Mossbauer Isomer Shift Research Articles

ChemInform Abstract: Oxy‐ and Thio‐Phosphorus Acid Derivatives of Tin(IV). The Crystal and Molecular Structure of O‐Methylmethylphosphatotriphenyltin(IV).

Abstract O-Methylmethylphosphatotriphenyltin(IV) (II) is synthesized by treating stoichiometric amounts of O,O′-dimethylmonothiophosphatotriphenyltin(IV) (I) with mercury. The negative 119Sn chemical shift in solution relative to tetramethyltin for I corresponds to higher than four coordination (at tin) in solution. The values of the 119Sn Mossbauer isomer shift for I and II confirm the presence of tin as tin(IV). The magnitudes of the quadrupole splitting and the ratios of the QS to the IS of both compounds reveal that tin is higher than four coordinated. The solid state structure of II was determined by X-ray diffraction. The compound crystallized in the trigonal space group R 3 with a, b 31.433(15), c 10.259(3) A, V 8778.2 A3, Z = 18, Dx 1.56 g cm−3 at 138 K. In the solid state compound II exists as a highly symmetric cyclic hexamer in which planar trimethyltin(IV) groups are axially bridged by O-P-O linkages. The geometry about tin is trigonal bipyramidal with an axial O-Sn-O angle of 177.1(2)° and average C-Sn-C angle of 120.0(2)°. The structure of the hexameric ring is linear at tin but bent at phosphorus with an O-P-O angle of 117.1(2)°.

M�ssbauer study of mixed-ligand complexes of europium(III)

A series of europium(III) mixed complexes was prepared containing 1,10-phenanthroline or 2,2′-bipyridine as neutral ligand and simple inorganic anions or benzoate and its derivatives as ionic ligands. The Mossbauer spectra of the products indicated that both types of ligands were directly coordinated to the central europium(III) atom. The Mossbauer isomer shift values reflected both electron shifting and steric effects of the substituents on the ligand. The quadrupole splitting values pointed to the similar symmetry of the electron shell of the europium in almost all complexes with analogous composition. The IR spectra provided evidence of the binding mode of the carboxylate groups in the complexes containing benzoate or its derivatives. The x-ray photoelectron spectra of the compounds demonstrated the effects of electrophilic and nucleophilic substituents on the electron binding energies in the europium, oxygen, and nitrogen orbitals.

Study of substituted pentacyanoferrate /II/ complexes by Mössbauer spectroscopy

The57Fe Mossbauer spectra of substituted pentacyanoferrate /II/ complexes [/CN/5 FeII L]3− have been obtained for L=adenine, guanine, purine, caffeine, ethylene sulphide, dimethyl sulphoxide, and ammonia. The Mossbauer parameters are utilized to classify the various ligands according to their σ and π bonding abilities. A linear correlation between the Mossbauer isomer shift and the tetragonal distortion caused by the ligand L is proposed. Comparison between this linear correlation and the approximative method of Wentworth and Piper1 is made.

Systematic trends in the237neptunium Mössbauer isomer shift: Overlap of IV, V and VI neptunium oxidation states and correlation between isomer shift and crystal structure

The influence of the neptunium ion environment on the237Np Mossbauer isomer shifts has been studied in various metal coordination complexes: fluorides, oxides, oxide fluorides and polycarboxylates. A linear dependence between the isomer shift and the mean neptunium-ligand distance in a series of hexavalent Np compounds has been evidenced and the feasibility of overlapped isomer shift areas, namely Np(IV), Np(V) and Np(VI) has been established.

Mössbauer and photochemical study of tetrakis/ o, o′-diethyldithiophosphato/europium/III/complexes

Eu-151 Mossbauer spectra of tetrakis/diethyldithiophosphato/europium/III/ compounds suggests substantial electron transfer from dithiophosphate ligand to europium. The photochemical reaction of these complexes was investigated in organic solutions, and it was demonstrated that the Mossbauer isomer shift was correlated with their 420 nm band.

