Distinct Iron Sites Research Articles

Fabrication and magnetic performance of rare-earth dysprosium doped Ca-Mg hexaferrite nanoparticles

In this research, dysprosium-substituted M-type Ca-Mg hexaferrites are successfully fabricated via sol–gel combustion technique, employing a nominal composition of Ca0.5Mg0.5Fe12-xDyxO19 (x = 0.0, 0.04, 0.08, 0.12, and 0.15). Analysis of XRD patterns reveals exclusive peaks corresponding to the M-type hexaferrite, with no indications of additional phases, underscoring the high purity and crystalline integrity of the synthesized material. The Rietveld refinement is used to evaluate lattice parameters ’a’ and ’c’ along with the cell volume ’V ’ using the distinct iron sites. Notably, a discernible upward trend in both lattice constants and cell volume emerged with the gradual introduction of dysprosium Dy+3 ion. Hexaferrite phase formation was also confirmed through the ‘c/a’ ratio which lies below the critical value of 3.98. Synthesized samples shows a composite surface morphology with adherent clusters of small crystals and well-formed individual crystals. VSM analysis suggests, remarkable results, particularly at the x = 0.15 doping level, which exhibits the highest magnetic parameters, including Ms = 32.13 emu g−1, Mr = 20.34 emu g−1, and Hc = 1.418 kOe. These results suggest that the synthesized samples hold promise as suitable contenders for optimizing recording media, offering the potential for improved data storage performance.

Spin dynamics of S=2 triangular and honeycomb lattices

Spin dynamics of $S=2$ triangular and honeycomb lattices realized in ${\mathrm{BaNa}}_{2}\mathrm{Fe}{[{\mathrm{VO}}_{4}]}_{2}$ and ${\mathrm{BaNa}}_{2}{\mathrm{Fe}}_{2/3}{\mathrm{Mg}}_{1/3}{[{\mathrm{VO}}_{4}]}_{2}$, respectively, are investigated by magnetization, heat capacity, and particularly $^{57}\mathrm{Fe}$ M\"ossbauer spectroscopy measurements from which spin fluctuation rates are derived. Slight distortions of the $[{\mathrm{FeO}}_{6}]$ polyhedra from ${D}_{3d}$ symmetry probe the respective anisotropy of the electronic ground state. ${\mathrm{BaNa}}_{2}\mathrm{Fe}{[{\mathrm{VO}}_{4}]}_{2}$ and ${\mathrm{BaNa}}_{2}{\mathrm{Fe}}_{2/3}{\mathrm{Mg}}_{1/3}{[{\mathrm{VO}}_{4}]}_{2}$ reveal magnetic decoupling of the underlying magnetic lattices. Down to the lowest temperature of our experiments (3.3 K) only the triangular lattice exhibits long-range order (${T}_{c}=6.5$ K). The long-range order involves only 2/3 of the Fe sites (static, ${H}_{hf}\ensuremath{\approx}190$ kOe), and 1/3 of the Fe sites fluctuate faster than the observation time window of the M\"ossbauer spectroscopy experiment. For the honeycomb lattice two distinct iron sites alternate. The moments oriented within the plane slow down considerably with ${H}_{hf}\ensuremath{\approx}120$ kOe whereas the moments on Fe sites aligned along the $c$ axis retain a three times larger fluctuation frequency. In both cases the orthogonal moments, obtained from the orientation of the hyperfine fields with respect to the electric field gradient, couple through Dzyaloshinskii-Moriya interaction. Magnetization data for both compounds show signs of ferromagnetic alignment of the reduced moments, $M$(0 kOe) $\ensuremath{\approx}2\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{B}$ per Fe. The evaluation of the specific heat data and M\"ossbauer spectra for the ${\mathrm{BaNa}}_{2}\mathrm{Fe}{[{\mathrm{VO}}_{4}]}_{2}$ supports the presence of short-range antiferromagnetic interactions below 50 K and suggests a crossover into competing ferromagnetic exchange below 10 K preceding the emerging long-range order.

