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The possibility of PO(4)(3-) for MoO(4)(2-) partial substitution in the langbeinite framework has been studied by exploration of the K-Fe(Sc)-Mo(W)-P-O systems using the high-temperature solution method. It was shown that 1/3PO(4)(3-) for MoO(4)(2-) substitution leads to formation of three novel compounds K(2)Fe(MoO(4))(PO(4))(2), K(2)Sc(MoO(4))(PO(4))(2), and K(2)Sc(WO(4))(PO(4))(2) with slightly increased lattice parameters and significant distortion of the anion tetrahedra without structure changes. In contrast, the antiferromagnetic structure is modified by substitution in the low-temperature region. The structural peculiarities are discussed in light of bond-valence sums calculations.

We have studied the high spin and low spin structural phases of the two-step spin transition compound [Fe(bpp) 2 ](NCS) 2 ·2H 2 O (bpp: 2,6-bis(pyrazol-3-yl)pyridine) of different sample history with X-ray synchrotron ( λ ∼ 0.32 A) powder diffraction at different temperatures as well as with usual X-ray ( λ ∼ 1.54 A) powder diffraction at room temperature by performing rapid repeated scans at regular intervals of time. A structural phase transition occurs only during the first step of the spin transition. Time dependent interconversions of the two crystallographic phases have been observed at room temperature for this material. The results indicate that a soft X-ray (Cu-K α )-induced structural change occurs in this material at room temperature. Possible mechanisms for this unusual observation are discussed.

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 temperatures demonstrates the presence of both FeIII and FeII in low-spin states in this compound. Quantitative analysis of the FeIII and FeII ions, and their temperature dependence exhibits the existence of an electron transfer phenomenon between Mn and Fe ions [FeIII ( t 2 g 5 , S = 1 / 2 )–CN–MnII ( t 2 g 3 e g 2 , S = 5 / 2 )]⇔[FeII ( t 2 g 6 , S = 0 )–CN–MnIII ( t 2 g 3 e g 1 , S = 2 )]. This electron transfer has been remarkably enhanced in the magnetically ordered region.

In the present article, we discuss the results of investigations of the influence of hydrostatic pressure (up to 1.2 GPa) on the spin transition behavior in [Fe(phen)2(NCS)2] polymorph II and [CrI2(depe)2]. It is demonstrated that pressure effect studies are very helpful in elucidating the mechanism of cooperative dynamic electronic structure phenomena accompanied by significant volume changes. Application of hydrostatic pressure serves as a tool for modifying the ligand field strength in a controlled manner.

57Fe Mossbauer spectroscopy of the metal-assembled layered ferrimagnetic material {N(n-C4H9)4[FeIIFeIII(C 2O4)3]}∞ was performed under a high external magnetic field at 4.2 K. This study successfully detected different types of FeII and FeIII high spin sites in the lattices. The hyperfine field values estimated for all the FeIII high spin sites are close to the expected values, whereas those for FeII high spin sites are substantially different to those expected usually. A large fraction of magnetically disordered FeIII high spin sites were found. It is understood that the layered structure of the compound may have two kinds of sub-lattices—(FeAII–ox–Fe AIII–···) and (FeBII–ox–Fe BIII–···)—along with randomly placed magnetically disordered FeCIII sites, where the FeAIII and FeBIII are antiferromagnetically aligned. The existence of the magnetically and/or structurally non-equivalent spin sites in the same lattice has been held responsible for the coexistence of different ferrimagnetic interactions leading to the spin glass state, as observed through earlier magnetic and calorimetric measurements.

A miniaturized Mossbauer spectrometer (MIMOS II) which was designed and constructed for extraterrestrial applications, such as iron-containing rock and soil analysis on the planet Mars, has been employed to investigate the iron-containing constituents in the paintings of a Lekythos Greek vase. Greek pottery is commonly painted with black figures. The Mossbauer backscattering spectra recorded with MIMOS II at room temperature show that the hand-painted black figures contain hematite.

