Mössbauer study of the vibrational anisotropy and of the light-induced population of metastable states in single-crystalline guanidinium nitroprusside

M?ssbauer studies were performed on single crystals of guanidinium nitroprusside with different orientations of their principal crystallographic axes (a, b, c) with respect to the incident radiation. The markedly anisotropic Lamb-M?ssbauer factor f _LM , i.e. f _LM ^(a) = 0.118(8), f _LM ^(b) = 0.174(8), f _LM ^(c) = 0.202(8) is in contrast to that of nitroprussides with inorganic anions. The observed anisotropy is ascribed to the anisotropic vibrational mean-square displacement of the nitroprusside anions as a whole which is due to the specific packing of both, anions and cations, as well as the very weak chemical bonding between the ions, typical only for guanidinium nitroprusside. The vibrational anisotropy of iron atoms in barium nitroprusside that has been observed by X-ray structural investigations has a different origin and therefore does not result in an anisotropic Lamb-M?ssbauer factor. We have also investigated metastable states in guanidinium nitroprusside that have been populated by means of incoherent irradiation from light-emitting diodes. With a specific orientation of the guanidinium nitroprusside single crystal a population of the metastable states up to 26% could be achieved. Populations of comparable size on lithium, sodium and potassium nitroprussides have only been reached using coherent laser irradiation. Received 15 December 1998 and Received in final form 3 March 1999     

Mössbauer spectroscopy in the time domain applied to the study of single‐crystalline guanidinium nitroprusside

Guanidinium nitroprusside GNP, (CN₃H₆)₂[Fe(CN)₅NO] has been investigated in single‐crystalline form by nuclear resonant forward scattering (NFS) using synchrotron radiation (Mössbauer spectroscopy in the time domain). This method provides a direct measure of effective absorber thickness and therefore also of the Lamb–Mössbauer factor f _LM. GNP has the advantage that all [Fe(CN₅)NO]^2- anions are practically aligned within the crystal. For the two different crystal orientations, with the crystallographic a- and c-direction parallel to the synchrotron beam, respectively, we have obtained f _LM ^(a)= 0.122(10) and f _LM ^(c)= 0.206(10), i.e., GNP exhibits significant anisotropic vibrational behavior. The quantum beat pattern of the NFS spectra obtained for the two different crystal orientations is discussed on the basis of radiation characteristics of the polarized synchrotron beam and the multipole transitions of oriented ⁵⁷Fe nuclei.     

Terahertz Cyclotron Photoconductivity in a Highly Unbalanced Two-Dimensional Electron–Hole System

The magnetic properties of the α-Fe₂O₃ hematite at a high hydrostatic pressure have been studied by synchrotron Mössbauer spectroscopy (nuclear forward scattering (NFS)) on iron nuclei. Time-domain NFS spectra of hematite have been measured in a diamond anvil cell in the pressure range of 0–72 GPa and the temperature range of 36–300 K in order to study the magnetic properties at a phase transition near a critical pressure of ~50 GPa. In addition, Raman spectra at room temperature have been studied in the pressure range of 0–77 GPa. Neon has been used as a pressure-transmitting medium. The appearance of an intermediate electronic state has been revealed at a pressure of ~48 GPa. This state is probably related to the spin crossover in Fe³⁺ ions at their transition from the high-spin state (HS, S = 5/2) to a low-spin one (LS, S = 1/2). It has been found that the transient pressure range of the HS–LS crossover is extended from 48 to 55 GPa and is almost independent of the temperature. This surprising result differs fundamentally from other cases of the spin crossover in Fe³⁺ ions observed in other crystals based on iron oxides. The transition region of spin crossover appears because of thermal fluctuations between HS and LS states in the critical pressure range and is significantly narrowed at cooling because of the suppression of thermal excitations. The magnetic P–T phase diagram of α-Fe₂O₃ at high pressures and low temperatures in the spin crossover region has been constructed according to the results of measurements.     

Site-selective detection of vibrational modes of an iron atom in a trinuclear complex

Nuclear inelastic scattering (NIS) experiments on the trinuclear complex [⁵⁷Fe{L-N₄(CH₂Fc)₂} (CH₃CN)₂](ClO₄)₂ have been performed. The octahedral iron ion in the complex was labelled with ⁵⁷Fe and thereby exclusively the vibrational modes of this iron ion have been detected with NIS. The analysis of nuclear forward scattering (NFS) data yields a ferrous low-spin state for the ⁵⁷Fe labelled iron ion. The simulation of the partial density of states (pDOS) for the octahedral low-spin iron(II) ion of the complex by density functional theory (DFT) calculations is in excellent agreement with the experimental pDOS of the complex determined from the NIS data obtained at 80 K. Thereby it was possible to assign almost each of the experimentally observed NIS bands to the corresponding molecular vibrational modes.     

