FeO4 Tetrahedra Research Articles

Structural and Magnetic Properties in Spinel Type Fe1−xZnxV2O4

Structural and magnetic properties in spinel type Fe1−xZnxV2O4 have been investigated for x ≤ 0.4. Successive structural transitions of cubic-to-tetragonal (c < a), tetragonal-to-orthorhombic and orthorhombic-to-tetragonal (c > a) are observed for x ≤ 0.2. On the other hand, the only cubic-to-tetragonal (c > a) structural transition is observed for 0.3 ≤ x ≤ 0.4. The result of the magnetic measurements indicates that the ferrimagnetic transition is observed for x ≤ 0.4 and the transition temperature is close to the tetragonal-to-orthorhombic transition temperature for x ≤ 0.2, and the cubic-to-tetragonal transition one for 0.3 ≤ x ≤ 0.4. These results are discussed in terms of Jahn-Teller distortions of FeO4 tetrahedron and VO6 octahedron, and spin-orbital interaction at Fe2+ ion.

Complex Cation Order in Anion-Deficient BanYFen-1O2.5n Perovskite Phases

The synthesis and structural characterization of BanYFen-1O2.5n (n = 2, 3, 4) anion-deficient perovskite phases is reported. The n = 2 and n = 3 members of the series, Ba2YFeO5 and Ba3YFe2O7.5, adopt structures which consist of ordered arrays of corner-linked YO6 octahedra and FeO4 tetrahedra on the perovskite B-sites and are described in unit cells related to a simple cubic perovskite cell by respective 2√2 × √2 × 2 and 3√2 × √2 × 2 geometric expansions. The complex cation ordering schemes observed in the two phases can be described on the basis of Y2Fe2O10□2 secondary building units. A rationalization for the observed structures is presented on the basis of the need to minimize lattice strain while maintaining the integrity of the local YO6 and FeO4 coordination polyhedra. The n = 4 member of the series, Ba4YFe3O10, adopts a cation and anion-vacancy disordered structure, which is attributed to dilution of the structure directing effects at extreme Y:Fe stoichiometric ratios. Magnetization data indicate Ba2YFeO5 and Ba3YFe2O7.5 exhibit paramagnetic behavior, consistent with the lack of a long-range Fe-O-Fe network. Ba4YFe3O10 adopts an antiferromagnetic state below 50 K.

Electrical conductivity and local structure of lithium tin iron vanadate glass

A relationship between electrical conductivity (σ) and local structure of 15Li2O·10Fe2O3·xSnO2·(70–x)V2O5·5P2O5 glass (x = 0–20 mol%), abbreviated as xLFSVP glass, was investigated by 57Fe- and 119Sn-Mossbauer spectroscopies, differential thermal analysis (DTA) and dc-four probe method. A small increase in quadrupole splitting (Δ) for FeIII was observed from 0.70 to 0.74± 0.02 mm s − 1 with an increase of “x”, whereas isomer shift (Δ) values of 0.40±0.01 mm s − 1 were independent of “x”. This result suggests that local distortion of FeIIIO4 tetrahedra was slightly increased in SnO2-containing vanadate glasses, which was reflected as an increase in glass transition temperature (Tg) from 266 to 285±5 °C. A slope of 675 K / (mm s − 1) obtained in ‘Tg vs. Δ plot’ proved that FeIII occupied the site of network former (NWF). An isothermal annealing of 10LFSVP glass at 500 °C for 100 min resulted in a marked decrease of Δ from 0.72 to 0.56±0.02 mm s − 1, indicating that local distortion of FeO4 tetrahedra was reduced by the structural relaxation of 3D-network. In contrast, identical δ and Δ values of 0.07±0.01 and 0.53±0.02 mms − 1, respectively, were observed in 119Sn-Mossbauer spectra of 10LFSVP glass before and after the annealing. These results indicate that SnIVO6 octahedra are loosely bound in the glass matrix as a network modifier (NWM). A marked increase in σ from 7.4 × 10 − 7 to 9.1 × 10 − 3 S cm − 1 was observed in 20LFSVP glass after the isothermal annealing, indicating that structural relaxation of 3D-network evidently causes a marked increase in σ.

