151Eu-Mössbauer study of pressure-induced valence transitions in EuM2Ge2 (M=Ni,Pd,Pt)

¹⁵¹Eu-Mössbauer spectroscopy is used to study the ternary intermetallics EuM₂Ge₂ (M=Ni, Pd, Pt), in which Eu is divalent, and EuNi₂Si₂ as a trivalent reference system in the pressure range 0–31 GPa. In EuNi₂Ge₂ and EuPd₂Ge₂, one observes a valence transition from Eu²⁺ to Eu³⁺ around 5 and 25 GPa, accompanied by a change in isomer shift of 9.3 and 10.1 mm/s, respectively. These are the largest pressure-driven changes in isomer shift for¹⁵¹Eu-spectroscopy observed up to now.     

On the nature of the valence transition in Eu(Pd1−xAux)2Si2

The intermediate valent compound series Eu(Pd_1−xAu_x)₂Si₂ has been studied by Mössbauer effect measurements on ¹⁵¹Eu(T = 4.2−300K) and ¹⁹⁷Au (4.2K) and by X-ray diffraction (10K, 300K). The temperature induced valence transition Eu²⁺ → Eu³⁺ for x < 0.175 is not of first order type, as suggested in a previous phase diagram [1]. The valence change of the Eu-ion is reflected also in the isomer shift of the ¹⁹⁷Au Mössbauer-resonance.     

Mössbauer study of (NH4)2FeCl5·H2O

Mössbauer spectra of the compound (NH₄)₂FeCl₅·H₂O have been studied as a function of temperature. Two phase transitions are observed in the temperature range between 7 K and 9 K. The transition at 9 K is structural and presents an unusually high thermal hysteresis. AroundT=8 K the substance orders magnetically and different Fe³⁺ contributions are present.     

Intermediate valence of europium in EuxLa1−xBe13

Mössbauer studies of the 21.6 keV transition of¹⁵¹Eu in cubic EuBe₁₃ and Eu_1/2La_1/2Be₁₃ in the temperature range 100 K to 600 K have been performed. The position of the absorption line in EuBe₁₃ moves from 0.60 mm/s at 100 K to 0.04 mm/s at 575 K. In Eu_1/2La_1/2B₁₃ the line moves from 0.50 mm/s at 100 K to –0.03 mm/s at 500 K. We conclude that Eu in Eu_xLa_1–xB₁₃ is in an intermediate valence state and we analyze the temperature dependence of the isomer shift in terms of an interconfigurational fluctuation model. The model contains temperature independent parameters E_exc, the interconfiguration excitation energy, T_f, the valence fluctuation temperature, and S₃, the isomer shift of pure Eu³⁺ in EuBe₁₃. Assuming S₃–S₂= 13 mm/s the analysis yields S₃=0.70 mm/s, T_f=400 K and E_exc=3000 K for EuBe₁₃, whereas T_f=460 K and E_exc=2750 K for Eu_1/2La_1/2B₁₃.     

Magnetic properties, magnetoresistance, and magnetic-field-induced phase transitions in Tb0.95Bi0.05MnO3 and Eu0.8Ce0.2Mn2O5 multiferroics

This paper reports on a study of the magnetic properties, magnetoresistance, and phase transitions in the semiconducting manganite multiferroics Tb_0.95Bi_0.05MnO₃ and Eu_0.8Ce_0.2Mn₂O₅ whose dielectric properties have been a subject of an earlier study. An analysis of these properties has led us to the conclusion that the above crystals at temperatures T ≥ 180 K undergo phase separation with the formation of a dynamic periodic alternation of quasi-2D layers of manganese ions in different valence states, i.e., charge-induced ferroelectricity. This state exhibits a giant permittivity and ferromagnetism in the layers containing Mn³⁺ and Mn⁴⁺ ions. At low temperatures (T < 100 K), the phase volume is occupied primarily by the dielectric phase. Studies of the magnetic properties and the effect of the magnetic field on the dielectric properties of crystals substantiate the scenario of the formation of a state with giant permittivity put forward by us. At low temperatures, Tb_0.95Bi_0.05MnO₃ exhibits features at the points of phase transitions in pure TbMnO₃. A ferromagnetic moment is observed to exist at all the temperatures covered. At the boundaries of the quasi-2D layers, magnetic-field-induced jumps of the electrical resistivity caused by metamagnetic transitions in the layers with Mn³⁺ and Mn⁴⁺ ions are observed. At temperatures T ≥ 180 K, the electrical resistivity undergoes a periodic variation in a magnetic field which is a manifestation of carrier self-organization. A high magnetic field is capable of shifting the phase transition from 180 K to higher temperatures and inducing additional phase transitions.     