Investigation of the decay rate for71Ge in chemical compounds and calibration of the Mössbauer isomer shift of the 67 keV γ-transition of73Ge

Changes of the radioactive decay rate of71Ge (T1/2=11.43 days) have been studied experimentally for71Ge in some compounds of bivalent and quadrivalent germanium. Relative changes Δλ/λ of the electron capture probability have been measured, and the chemical changes Δρ(0) of the electron density at the germanium nuclei have been determined from these measurements. Values for the Mossbauer isomer shift calibration constants for the 67 keV γ-transition of73Ge have been estimated:C=δ/Δρ(0)=(0.061±0.020) mm·s−1/a.u., Δ〈r2〉=(21±7)·10−3 fm2, ΔR/R=(6.9±2.3)·10−4.

Isomer shift calibration of61Ni by lifetime variation measurements in the electron capture decay of57Ni

In the present work, differential ionization chambers are used in order to measure the difference in the electron capture decay rate of57Ni nuclei in several pairs of different chemical compounds. Combination of these values with the corresponding Mossbauer isomer shifts provides a reliable experimental method to determine differences in electron densities at the nucleus. This allows to calibrate the isomer shift scale of the concerned nuclide. By using61Ni isomer shifts given in the literature, this work leads to the first experimental determination of the61Ni isomer shift calibration constant α=−(1.8±0.9)·10−3.a03 mm.s−1. This corresponds to a change in the mean-square charge radius Δ(r2)=−(7.2±3.4)·10−4 fm2 during the 67.4 keV Mossbauer transition of61Ni.

The Mossbauer effect and electron configuration for tin metal

The electron contact densities for the various electron configurations of tin and its oxidation states have been computed using the relativistic multi-configuration Dirac-Fock program of Desclaux (1975). The interactions between the configurational states obtained using jj-coupling and the seniority scheme have been investigated using Grant's program (1976) MCP75. All the off-diagonal matrix elements of the inter-electronic Coulomb interaction between the configurational states are found to be vanishingly small. The dependence of the computed electron contact density on the valence shell occupation numbers has been investigated and a phenomenological formula obtained. Using the formula and the measured Mossbauer isomer shift, the electron configuration of beta -tin has been found to be 5s1.785p2.22 in good agreement with the literature value.

Europium valency in EuCu2−x Fe x Si2, EuCu2Si2−x Ge x , EuCu2−x Si2+x and EuNi2−x Si2+x

Mossbauer spectroscopy, X-ray photoemission spectroscopy and magnetization studies of EuCu2−x Fe x Si2, EuCu2Si2−x Ge x , EuCu2−x Si2+x , and EuNi2−x Si2+x have been performed. In EuFe2Si2 and EuNi2Si2 the Eu ion is trivalent, in EuCu2Si2 it is of intermediate valency. In all systems withx=0, many inequivalent Eu sites of intermediate valency are formed. In EuCu2−x Fe x Si2, the average valency of Eu moves nonmonotonically toward Eu3+. In EuCu2Si2−x Ge x , EuCu2−x Si2+x and EuNi2−x Si2+x , the Eu average valency moves quickly towards Eu2+. In all three systems, whenx=1, all Eu ions are already stable divalent. The systems EuCuSi3 and EuNiSi3 order magnetically at 40 K and 34 K, and exhibit hyperfine fields of 323 kOe and 458 kOe, respectively. XPS and Mossbauer isomer shift determinations of the average Eu valence as a function of temperature in EuCu1.5Fe0.5Si2 are in good agreement. The experimental observations concerning the Eu valencies can not be explained solely in terms of the local volume available to the Eu ion; the nature of chemical environment plays a dominant role.

