Interplay of Antiferromagnetic Coupling and Spin Crossover in Dinuclear Iron(II) Complexes

This article reports on the study of the interplay between magnetic coupling and spin transition in 2,2′-bipyrimidine (bpym)-bridged iron(II) dinuclear compounds. Coexistence of both phenomena has been observed in [Fe(bpym)(NCS)2]2bpym, [Fe(bpym)(NCSe)2]2bpym and [Fe(bt)(NCS)2]2bpym (bpym = 2,2′-bipyrimidine, bt = 2,2′-bithiazoline) by the action of external physical factors namely pressure or electromagnetic radiation. Competition between magnetic exchange and spin crossover has been studied in [Fe(bpym)(NCS)2]2bpym at 6.3 kbar. LIESST experiments carried out in [Fe(bpym)(NCSe)2]2bpym and [Fe(bt)(NCS)2]2bpym at 4.2 K have shown that is possible to achieve dinuclear molecules with different…

The Two‐Step Spin Conversion in a Supramolecular Triple Helicate Dinuclear Iron(II) Complex Studied by Mössbauer Spectroscopy

The triple helicate dinuclear iron(II) complex, [Fe-2(L)(3)](ClO4)(4)center dot 2H(2)O (1), previously reported by Tuna et al. (Chem. Eur. J. 2004, 10, 5737), was prepared and characterised by detailed SQUID and Fe-57 Mbssbauer measurements. Compound 1 exhibits a thermochromic two-step spin conversion at T-SC((1)) ca. 240 K and T-SC((2)) ca. 120 K, but does not switch its spin state further below 20 K as proven by Mossbauer spectroscopy. The sharp variation of the susceptibility below 20 K is due to zero-field splitting of the remaining iron(II) high-spin species. Applied field Fe-57 Mossbauer spectroscopy experiments at 4.2 K indicate that the gradual thermal spin conversion from [HS-HS] p…

57Fe Mössbauer spectroscopy predicts superstructure for K0.08[Cu(II)(N,N'app)Cl]2[Fe(III)(CN)6].0.92H3O.3H2O.

The compound [Cu(N,N'app)Cl](2)[Fe(CN)(6)].xH(2)O, with N,N'app being bis(N,N'-3-aminopropylpiperazine), was prepared and its structure determined by single crystal X-ray analysis, confirming a ratio of two copper complexes to one iron complex; (57)Fe Mössbauer spectra showed three quadrupole doublets typical of iron(iii) low spin species which call for the presence of a superstructure.

Magnetic transitions in double perovskiteSr2FeRe1−xSbxO6(0⩽x⩽0.9)

The double perovskites ${\mathrm{Sr}}_{2}\mathrm{Fe}M{\mathrm{O}}_{6}$ $(M=\mathrm{Re},\mathrm{Mo})$ belong to the important class of half-metallic magnetic materials. In this study we explore the effect of replacing the electronic $5d$ buffer element Re with variable valency by the main group element Sb with fixed valency. X-ray diffraction reveals ${\mathrm{Sr}}_{2}{\mathrm{FeRe}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}{\mathrm{O}}_{6}$ $(0lxl0.9)$ to crystallize without antisite disorder in the tetragonally distorted perovskite structure (space group $I4∕mmm$). The ferrimagnetic behavior of the parent compound ${\mathrm{Sr}}_{2}{\mathrm{FeReO}}_{6}$ changes to antiferromagnetic upon Sb subst…

Structure and properties of CoMnSb in the context of half-metallic ferromagnetism

Although its X-ray powder diffraction patterns show a superstructure, the compound CoMnSb, like the well-known half-Heusler alloy NiMnSb, is often referred to the category of half-metallic ferromagnets with $C{1}_{b}$ structure. Our study assigns CoMnSb to space group $Fm\overline{3}m$. The crystal structure of CoMnSb can be represented as an alternation of ${\mathrm{Co}}_{2}\mathrm{Mn}\mathrm{Sb}$ and MnSb structural units, and, in contrast to NiMnSb, displays three Mn and two Sb positions in the elementary cell. The presence of nonequivalent antimony and manganese positions was verified using NMR and M\"ossbauer spectroscopic measurements. Band-structure calculations based on a proposed s…

On the Nature of the Plateau in Two-Step Dinuclear Spin-Crossover Complexes

A remarkable feature of the spin-crossover process in several dinuclear iron(II) compounds is a plateau in the two-step transition curve. Up to now, it has not been possible to analyse the spin state of dinuclear pairs that constitute such a plateau, due to the relative high temperatures at which the transition takes place in complexes investigated so far. We solved this problem by experimentally studying a novel dinuclear spin-crossover compound [[Fe(phdia)(NCS)(2)](2)(phdia)] (phdia: 4,7-phenanthroline-5,6-diamine). We report here on the synthesis and characterisation of this system, which exhibits a two-step spin transition at T(c1)=108 K and T(c2)=80 K, displaying 2 K and 7 K wide therm…