High-pressure in situ investigation of cubanite (CuFe2S3): Electronic structure

An in situ study of cubanite (CuFe₂S₃) was performed using energy dispersive X-ray diffraction and Mössbauer spectroscopy in a diamond anvil cell at room temperature and pressures up to 5 GPa. Mössbauer spectra of orthorhombic cubanite show a single iron site with a hyperfine magnetic field that is relatively insentive to pressure, and a centre shift which decreases with pressure at a rate consistent with no significant changes in bonding. Above 3 GPa, however, a nonmagnetic component appears that can be fitted to a single asymmetric quadrupole doublet with a centre shift corresponding to valence between Fe²⁺ and Fe³⁺. This is consistent with X-ray diffraction data that show an accompanying transition from the orthorhombic structure to the NiAs structure, where localised electron transfer could occur across pairs of face-shared octahedra or extended electron delocalisation could occur along sheets of face- and edge-shared octahedra.     

X-ray absorption spectroscopic study of mixed valence systems EuCu2Si2, YbCu2Si2 and Sm4Bi3

X-ray absorption spectroscopy technique is employed to determine the valence of the rare earth ions in EuCu₂Si₂, YbCu₂Si₂ and Sm₄Bi₃. In each case, two absorption peaks corresponding to two different valence states of respective rare earth ions have been observed. Low temperature (77 K) study of EuCu₂Si₂ indicates distinct change in the relative intensities of the absorption peaks compared to those registered at room temperature (300 K). It is inferred from the change in the relative intensities that the population of Eu²⁺ in EuCu₂Si₂ decreases at liquid nitrogen temperature compared to Eu³⁺. Conclusions drawn from these results agree well with those reported by others using different experimental techniques. In Sm₄Bi₃, Sm²⁺ and Sm³⁺ are found to occur in the ratio of 3:1.     

Magnetic collapse in yttrium iron garnet Y<Subscript>3</Subscript>Fe<Subscript>5</Subscript>O<Subscript>12</Subscript> at high pressure

The effect of high pressures up to 70 GPa on single-and polycrystalline samples of yttrium iron garnet Y₃⁵⁷Fe₅O₁₂ is studied by Mössbauer absorption spectroscopy (for the ⁵⁷Fe nucleus) in a diamond-anvil cell. It is found that the hyperfine magnetic field H_hf at ⁵⁷Fe nuclei vanishes abruptly at a pressure of 48 ± 2 GPa, which indicates the transition of the crystal from the ferrimagnetic state to nonmagnetic one. The magnetic transition is irreversible. When the pressure decreases, the magnetic state is not recovered and the garnet remains nonmagnetic until zero pressure. The behavior of the quadrupole splitting and isomer shift shows that, simultaneously with the magnetic transition, irreversible electron and possibly spin transitions occur with changes in the local crystalline structure. The mechanisms of the magnetic collapse are discussed.     

High-Pressure Mössbauer Measurements on Nanocrystalline Perovskite (La,Sr)(Mn,Fe)O3

We report here the Mössbauer measurements on nanocrystalline perovskite structured manganite La_0.8Sr_0.2Mn_0.8Fe_0.19 ⁵⁷Fe_0.01O₃ as a function of pressure up to 10 GPa at room temperature. The nanocrystalline sample, prepared by sol–gel technique found to have crystallite sizes of ∼138 ± 10 Å. Zero-field electrical resistivity measurements with temperature support the nanocrystalline nature. At ambient pressure, Fe³⁺ as well as Fe⁴⁺ ions are distributed in two different environments – Fe³⁺ in low symmetric site surrounded by Mn³⁺ ions only while Fe⁴⁺ in high symmetric site with at least one Mn³⁺ ion. Pressure seems to affect the higher symmetric site. A sudden increase in isomer shift at 0.52 GPa indicates the first order phase transition representing the transformation of Fe⁴⁺ to Fe³⁺. Another transition at 3.7 GPa, represents the presence of Fe³⁺ in single kind of environment. Pressure dependence of electrical resistivity measurements verifies the transitions attributing the first order transition to the cross over of localized-electron to band magnetism.     

