Unusual spin-transition anomaly in the crossover system [Fe(2-pic)3]Cl2·EtOH

The spin transition in [Fe(2-pic)₃]Cl₂·EtOH (2-pic = 2-picolylamine) has been reinvestigated by Mössbauer spectroscopy and magnetic susceptibility measurements. With both techniques we see a “two-step” spin conversion in the crossover region with transition temperatures at 120.7 and 114.0 K. This has never been reported for any other spin crossover system. The quadrupole splittings for both spin states change abruptly in the range of the transition.     

Electronic relaxation phenomena following (57)Co(EC)(57)Fe nuclear decay in [Mn(II)(terpy)2](ClO4)2.(1/2)H2O and in the spin crossover complexes [Co(II)(terpy)2]X2.nH2O (X = Cl and ClO4): a Mössbauer emission spectroscopic study

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 M?ssbauer 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 M?ssbauer 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 M?ssbauer 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.     

Pressure-induced change of the hyperfine interaction in single crystals of FeCl2·4H2O in external magnetic field at 4.2 K

Pressure of ≈ 7 kbar applied to single crystals of paramagnetic FeCl₂·4H₂O at 4.2 K and 40 kOe reduces the effective magnetic field H^eff. at the Fe nucleus from 229.5 ± 1 to 223.5 ± 1 kOe. A decrease of the ligand field splitting with pressure is indicated.     

Polarization effects in Mössbauer absorption by the diamagnetic powder of sodium-nitroprusside in external magnetic fields

The Mössbauer spectra of the powder sample of sodium nitroprusside have been measured in external magnetic fields at 4.2 K. The dependence of the absorption on thickness of the absorber has been studied. The fits including the polarization of the absorber by means of the applied field and the calculations of the convolution integral without polarization effects have been compared by a least squares value x². The asymmetry parameter of the EFG is (4.2 K)=0.20±0.05.     

Inkjet printed Cu(In,Ga)S<Subscript>2</Subscript> nanoparticles for low-cost solar cells

Cu(In,Ga)Se₂ (CIGSe) thin film solar cells were fabricated by direct inkjet printing of Cu(In,Ga)S₂ (CIGS) nanoparticles followed by rapid thermal annealing under selenium vapor. Inkjet printing is a low-cost, low-waste, and flexible patterning method which can be used for deposition of solution-based or nanoparticle-based CIGS films with high throughput. XRD and Raman spectra indicate that no secondary phase is formed in the as-deposited CIGS film since quaternary chalcopyrite nanoparticles are used as the base solution for printing. Besides, CIGSe films with various Cu/(In + Ga) ratios could be obtained by finely tuning the composition of CIGS nanoparticles contained in the ink, which was found to strongly influence the devices performance and film morphology. To date, this is the first successful fabrication of a solar device by inkjet printing of CIGS nanoparticles.     

Quadrupole splitting of Fe(II) spin crossover compounds study of temperature and pressure dependence and the implication for the interaction mechanism

The temperature and pressure dependence of the quadrupole splitting ΔE _Q of the 3/2→1/2 nuclear Mössbauer transition of⁵⁷Fe in the spin crossover compounds [Fe _x Zn_1?x (2-pic)₃]Cl₂?EtOH and the deuterated analog [Fe _x Zn_1?x (2-pic-ND₂)₃] Cl₂?EtOD (2-pic=2-picolylamine) have been studied. The change of ΔE _Q can be linearly related to the defomation of the lattice as observed by X-ray measurements. The dependence of ΔE _Q on temperatureT, pressurep, and the fraction γ of molecules in the electronic high spin state in the pure iron (x=1) compounds is therefore interpreted as a result of the change of the lattice contribution to the electric field gradient (EFG) at the nucleus only. The intrinsic EFGs of the molecules remain unchanged under the cooperative interaction of the spin crossover complex molecules. This fact is consistent with a simple elastic interaction between the complexes rather than a Jahn-Teller-type of interaction also discussed in the literature.     

Determination of Spin State in Dinuclear Iron(II) Coordination Compounds Using Applied Field Mössbauer Spectroscopy

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 Mössbauer spectroscopy data irrespective of whether they belong to LS-LS, LS-HS and HS-HS pairs. However, the distinction of pairs becomes possible if Mössbauer 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.

     

Lamb–Mössbauer factor of electronically excited molecular states measured by time‐differential Mössbauer emission spectroscopy

The Lamb-Mössbauer Factor (LMF) of molecular crystals is expected to depend on the electronic molecular states by their different intramolecular vibrational frequencies. Revisiting Mössbauer spectra obtained by time differential Mössbauer emission spectroscopy of the low spin compound [⁵⁷Co/Mn(bipy)₃](PF₆)₂ (bipy= 2,2'-bipyridine) a ratio of 1.25 for the LMFs of the low spin ground state and of an excited high spin state decaying in the Mössbauer time window could be evaluated. The difference found is in line with the change of LMF observed for spin crossover compounds where the excited high spin state is populated by the so‐called LIESST effect. The initial population of the high spin state is close to 100%.