Study of Ga3+-Induced Hydrothermal Crystallization of an α-Quartz Type Ga1-xFexPO4 Single Crystal by in Situ X-ray Absorption Spectroscopy (XAS)

The dissolution of α-FePO(4) and the α-Ga(0.75)Fe(0.25)PO(4) solid solution with α-quartz-type structures under hydrothermal conditions in 1 M HNO(3) aqueous solution was investigated by in situ X-ray absorption spectroscopy (XAS) at the Fe K-edge. The solubility of α-FePO(4) increases with temperature and is higher at 25 MPa than at 50 MPa. The Fe(3+) cation in solution is 6-fold coordinated with an average Fe-O distance close to 2.0 Å. A similar experiment was performed with a solid solution of α-quartz-type Ga(0.75)Fe(0.25)PO(4) as the starting phase under a pressure of 25 MPa. By varying the temperature from 303 K up to 573 K a single crystal was grown with 23% Fe(3+) with the α-quartz-type structure. These results show that the crystallization of pure α-quartz-type FePO(4) by the hydrothermal method is not possible due to the formation of very stable Fe(3+) hexa-aquo complexes [Fe(H(2)O)(6)](3+) and to the absence of FeO(4) tetrahedra in solution. Ga(3+) cations in solution induce the formation of gallophosphate complexes at the solid-liquid interface, which are at the origin of the nuclei for crystallization. We propose a crystallization mechanism in which the Fe(3+) substitutes Ga(3+) with a 4-fold coordination in mixed (iron/gallo)-phosphate complexes that leads to the growth of an α-quartz-type Ga(0.77)Fe(0.23)PO(4) single crystal.

Structural changes on cycling Li2FeSiO4 polymorphs from DFT calculations

Cation mixing has been demonstrated experimentally in the Li-ion battery cathode material Li2FeSiO4. This feature is investigated here using DFT calculations. It is shown that full reversal of Li/Fe site occupations is energetically favoured on delithiation for all three electrochemically active Li2FeSiO4 polymorphs. The common layered topology in the arrangement of SiO4 and FeO4 tetrahedra in all three polymorphs transforms into a 3D-framwork. Calculations show here that such a change in structure leads to a lowering of electrochemical insertion potential from ~3.1 to ~2.8V, in good agreement with experimental data. Calculations also predict the correct anisotropy in the cell expansion on delithiation on Li/Fe site reversals. Partial mixing of Li and Fe site occupations is energetically less favourable, which supports a two-phase transformation mechanism.

Local Oxygen-Vacancy Ordering and Twinned Octahedral Tilting Pattern in the Bi0.81Pb0.19FeO2.905 Cubic Perovskite

The structure of Bi0.81Pb0.19FeO2.905 was investigated on different length scales using a combination of electron diffraction, high-resolution scanning transmission electron microscopy, synchrotron X-ray powder diffraction, and Mo spectros- copy. In the 80−300 K temperature range, the average crystal structure of Bi0.81Pb0.19- FeO2.905 is a cubic Pm3 perovskite with a = 3.95368(3) A at T = 300 K. The (Pb 2+ , Bi 3+ ) cations and O 2− anions are randomly displaced along the ⟨110⟩ cubic directions, indicating the steric activity of the lone pair on the Pb 2+ and Bi 3+ cations and a tilting distortion of the perovskite framework. The charge imbalance induced by the hetero- valent Bi 3+ → Pb 2+ substitution is compensated by the formation of oxygen vacancies preserving the trivalent state of the Fe cations. On a short scale, oxygen vacancies are located in anion-deficient (FeO1.25) layers that are approximately 6 perovskite unit cells apart and transform every sixth layer of the FeO6 octahedra into a layer with a 1:1 mixture of corner-sharing FeO4 tetrahedra and FeO5 tetragonal pyramids. The anion-deficient layers act as twin planes for the octahedral tilting pattern of adjacent perovskite blocks. They effectively randomize the octahedral tilting and prevent the cooperative distortion of the perovskite framework. The disorder in the anion sublattice impedes cooperative interactions of the local dipoles induced by the off-center displacements of the Pb and Bi cations. Magnetic susceptibility measurements evidence the antiferromagnetic ordering in Bi0.81Pb0.19FeO2.905 at low temperatures.