The spin-crossover complexes [Fe(teec)6](X)2 (with teec=1-(2-chloroethyl)-tetrazole and X=BF4, ClO4, PF6) possess two iron(II) sites each having a different spin-crossover behaviour. The magnetic behaviour of the complexes obtained by slow crystallisation differs strongly from those obtained by precipitation. This difference results in either a spin crossover of 50% (X=ClO4, crystalline), two gradual steps (X=BF4, PF6), or two steps, of which one is gradual and the other having a thermal hysteresis (X=ClO4 precipitated). Both 57Fe Mossbauer spectroscopy and magnetic susceptibility measurements have been recorded as a function of T. The crystal structure of [Fe(teec)6](BF4)2 has been determined using X-ray synchrotron powder diffraction. Of [Fe(teec)6](ClO4)2 only the cell parameters could be determined. The ligand field spectra of the low-spin and high-spin states clearly indicate a 1A1→5T2 spin transition with 10 DqLS/10 DqHS=1.72.

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.

The compound [Fe(phy)2](BF4)2 (phy = 1,10-phenanthroline-2-carbaldehyde phenylhydrazone) shows spin-transition with a hysteresis of 10 K width around room temperature. Continuous irradiation of the compound during heating and cooling with green light (514 nm) shifts the hysteresis in temperature.

19 F and 11 B spin-lattice relaxation times were measured in [ Zn ( ptz ) 6 ] ( BF 4 ) 2 and in the spin-crossover compound [ Fe ( ptz ) 6 ] ( BF 4 ) 2 . For both compounds BF 4 - anion reorientation is active above 50 K. For [ Zn ( ptz ) 6 ] ( BF 4 ) 2 , the anion-reorientation dynamics is different in the temperature regions of 50-90 K, 90-120 K, and above 150 K; between 120 and 150 K it changes rapidly reflecting a structural change. In [ Fe ( ptz ) 6 ] ( BF 4 ) 2 the mechanism for the paramagnetic relaxation involving the 19 F nuclei is found to be of the diffusion-limited type according to the theory of Lowe and Tse. The present results prove that the spin-crossover takes place in a dynamic surrounding and not in a static crystal lattice.

Tris(2,2'-bipyridyl)-57CoII and bis(2,2':6',2'-terpyridine)-57CoII complexes were synthesised in the supercages of zeolite‐Y in order to study the effect of molecular isolation on the aftereffects of the 57Co(EC)57Fe decay. As compared to the regular crystalline salts of the complex ions where, according to the emission Mossbauer spectra, the most abundant species is low‐spin FeII, the molecular isolation in the zeolite resulted in a larger fraction of low‐spin FeIII and a varying amount of high‐spin Fe2+ species. In the investigated temperature range, 20 K to 295 K, the majority of the changes was observed above 80 K. In the case of tris (2,2'-bipyridyl)-57CoII-Y, the most characteristic change occurred in valence states, while for bis (2,2':6',2'-terpyridine)-57CoII‐Y, the temperature dependence of the spin states was more prominent. The change in the low spin valence states is explained partly by donor-acceptor properties of the zeolite lattice. The variation in the high spin fraction is explained by radiation damage of the ligand sphere and/or fragmentation of the complex ion followed by incomplete recombination in the supercage. Molecular isolation itself did not seem to increase the chance of fragmentation (as a consequence of charge neutralization following Auger ionization) of these highly conjugated complex molecules.

The Monte Carlo method has been applied to reproduce the two-step spin transition in [FexZn1−x(2-pic)3]Cl2 · EtOH (2-pic = 2-picolylamine). The partition function of the Hamiltonian has been factorized in two parts describing the inner degrees of freedom of the spin transition molecules and the interaction between them. The factorization is equivalent to an effective Hamiltonian which can be mapped on an Ising system in an applied magnetic field. The interaction consists of an infinite range and a short range interaction part. Qualitatively the two-step transition under pressure and the Zn diluted system have been well reproduced with infinite range and the next nearest neighbour interaction of ferromagnetic type and the nearest neighbour interaction of antiferromagnetic type in a simple cubic lattice.