Magnetic structure and anisotropy of YFe 6 Ga 6 and HoFe 6 Ga 6

The magnetic structure of RFe₆Ga₆ intermetallic compounds with R = Y, Ho have been determined by neutron powder diffraction, ⁵⁷Fe M?ssbauer spectroscopy, AC susceptibility, TGA (Thermo-Gravimetric Analysis) and magnetization measurements. Both compounds crystallize in the tetragonal ThMn₁₂ structure (space group I4/mmm) with the magnetic structure of YFe₆Ga₆ consisting of a simple ferromagnetic alignment of Fe moments in the basal plane with a Curie temperature of 475(5) K. Gallium atoms are found to fully occupy the 8i site, with Fe and Ga atoms equally distributed over the 8j site, whilst Fe atoms fully occupy the 8f site. The average Fe moments are 1.68(10) and 1.46(10) at 15 and 293 K, respectively. The average room temperature Fe magnetic moments determined by neutron diffraction are in overall agreement with the average Fe moment deduced from M?ssbauer spectroscopy and bulk magnetization measurements on this compound. The magnetic anisotropy of the compound HoFe₆Ga₆ is also planar in the temperature range 6-290 K, with Ho magnetic moments of 9.28(20) and 2.50(20) at 6 K and 290 K, respectively, coupled anti-ferromagnetically to the Fe sublattice and a Curie temperature of 460(10) K. The magneto-crystalline anisotropies of both compounds are comparable at low temperatures. Received 8 March 2001 and Received in final form 18 June 2001     

Mössbauer and exafs investigation of spin-equilibrium in a model-compound for heme proteins

We have used temperature dependent magnetic susceptibility, Mössbauer and EXAFS measurements to study the high spin (HS) — low spin (LS) conversion in Fe(II) (bptn) (NCS)₂, and we have combined consistently the information obtained from the different methods.     

Tin–DNA complexes investigated by nuclear inelastic scattering of synchrotron radiation

Nuclear inelastic scattering (NIS) of synchrotron radiation has been used to investigate the dynamics of tin ions chelated by DNA. Theoretical NIS spectra have been simulated with the help of density functional theory (DFT) calculations using 12 models for different binding sites of the tin ion in (CH₃)Sn(DNA_Phosphate)₂. The simulated spectra are compared with the measured spectrum of the tin–DNA complex.     

Incommensurate modulated magnetic structure in orthorhombic EuPdSb

Orthorhombic EuPdSb is known to undergo two magnetic transitions, at 12 K and at T _N≃ 18 K, and in phase III (T < 12 K), single crystal magnetisation data have shown that the spin structure is collinear antiferromagnetic, with magnetic moments along the crystal a axis. From a ¹⁵¹Eu M?ssbauer absorption study, we show that, at any temperature within phase III, all the moments have equal sizes, and that in phase II (12 K< T <18 K) the magnetic structure is modulated and incommensurate with the lattice spacings. The modulation is close to a pure sine-wave just below T _N = 18 K, and it squares up as temperature is lowered. We measured the thermal variations of the first and third harmonics of the moment modulation, and we could determine the first and third harmonics of the exchange coupling. We furthermore show that the antiferromagnetic-incommensurate transition at 12 K is strongly first order, with a hysteresis of 0.05 K, and that the incommensurate-paramagnetic transition at 18 K is weakly first order. Finally, we present an explanation of the spin-flop transition observed in the single crystal magnetisation data in phase III when || in terms of an anisotropic molecular field tensor. Received 17 January 2001 and Received in final form 20 March 2001     

Ferromagnetism in lacunar perovskite manganites Pr 0.7 Sr 0.3 - x x MnO 3 and Pr 0.7 - x x Sr 0.3 MnO 3