The influence of the Jahn–Teller effect at Fe2+ on the structure of chromite at high pressure

The crystal structure of chromite FeCr2O4 was investigated to 13.7 GPa and ambient temperature with single-crystal X-ray diffraction techniques. The unit-cell parameter decreases continuously from 8.3832 (5) to 8.2398 (11) Å up to 11.8 GPa. A fit to the Birch–Murnaghan equation of state (EoS) based on the P–V data gives: K 0 = 209 (13) GPa, K′ = 4.0 (fixed), and V 0 = 588 (1) Å3. The FeO4 tetrahedra and CrO6 octahedra are compressed isotropically with pressure with their Fe–O and Cr–O bond distances decreasing from 1.996 (6) to 1.949 (7) Å and from 1.997 (3) to 1.969 (7) Å, respectively. The tetrahedral site occupied by the Fe2+ cation is more compressible than the octahedral site occupied by the Cr3+ cation. The resulting EoS parameters for the tetrahedral and the octahedral sites are K 0 = 147 (9) GPa, K′ = 4.0 (fixed), V 0 = 4.07 (1) Å3 and K 0 = 275 (24) GPa, K′ = 4.0 (fixed), V 0 = 10.42 (2) Å3, respectively. A discontinuous volume change is observed between 11.8 and 12.6 GPa. This change indicates a phase transition from a cubic (space group Fd-\({\overline{3}}\) m) to a tetragonal structure (space group I41 /amd). At the phase transition boundary, the two Cr–O bonds parallel to the c-axis shorten from 1.969 (7) to 1.922 (17) Å and the other four Cr–O bonds parallel to the ab plane elongate from 1.969 (7) to 1.987 (9) Å. This anisotropic deformation of the octahedra leads to tetragonal compression of the unit cell along the c-axis. The angular distortion in the octahedron decreases continuously up to 13.7 GPa, whereas the distortion in the tetrahedron rises dramatically after the phase transition. At the pressure of the phase transition, the tetrahedral bond angles along the c-axis direction of the unit cell begin decreasing from 109.5° to 106.6 (7)°, which generates a “stretched” tetrahedral geometry. It is proposed that the Jahn–Teller effect at the tetrahedrally coordinated Fe2+ cation becomes active with compression and gives rise to the tetrahedral angular distortion, which in turn induces the cubic-to-tetragonal transition. A qualitative molecular orbital model is proposed to explain the origin and nature of the Jahn–Teller effect observed in this structure and its role in the pressure-induced phase transition.

Electrical conductivity and local structure of barium manganese iron vanadate glass

Local structure and electrical conductivity of semiconducting 20BaO·10Fe2O3·x MnO2·(70 − x)V2O5 glass (x = 0 − 30), abbreviated as xBFMV, were investigated by means of 57Fe-Mossbauer spectroscopy, differential thermal analysis (DTA) and DC four-probe method. Mossbauer spectrum of these vanadate glasses consists of a doublet with an identical isomer shift (δ) of 0.38±0.01 mm s − 1, indicating that distorted FeO4 tetrahedra constitute the structural units with distorted VO4 tetrahedra and VO5 pyramids. Quadrupole splitting (Δ) gradually increases from 0.70±0.02 to 0.87±0.02 mm s − 1 with an increase in the MnO2 content, indicating an increased local distortion of FeIIIO4 tetrahedra. DTA study of these glasses showed a gradual increase of glass transition temperature (T g) from 329±5 to 411±5°C, showing an improved thermal durability. ‘T g vs. Δ plot’ yielded a straight line with a large slope of 707°C(K)/mm s − 1, proving that FeIII played a role of network former (NWF). An isothermal annealing of 10BFMV glass at 500°C for 1000 min resulted in a marked increase in the electrical conductivity (σ) from (4.5±3.9) × 10 − 7 to (1.4±0.3) × 10 − 2 S cm − 1 and a decrease in the activation energy for the electrical conduction (E a) from 0.33 ± 0.07 to 0.11±0.01 eV, while Δ of FeIII decreased from 0.76±0.02 to 0.49±0.02 mm s − 1. These results suggest that decrease in the distortion of FeIIIO4 tetrahedra involved with the structural relaxation causes an increase in the probability of electron hopping from VIV or VIII to VV.

Local electronic structure analysis for brownmillerite Ca(Sr)FeO2.5 using site-resolved energy-loss near-edge structures