Mossbauer spectroscopy and magnetic susceptibility measurements for the complex Fe(ppi) 2 (NCS) 2 in the temperature range 11–300 K indicate that the spin transition of this compound is incomplete until 11 K. The similar Debye-Waller factors in high spin (HS) and low spin (LS) states is shown by the comparison of the HS fractions from the two different methods. The Debye temperature of the compound was calculated from the defined Debye-Waller factors obtained from Mossbauer measurement at each experimental temperature and was found to depend on the temperature as well as on the HS fraction. The modification of the Debye-Waller factors in the HS and LS states for the HS fraction from Mossbauer measurements should be considered to result from the temperature dependence of the Debye temperature.

The cooperativity of the thermal spin transition in the Fe(II) spincrossover compound [Fe(ptz)6](BF4)2 (ptz = 1-propyltetrazole) and in isomorphous mixed crystals with the isostructural zinc complex is investigated. From powder X-ray measurements the lattice deformation (tensor e) accompanying the spin transition is determined. For diluted mixed crystals with x 0.44 a first order crystallographic phase transition (R3i → P1i) is observed on cooling, which is triggered by the spin transition and can be suppressed by cooling rapidly. The thermal spin transition is measured with UV/VIS optical absorption spectroscopy on mixed single crystals in the R3i structure. From this metal dilution experiment an interaction constant of 169 cm − 1 for the cooperativity of the spin transition is determined. Furtheron, this interaction constant is calculated on the grounds of elasticity theory: The lattice deformation due to the spin transition is traced back to anisotropic elastic point defects, which directly interact with each other via their stress fields and indirectly via the surface of the elastic crystal by an image pressure. The elastic properties of the crystalline matrix are taken in the isotropic approximation. They are derived from the complete sets of anisotropic elastic constants of the pure iron and zinc compounds, which have been measured previously by Brillouin spectroscopy. The contribution of elastic energy calculated this way is ≃ 80% of the experimental value of the interaction constant, i.e. the cooperativity in crystalline spincrossover compounds is quantitatively of elastic nature.

The complete set of seven elastic constants of the Fe(II) spincrossover compound [Fe(ptz)6] (BF4)2 (ptz = 1-propyltetrazole) with crystal symmetry R3i was measured by Brillouin spectroscopy at 300K. The measurement on the plate-like single crystals were performed using an oriented sample rotation technique and the combination of two scattering geometries. For comparison the elastic constants of two isostructural compounds were also measured at 300K: the analogous perchlorate spin-crossover compound [Fe(ptz)6] (ClO4)2 and the zinc complex [Zn(ptz)6] (BF4)2. The knowledge of the elastic constants is neccessary for the calculation of the elastic interaction between the spincrossover molecules, and this is the first time that the complete set of elastic constants of a spincrossover compound has been measured directly.

Oxidic and metallic iron and iron/nickel multilayers, formed after thermal and chemical treatment of Langmuir-Blodgett films, were investigated. Reduced iron-stearate multilayers were compared with evaporated films concerning their phase composition and their lateral homogeneity. The sequence of metallic and oxidic iron in the films after reduction was determined. Oxidic iron/nickel mixed layers can be prepared from Langmuir-Blodgett films, in which a magnetically non-ordered Fe3+ phase exhibiting a distribution of the quadrupole splitting is observed. After reduction of such layers, two metallic iron phases appear dependent on the nickel concentration. The hyperfine field distribution of the magnetically ordered metallic phase can be evaluated using a method proposed by Rancourt.