Structure and Properties of GdAuSn and the GdAuSn/MnAuSn System

The crystal structure of GdAuSn was refined by means of single crystal X-ray diffraction. Band structure calculations based on the structural data confirmed the antiferromagnetic ground state and the metallic behaviour of GdAuSn. 119mSn, 155Gd and 197Au Mossbauer spectroscopic studies were used to verify the values of the hyperfine parameters that were given by the band structure calculations. Band structure calculations of MnAuSn confirmed that this half-Heusler compound belongs to the family of half-metallic ferromagnets. Magnetic susceptibility, conductivity and Mossbauer studies were used to characterize granular material based on the half-Heusler ferromagnet MnAuSn in the antiferromagn…

Mixed spin-state [HS-LS] pairs in a dinuclear spin-transition complex: confirmation by variable-temperature 57Fe Mössbauer spectroscopy.

Exquisite sensitivity of Mossbauer spectroscopy for tiny local molecular distortion is demonstrated in [FeII2(pmat)2](BF4)4?DMF: high-spin (HS) iron(II) in [HS-HS] and in [LS-HS] (low-spin–high-spin) pairs is clearly distinguished (see picture) for the first time without the need to apply a magnetic field. This dinuclear complex clearly shows that spin crossover via a [LS-HS] species is promoted by the use of a highly constrained bridging ligand (the bis-terdentate pmat).

Stoichiometry of LiNiO2 Studied by Mössbauer Spectroscopy

From the 61Ni and 57Fe Mossbauer spectroscopy data follows the cationic site assignment in Li1−x Ni1+x O2. Our data explain the ferromagnetic properties of this material because of the appearance of Ni2+ (S = 1) among Ni3+ (S = 1/2) in Ni3+O2 hexagonal planes. We have no evidence for the ferromagnetic interaction between the NiO2 layers through the excess Ni2+ ions substituting the Li+ ions. The presence of Ni2+ found in the Ni3+O2 planes explains the absence of the Jahn-Teller distortions probably because of the electronic transfer between the Ni3+ and Ni2+ ions.

Titelbild: Mixed Spin-State [HS-LS] Pairs in a Dinuclear Spin-Transition Complex: Confirmation by Variable-Temperature57Fe Mössbauer Spectroscopy (Angew. Chem. 16/2008)

Substitution Effects in Double Perovskites: How the Crystal Structure Influences the Electronic Properties

We systematically studied substituted Sr2FeReO6 with respect to experimental characterization and theoretical band structure calculations. In the framework of the tight-binding approach, hole- or electron-doping of Sr2MM’O6 were performed at the M or M’ positions either by transition or main group metals. Hole-doping, rather than electron-doping, has a favorable effect to improve the half-metallicity (Curie temperature and saturation magnetization) of the parent compound. When M is substituted by another metal, the original M’ metal will serve as a redox buffer (and vice versa). Substituting M by another metal with a size similar to that of the metal at M’ position causes disorder, which ha…

Pressure-induced electron transfer in ferrimagnetic Prussian blue analogs

M\"ossbauer and magnetic susceptibility measurements were performed under pressure on three Prussian blue analogs, ${\mathrm{K}}_{0.1}{\mathrm{Co}}_{4}[{\mathrm{Fe}(\mathrm{CN})}_{6}{]}_{2.7}\ensuremath{\cdot}18{\mathrm{H}}_{2}\mathrm{O},$ ${\mathrm{K}}_{0.28}{\mathrm{Co}}_{4}[{\mathrm{Fe}(\mathrm{CN})}_{6}{]}_{2.76}\ensuremath{\cdot}18{\mathrm{H}}_{2}\mathrm{O},$ and ${\mathrm{Cs}}_{0.7}{\mathrm{Co}}_{4}[{\mathrm{Fe}(\mathrm{CN})}_{6}{]}_{2.9}\ensuremath{\cdot}16{\mathrm{H}}_{2}\mathrm{O}.$ A pressure-induced electron transfer ${\mathrm{Co}}^{2+}(S=\frac{3}{2})\ensuremath{-}{\mathrm{Fe}}^{3+}(S=\frac{1}{2})\ensuremath{\rightarrow}{\mathrm{Co}}^{3+}(S=0)\ensuremath{-}{\mathrm{Fe}}^{2+}(S=0)…

Effect of cation disorder on the magnetic properties ofSr2Fe1−xGaxReO6(0<x<0.7)double perovskites

The effect of diamagnetic dilution of the Fe sublattice on the structural and magnetic properties of the double perovskite Sr{sub 2}Fe{sub 1-x}Ga{sub x}ReO{sub 6} (0 =}0.4 is detected by x-ray structural analysis accompanied by the observation of a magnetically ordered and a paramagnetic phase in the corresponding Moessbauer spectra. Below 20% Ga content, Ga statistically dilutes the -Fe-O-Re-O-Fe- double-exchange pathways. Phase separation begins at 20% Ga substitution; between 20% and 40% ofmore » Ga content, the paramagnetic Ga-based phase does not contain any Fe. The Fe-containing, paramagnetic cubic phases which can be detected by Moessbauer spectroscopy first appear for x=0.4.« less