Oxygen K-edge and Fe L2,3-edge electron energy-loss near-edge structures (ELNES) were measured for FeO6 octahedra and FeO4 tetrahedra in the brownmillerite Ca(Sr)FeO2.5 by focusing an electron probe at individual Fe sites using scanning transmission electron microscopy combined with electron energy-loss spectroscopy. The observed site-resolved oxygen K-ELNES showed different features reflecting the local chemical bonding around the FeO6 octahedra and FeO4 tetrahedra. A pre-peak in the O K-edge spectra, which is attributed to a transition to an unoccupied O 2p band hybridized with the Fe-3d band, shows splitting in the spectrum of the FeO6 octahedral site. Additionally, for the oxygen linking the octahedral and tetrahedral Fe sites in CaFeO2.5, charge transfer was found to preferentially occur toward the tetrahedral Fe ions. In the case of SrFeO2.5, charge transfer from the oxygen located in the ac plane was biased toward the tetrahedral Fe atoms. Based upon an analysis of the pre-peak intensity of the O K-ELNES, it was concluded that bonding between the oxygen and iron atoms at the tetrahedral site was more covalent in character than at the octahedral site. The strong covalent character of the tetrahedral sites would be one of the reasons for distortion in the FeO6 octahedra in Ca(Sr)FeO2.5, as exhibited by an extension along the b-axis.

Framework Fe Ions in Fe-ZSM-5 Zeolite Studied by UV Resonance Raman Spectroscopy and Density Functional Theory Calculations

The transition metal containing zeolites show high selectivity and activity in many catalytic reactions under mild conditions; however, detailed characterization of transition metal site ions and the nature of the active sites remain elusive. Here, we present a study of the framework Fe ions in Fe-ZSM-5 zeolite via interplay between resonance Raman spectroscopy excited by 244 and 532 nm laser lines and density functional theory (DFT) calculations. Raman bands at 1165, 1115, 1005, and 516 cm−1 from the framework iron ions in the framework of Fe-ZSM-5 were observed. On the basis of the DFT calculations, the bands at 1165, 1115, and 516 cm−1 were attributed to ν0(A1), ν1(A1), and ν2(A1) modes with totally symmetric vibration of FeO4 tetrahedron in Fe-ZSM-5, originating from resonant enhanced Raman scattering, while the band at 1005 cm−1 for ν6(T2) was identified as a nontotally symmetric vibration of the FeO4 tetrahedron from normal Raman scattering. The present work indicates that the resonance Raman bands at 500 and 1100 cm−1 can be used as the spectral characteristics of the transition metal ions substituted in the framework of zeolites.

The effect of oxygen vacancies and aluminium substitution on the high-pressure properties of brownmillerite-structured Ca2Fe2−x Al x O5

The structural evolution with pressure and the equations of state of three members of the brownmillerite solid solution, Ca2(Fe2−x Al x )O5, have been determined by single-crystal X-ray diffraction up to a maximum pressure of 9.73 GPa. The compositions of the samples were x = 0.00 and x = 0.37 (with Pnma symmetry) and x = 0.55 (with I2mb symmetry). No phase transitions were observed in the experiments. The equation of state parameters determined from the pressure-volume data are K 0T = 128.0 (7) GPa, K′0 = 5.8 (3) for the sample with x = 0.00, K 0T = 131 (2) GPa, K′0 = 5.5 (4) for x = 0.37, and K 0T = 137.5 (6) GPa, K′0 = 4 for x = 0.55. The bulk modulus therefore increases with Al content, being 11% higher in the x = 0.55 sample than in the Al-free sample. The unit-cell compression is anisotropic, with the c-axis being stiffer than a or b, and the anisotropy increases with increasing Al content of the structure. The structural response to pressure of all samples is similar. The (Al,Fe)O4 tetrahedra and the (Al,Fe)O6 octahedra undergo approximately isotropic compression. There is an increase in the twists of the chains of corner-sharing (Al,Fe)O4 tetrahedra, and an increase in the tilts of the (Al,Fe)O6 octahedra, because these framework polyhedra are stiffer than the Ca–O bonds to the extra-framework Ca site. The alignment of the two shortest Ca–O bonds sub-parallel to [001] accounts for the relative stiffness of the c-axis and thus the elastic anisotropy.

Structural role and coordination environment of Fe in Fe2O3–PbO–SiO2–Na2O composite glasses

The structural role, coordination geometry and valence of Fe in a series of Fe2O3–PbO–SiO2–Na2O glasses are studied by means of Fe-K-NEXAFS and EXAFS spectroscopies. Parameters for the study are the concentration of the Fe and Pb-oxides, the SiO2/Na2O ratio and the cast temperature. The EXAFS and NEXAFS results reveal that the role of Fe3+ depends on the concentration of Fe2O3. More specifically, in most of the studied quaternary systems, the Fe3+ ion is a glass former, i.e. the Fe atoms belong to FeO4 tetrahedra that participate in the formation of the glassy network. The role of Fe as an intermediate oxide is identified only in one sample with 20wt% Fe2O3, where ∼80at.% of the Fe atoms are tetrahedrally coordinated with O atoms, while the remaining ∼20at.% of the Fe atoms occupy octahedral sites. It is also revealed that the tetrahedral coordination of Fe in the vitreous matrix is destroyed when a number of parameters is altered, such as the Tcast, the (Fe+Si)/O and the SiO2/Na2O ratio.