By means of the Langmuir–Blodgett (LB) technique, mono- and multilayers of Ni2+ and Fe3+ containing fatty acid salts are transferred onto silicon wafers. After thermal treatment, oxidic layers can be produced. The absolute amounts of Fe3+ and Ni2+ ions per monolayer of LB film were measured and calculated by three different methods. In the case of nickel, the amount expected from theory and preparation conditions was obtained experimentally. In contrast, about twice the number of Fe3+ ions as expected is transferred. An explanation can be given by the presence of hydroxide groups bound to iron. The mean film thickness of the oxidic layers after thermal treatment was estimated where the thicknesses were found to be

The magnetism of a series of tetranuclear complexes of the [ Fe II 4 L 4 ]( BF 4 ) 8 [2×2] -grid-type was investigated, revealing the occurrence of spin-transition (ST) behavior within this class of compounds. The phenomenon depends directly on the nature of the substituent R1 of the ligand L . All FeII ions in compounds with R1 substituents favoring strong ligand fields ( R 1 = H ; OH ) remain completely in the diamagnetic low-spin state. Only the complex bearing R1=Ph exhibits thermally induced ST behavior.

In a first approximation the Debye-Waller factor (DWF) of molecular crystals is factorized into a molecular and a lattice part:f=f m f l. In the case of spincrossover compounds there is the unique possibility to measure differences of both parts in the two spin states by switching the spin states with the LIESST effect. Measurements of the DWF depending on temperature, γ-ray direction and spin state were performed with57Fe-Mossbauer spectroscopy on three single crystals (R¯3) of the spincrossover system [FexZn1−x(ptz)6](BF4)2 (ptz=1-propyltetrazole), a concentrated sample withx=1 and diluted ones withx=0.30 andx=0.005. The DWF decreases from 10 to 300 K by a factor of ∼ 100 and its anisotropyf ⊥/f ∥ increases from ∼ 1 to 5.4. The differences of the DWF in the low spin and the high spin state (after LIESST) for the diluted and the concentrated samples were measured at low temperatures (≤ 50 K). From these data both the Gruneisen constant of the lattice γG=2.9 and the lowest IR active intramolecular frequency (45 cm−1 (HS), 58 cm−1 (LS)) have been determined. The contribution of the DWF from the acoustic modes of the lattice is calculated from the elastic constants of rhombohedral [Fe x Zn1−x (ptz)6](BF4)2. The contribution of the optical lattice vibrations is estimated. The influence of different DWF in the two spin states on the evaluation of the fraction of molecules in either spin state from Mossbauer area data is discussed.

The Lamb-Mossbauer factor in spin crossover compounds has been the subject of controversial discussions in the literature. For several compounds, a significantly larger Lamb-Mossbauer factor for the complex molecules in the low-spin (fls) than in the high-spin (fhs) state has been reported, while for compounds studied in our laboratory such differences could not be observed. We prepared the compound Fe(tpa)(NCS)2, thef factors of which were reported to be very different. We could not prove thef factors of this compound to be different. The implications of a different structural phase of compound prepared by us are discussed.

57Fe and119Sn Mossbauer measurements have been carried out on powder samples of three differently substituted M-type barium hexaferrites. By57Fe Mossbauer measurements in an external magnetic field applied parallel to the γ-ray direction, we found a canted spin structure for all samples. Furthermore, we detected a strong preference of the Sn4+ ions for the 4f2 sites. From57Fe Mossbauer measurements aboveT N, we conclude that the substitution does not influence the 2b sites. The analysis of the magnetically split119Sn Mossbauer spectra at room temperature in the case of the Co-Sn and Zn-Sn substituted samples shows a strong difference between the two. The spectra were interpreted due to the different surroundings of the Sn4+ ions.

The time integral Mossbauer emission spectrum of a57Co/Co1−xO source (x ≈ 10−5) at RT consists of two single Lorentzian lines of an Fe2+ (76%) charge state and an Fe3+ (24%) aliovalent charge state. The spectrum measured by γ-X-ray coincidence spectrpscopy shows the same fraction of the aliovalent charge state, contrary to the expectation derived from the competing acceptor model as successfully applied by Tejada and Parak [1], who could explain the dependence of the formation of aliovalent charge states after the nuclear transformation on the stoichiometric parameterx. The consequences of this unexpected behaviour for the competing acceptor model are discussed.