The isomer shift of elemental iodine under high pressure

The Mossbauer isomer shift (IS) in elemental 129I was measured as a function of pressure to 30 GPa unsing diamond anvil cells. The IS increases gradually with pressure reaching a value of ΔIS = 0.56(3) mm sec −1 at P > 16 GPa. No discontinuity was observed at P = 21 GPa where a crystallographic phase transition has been reported. Using the experimental V(P) relation, the IS dependence on − ℓnV was found to be linear to the highest pressure. The dϱ(0) dℓnV value for elemental I 2 was found to be -6.6(2) a −3 0. The volume dependence of the p-holes is deduced resulting in a 5s 25p 4.62 configuration at the highest pressure. The systematics of dϱ(0) dℓnV for the 5sp elements from βSn to Xe are discussed.

Relation of contact densities with Mössbauer isomer shifts for 35.46 keV M1 transition of125Te

The internal conversion electron and Mossbauer isomer shifts associated with the 35.46 keV M1 transition of125Te were observed for different metal samples into which radioactive125I as probe atoms were introduced by means of ion-implantation. From the correlation between the Mossbauer isomer shifts and intensity ratios of O shell to NI shell conversion electrons, a relation between 5s-electron contact densityp5s(0) in a.u. and isomer shift δ in mm/s was deduced to bep5s(0)=106+30.3 δ±4.3[(δ−0.30)2+0.069]1/2, where δ was measured relative to ZnTe in mm/s. The change of the nuclear charge radius in the 35.46 keV M1 transition of125Te was found to be ΔR/R=(0.85±0.12)×10−4 (corresponding to Δ =(3.7±0.5)×10−3 fm2) when a theoretical 4s-electron contact density of 928 a.u. was used.

Modelling of metal clusters as fragments of crystalline solids

The structural and electronic properties of pure and hydrogenated Li metal have been calculated by modelling the crystalline solid in terms of small clusters of constituent atoms. Using the self-consistent molecular orbital method in the Hartree-Fock approximation, the authors have investigated the minimum number of atoms necessary to distinguish among various crystalline phases. They find that with as few as three atoms in the molecular cluster, the BCC phase can be shown to be the most stable structure. They have calculated the dependence of the optimised lattice parameter as a function of cluster size for both FCC and BCC phases of Li. While the calculated lattice constants converge to the bulk value with only about 20 atoms in the cluster, the binding energy per atom shows no sign of saturation. Thus how well the properties of molecular clusters represent bulk characteristics depends on the properties being investigated. This aspect is further illustrated by calculating the equilibrium site of hydrogen in Li. It is shown that for small clusters consisting of less than 20 atoms, the preferential site of hydrogen depends not only on the cluster size, but also on the way the cluster is constructed. The distortion of the nearest neighbour atoms around the hydrogen atom is studied by minimising the total energy with respect to the nearest-neighbour Li-hydrogen distance. Contrary to the conventional wisdom, they find that the metal atoms surrounding hydrogen are not always displaced outward. For example, the Li atoms around an octahedral hydrogen atom relax outward, while those around a tetrahedral hydrogen atom relax inward. In both cases, the equilibrium distance between hydrogen and nearest Li atom is 3.48 au. This is comparable with the corresponding distance of 3.6 au in stoichiometric lithium hydride. They have also calculated the charge transfer between Li and hydrogen atoms and the population of various orbital states for all the Li atoms. The results are used to analyse the screening of impurities in the metallic environment and to shed light on Mossbauer isomer shifts in metal-hydrogen systems.

Extracting structural information from mössbauer isomer shift data in amorphous iron-boron alloys

The semi-empirical model of Miedema and Van der Woude is used to extract information from Mossbauer isomer shift data about the packing in amorphous FexB1-x-alloys (10at%<x<90at%). At the iron-rich side the samples have an open structure, which changes to a strained one at the boron-rich side. The predicted increased Fe-Fe distance in a-Fe50B50 was verified by energy-dispersive X-ray diffraction.

Contact density and number of 5s electrons in?-Sn

From a relation between the 5s (valence) electron contact density and the Mossbauer isomer shift, the contact density and the number of 5s electrons of Sn in \-Sn were evaluated to be 72.3±4.1 a.u. and 1.87±0.13, respectively, where the renormalization of solid-state effect was considered.

