Quadrupole Splitting Distribution Research Articles

A study of defect structures in Fe-alloyed ZnO: Morphology, magnetism, and hyperfine interactions

In order to study the effect of Fe cation substitution on the local structure, defect formation, and hyperfine interactions in ZnO, Mössbauer spectroscopy measurements of the microwave processed Zn1−xFexO (x=0.05, 0.10, 0.15, and 0.20) nanoparticles, together with ab initio calculations, were performed. Complementary information on the distribution of particle size and morphology, as well as magnetic properties, were obtained by X-ray diffraction, transmission electron microscopy, and squid-magnetometry. The selected model for analyzing the Mössbauer spectra of our samples is a distribution of quadrupole splittings. The fitting model with two Lorentz doublets was rejected due to its failure to include larger doublets. The Fe3+ ions do not yield magnetic ordering in the samples at room temperature. The results from first-principles calculations confirm that the major component of the Mössbauer spectra corresponds to the Fe-alloyed ZnO with Zn vacancy in the next nearest neighbor environment. The magnetic measurements are consistent with the description of the distribution of iron ions over the randomly formed clusters in the ZnO host lattice. While at room temperature all the samples are paramagnetic, magnetic interactions cause a transition into a cluster spin-glass state at low temperatures.

Phase separation in Ca1−xLaxFe2As2 superconductors: a 57Fe Mössbauer study

We report a detailed 57Fe Mössbauer study of lanthanum doped CaFe2As2 superconductors. The quadrupole splitting distribution (QSD) method was adopted to analyze the Mössbauer spectra of Ca1−xLaxFe2As2 (x = 0.2, 0.3) single crystals. For both compounds we observed two QSD contributions centered at 0.31 mm s−1 and −0.32 mm s−1 at room temperature. The first principles calculations of the electronic structures and the electric field gradient (EFG) of Ca1−yLayFe2As2 model systems reveal that the EFG changes from positive to negative with increasing dopant concentration, indicating that the La atoms distribute heterogeneously in the compounds. The two QSD components behave differently with decreasing temperature. The minority La-rich phase undergoes superconducting transition, while short range spin fluctuations and/or spin-phonon coupling appear in the majority La-poor phase. Our experiments provide new evidence of the phase separation picture at low temperatures in Ca1−xLaxFe2As2 superconductors.

The Mössbauer study of atmospheric iron-containing aerosol in the coarse and PM2.5 fractions measured in rural site

The size-resolved iron-containing atmospheric aerosol was measured by Mössbauer spectroscopy. The particulate matter (PM) was size-segregated into aerosol <2.5μm in diameter (fine fraction, PM2.5) and aerosol >2.5μm (coarse fraction).The Mössbauer spectra at room temperature show components assigned to mono- and polymeric species which are products related to hydrolysis of ferric solutions in the atmosphere. In order to extract reliable information regarding the quadrupole splittings necessary for identification of iron-containing compounds, the histogram method was used for evaluation of the Mössbauer spectra. The value and distribution of quadrupole splitting (QS) parameters obtained by this method suggests that the ultra fine particles (<10nm) identified as a ferrihydrite, hematite and goethite were observed in the spectra both in fine and coarse fraction of the collected aerosol. However, the content of the individual chemical components was different for each fraction. The least crystalline form of ferrihydrite (QS≅0.85mm) was observed in PM2.5 fraction more frequently than in the coarse fraction. The content of the ultra fine hematite particles which presence was confirmed in low temperature measurements was also larger in PM2.5 fraction than in the coarse fraction. Superparamagnetic behavior of iron-containing atmospheric aerosol was observed in low temperature Mössbauer spectra.