The Mossbauer emission spectra of nucleogenic iron(II) complexes with a low spin (LS) ground state show two metastable iron(II) high spin (HS) states at low temperatures. In order to identify these metastable HS states, the compound [57Co/Mn(bipyridine)3](PF6)2 has been studied by time differential Mossbauer emission spectroscopy (TDMES) and optical lifetime measurements of excited electronic states in the corresponding Fe-doped Mn compound. The lifetime of one of the HS states of the nucleogenic iron(II) determined by TDMES has been measured to be the same as the lifetime of the laser-excited iron(II) electronic state.

The applicability of a low-noise scintillation detector (ScD) for the registration of electrons of several keV energy has been studied employing photomultipliers (PM) of different types and sizes. With the application of a sedimented P-47 scintillation screen, the values of the low-energy sensitivity limit and those of the light conversion coefficient were determined as about 2.7–4.7 keV and 2.8–6.6 photoelectrons/keV, respectively, for the set of PM's (Philips-Valvo XP 2020, Philips-Valvo XP 2052, Philips-Valvo XP 2972, EMI 9124a) studied. It is concluded that such scintillation detectors might be used advantageously as electron counters in the range of E > 5 keV. Applications below this kinetic energy value are also feasible when applying a floating acceleration of several kV to the ScD — a voltage much lower than the values required for Everhart-Thornley detectors.

In the spin-crossover compound [Fe(ptz)6](BF4)2 (where ptz=1-n-propyltetrazole) six different phases are observed. When a single crystal is slowly cooled from high temperatures to those below 125 K, the reflections broaden into diffuse maxima and split into two maxima along the c* direction [Kusz, Gütlich & Spiering (2004). Top. Curr. Chem. 234, 129–153]. As both maxima are broad along the c* direction, the short-range order exists only along the c direction and in the ab plane the structure remains long-range ordered. In this disordered phase additional satellite reflections appear. Upon heating above 135 K, the diffuse maxima return to their previous shape and this process is completely reversible. Rapidly cooled samples, on the other hand, do not show such splitting and the symmetry remains R\bar 3, despite a jump in lattice parameters. We use a special technique to analyse the disorder model of the slowly cooled samples, which consists of layered domains shifted in the hexagonal ab plane. The low-spin disordered phase was solved in a novel approach to accommodate the very unusual twinning and refined in the non-standard space group C\bar 1. In contrast to the ordered low-spin phase, the Fe ion is in a non-centrosymmetric coordination polyhedron and two of the six propyl groups change their conformation.

Studies of the spin transition behavior of the mononuclear compound [Fe(pythiaz)(2)](BF(4))(2) have been carried out under hydrostatic pressures up to 9.13 kbar in the 5-300 K temperature range. Under ambient pressure this compound exhibits an approximately half-step (incomplete) HS [Formula: see text] LS transition with T(1/2) = 146 K without any thermal hysteresis. At pressures up to 4.5 kbar the behavior remains similar but with an upward displacement of T(1/2) and a slight decrease in the residual high spin fraction at low T. Application of higher pressures resulted in an almost complete two-step spin transition with several unusual pressure effects. Along with the expected pressure dependence of T(1/2) the surprising appearance of hysteresis in the spin transition curves was observed. It is suggested that the likely origin of this unprecedented behavior is a pressure-induced structural change.

Systematic studies were performed to investigate the distorting effect of the electron energy dependence of the channeltron detector efficiency on the shape of data distributions in electron spectroscopy. In agreement with earlier results for electron energies up to 3 keV, it was found that a virtually energy-independent shape of the electron distribution might be achieved even in a very broad range of electgron energies (up to 15 keV) by appropriate selection of the operational high voltage of a channeltron. However, the efficieny of channeltrons was also found to depend on count rates, indicating one of the principal difficulties in comparing distribution recorded under different experimental conditions.