Deficiency effects in the A site upon the structural, magnetic and electrical properties in the lacunar perovskite manganite oxides Pr_0.7Sr_{0.3-x x}MnO₃ ( 0 ? x ? 0.3) and Pr_{0.7-x x}Sr_0.3MnO₃ ( 0 ? x ? 0.23) have been investigated. This study focuses on the different parameters which govern the magnetic and electrical properties in such samples. The powder X-ray diffraction patterns for all samples could be indexed either with a rhombohedral perovskite structure and R c space group for x ? 0.2 in strontium deficient samples and for x ? 0.1 for praseodymium deficient ones. For other values of x the samples could be indexed in the orthorhombic structure with Pbnm space group. Magnetic and electrical investigations show that praseodymium and strontium vacancies do not have similar effects on the lacunar compounds. Magnetization measurements versus temperature show that all our samples exhibit a magnetic transition when the temperature decreases. All the praseodymium deficient samples exhibit a paramagnetic-ferromagnetic transition when the temperature decreases while the strontium deficient ones exhibit this transition only for low x values. The magnetic transition temperature shifts to lower values as the strontium deficiency increases (from 265 K for x = 0 to 90 K for x = 0.3) and to higher values with the praseodymium deficiency increase (from 265 K for x = 0 to 315 for x = 0.23). Resistivity measurements as a function of temperature show a semiconducting-metallic transition for all x values in the praseodymium lacunar samples and only for low x values ( 0 ? x ? 0.1) in the strontium lacunar ones when the temperature decreases. Received 12 April 2000 and Received in final form 8 January 2001     

Pressure-induced electron spin transition in the paramagnetic phase of the GdFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> Heisenberg magnet

The HS → LS spin crossover effect (high-spin → low-spin transition) induced by high pressure in the range 45–53 GPa is observed in trivalent Fe³⁺ ions in the paramagnetic phase of a Gd⁵⁷Fe₃(BO₃)₄ gadolinium iron borate crystal. This effect is studied in high-pressure diamond-anvil cells by two experimental methods using synchrotron radiation: nuclear resonant forward scattering (NFS) and Fe K _β high-resolution x-ray emission spectroscopy (XES). The manifestation of the crossover in the paramagnetic phase, which has no order parameter to distinguish between the HS and LS states, correlates with the optical-gap jump and with the insulator-semiconductor transition in the crystal. Based on a theoretical many-electron model, an explanation of this effect at high pressures is proposed.     

Mössbauer, nuclear inelastic scattering and density functional studies on the second metastable state of Na2[Fe(CN)5NO]·2H2O

The structure of the light-induced metastable state SII of Na₂[Fe(CN)₅NO]·2H₂O was investigated by transmission Mössbauer spectroscopy (TMS) in the temperature range between 85 and 135 K, nuclear inelastic scattering (NIS) at 98 K using synchrotron radiation and density functional theory (DFT) calculations. The DFT and TMS results strongly support the view that the NO group in SII takes a side-on molecular orientation and, further, is dynamically displaced from one eclipsed, via a staggered, to a second eclipsed orientation. The population conditions for generating SII are optimal for measurements by TMS, yet they are modest for accumulating NIS spectra. Optimization of population conditions for NIS measurements is discussed and new NIS experiments on SII are proposed.     

Effective hyperfine temperature in frustrated Gd 2Sn 2O 7: two level model and 155Gd Mössbauer measurements

Using ¹⁵⁵Gd M?ssbauer spectroscopy down to 27 mK, we show that, in the geometrically frustrated pyrochlore Gd₂Sn₂O₇, the Gd³⁺ hyperfine levels are populated out of equilibrium. From this, we deduce that the hyperfine field, and the correlated Gd³⁺ moments which produce this field, continue to fluctuate as T ↦ 0. With a model of a spin 1/2 system experiencing a magnetic field which reverses randomly in time, we obtain an analytical expression for the steady state probability distribution of the level populations. This distribution is a simple function of the ratio of the nuclear spin relaxation time to the average electronic spin-flip time. In Gd₂Sn₂O₇, we find the two time scales are of the same order of magnitude. We discuss the mechanism giving rise to the nuclear spin relaxation and the influence of the electronic spin fluctuations on the hyperfine specific heat. The corresponding low temperature measurements in Gd₂Ti₂O₇ are presented and discussed. Received 17 October 2001 Published online 6 June 2002     

Magnetic properties of ErFe6Fa6 studied by magnetization and neutron diffraction

Neutron-diffraction experiments reveal that ErFe₆Ga₆ forms in the tetragonal ThMn₁₂-type of structure (space group I₄/mmm). The Fe sublattice orders ferromagnetically below K. The Er moments order antiparallel to the Fe moments which, below about 250 K, leads to a decrease of the total magnetization. The easy magnetization direction of ErFe₆Ga₆ is perpendicular to the c-axis in the whole temperature range. Refinement at 2 K shows that ErFe₆Ga₆ orders ferrimagnetically with Er moments of 8.5 (2) and Fe moments at the 8(j) site of 1.9 (1) and at the 8(f) site of 1.7 (1) , respectively. At room temperature, ErFe₆Ga₆ exhibits the same type of magnetic order, however with substantially lower Er moments of 1.0 (4) and Fe moments at the 8(f) site of 1.2 (2) . The Fe moments at the 8(j) site amount to 1.9 (5) /Fe. Received 24 November 1999 and Received in final form 27 March 2000     

Time integral and time differential Mössbauer measurements on [57Co/Mn(bipy)3](PF6)2

The Mössbauer 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 Mössbauer 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.     