Tailoring the Composition of a Mixed Anion Iron-Based Fluoride Compound: Evidence for Anionic Vacancy and Electrochemical Performance in Lithium Cells

Microwave-assisted synthesis allows stabilizing Fe-based fluoride compounds with hexagonal tungsten bronze (HTB) network. The determination of the chemical composition, i.e., FeF2.2(OH)0.8·(H2O)0.33, revealed a significant deviation from the pure fluoride composition, with a high content of OH groups substituting fluoride ions. Rietveld refinement of the X-ray diffraction data and Mössbauer spectroscopy showed that the partial OH/F substitution impact on the structure (interatomic distances, angles, and so on) and the local environment of iron (isomer shift and quadrupole splitting distribution). The thermal behavior of the hydroxyfluoride compound has been thoroughly investigated. From room temperature to 350 °C under Ar flow, the HTB-type structure remains stable without any fluorine loss and only water departure. At T > 350 °C, the structure started to collapse with a partitioning of anions leading to α-FeF3 and α-Fe2O3. Within 200 °C ≤ T ≤ 350 °C, the chemical composition can be tuned with different contents of OH–/O2– and structural water. By an adequate thermal treatment, it has been shown that anionic vacancies formed by dehydroxylation reaction could be stabilized within the HTB network yielding a compound containing three different anions, i.e., FeF2.2(OH)0.8–xOx/2□x/2. XRD Rietveld analysis, atomic pair distribution function, and Mössbauer spectroscopy confirmed the formation of under-coordinated iron FeX5□1 (X = O2–, F–, and OH–) atoms. Different compositions have been prepared by thermal treatment at T ≤ 350 °C and their electrochemical properties evaluated in lithium cell. Structural water seems to block the diffusion of lithium within the hexagonal cavities. Increasing the content of anionic vacancies significantly improves the reversible capacity emphasizing a peculiar role on electrochemical properties. Pair distribution functions obtained on lithiated and delithiated samples indicated that the HTB network was maintained (in the 2–4.2 V voltage range) during the intercalation processes.

Octahedral cation distribution in glauconites from Southern Urals by combination of crystal-chemical model and quasi-continuous model-independent quadrupole splitting distributions (QSD) fitted to their Mössbauer spectra

Mossbauer spectra of two glauconites, BSH-11 and KUL-1, from the Southern Urals are interpreted by simulation using crystal-chemical modelling and by fitting of quadrupole splitting distribution (QSD). The crystal-chemical modelling is based on X-ray diffraction (XRD) data and includes, as a priori information, individual quadrupole splittings for Fe 3+ and for Fe 2+ that correspond to specific local cation arrangements (LCA) having certain occurrence probabilities. The latter are provided by two-dimensional octahedral cation distribution using integrated infrared optical densities for bands corresponding to the cation pairs bound to the OH groups. Combination of crystal-chemical modelling and quasi-continuous model-independent QSDs fitted to Mossbauer spectra resulted in cation distributions for which R 2+ (Mg, Fe 2+ ) and R 3+ (Al, Fe 3+ ) cations occupy one of two structurally independent cis- sites with equal probabilities. The cation distributions obtained are characterized by Fe-clustering, domain structure, and different trends to homogeneous charge dispersion.

Local structure in C2/c clinopyroxenes on the hedenbergite (CaFeSi2O6)-ferrosilite (Fe2Si2O6) join: A new interpretation for the Mossbauer spectra of Ca-rich C2/c clinopyroxenes and implications for pyroxene exsolution

Three synthetic Ca-rich clinopyroxenes on the hedenbergite-ferrosilite join, with compositions (Hd 50 Fs 50 ), (Hd 60 Fs 40 ), and (Hd 70 Fs 30 ), were investigated by single-crystal X-ray diffraction and Mossbauer spectroscopy at room temperature. For all samples, there is no evidence for the presence of phases with symmetry other than C 2/ c . Structure refinement, using a split-atom model, shows the existence of two different local configurations in the M2 cavity, a hedenbergite-like arrangement and a C 2/ c clinoferrosilite-like arrangement that are centered by Ca and Fe 2+ , respectively. The Mossbauer spectra were analyzed by a quadrupole splitting distribution (QSD) method, and the hyperfine parameters of the QSD fit indicate the presence of Fe 2+ in three local environments that are similar to those of Fe 2+ at the M1 site in hedenbergite, and the M1 and M2 sites in C 2/ c clinoferrosilite, in agreement with the structure-refinement results. Our results are consistent with the calculated phase diagram for hedenbergite-ferrosilite, where a miscibility gap and a two-phase field of metastable hedenbergite and metastable pigeonite occur at low temperature. The coexistence of a hedenbergite-like configuration around Ca and a C 2/ c clinoferrosilite-like configuration around Fe can be considered as a precursor for pyroxene exsolution at low temperature. Our results support previous structure-refinement work on Ca-rich clinopyroxenes along the diopside-enstatite join, and provide a new insight in the interpretation of Mossbauer spectra of Ca-rich C 2/ c clinopyroxenes.