Silicon steel (Fe-3wt%Si), as used for transformers and generators, has been annealed in wet hydrogen at elevated temperatures. The composition, sequence, and thicknesses of the layers found by conversion electron Mossbauer spectroscopy (CEMS) and Auger electron spectroscopy (AES) depth profiling for a 10 minutes anneal in different atmospheres are reported. In the range from 500°C to 720°C we observed carbide formation, indicating that the decarburization is hindered. Above 800°C, the layers consist of fayalite or of fayalite and iron oxides, depending on the oxygen potentialaO. At 843°C, the onset of iron oxide formation was found ataO=0.33.

The passivation of low carbon steel was studied in aqueous solution of 0.5 M Na2SO4+0.001 M NaHSO3 at pH=3.5 and 6.5. The found major components at pH-3.5 were: γ-FeOOH and Fe3C, and also FeSO4.H2O could be identified on the surface of the low carbon steel as a minor component. At pH-6.5, the passive film contained only amorphous iron(III)-oxide or oxyhydroxide.

A new detector for CEMS has been built and optimized with respect to the statistical quality of spectra obtained. The optimization has been performed by measuring Mosbauer and pulse height spectra at in- and off-resonance. Single channel analyzer settings were calculated by a new optimization routine. A comparison of different detector designs has been performed using the statistical utility rate of spectra obtained from a stainless steel foil. A procedure for determining optimal operating parameters for ICEMS gas flow proportional counters is proposed.

Five novel two-dimensional coordination polymers {Fe(4PhPy)(2)[M(II)(CN)(4)]}.sH(2)O (4PhyPy = 4-phenylpyridine; 1: M(II) = Pd, s = 0; 2: M(II) = Ni, s = 0; 3: M(II) = Pt, s = 1) and {Fe(4PhPy)(2)[M(I)(CN)(2)](2)}.sH(2)O (4: M(I) = Ag, s = 1; 5: M(I) = Au, s = 0.5) exhibiting spin-crossover properties have been synthesized. They were characterized at various temperatures using X-ray absorption spectroscopy (XAS), powder X-ray diffraction (PXRD), differential scanning calorimetry (DSC), and magnetic susceptibility measurements. The occurrence of a cooperative thermal spin transition detected by the magnetic method is located at critical temperatures T(c)( downward arrow)/T(c)( upward arrow) = 163 K/203 K (1), 135 K/158 K (2), and 172 K/221 K (3), and a less cooperative one is located at T(c) = 188 K (4) and 225 K (5). Compounds 1-5 show an abrupt color change from yellow (high-spin (HS) state) to red (low-spin (LS) state) upon spin-state conversion. The dehydration of the compounds changes the type of the spin transition, making it more abrupt and shifting the critical temperature to higher temperatures. For 1 and 2, XAS provides local structural information on the contraction of the FeN(6) coordination sphere upon the HS-to-LS transition, in line with the magnetic results. Variable-temperature characterization of 1 by X-ray diffraction evidences the very abrupt phase transition with a large hysteresis. A light-induced spin conversion (LIESST effect) is detected by magnetic measurements in 1-5 below 70 K.

Using our new spectrometer for time-differential Mossbauer emission spectroscopy we reinvestigated the high-spin→low-spin relaxation following the57Co(EC)57Fe process in [57Co/Co(phen)3](ClO4)2 (phen=1,10-phenanthroline). In contrast to former investigations we found a temperature dependent initial population less than 1 for the metastable high-spin states. The results and the data evaluation method are presented.

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.

