C. R. Abeledo

Orbital ground state inversion in [Fe(H2O6](CIO4)2

Abstract Mossbauer spectroscopy in external magnetic fields has been used to measure the signs of the principal components of the electric field gradient V zz in the two forms of [Fe(H 2 O) 6 )(C10 4 ) 2 . The signs are V zz negative for the (B) with Δ E B = 1.4 mm/sec. This implies a trigonally distorted ligand field and the transition (A) → (B) corresponds to compression → elongation with respect to the trigonal axis.

Orbital ground state reversal in the high‐spin complex Fe(II) (N, N′‐dicyclohexylthiourea)6(ClO4)2

Mossbauer spectroscopy of iron (II)(N, N′‐Dicyclohexylthiourea)6 (ClO4)2 shows the existence of two isomers. At low temperatures (T < 200°K) a form A with quadrupole splitting ΔE = 3.31 mm/sec is stable. At and above room temperature a form B with ΔE = 1.32 mm/sec is stable. For 200 °K < T < 300°K both forms are observed with the B/A ratio increasing with increasing temperature. The values of ΔE are typical of Fe2+ with an orbital singlet and an orbital doublet ground state for A and B, respectively. The isomer shift and magnetic susceptibility results indicate that both forms of the molecule contain high‐spin ferrous ions. Infrared spectra show that in both forms in the iron is bound to the sulphur atoms of each of the six dicyclohexylthiourea ligands. Mossbauer spectra in external magnetic fields show that the sign of the principal component of the electric field gradient is negative for form A and positive for form B, and that the asymmetry parameter of the electric field gradient tensor is negligible ...

Mössbauer spectroscopy of magnetic phases in FeCl2 · 2H2O

Abstract Mossbauer spectroscopy of the three magnetic phases of single crystal FeCl2 · 2H2 in external magnetic fields at 4.2 K is reported.

Distortion isomerism in ferrous thiourea complexes—II(1) Thermodynamics and kinetics

Mossbauer spectroscopy as a function of temperature of Fe(II)(N,N′-dicyclohexyl-thiourea)6(ClO4)2(FeDCTU) shows the existence of two isomeric forms A and B which coexist in the temperature range 200 < T < 300 K. The relative concentrations of A and B were studied as a function of time at different temperatures in the coexistence region. From the equilibrium constant as a function of temperature we obtain the following thermodynamic parameters: ΔH = −4520 ± 120 cal/mol and δS = 20 ± 0.1 e.u. The kinetics seem to follow an ordinary first order law but with an unusual temperature dependence of the rate constant k1. The results are discussed in terms of hydrogen bonded interactions between the perchlorate anions and the cyclohexylthiourea moieties.

Mössbauer Spectroscopy of PdH¯ Fe Alloys

Large changes in the electron susceptibility of PdH alloys have been observed with increasing H concentration, with important consequences for the magnetic and superconducting properties of these alloys. (1) To observe the effect of the changing spin susceptibility, we have studied PdH Fe alloys with 2, 1 and < 0.1 at. % Fe by Mossbauer spectroscopy in external magnetic fields. Mydosh(2) has reported spin glass behavior in the high Fe concentration alloys and Kondo phenomena in the dilute Fe alloys. For hydrogen‐metal ratios < 0.5, α and β phases, with low and high hydrogen content respectively, coexist. For temperatures below Tc of the α phase but above Tc of the β phase, we observe a superposition of a six line spectrum and a single line corresponding to the α and β phases respectively. (3) In an applied field, the β phase magnetizes and both phases have hyperfine fields which extrapolate to the same saturation value ( H hf s  = −308  kOe   for  1%  Fe ). For H/metal ratios > 0. 5, only the β phase is present and slightly lower Hhf s values are obtained. These results indicate that hydrogenation of Fe Pd ¯ alloys has little effect on the iron moment but has a marked effect on the polarization of the Pd spins and consequently on the coupling between spins. The low Fe concentration samples were studied as Co57 in Pd sources. For a 10 mCi source (∼0.1 % Fe in Pd) with H/Pd  > 0.5 , a Brillouin function with parameters J  = 2.5 , g  = 2 and H hf s  = −300  kOe gave the best fit to the data. The hyperfine field is the same obtained in the nonhydrogenated case. For a 1 mCi source (∼0.01% Fe in Pd) with H/Pd, lower moments and hyperfine fields are observed suggesting Kondo quenching of the moment. These results suggest competition between Kondo and exchange effects in determining the Fe moment in dilute alloys.

Abstract: Mössbauer Spectroscopy of PdH Fe Alloys

Large changes in the electron susceptibility of PdH alloys have been observed with increasing H concentration, with important consequences for the magnetic and superconducting properties of these alloys. (1) To observe the effect of the changing spin susceptibility, we have studied PdH Fe alloys with 2, 1 and < 0.1 at. % Fe by Mossbauer spectroscopy in external magnetic fields. Mydosh(2) has reported spin glass behavior in the high Fe concentration alloys and Kondo phenomena in the dilute Fe alloys. For hydrogen‐metal ratios < 0.5, α and β phases, with low and high hydrogen content respectively, coexist. For temperatures below Tc of the α phase but above Tc of the β phase, we observe a superposition of a six line spectrum and a single line corresponding to the α and β phases respectively. (3) In an applied field, the β phase magnetizes and both phases have hyperfine fields which extrapolate to the same saturation value ( H hf s  = −308  kOe   for  1%  Fe ). For H/metal ratios > 0. 5, only the β phase is present and slightly lower Hhf s values are obtained. These results indicate that hydrogenation of Fe Pd ¯ alloys has little effect on the iron moment but has a marked effect on the polarization of the Pd spins and consequently on the coupling between spins. The low Fe concentration samples were studied as Co57 in Pd sources. For a 10 mCi source (∼0.1 % Fe in Pd) with H/Pd  > 0.5 , a Brillouin function with parameters J  = 2.5 , g  = 2 and H hf s  = −300  kOe gave the best fit to the data. The hyperfine field is the same obtained in the nonhydrogenated case. For a 1 mCi source (∼0.01% Fe in Pd) with H/Pd, lower moments and hyperfine fields are observed suggesting Kondo quenching of the moment. These results suggest competition between Kondo and exchange effects in determining the Fe moment in dilute alloys.