The turquoise-chalcosiderite Cu(Al,Fe3+)6(PO4)4(OH)8{middle dot}4H2O solid-solution series: A Mossbauer spectroscopy, XRD, EMPA, and FTIR study

Eight turquoise samples covering a wide range of compositions in the turquoise-chalcosiderite solid-solution series were analyzed by Mossbauer spectroscopy, X-ray diffraction (XRD), electron microprobe analysis (EMPA), and Fourier transform infrared (FTIR) spectroscopy. Two of the turquoise samples display evidence of alteration from weathering processes. The unit formulas were calculated on the basis of 24 (O,OH) anions and 11 cations using the results of EMPA, assuming all Fe as Fe3+, as confirmed by Mossbauer spectroscopy. The altered turquoise samples show deficiencies in both cations and anion groups, indicated by EMPA, but they preserve the crystal structure of turquoise, as verified by XRD. They also show large amounts of Si and Ca in their microprobe data, due to the presence of kaolinite and Ca carbonate, respectively, which are identified by FTIR spectroscopy. The isomorphous substitution of Fe3+ for Al in the turquoise structure broadens and shifts the IR bands to lower frequencies, in particular the OH-stretching bands. The Mossbauer spectra, collected at room temperature, are fitted with two generalized Fe3+ sites, using a Voigt-based quadrupole-splitting distribution method, which are assigned to the M3 (smaller quadrupole splitting) on the one hand and M1 and M2 octahedral sites on the other hand. The Fe3+ distribution over the M3 and M1,2 sites, calculated from the Mossbauer relative areas and EMPA, indicates that Fe3+ prefers the larger M3 octahedron in the turquoise-chalcosiderite solid-solution series.

Effects of particle size on the structural and hyperfine properties of tin dioxide nanoparticles

In this work, we report on the study of SnO2 nanoparticles prepared by a polymer precursor method. X-ray diffraction (XRD) data analysis evidenced the formation of only the tetragonal rutile-type phase for the as-grown and thermally annealed samples. A mean grain size of about 11 nm for the as-prepared sample has been determined. This mean size increases after the thermal annealing and with the annealing temperature. The room temperature Mossbauer spectra (MS) were well fitted using a quadrupole splitting (QS) distribution. The isomer shift (IS) tends to increase when the grain size decreases. That increase has been associated to the extra s-electron density generated by the oxygen vacancies.

Recrystallization in fully metamict gadolinite from Ytterby (Sweden), annealed in air and studied by 57Fe Mossbauer spectroscopy