Short-range antiferromagnetic correlations have been studied in the layered compounds (PPh 4 ) [Fe I I Fe I I I (ox) 3 ] and (NBu 4 ) [Fe I I Fe I I I (ox) 3 ] by neutron polarization analysis and Mossbauer spectroscopy. Polarized neutron diffraction profiles obtained between 2 and 50 K on (d 2 0 -PPh 4 ) [Fe I I Fe I I I (ox) 3 ] show no magnetic Bragg scattering; the lack of such scattering indicates the absence of long-range magnetic order. However, a broad asymmetric feature observed at a Q of ca. 0.8 A - 1 is attributed to two-dimensional short-range magnetic correlations, which are described by a Warren function. The correlation length is ca. 50 A between 2 and 30 K and then decreases to ca. 20 A at 50 K. The Mossbauer spectra of (PPh 4 ) [Fe I I Fe I I I (ox) 3 ] and (NBu 4 ) [Fe I I Fe I I I (ox) 3 ] have been measured between 1.9 and 293 K and 1.9 and 315 K, respectively, and are very similar. The paramagnetic spectra exhibit both high-spin Fe I I and Fe I I I doublets with relative areas which indicate a 5% and 2% excess, respectively, of Fe I I I . The coexistence in (PPh 4 ) [Fe I I Fe I I I (ox) 3 ] between 10 and 30 K of broad sextets and doublets in the Mossbauer spectra and the paramagnetic scattering observed in the polarized neutron measurements indicate the coexistence of spin-correlated and spin-uncorrelated regions in the layers of this compound. The polarized neutron scattering profiles and the Mossbauer spectra yield the magnetic exchange correlation length and lifetime, respectively, and the combined results are best understood in terms of layers composed of random frozen, but exchange correlated domains of ca. 50 A diameter at the lowest temperatures, of spin-correlated domains and spin-uncorrelated regions at intermediate temperatures, and of largely spin-uncorrelated regions above the Neel temperature as determined from magnetometry. The similarity of the Mossbauer spectra of (PPh 4 ) [Fe I I Fe I I I (ox) 3 ] and (NBu 4 ) [Fe I I Fe I I I (ox) 3 ] leads to the conclusion that similar magnetic exchange correlations are present in the latter compound.

The Mossbauer spectra of the layer ferrimagnets PPh4FeIIFeIII(ox)3 and NBu4FeIIFeIII(ox)3 have been measured between 1.9 and 315 K. The paramagnetic spectra exhibit both high-spin iron(II) and iron(III) doublets. The iron(II) quadrupole splittings are negative and decrease substantially upon cooling as a result of a low-symmetry crystal field splitting. In contrast, the iron(III) splittings are small, positive, and virtually independent of temperature. The respective magnetic ordering temperatures of 30 and 42.5 K agree well with the phase observed by bulk magnetometry and the magnetic sextets yield hyperfine fields of ca. 60 kOe for iron(II) and 540 kOe for iron(III). The unusually low iron(II) field results from the exceptionally large orbital and dipolar contributions, while the iron nuclei experience a static hyperfine field at 1.9 K and a relaxing hyperfine field between ca. 10 and 30 K.

The valence states of the nucleogenic (57)Fe arising from the nuclear disintegration of radioactive (57)Co by electron capture decay, (57)Co(EC)(57)Fe, have been studied by Mössbauer emission spectroscopy (MES) in the (57)Co-labeled systems: [(57)Co/Co(terpy)(2)]Cl(2).5H(2)O (1), [(57)Co/Co(terpy)(2)](ClO(4))(2).(1)/(2)H(2)O (2), and [(57)Co/Mn(terpy)(2)](ClO(4))(2). (1)/(2)H(2)O (3) (terpy = 2,2':6',2' '-terpyridine). The compounds 1, 2, and 3 were labeled with ca. 1 mCi of (57)Co and were used as the Mössbauer sources at variable temperatures between 300 K and ca. 4 K. [Fe(terpy)(2)]X(2) is a diamagnetic low-spin (LS) complex, independent of the nature of the anion X, while [Co(terpy)(2)]X(2) complexes show gradual spin transition as the temperature is varied. The Co(II) ion in 1 "feels" a somewhat stronger ligand field than that in 2; as a result, 83% of 1 stays in the LS state at 321 K, while in 2 the high-spin (HS) state dominates at 320 K and converts gradually to the LS state with a transition temperature of T(1/2) approximately 180 K. Variable-temperature Mössbauer emission spectra for 1, 2, and 3 showed only LS-(57)Fe(II) species at 295 K. On lowering the temperature, metastable HS Fe(II) species generated by the (57)Co(EC)(57)Fe process start to grow at ca. 100 K in 1, at ca. 200 K in 2, and at ca. 250 K in 3, reaching maximum values of 0.3 at 20 K in 1, 0.8 at 50 K in 2, and 0.86 at 100 K in 3, respectively. The lifetime of the metastable HS states correlates with the local ligand field strength, and this is in line with the "inverse energy gap law" already successfully applied in LIESST relaxation studies.