Metal-to-metal electron transfer and magnetic interactions in a mixed-valence Prussian Blue analogue

Abstract In search of a new Prussian Blue analogue exhibiting fascinating magnetic properties, potassium manganese hexacyanoferrate, K 0.2 Mn 0 . 66 II Mn 1.44 III [ Fe 0.2 II Fe 0.8 III ( CN ) 6 ] O 0.66 ( CH 3 COO ) 1.32 ] , 7.6H2O, has been synthesized. This compound undergoes a paramagnetic to ferrimagnetic transition at 10 K. Temperature and magnetic field-dependent magnetization studies of this compound have revealed different spin alignments below and above 3 K. The nature of possible magnetic interactions between the nearest neighbor magnetic centers has been discussed in order to explore the origin of the observed magnetic interactions. Mossbauer spectroscopic study at different te…

Weak itinerant ferromagnetism and electronic and crystal structures of alkali-metal iron antimonides: NaFe4Sb12andKFe4Sb12

The synthesis, chemical, structural, and magnetic properties of alkali-metal compounds with filled-skutterudite structure, $\mathrm{Na}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$ and $\mathrm{K}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$, are described. X-ray and neutron diffraction and elemental analysis established the crystal structure without defects and disorder on the cation site. The temperature and pressure dependence of the cubic unit cell of $\mathrm{Na}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$ and the displacement parameter of Na are investigated. The electronic structure is calculated by density functional methods (LMTO, FPLO). Quantum chemical calculations (electron localization function) reveal …

Cover Picture: Mixed Spin-State [HS-LS] Pairs in a Dinuclear Spin-Transition Complex: Confirmation by Variable-Temperature57Fe Mössbauer Spectroscopy (Angew. Chem. Int. Ed. 16/2008)

Direct monitoring of spin state in dinuclear iron(II) coordination compounds

So far there has been no direct method to determine the spin state of molecules in dinuclear iron(II) compounds. The molecular fractions of high-spin (HS) and low-spin (LS) species have been deduced from magnetic susceptibility and zero-field Mossbauer spectroscopy data irrespective of whether they belong to LS–LS, LS–HS and HS–HS pairs. However, the distinction of pairs becomes possible if Mossbauer measurements are carried out in an external magnetic field. The proposed method opens new possibilities in the study of spin crossover phenomena in dinuclear compounds.

Mössbauer investigation of the photoexcited spin states and crystal structure analysis of the spin-crossover dinuclear complex [{Fe(bt)(NCS)(2)}(2)bpym] (bt=2,2'-bithiazoline, bpym=2,2'-bipyrimidine).

The crystal structure of the complex [{Fe(bt)(NCS)(2)}(2)bpym] (1) (bt=2,2'-bithiazoline, bpym=2,2'-bipyrimidine) has been solved at 293, 240, 175 and 30 K. At all four temperatures the crystal remains in the P space group with a=8.7601(17), b=9.450(2), c=12.089(3) A, alpha=72.77(2), beta=79.150(19), gamma=66.392(18) degrees , V=873.1(4) Angstrom(3) (data for 293 K structure). The structure consists of centrosymmetric dinuclear units in which each iron(II) atom is coordinated by two NCS(-) ions in the cis position and two nitrogen atoms of the bridging bpym ligand, with the remaining positions occupied by the peripheral bt ligand. The iron atom is in a severely distorted octahedral FeN(6) e…

Spin Crossover in Fe(II) Molecular Compounds — Mössbauer and µSR Investigations

The compound [Fe(ptz)6](C104)2 (ptz = 1-propyl-tetrazole) displays a complete and gradual spin crossover centred around 125 K as evidenced by magnetic and muon measurements over the temperature range ∼ 4.2–300 K. Although the crystal structure reveals only one crystallographic site, line broadening is observed in the Mossbauer spectra in the vicinity of the spin transition. The muon spin relaxation behaviour of this compound indicates that a structural transformation rather than dynamic processes may account for the observed spectral features. Both the Mossbauer and muon measurements are consistent with a mixture of high and low spin Fe ions in the transition region.

In situ— High Temperature Mössbauer Spectroscopy of Iron Nitrides and Nitridoferrates

The stoichiometric iron nitrides γ′-Fe4N, e-Fe3N and ζ-Fe2N were characterized by Mossbauer spectroscopy. The thermal decomposition of e-Fe3N was studied in-situ by means of a specially developed Mossbauer furnace. We found e-Fe3N to γ′-Fe4N and e-Fe3Nx (x ≥ 1.3) as decomposition products and determined the border of γ′/e transformation at T ≅ 930 K. Mossbauer spectroscopy was applied to study in-situ the thermal decomposition of the nitridometalate Li3[FeIIIN2] and the formation of Li2[(Li1-xFeIx)N], the compound with the largest local magnetic field ever observed in an iron containing material. The kinetics of formation and the stability of Li2[(Li1-xFeIx)N] was of particular interest in …

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