57 Fe Mossbauer annealing studies of fully metamict gadolinite from Ytterby (Sweden) have been carried out in air from 373 to 1473 K. The Mossbauer spectrum of the untreated sample, dated at 1795 Ma and absorbed α-dose 1.07 × 10 16 α-decay/mg, is characterized by two broad peaks solely from Fe 2+ components in octahedral coordinations. The corresponding quadrupole splitting distribution (QSD) shows two distinct maxima at 1.59 and 2.16 mm/s, which reflect the post-metamictization distributions of Fe 2+ positions up to heating at 773 K. Changes in the Mossbauer hyperfine parameters are observed only after heating to 873 K, at which point the first Fe 3+ component from the oxidation of Fe 2+ appears. The relative contribution of Fe 3+ /∑Fe reaches a maximum of 0.54 at 1073 K and then decreases with increasing temperature. A new Fe 3+ doublet is observed in the samples heated from 1173 to 1473 K. This doublet is characterized by extremely high quadrupole splitting ranging from 2.51 to 2.66 mm/s, implying extreme distortion of the coordination octahedra in partially and fully crystalline gadolinite. The transition from metamict to crystalline structure begins at 1133 K where the Mossbauer spectrum is represented by one Fe 2+ doublet and one Fe 3+ doublet. Both doublets suggest a uniform transitional octahedral site over the whole structure. With increasing annealing temperature, the line widths of the Fe 2+ doublet decrease, and, at high temperatures, an asymmetry of the absorption peaks is observed. The sample becomes completely recrystallized after annealing at 1373 K, and at this point the Mossbauer spectra are nearly exact superpositions of the pure Fe 2+ and Fe 3+ (present in minor proportion) in synthetic gadolinites.

An improved model for the interpretation of Mossbauer spectra of dioctahedral 2:1 trans-vacant Fe-rich micas: refinement of parameters

Two Upper Riphean glauconite samples of the Southern Urals with unusually low Fe3+/Fe2+ ratios were studied by 57Fe Mossbauer and IR spectroscopy. Room and liquid-nitrogen temperature Mossbauer spectra were analyzed with quasi-continuous model-independent quadrupole splitting distributions (QSDs). The obtained QSDs were interpreted using a crystal-chemical model taking account of the effects of the local structural and chemical heterogeneity of layer silicates. The main parameters of the crystal-chemical model are individual quadrupole splittings, Δ i pred and Δ i tent, for Fe3+ and Fe2+, respectively, in different local cationic arrangements (LCAs) with their occurrence probabilities, w i , provided by two-dimensional simulation of the cation distribution (CD). Using the parameters of the crystal-chemical model, QSDsim profiles were simulated and compared with the experimental QSDs. To obtain agreement between QSDexp and QSDsim profile, w i and Δ i values were adjusted using the method of successive approximations. As a result, the tentative Δ i tent values for Fe2+ contributions to Mossbauer spectra were refined. The sequence of refined Δ i ref values corresponding to specific LCA types remained the same as that for the Δ i tent values. For the studied glauconites, the CD reconstructions satisfying the experimental QSDs are characterized by similar local distribution of cations around Fe3+ and by different local cation distribution around Fe2+.

Quasicrystalline phase formation in the mechanically alloyed Al70Cu20Fe10

In the present work, the formation of the Al70Cu20Fe10 icosahedral phase by mechanical alloying the elemental powders in a high-energy planetary mill was investigated by X-ray diffraction and Mossbauer spectroscopy. It was verified that the sample milled for 80 h produces an icosahedral phase besides Al(Cu, Fe) solid solution (β-phase) and Al2Cu intermetallic phase. The Mossbauer spectrum for this sample was fitted with a distribution of quadrupole splitting, a doublet and a sextet, revealing the presence of the icosahedral phase, β-phase and α-Fe, respectively. This compound is not a good hydrogen storage. The results of the X-ray diffraction and Mossbauer spectroscopy of the sample milled for 40 h and annealed at 623°C for 16 h shows essentially single i-phase and tetragonal Al7Cu2 Fe phase.

Mössbauer study of tektites

Room temperature 57Fe Mossbauer spectroscopy has been used to investigate the structural and oxidation state of Fe in tektites from different strewn fields. Spectra have been analyzed in terms of two quadrupole splitting distributions corresponding to Fe3 + and Fe2 + . All tektites show similar distribution of quadrupole splitting. Each distribution has one peak. The Fe2 + sites show a narrow region of Mossbauer line shift (δ) and quadrupole splitting (e), δ = 1.02–1.10 mm/s and e = 0.85–1.00 mm/s relative to α-Fe. These values have been assigned to intermediate coordination between tetrahedral and octahedral. The Fe3 + sites show wider regions of hyperfine parameters: δ = 0.25–0.45 mm/s and e = 0.65–0.90 mm/s. The Fe3 + /Fe2 + ratio was found to be 0.05–0.15.