Studies of the spin transition behavior of the mononuclear compound [Fe(pythiaz)2](BF4)2 have been carried out under hydrostatic pressures up to 9.13 kbar in the 5–300 K temperature range. Under ambient pressure this compound exhibits an approximately half-step (incomplete) HS ↔ LS transition with T1/2 = 146 K without any thermal hysteresis. At pressures up to 4.5 kbar the behavior remains similar but with an upward displacement of T1/2 and a slight decrease in the residual high spin fraction at low T . Application of higher pressures resulted in an almost complete two-step spin transition with several unusual pressure effects. Along with the expected pressure dependence of T1/2 the surprising appearance of hysteresis in the spin transition curves was observed. It is suggested that the likely origin of this unprecedented behavior is a pressure-induced structural change.

New nonstoichiometric cesium bromo aurates(I,III) Cs1+x(AuBr4)(AuBr2, Br3,Br)x have been synthesized, in which the almost equally shaped anions AuBr3- and Br3- can interchange themselves. The compounds crystallize in a disordered cubic perovskite structure with a = 547.5 pm or in a tetragonal superstructure with a 16-fold perovskite cell (a = 1553, c = 1099 pm). The investigations show that other bromoaurates as the previously reported "Cs2AgAuBr6" can contain perceptible amounts of Br3- ions also. The IR and 197Au Mössbauer spectra of the new bromoaurates and of Cs2AuoBr6 are discussed

61Ni Mossbauer measurements have been performed at 4.2 K on spherical Ni particles with an average diameter of 100 and 30 A, covered with a protective layer of SiO. Their spectra contain a surface component with a significantly reduced hyperfine magnetic field as compared with the field in the bulk. This result confirms recent theoretical predictions.

Earlier work in our laboratory on the effect of metal dilution on the 5T2(Oh ⇌ 1A1(Oh) equilibrium in polycrystalline spin crossover systems of iron(II) has shown that the relative stability of the high spin state, 5T2(Oh), at a given temperature, increases markedly with decreasing iron concentration. These results have initiated the present work. Using 57Fe Mossbauer spectroscopy, we have investigated the electronic ground state of iron(II) in the highly diluted solid solutions [FexM1−xL3] with L = bipy, M = Mn, Ni, Zn, x ≲ 0.005 and L = phen, M = Ni, Zn, x ≲ 0.005. Although the critical field potential (Vc) of the pure iron complexes, which are known to be low spin, is reported to be not far from the crossover point Vc = P (P, the mean spin pairing energy), the effect of metal dilution does not reduce the difference ¦ Vc - P ¦ sufficiently as to thermally populate the 5T2(Oh) state at room temperature to any noticeable extent. ¦ Vc - P ¦ has been estimated to be ≳ 1200 cm−1. The quadrupole splitting and the resonance line width are nearly the same in the pure compounds and the solid solutions. The isomer shift, however, is found to increase slightly in the order M = Fe < Ni < Zn < Mn, and parallels the increase in the ionic radii of the M++ host ions.