Mössbauer Spectroscopy of PdH¯Fe Alloys

Large changes in the electron susceptibility of PdH alloys have been observed with increasing H concentration, with important consequences for the magnetic and superconducting properties of these alloys. (1) To observe the effect of the changing spin susceptibility, we have studied PdH Fe alloys with 2, 1 and < 0.1 at. % Fe by Mossbauer spectroscopy in external magnetic fields. Mydosh(2) has reported spin glass behavior in the high Fe concentration alloys and Kondo phenomena in the dilute Fe alloys. For hydrogen‐metal ratios < 0.5, α and β phases, with low and high hydrogen content respectively, coexist. For temperatures below Tc of the α phase but above Tc of the β phase, we observe a superposition of a six line spectrum and a single line corresponding to the α and β phases respectively. (3) In an applied field, the β phase magnetizes and both phases have hyperfine fields which extrapolate to the same saturation value ( H hf s  = −308  kOe   for  1%  Fe ). For H/metal ratios > 0. 5, only the β phase is present and slightly lower Hhf s values are obtained. These results indicate that hydrogenation of Fe Pd ¯ alloys has little effect on the iron moment but has a marked effect on the polarization of the Pd spins and consequently on the coupling between spins. The low Fe concentration samples were studied as Co57 in Pd sources. For a 10 mCi source (∼0.1 % Fe in Pd) with H/Pd  > 0.5 , a Brillouin function with parameters J  = 2.5 , g  = 2 and H hf s  = −300  kOe gave the best fit to the data. The hyperfine field is the same obtained in the nonhydrogenated case. For a 1 mCi source (∼0.01% Fe in Pd) with H/Pd, lower moments and hyperfine fields are observed suggesting Kondo quenching of the moment. These results suggest competition between Kondo and exchange effects in determining the Fe moment in dilute alloys.

Hyperfine interactions in MgF2:Fe2+ and ZnF2:Fe2+ by Mössbauer spectroscopy

Abstract Measurement of the magnetic hyperfine interaction in paramagnetic Fe 2+ in ZnF 2 and MgF 2 by Mossbauer spectroscopy is reported. The results, −275 ± 3 kOe and −270 ± 3 kOe for ZnF 2 :Fe 2+ and MgF 2 :Fe 2+ respectively are compared with a previous analysis of hyperfine interactions in the rutile fluorides.

Abstract: Phase Transitions in FeCl 2 ·2H 2 O in External Magnetic Fields: Mössbauer Spectroscopy

Monoclinic FeCl 2 ·2 H 2 O orders antiferromagnetically at T N = 23 K , and the magnetic structure consists of two sublattices of ‐ FeCl 2 ‐chains lying along the c‐axis. The coupling along the chains is ferromagnetic with weak antiferromagnetic coupling between chains. Application of an external magnetic field along the easy axis (a) induces phase transitions at H 1 = 39 kOe and at H 2 = 46 kOe . We report the observation of the three phases using the Mossbauer effect in a single crystal of FeCl 2 ·2 H 2 O cut parallel to the c‐axis and placed at 32° to the γ ray beam and magnetic field Ho so that Ho was parallel to the easy axis α. The results may be summarized as follows: (1) For H 0 H 1 we observe two superposed spectra with equal intensities due to the external field adding and subtracting to the hyperfine field in the spin down and spin up sublattices respectively; (2) For H 1 H o H 2 the relative intensities of the spin up to spin down spectra are roughly 3:1. (3) For H 2 H o , only one spectrum is observed. These observations are consistant with the antiferromagnetic⇒ferrimagnetic⇒paramagnetic model of Narath1 from susceptibility measurements.

Phase Transitions in

Monoclinic FeCl 2 ·2 H 2 O orders antiferromagnetically at T N = 23 K , and the magnetic structure consists of two sublattices of ‐ FeCl 2 ‐chains lying along the c‐axis. The coupling along the chains is ferromagnetic with weak antiferromagnetic coupling between chains. Application of an external magnetic field along the easy axis (a) induces phase transitions at H 1 = 39 kOe and at H 2 = 46 kOe . We report the observation of the three phases using the Mossbauer effect in a single crystal of FeCl 2 ·2 H 2 O cut parallel to the c‐axis and placed at 32° to the γ ray beam and magnetic field Ho so that Ho was parallel to the easy axis α. The results may be summarized as follows: (1) For H 0 H 1 we observe two superposed spectra with equal intensities due to the external field adding and subtracting to the hyperfine field in the spin down and spin up sublattices respectively; (2) For H 1 H o H 2 the relative intensities of the spin up to spin down spectra are roughly 3:1. (3) For H 2 H o , only one spectrum is observed. These observations are consistant with the antiferromagnetic⇒ferrimagnetic⇒paramagnetic model of Narath1 from susceptibility measurements.