The hybridization contribution to the Mössbauer isomer shift in intermetallic compounds containing 151Eu

Abstract The Mossbauer isomer shift in intermetallic compounds analysed with the cellular model of Miedema and Van der Woude is composed of three contributions. One of these contributions (R′) is the hybridization of the s and p electrons of a non-transition metal with the d electrons of the Mossbauer atom. In Gd-based compounds R′ is regarded as a constant. In a previous work, on the other hand, we found in Eu-based compounds a strong correlation between R′ and the number of outer p electrons of the non-transition metal. We will show in this study that this correlation is artificial and can be attributed to a volume contraction of the Eu atoms.

Valence state of Eu in EuPd2P2

Abstract EuPd2P2 has been classified as mixed-valent from its lattice volume anomaly, while our Mossbauer isomer-shifts and magnetic measurements suggest a divalent-4f7 state. The Eu-LIII-edge X-ray absorption spectrum, however, exhibits a double-peaked structure typical for mixed-valent materials. Various reasons for this apparent discrepancy are discussed including the possible existence of a partly extended 4f radius.

Determination of the change of nuclear charge radius of the119Sn Mössbauer transition by internal conversion

Radioactive119Sb was implanted into six host matrices (CaSnO3, Pt, Y, Au,Β-Sn, Pb) and internal conversion electrons of the 23.87 keV transition in119Sn were measured with an iron-free magnetic spectrometer as well as Mossbauer spectra. In the analysis of the conversion spectra of outermost electrons, the overlapping K-LM Auger lines were subtracted using the Auger spectrum of tin measured with another source of117mSn, and the shake-off effect accompanying the conversion process was considered. From the correlation between the Mossbauer isomer shifts and the intensity ratios of O-shell to N1-shell conversion electrons, the change of the nuclear charge radius of the 23.87 keV transition of119Sn was deduced to be δR/R=(0.87 ± 0.25) × 10−4 for a uniform charge distribution ofR= 1.2 ×A1/3 fm or, equivalently, δ 〈r2>—=(3.6 ± 1.0) ×10−3 fm2.

Correlation of isomer shift and quadrupole splitting and the sign of EFG

Mossbauer isomer shift /δ/ and quadrupole splitting /ΔEQ/ values in a series of isostructural compounds correlate well to give linear relationship provided the sign of electric field gradient /EFG/ is assigned to the ΔEQ. The correlation has been used in predicting the sign of EFG for some monosubstituted pentacyanoferrate/II/ complexes. Usefulness of such correlations and its limitations are presented.

Valence state of Eu and unit-cell volume anomaly in EuPd2P2

Abstract Mossbauer isomer shift and magnetic susceptibility measurements clearly prove the divalent nature of Eu in EuPd 2 P 2 , a new compound recentlu classified as intermediate valent on grounds of its unit-cell volume anomaly. We explain the relatively small positive deviation in unit-cell volume from the systematic trend in trivalent REPd 2 P 2 compounds as a consequence of strong chemical bonding between the divalent Eu ion and the ligands.

A simple potential model for estimates of exchange effects in electron capture decay

In a central field description the use of hydrogen wavefunctions for computation of exchange and overlap integrals can be justified as long as relative potential changes are small. The simple procedure leads to linear and quadratic forms, respectively, in the relative potential changes Delta Z/Z from the initial to the final state of the orbitals. Bahcall factors Bn are derived which only contain potential ratios of the nth orbital to the 1s orbital in first order and the relative potential alteration of the nth orbital during the electron capture (EC) process. Their contribution to the variation of the EC decay constant lambda epsilon leads to mod ( Delta lambda epsilon / lambda epsilon )Delta B mod approximately=0.2/Z2 for a single charge ionisation of an element (or ion) of atomic number Z. As far as it is justified to model chemical variations in a single-centre consideration by statistical occupation number changes in the valence shell, this allows an estimate of possible disturbances in the interpretation of lambda epsilon variations in terms of electron density variations near the nucleus and thus of contributions to Mossbauer isomer shift calibration from this method.