Observation of photoexcitation of Fe-oxide grown on TiO<Subscript>2</Subscript>(100) by visible light irradiation

Electronic excitation of materials is of fundamental and technological importance and interest in terms of photoinduced phase transition, photovoltaics, and photocatalysis. In the present study, photoexcitation of Fe₂ O ₃ epitaxially grown on rutile TiO₂(100) was investigated with conversion electron Mössbauer spectroscopy (CEMS) under dominantly visible-light irradiation. ⁵⁷Fe was deposited on the substrate at a substrate temperature of 973 K, and the resulting film was characterized by RHEED and XPS. After deposition of Fe on TiO₂(100), it was found that Fe was oxidized to Fe ³⁺, and the structure was analyzed to be the rhombohedral phase of Fe₂ O ₃. While the CEMS spectrum without light irradiation showed a quadrupole splitting of 0.80 mm/s with an isomer shift of +0.25 mm/s, an additional component with a quadrupole splitting of 0.85 and an isomer shift of +0.67 mm/s was observed under light irradiation. The latter component corresponds to a reduced state of Fe at the octahedral site surrounded by oxygen atoms. The lifetime of this photoexcited state is discussed.     

Optical absorption and emission from irradiated RbMgF3:Eu2+ and KMgF3:Eu2+

The optical properties of irradiated RbMgF₃:Eu²⁺ and KMgF₃:Eu²⁺ have been investigated. Previous research has shown that Eu²⁺ ions in unirradiated RbMgF₃ give rise to broad band absorption around 250 nm and sharp intense line emission at 360 nm. When this material is irradiated little or no change occurs in the 250 nm absorption, but the lifetime of the Eu²⁺ 360 nm transition is reduced. In addition, new emission is observed at 680 nm. In the case of irradiated KMgF₃:Eu²⁺ two new emission bands are observed at 600 and 800 nm. All of these transitions have short lifetimes and are not due to Eu³⁺ ions.     

Strontium ferrates(IV): transition metal oxides at the insulator-metal borderline

Oxoferrates with iron in the high formal oxidation state of 4+ show a variety of electronic properties including insulating behavior, metallic conductivity, and a valence disproportionation of Fe⁴⁺. Here, we report investigations of Sr₂FeO₄ which crystallizes in the two-dimensional K₂NiF₄-type structure. From resistivity and magnetic susceptibility measurements it is found that Sr₂FeO₄ is an antiferromagnetic semiconductor with a Néel temperatureT _N of about 60 K. The Mössbauer spectra of the paramagnetic phase of Sr₂FeO₄ reveal a single Fe⁴⁺ quadrupole doublet. Those of the ordered phase consist of a complicated magnetic hyperfine pattern with at least four inequivalent Fe⁴⁺ sites. These may arise from structural distortions and/or from a complicated spin structure. External pressures above 6 GPa lead to an increase in near-infrared oscillator strength indicating a gap-narrowing and possibly an insulatormetal transition. Raman spectra between 20 and 300 K show, in addition to the normal Raman-active phonon modes of the K₂NiF₄-type crystal structure, a further oxygen-derived phonon mode. The additional Raman band vanishes near 5.5 GPa. The electronic behavior of strontium ferrates(TV) is discussed within the general systematics for the electronic structure of transition metal compounds.On leave from: Institute of Crystallography, Russian Academy of Science, Moscow, Russia.     

TDPAC andemission Mössbauer studies of Fe3O4(99Ru)

Hyperfine interactions of⁹⁹Ru(»⁹⁹Rh) nuclei in Fe₃O₄ were studied by means of TDPAC andemission Mössbauer spectroscopy. The isomer shift obtained from an emission Mössbauer spectrum andthe temperature dependence of the hyperfine magnetic field obtained from TDPAC measurements indicate that⁹⁹Ru ions arising from⁹⁹Rh nuclei dilutely doped in Fe₃O₄ exist as a mixed state of Ru²⁺ andRu³⁺, which is not common in ruthenium oxides. Fe₃O₄(⁹⁹Ru) is the first example where a Ru ion in a low valence state exhibits its own magnetization in oxides.     

I.R.-Absorption peculiarities of GaP:Fe crystals irradiated by electrons

Using photo-ionization transitions theory by Allen and Langer the existence of two kinds of transitions was demonstrated for GaP:Fe crystals, namely, transition from valence band to local level (E_{2 = 1.27eV}). The effect of that form local level to conduction band (E_{2 = 1.27eV}). The effect of electron irradiation (E_{el = 3 MeV}) on both intracenter absorption and absorption in the 0.7?1.6 eV spectral region was investigated and the conclusion was made that absorption E₁ is probably due to Fe²⁺+e → Fe³⁺ photoionization transition while E₂-absorption is due to Fe³⁺ → Fe⁴⁺ +e transition.     