Mössbauer spectroscopy of phyllosilicates: effects of fitting models on recoil-free fractions and redox ratios

AbstractClay minerals are ubiquitous constituents in soils on Earth, are occasionally found in meteorites, and may also occur on planetary surfaces in the presence of water. However, little is known about the fundamental Mössbauer parameters (the intrinsic isomer shift, δI, the characteristic Mössbauer temperature, θM, and the recoil-free fraction, f) that are characteristic of clay minerals and critical to the correct interpretation of the Fe3+/ΣFe ratios as well as the mineral modes. Spectra of well characterized single mineral samples at multiple temperatures may be used for the determinations of f. Hence, measurements of five-layer silicates with a range of layer types are presented here: nontronite, Fe-smectite, glauconite, annite and biotite. The spectra were fitted using three different software packages: WMOSS from Science, Engineering &amp; Education Co. in Minnesota; Recoil, from the University of Ottawa in Canada; and two programs used at the University of Ghent in Belgium. Four different approaches to modelling line shapes were used: (1) Lorentzian; (2) pseudo-Voigt (convolution of Lorentzian and Gaussian curves); (3) quadrupole-splitting distributions (QSD); and (4) a technique that does not assume a particular line shape (subsequently referred to as ‘model-independent’). Values of δI, θM and f were determined using the method of De Grave &amp; Van Alboom (1991).Results show that multiple doublets are routinely required by all models to represent Fe-site occupancy, even when all the Fe atoms of the same valence are in the same site, as is the case for dioctahedral smectite, nontronite, mica and glauconite. Consistent values of centre shift (δ) and quadrupole splitting (Δ) were obtained for the two distributions of M2Fe3+ in the smectites. In glauconite, a single Fe2+ doublet was clearly resolved and gave systematic values for δ, Δ and area, but the two Fe3+ doublets were less defined. In annite, two Fe2+ and two Fe3+ doublets were modelled, while three Fe2+ and one Fe3+ doublet were used for biotite. Three different programs that use Lorentzian line shapes gave very similar results for δ, Δ and area. The two different implementations of QSD line shapes gave similar but sometimes slightly different results, and the pseudo-Voigt and model-independent fits usually fell between the ranges for Lorentzian and QSD results.The value of δI is ~0.58 mm/s for Fe3+ and ~1.31 mm/s for Fe2+ across all models and line shapes, which is expected because the Fe3+ has an additional shielding 3d electron. Values for θM data are nearly identical for Fe3+ in nontronite and Fe-smectite (~450 K), somewhat varied for Fe3+ in glauconite and biotite (θM = ~730 K and ~615 K, respectively), and relatively distinct for Fe2+ (~350 K). Some values for θM and f could not be determined due to the non-monotonic behaviour of the fitted values for δ as a function of temperature. Values of f295 were 0.821–0.917 for Fe3+ and 0.662–0.743 for Fe2+, consistent with previous studies of the recoil-free fraction in micas and other silicates. Calculated scatter in δ, Δ, area and f values as a function of different line shapes and computer software was significantly reduced at lower temperatures. Sources of error in each of the calculated parameters are discussed.

Mössbauer and IR analysis of iron sites in four ferruginous smectites

Abstract57Fe Mössbauer spectra of the Na+- and Ca2+-saturated forms of four ferruginous smectites,displaying a range of compositions, have been fitted with Voigtian profiles to represent (principally) two correlated distributions of isomer shifts and quadrupole splittings for the Fe3+ sites. Correlating the areas and hyperfine parameters of these distributions with simulations of the octahedral array of cations based on chemical analyses and infrared spectroscopy has enabled identification of the Fe3+ ions that contribute to the outer quadrupole doublet. The octahedral Fe3+ sites are characterized as having at least one Mg2+ nearest-cation neighbour and probably a cisconfiguration with respect to the OH– ions.

