetic equivalence between the Fe 3+ sites at low temperature as a consequence of the reported structural phase transition. This hypothesis is strongly supported by previous Mössbauer measurements performed on related compounds, β-(NH 4 ) 2 FeF 5 , and (NH 4 ) 2 FeCl 5 .H 2 O. , These experiments indicate the presence of two different Fe 3+ magnetic contributions, two corresponding hyperfine fields, and two magnetic critical temperatures. In passing, we note that none of the related complexes, K 2 FeF 5 ·H 2 O, Cs 2 FeF 5 ·H 2 O, and Rb 2 FeF 5 ·H 2 O, exhibits a persistent rapidly relaxing phase below T N in the zero field Mössbauer spectra. , It is clear that more information concerning the structural phase transition and the low temperature structure is necessary in order to achieve complete understanding of the magnetic properties of (NH 4 ) 2 FeF 5 ·H 2 O at low temperatures.…”

“…Owing to the structural phase transition suggested by the DSC experiments and the relevance of the thermal history of the sample reported for similar compounds, − it is appropriate to mention that two sample cooling rates have been used to decrease the temperature from ambient to 5 K. The first consists of liquid helium quenching for about 15 s. The temperature is slowly decreased over about 3 h in the second procedure. From 5 K to the lowest reached temperature, the cooling rate was the same for both procedures.…”

Mössbauer and muon studies ofβ-(NH4)2Attenborough1, Hall2, Nikolov3 et al. 1996Phys. Rev. B3000View full textAdd to dashboardCite

“…This possibility was prompted by the case of another NH 4 -containing antiferromagnet ͑NH 4 ͒ 2 FeCl 5 •H 2 O. Among the A 2 FeX 5 •H 2 O family ͑Aϭalkali or NH 4 , Xϭhalogen͒ this compound is unique in various ways but, for the present, we mention only the existence of two closely spaced heat-capacity cusps 2 in the region of T N Ϸ7 K. Various 57 Fe Mössbauer spectroscopy measurements [3][4][5] showed that spectra in the antiferromagnetic region could not be satisfactorily fitted with only one magnetic component and it was suggested by two of us 5 that the two heat-capacity cusps were associated with two magnetic ordering temperatures. The following questions thus arose: Does ␤-͑NH 4 ͒ 2 FeF 5 have more than one ordering temperature?…”

“…Indeed it had been earlier suggested that the lower-temperature heatcapacity cusp in ͑NH 4 ͒ 2 FeCl 5 •H 2 O might be due to a spin reorientation but single-crystal Mössbauer experiments 5 showed that the axis of Fe spin alignment remained unchanged through that temperature. In the case of ␤-͑NH 4 ͒ 2 FeF 5 we did not have single crystals and do not know the orientation of the magnetic easy axis, but the Mössbauer and SR data reported here would not be explained by a spin reorientation.…”

Mossbauer spectra from (NH4)2FeCl5.H2O in applied magnetic fields

“…In the present article we report a further study of this compound in its antiferromagnetic phase, specifically the spin reorientation (SR) induced by applied magnetic fields. Single-clystal samples of area approximately 25 mm2 were prepared for Mossbauer absorption experiments as described earlier [4]. Both ab-and ac-plane absorbers were used.…”