Cationic Order in Double Perovskite Oxide, Sr2Fe1−x Sc x ReO6 (x = 0.05, 0.1)

We have synthesized by sol–gel method the following polycrystalline double perovskite samples: Sr₂Fe_1?x Sc _x ReO₆ (x = 0, 0.05, 0.1). The results of the Rietveld refinements presented single double perovskite phases with orthorhombic symmetry for the system Sr₂Fe_1?x Sc _x ReO₆, the differences in atomic radii between Fe³⁺ and Sc³⁺ cause a lowering in symmetry with respect to the parent Sr₂FeReO₆ tetragonal compound. The Curie temperatures are found at about 426 and 436 (±5) K for Sr₂Fe_0.9Sc_0.1ReO₆ and Sr₂Fe_0.9Sc_0.05ReO₆, respectively. The Mössbauer spectra measured at 77 K show complex hyperfine structures resulting from different magnetic contributions at Fe³⁺ sites; the average hyperfine field is estimated 50 T and the isomer shift at 0.5 mm/s. At room temperature an intermediate valence state for Fe is also observed.     

Local and itinerant Magnetism AND Crystal structure of RRh2Ge2 and RRu2Ge2 (r = rare earth)

The compounds RRh₂Ge₂; and RRu₂Ge₂ were synthesized X-ray studies show that they have the expected ThCr₂Si₂; tetragonal-type structure Magnetization studies at 1 8–300 K, ¹⁵¹Eu Mossbauer studies at 4 l, 77 and 300 K and the crystallography studies show the following- All RRh₂Ge₂; like RRh₂Si₂; exhibit two magnetic phase transitions, one corresponding to the antiferromagnetic ordering of the local rare earth moments. T_N = 8–90 K, the other corresponding to the itinerant electron ordering of the Rh sublattice. T_M = 3–9 K The heavy rare earths in RRu₂G₂; order antiferromagnetically and undergo a spin-flop transition in a relatively low magnetic field, <10 kOe The light elements in RRu₂Ge₂; order in a ferromagnetic, somewhat unclear structure NdRu₂Ge₂, like NdRu₂Si₂, exhibits two peaks in the magnetization curves Again, the lower may correspond to itinerant electron ordenng or, alternatively, to spin reorientation of the rare earth sublattice Eu in both EuRh₂Ge₂ and EuRu₂Ge₂ is divalent, whereas Ce in both CeRh₂Ge₂ and CeRu₂Ge₂ is trivalent For all rare earths the ordenng transition in RRh₂Ge₂ is higher than in RRu₂Ge₂. This fact can be associated with the smaller R-R distances in RRh₂Ge₂ and/ or due to the stronger magnetic character of the Rh 4d conduction electrons Companson of the magnetic properties and ¹⁵¹Eu hyperfine interactions of Eu²⁺Rh₂Ge₂, Eu²⁺Ru₂Ge₂, Eu²⁺Rh₂Si₂ and Eu³⁺Ru₂Si₂ with all the other systems leads to the conclusion that the conduction electrons play the dominant role in determining the magnetic properties of these systems Crystal-field effects are also of considerable importance, since the Mossbauer studies yield for the second-order crystal-field parameter A⁰₂〈r²_4f〉 the huge values +385 and +282 K for EuRu₂Ge₂; and EuRh₂Ge₂, respectively The easy axis of magnetization in the Eu compounds is in the basal plane The large second-order crystal field predicts well the direction of the easy axis for all other rare earths No superconductivity has been observed in any of the compounds, down to 1 8 K A companson of the magnetic properties of the germanides with those of the silicides shows great similanties, the differences being accounted for by the different unit cell sizes and c/a ratios.     