Application of μ-XAFS for the determination of the crystallization ratio in a series of vitro-ceramic materials containing industrial waste

The distribution and microstructure of Fe in a series of vitrified Pb- and Fe-rich industrial waste samples is studied by means of X-ray fluorescence mapping (XRF), conventional and micro-X-ray absorption fine structure (μ-XAFS and XAFS) spectroscopies. The under study samples contain toxic ash, that is vitrified with appropriate quantities of vitrifying (SiO2) and flux (Na2O) agents and are studied in the as-casted state as well as after annealing. XRF mapping in combination with μ-XAFS applied to samples containing 50% and 60% ash, demonstrated that annealing at temperatures above 600°C induces loss of homogeneity and formation of Fe-rich microcrystalline islands. By appropriate fitting of the EXAFS spectra it is found that X% (where X depends on the sample composition and annealing temperature) of the Fe atoms form Fe-rich islands where Fe is octahedrally coordinated in FeO6 polyhedra (RFe–O=0.193nm). Contrary to that, (100−X)% of the Fe atoms are incorporated in the vitreous matrix (which has a lower Fe content), and are coordinated in FeO4 tetrahedra (RFe–O=0.188nm). The crystallization ratio X/(100−X) is found to increase with the annealing temperature and the loss of homogeneity becomes more extensive.

Mössbauer study of semiconducting and ferrimagnetic fly ash-recycled glass

Room temperature Mossbauer spectrum of fly ash-recycled glass (FARG), prepared with more than 86 mass% of fly ash and less than 14 mass% of Fe2O3, comprises two types of doublets due to Fe(II) and Fe(III) of magnetite nanoparticles. Isothermal annealing of fly ash-recycled glass at 1100 °C for 60 minutes results in a precipitation of ferrimagnetic magnetite phase having an internal magnetic field of 46.4-48.2 T. When the Fe2O3 content is equal to or more than 14%, room temperature Mossbauer spectrum of FARG shows a magnetic hyperfine structure due to a magnetite phase, in addition to two doublets due to Fe(II) and Fe(III). An increase in the electric conductivity is observed from the order of 10-8 to 10-6 S . cm-1 after heat treatment at around the crystallization temperature. This can be ascribed to an improved step-by-step electron hopping from Fe(II) to Fe(III) of distorted FeO4 tetrahedra in the three-dimensional glass network.

Redox energetics of SrFeO 3− δ — a coulometric titration study

The variation of the oxygen non-stoichiometry (3− δ) in SrFeO 3− δ with the partial pressure of oxygen was measured by coulometric titration at 1223, 1248 and 1273 K. An electrochemical cell that could be heated and cooled repeatedly without introducing leakages was developed for the experiments. The Gibbs energy, the standard entropy and the partial molar enthalpy and entropy of oxygen of SrFeO 3− δ are derived from the measured isotherms and compared with earlier reported data. Solid solution models are presented and used in the analysis of the observed redox properties. It is shown that the redox properties can be interpreted by a non-ideal solution model that includes excess configurational entropy as the only excess term. A reduced structural disorder on the O-sub-lattice is suggested from the formation of preferred structural entities such as FeO 6-octahedra, FeO 5-square pyramids and FeO 4-tetrahedra.

Atomic ordering in FeSr2LnCu2O6+ system (Ln=Nd, Y and Er)

Abstract FeSr2YCu2O6+δ exhibits superconductivity around 50 K, when it is properly annealed in N2 atmosphere and subsequently in O2 atmosphere under ambient and high pressure. Neutron powder diffraction study shows that N2-annealing causes ordering of Cu and Fe, and that O2-annealing and oxidizing under high pressure supply the charge on the CuO2 sheets. Recently, we have found a superstructure in the sample N2-annealed within a particular temperature range. Electron diffraction study indicates that it has an orthorhombic structure with the unit cell of 2 a× 2 a×2c as compared with the tetragonal FeSr2YCu2O6+δ structure with the unit cell of a×a×c. Neutron powder diffraction study indicates that it has the CoSr2YCu2O7-type structure with the FeO4 tetrahedron, due to not only ordering of Cu and Fe but also oxygen ordering on the FeOδ sheet, which is consistent with the electron diffraction study. We have also obtained the similar superstructure in FeSr2ErCu2O6+δ, but the situation is different in FeSr2NdCu2O6+δ which does not exhibit superconductivity even after O2-annealing and oxidizing. Atomic ordering in the FeSr2YCu2O6+δ system is influenced by the size of the ionic radius in the Y site, which is effective on the occurrence of superconductivity.