Mössbauer spectroscopy characterization of Zr–Nb–Fe phases

The aim of this work was the characterization of the ternary phases and of those coming from the corresponding binary systems in the Zr–Nb–Fe diagram by means of Mössbauer spectroscopy. This is part of a complete study involving a tentative isothermal section at 900 °C of the center of the Zr–Nb–Fe diagram which will be published elsewhere. Zr–Nb–Fe alloys with Nb contents between 5 and 50 at% and Fe contents between 10 and 60 at% were analyzed after a heat treatment at 900 °C for 4 month. Mössbauer characterization of the phases was complemented by optical and scanning electron microscopies, X-ray diffraction and electron microprobe analysis. From the obtained results it can be inferred that Fe in both of the Laves phases present in this system (Zr(FeNb) 2 and (ZrNb)Fe 2) sees different environments, producing quadrupole splitting and hyperfine field distributions, respectively. Two types of body centered cubic β phases (Zr-rich and Nb-rich) were found having noticeable differences in their Mössbauer parameters. Furthermore it was shown that the ternary Fe(NbZr) 2 compound would show magnetic character at low temperatures. Concentration dependence of the hyperfine parameters and their relations with the lattice parameters contributed to the structural characterization of the phases.

Quadrupole Splitting Distributions in Purpurite and Related Minerals

The quadrupole splitting distributions (QSDs) from the Mossbauer spectra of triphylite, ferrisicklerite and purpurite at 298 K and 80 K were obtained by the use of the Voigt-based quadrupole splitting distribution (QSD) method for the first time. QSDs of Fe2+ and Fe3+ are attributed to Fe2+ and Fe3+ at the corresponding octahedrally coordinated sites in the crystal structures of the three phosphate minerals. The influence on the distortion of the M2 site by different next-nearest neighbor (NNN) configurations was discussed based on the Jahn–Teller effect in purpurite, and the authors propose two M2 subsites with different distortions in purpurite. Two QSDs of Fe3+ in the Mossbauer spectra of purpurite are tentatively assigned to Fe3+ at the two M2 subsites, and next-nearest neighbor (NNN) effects were used to interpret the Mossbauer spectra of purpurite.

X-ray powder diffraction and 57Fe – Mössbauer spectroscopy of synthetic trioctahedral micas {K}[Me3] $\langle$ TSi3 $\rangle$ O10(OH)2, Me = Ni2+, Mg2+, Co2+, Fe2+; T = Al3+, Fe3+

Trioctahedral 1 M micas have been synthesized along (pseudo)binary joins using hydrothermal techniques and controlled oxygen fugacities. Octahedrally coordinated iron in annite {K}[Fe3]\(\langle\)AlSi3\(\rangle\)O10(OH)2 was successively replaced by Co2+, Mg2+ and Ni2+ and tetrahedrally coordinated aluminum by Fe3+. Unit cell parameters decrease almost linearly with decreasing average radius of the octahedral cation/average M–O bond length within the octahedral sheet. With increasing substitution of Fe2+ the octahedral sheet becomes more flattened, the ditrigonal distortion of the tetrahedral sheet increases up to a maximum value of ≈10° for micas with tetrahedral sheet compositions close to \(\langle\)AlSi3\(\rangle\) and up to ≈14° for those containing a \(\langle\)FeSi3\(\rangle\) tetrahedral sheet. All iron-bearing samples were studied by 57Fe Mossbauer spectroscopy. With increasing substitution of iron by smaller divalent cations the quadrupole splitting distribution (QSD) evolves from a broad bimodal distribution in annite to a smaller unimodal distribution in Mg2+ and Ni2+-rich samples so that for high substitution rates more regular local environments are dominating. These results, however, can not be interpreted in terms of an octahedral cation ordering scheme. For none of the micas investigated reliable Fe2+ M2/M1 area ratios can be extracted. fMoreover, the complete QSD is shifted towards higher quadrupole splitting values. Similar observations were obtained for substituting Fe2+ by Mg2+ and Ni2+ in tetra-ferri-annite free of octahedral coordinated trivalent cations. Unlike in the Al3+ bearing micas a third QSD component is missing which supports the claim that the appearance of this third QSD component is closely related to the presence of trivalent cations (Al3+, Fe3+) in octahedra coordination.