Luminescence properties of SrSi2O2N2 doped with divalent rare earth ions

The optical properties of SrSi₂O₂N₂ doped with divalent Eu²⁺ and Yb²⁺ are investigated. The Eu²⁺ doped material shows efficient green emission peaking at around 540 nm that is consistent with 4f⁷→4f⁶5d transitions of Eu²⁺. Due to the high quantum yield (90%) and high quenching temperature (>500 K) of luminescence, SrSi₂O₂N₂:Eu²⁺ is a promising material for application in phosphor conversion LEDs. The Yb²⁺ luminescence is markedly different from Eu²⁺ and is characterized by a larger Stokes shift and a lower quenching temperature. The anomalous luminescence properties are ascribed to impurity trapped exciton emission. Based on temperature and time dependent luminescence measurements, a schematic energy level diagram is derived for both Eu²⁺ and Yb²⁺ relative to the valence and conduction bands of the oxonitridosilicate host material.     

A Mössbauer effect study of the electronic and structural properties of Fe9(PO4)O8

Fe₉(PO₄)O₈ is a mixed valence compound with both layers of (FeO)₆, which are similar to those in stoichiometric wüstite, FeO, and layers of Fe₃PO₆, which are similar to those found in anhydrous iron(III) phosphate, FePO₄. A detailed Mössbauer effect study between 232 and 850 K of the electronic and structural properties of Fe₉(PO₄)O₈ has been undertaken for comparison purposes and to study any valence averaging electron delocalization or exchange that may be present. An earlier single crystal X-ray study has revealed that Fe₉(PO₄)O₈ crystallizes with five distinct iron sites in the ratio of 1∶1∶2∶4∶1. The differently distorted octahedral Fe(1), Fe(3), and Fe(4) sites contain divalent iron, the tetrahedral Fe(5) site contains divalent iron, and the octahedral Fe(2) site contains trivalent iron. Because of the variety of iron sites, the paramagnetic Mössbauer spectra of Fe₉(PO₄)O₈ are complex and exhibit many partially resolved lines. The logarithm of the Mössbauer spectral absorption area and the median isomer shift vary linearly with temperature and yield an effective Mössbauer temperature of 300 K for Fe₉(PO₄)O₈. The temperature dependence of the median isomer shift indicates electron delocalization into an unspecified conduction band above 630 K. The differing site degeneracies, site symmetries, and site valencies make it possible to fit the Mössbauer spectra of Fe₉(PO₄)O₈ with two different models, both of which yield a realistic temperature dependence of the hyperfine parameters, but which lead to different conclusions about the presence of valence averaging electron exchange. Hence, the Mössbauer spectra can not, unequivocally, demonstrate the presence of valence averaging in Fe₉(PO₄)O₈. However, the spectra do indicate the presence of structural changes, both above and below 295 K, which are consistent with a monoclinic space group as suggested by the presence of the weak superlattice reflections reported earlier. The relative component spectral areas indicate, in agreement with the relative Wigner-Seitz cell volumes, that the iron(III) on the Fe(2) site has a relatively low recoil-free fraction, whereas the six-coordinate iron(II) on the Fe(1) site has a relatively high recoil-free fraction.     

Electron hopping in Fe1−xCuxCr2S4 (0<x<0.5)

A theoretical expression for the line shape of the Mössbauer spectra in the presence of electron hopping between Fe²⁺ and Fe³⁺ is obtained by using a simple stochastic model. Analyses based upon this expression show that the origin of the complicated Mössbauer spectra observed in the magnetic semiconductors Fe_1?xCu_xCr₂S₄ (0<x<0.5) at 77 K is electron hopping between Fe⁺² and Fe³⁺ This hopping occurs at a rate of a few MHz. Quantitative estimates are given for some parameters; the isomer shifts, the internal magnetic fields, the quadrupole splittings and the proportions of Fe²⁺ and Fe³⁺. The valence distribution in this system is determined from the results. For example, the distribution Fe²⁺_0.69Fe³+_0.29Cu¹⁺_0.02Cr³⁺_1.72Cr²⁺_0.28S^2?₄ is obtained for x = 0.02. The existence of Cr²⁺ is concluded.     

Fe2+ spin transition in [Fe(trz)(Htrz)2](BF4) as evidenced by a variable temperature EPR study

The EPR of Fe³⁺ ions has been used for the first time to evidence a low-spin (S=0) to high-spin (S=2) transition of Fe²⁺ ions in an octahedral ferrous complex [Fe(trz)(Htrz)₂](BF₄). The temperature dependence of the intensity of the Fe³⁺ EPR line atg=4.3 reveals a spin transition which occurs for the Fe²⁺ ions, with hysteresis. The transition temperatures areT _c↑=374 K in the warming mode andT _c↓=345 K in the cooling mode. The analysis of the EPR spectral data indicates the presence of a structural phase transition accompanying the spin transition.