The Iron Phosphate NaZnFe2(PO4)3.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Microstructural Evolution and Magnetic Properties of NiFe2O4 Nanocrystals Dispersed in Amorphous Silica

NiFe2O4 nanocrystals were dispersed in silica by a sol−gel route. The dried gel was amorphous, in which isolated Fe3+ ions had a weak interaction with silica matrix, as characterized by a weak IR absorption at ca. 580 cm-1. Heat treatment at 400 °C resulted in nickel ferrite clusters being partially formed, and these clusters were observed to interact with the matrix through Si−O−Fe bonds. This interaction reached its maximum with the complete formation of NiFe2O4 clusters as the temperature was raised to 600 °C. Above this temperature, NiFe2O4 clusters grew larger into nanocrystals, while the interaction between the nanocrystals and silica matrix disappeared with breakage of Si−O−Fe bonds. The grain growth for magnetic nanoparticles was accompanied with rearrangement of amorphous silica network. The preference of forming NiFe2O4 nanocrystals eliminated the possibility of precipitation of crystallite component oxides, e.g., NiO, γ-Fe2O3, or Fe3O4 in amorphous silica matrix, or crystalline silica, e.g., cristobalite or quartz, even when the treatment temperature was 1100 °C. Fe ions in silica glasses were determined by Mössbauer spectroscopy to be present exclusively as Fe3+ ions in a high-spin state at octahedral coordination, and the chemical environment of the Fe3+ ions seemed to remain unchanged until the nickel ferrite clusters crystallized. The formation mechanism for NiFe2O4 nanocrystals can be explained in terms of Ni2+ ions shifting from the tetrahedral centers to undistorted octahedral sites in the spinel lattice and the partial transformation of FeO6 octahedron to FeO4 tetrahedron. The critical dimension for the NiFe2O4 nanocrystals in silica was detected as ca. 9 nm. Below the critical size, NiFe2O4 nanocrystals had a superparamagnetic single-domain structure, while the nanocrystals with particle sizes larger than the critical size exhibited bulklike behavior.

Effect of Fe2O3 Content on the Local Structure and Crystallization Mechanism of Calcium Gallate Glasses

57Fe Mossbauer and FT-IR of 60CaO·(40-x)Ga2O3·xFe2O3 glasses revealed that Fe(III)O4 tetrahedra occupy substitutional sites of Ga(III)O4 tetrahedra. The activation energy for crystallizaation (E a) obtained from DTA study decreases distinctly from 8.9 to 4.8 eV with an increase in the Fe2O3 content. In the case of 40CaO·(60-x)Ga2O3·xFe2O3 glasses, a week doublet due to Fe(II)O4 tetrahedra or Fe(III)O6 octahedra was observed in addition to Fe(III)O4 tetrahedra. The E avalue decreases gradually from 7.1 to 5.6 eV with an increasing Fe2O3 content. These results indicate that a large amount of Fe2O3 in the network structure is favorable for the crystallization of calcium gallate glass, since it reduces the chemical bond strength.

57Fe Mössbauer, XRD and DTA Study of the Crystallization Behavior of Calcium Aluminate Glass Containing Iron(III)

57Fe Mossbauer spectroscopy was applied to the crystallization study of IR-transmitting calcium aluminate glass, 60CaO·32Al2O3·5Fe2O3·3Eu2O3, in which Fe(III) atoms occupy tetrahedral Al(III) sites. Heat treatment of calcium aluminate glass below and above the crystallization temperature causes the formation of mayenite, 12CaO·7Al2O3, and brownmillerite, 4CaO·Al2O3·Fe2O3, respectively. Mossbauer spectra of the glass ceramics indicate that Fe(III) atoms substitute Al(III) sites, having less covalent Fe(III)-O bonds and more distorted Fe(III)O4 tetrahedra than they do in the original glass. DTA revealed that crystallization of calcium aluminate glass is triggered by simultaneous cleavage of Ca-O and Al-O bonds.