Impurity scattering in f-wave superconductor UPt 3

We study theoretically the effect of impurity scattering in f-wave (or E_2u) superconductors. The quasi-particle density of states of f-wave superconductor is very similar to the one for d-wave superconductor as in hole-doped high T _c cuprates. Also in spite of anisotropy in Δ( ), both the reduced superfluid density and the reduced electronic thermal conductivity is completely isotropic. Received 11 October 2000     

Molecular rotations studied by nuclear resonant scattering

Nuclear resonant scattering techniques can be used to study both fast and slow dynamics of Mössbauer nuclei. The influence of rotational dynamics in molecular systems is studied applying three types of scattering techniques: (1) Synchrotron radiation perturbed angular correlation (SRPAC) yields direct and quantitative evidence for rotational dynamics in the μs-ns regime. (2) Nuclear inelastic scattering (NIS) monitors the relative influence of intra- and intermolecular forces via the vibrational density of states, which can be influenced by the onset of molecular rotation. (3) In nuclear forward scattering (NFS), information both on rotational and on translational dynamics can be extracted. Results using SRPAC and NIS on a plastic crystal and NFS on ferrocene confined in a molecular sieve are presented.     

Structural features of the La-Sr-Fe-Co-O system

A structural study has been performed on the La_0.8Sr_0.2Fe_xCo_1-xO₃ (x = 0.025 to 0.3) system displaying large magnetoresistance (MR) at room temperature. A detailed analysis of the crystal structure and microstructure was done by X-ray diffraction (XRD), transmission and scanning electron microscopy (TEM and SEM). The atomic resolution TEM images and the appearing superreflections in the corresponding SAED patterns revealed that a superstructure is formed due to the presence of iron. The correlation between the ordered microstructure and the observed large MR ratio is discussed. ⁵⁷Fe M?ssbauer spectroscopy was utilized to gain information on the valence state of iron in the sample with x = 0.3. The lattice parameters of Fe- doped La_0.8Sr_0.2Fe_xCo_1-xO₃ compounds were found to increase monotonously with increasing Fe content. The valence state of iron was found to be Fe³⁺. Received 22 January 2001     

Mössbauer effect studies on the formation of iron oxide phases synthesized via microwave–hydrothermal route

Microwave–hydrothermal (MH) route was employed to synthesize various iron oxide phases in ultra-fine crystalline powders by using ferrous sulphate and sodium hydroxide as starting chemicals. All chemical reactions were carried out under identical MH conditions, namely, at 190°C, 154 psi, 30 min, by varying the molar ratio (MR) of FeSO₄/NaOH in the aqueous solutions. The variation of MR has a dramatic effect on the crystallization behavior of various phases of iron oxides under MH processing conditions. For example, spherical agglomerates of Fe₃O₄ powder were obtained if MR equal to 0.133 (pH?>?10 sample A). On the other hand non-stoichiometric Fe₃O₄ powders (Sample B) were obtained for all higher MR of FeSO₄/NaOH between 0.133 and 4.00 (6.6?2O₃ powders (sample C) were produced. Fe⁵⁷ Mössbauer spectra were recorded for all the three sets of samples at room temperature. In the case of sample B, temperature dependent Mössbauer spectra were recorded in the range of 77–300 K to understand the non-stoichiometric nature of Fe₃O₄ powders. All these results are discussed in the present paper.     

Mössbauer effect study of the temperature and pressure dependence of the singlet-quintet intersystem crossing dynamics in an iron(II) spin crossover complex

The lineshapes of Mössbauer spectra of the iron(II) spin crossover complex [Fe(6-mepy)₃ tren] (PF₆)₂ are affected by the dynamics of the HS?LS equilibrium. The lineshapes are reproduced with a stochastic two-state-relaxation-model yielding rate constants similar to those determined for related complexes in solution. Application of an external pressure of 150 MPa increases the relaxation rate.