Reason for line broadening in emission Mössbauer spectra

Resonant lines observed in source experiments are usually significantly broader thanthosein the corresponding absorber experiments. In order to explain this phenomenon, the model of a trapped electron has been proposed. Auger electrons emitted, for instance, after electron capture by 57Со or after the converted isomeric transition of119mSn, as well as secondary electrons, may be trapped in the vicinity of the nucleogenic ion. An asymmetric distribution of quadrupole splittings in the resulting emission Mossbauer spectra is thus a consequence of electron capture by lattice traps located at different distances from the daughter ion. Estimates of quadrupole splitting values, which may be causedby trapped electrons located at different distances from the nucleogenic atom, support the above mentioned model.

The Features of Mössbauer Spectra of Hemoglobins: Approximation by Superposition of Quadrupole Doublets or by Quadrupole Splitting Distribution?

Mossbauer spectra of hemoglobins have some features in the range of liquid nitrogen temperature: a non-Lorentzian asymmetric line shape for oxyhemoglobins and symmetric Lorentzian line shape for deoxyhemoglobins. A comparison of the approximation of the hemoglobin Mossbauer spectra by a superposition of two quadrupole doublets and by a distribution of the quadrupole splitting demonstrates that a superposition of two quadrupole doublets is more reliable and may reflect the non-equivalent iron electronic structure and the stereochemistry in the α- and β-subunits of hemoglobin tetramers.

Hyperfine electric field gradient tensors at Fe2+ sites in octahedral layers: Toward understanding oriented single-crystal Mossbauer spectroscopy measurements of micas

In this first systematic, theoretical study of the complete electric field gradient (EFG) tensor of 5 7 Fe Mossbauer spectroscopy as a function of chemistry and local structural distortion using electronic structure calculations, local EFG tensors were calculated at the central Fe 2 + sites in clusters of seven octahedra, where the neighbor octahedra were populated with a mixture of Mg 2 + and Al 3 + cations. The independent parameters of the EFG tensor-particularly the principal value V z z , the asymmetry parameter η, and the direction of the principal axis with regards to the octahedral sheet-were examined in detail as functions of octahedral flattening for each of the thirteen possible configurations of cations. We demonstrate that the argument that the EFG tensor at a particular site is largely determined by the point group symmetry of the corresponding crystallographic site is not correct. We find that in clusters with monoclinic or triclinic local point group symmetry, the value of V z z changes discontinuously from positive to negative as flattening increases. The principal axis changes from being in the plane of the octahedral sheet to being normal to it at this discontinuity, and the value of η reaches a maximum and begins to decline. This discontinuous behavior is caused by the continuous change of the EFG eigenvalues as the Fe(3d) character of the highest occupied spin-down orbital evolves with flattening. Taking EFG tensor results from all clusters, and using probabilities for the occurrence of each possible cation configuration in a chemically disordered octahedral sheet of a given bulk composition, we simulate averages and distributions of η, principal axis angles, and quadrupole splittings. While the value of η is broadly distributed from zero to one at all flattening angles and bulk compositions, there are two distinct populations of principal axes, normal to and in-plane with the octahedral sheet, as has been observed experimentally. We find these in-plane and normal populations of principal axes correspond exactly to populations with positive and negative values of V z z , respectively. Histograms representing quadrupole splitting distributions (QSDs) show features found in experimental QSDs, such as a constant high edge, a variable low edge, and a QSD width that changes dramatically with flattening. These results represent the first predictions relating average structural parameters, as would be obtained by X-ray diffraction, to characteristics of the Fe 2 + QSD, as obtained by 5 7 Fe Mossbauer spectroscopy, in a chemically disordered material.