Spin freezing into a disordered state in CaFeTi2O6 synthesized under high pressure

Double perovskites exhibit a variety of interesting phenomena due to numerous combinations of cation orderings and distortions. $\mathrm{CaMnT}{\mathrm{i}}_{2}{\mathrm{O}}_{6}$ and $\mathrm{CaFeT}{\mathrm{i}}_{2}{\mathrm{O}}_{6}$ are the only two discovered double perovskites that have the columnar type A-site ordering associated with the ${a}^{+}{a}^{+}{c}^{\ensuremath{-}}$ titling system. The former perovskite, synthesized recently, has a polar structure below ${T}_{c}$ and shows an unusual ferroelectricity. The latter, while synthesized decades ago, received less investigation. Here, we have synthesized $\mathrm{CaFeT}{\mathrm{i}}_{2}{\mathrm{O}}_{6}$ under high pressure and carried out thorough characterizations through measurements of x-ray powder diffraction (XRD), second harmonic generation (SHG), dc and ac magnetization, M\ossbauer spectra, resistivity, thermoelectric power, and specific heat. Although the structural refinement of the XRD pattern cannot distinguish two possible tetragonal phases, SHG measurements help to rule out the possibility of a polar structure observed in $\mathrm{CaMnT}{\mathrm{i}}_{2}{\mathrm{O}}_{6}$. Fitting the paramagnetic susceptibility to a Curie-Weiss law gives a negligible Weiss constant. The weak exchange interaction gives way to a dipole-dipole interaction and single-ion anisotropy. An anomaly at 1.2 K in the specific heat is too broad for a magnetic transition, but suggestive of a spin-ice-like state as in the pyrochlores. M\ossbauer spectroscopy shows two distinct iron sites, as expected for the crystal structure. Both sites are occupied by high-spin $\mathrm{F}{\mathrm{e}}^{2+}$ as indicated by the isomer shift. The sharply different temperature dependence of the quadrupole splitting at the two iron sites can be well explained based on the local structural distortions from the refinement of XRD patterns. The line broadening associated with long spin-relaxation times is observed at 6 K as the spin freezing transition is approached from high temperatures.

Site-selective electronic structure of pure and doped Ca2O3Fe3S2

Using density functional dynamical mean-field theory we investigate the site-selective electronic structure of ${\mathrm{Ca}}_{2}{\mathrm{O}}_{3}{\mathrm{Fe}}_{3}{\mathrm{S}}_{2}$. We confirm that the parent compound with two distinct iron sites is a multiorbital Mott insulator similar to ${\mathrm{La}}_{2}{\mathrm{O}}_{3}{\mathrm{Fe}}_{2}{\mathrm{S}}_{2}$. Upon electron/hole doping, carrier localization is found to persist in the two active iron channels because the chemical potential lies in a gap structure with anisotropic and almost vanishing states near the Fermi energy. This emergent behavior stems from large electronic reconstruction caused by dynamical spectral weight transfer involving states with distinct $d$-shell occupancies and orbital character at low energies. We detail the implications of our microscopic analysis and discuss the underlying physics which will emerge in future experiments on ${\mathrm{Ca}}_{2}{\mathrm{O}}_{3}{\mathrm{Fe}}_{3}{\mathrm{S}}_{2}$.

Spin and valence states of iron in (Mg0.8Fe0.2)SiO3 perovskite

The spin and valence states of iron in (Mg0.8Fe0.2)SiO3 perovskite were measured between 0 and 65 GPa using synchrotron Mössbauer spectroscopy. Samples were synthesized in situ in the laser‐heated diamond cell under reducing conditions. The dominant spin state of iron in perovskite is high spin at pressures below 50 GPa. Above 50 GPa, the spectra shows severe changes which can be explained by appearance of two distinct iron sites with similar site weightings. One site has Mössbauer parameters consistent with high spin Fe2+, while the other has the parameters previously interpreted as intermediate spin. The latter intermediate‐spin assignment is not unique, as similar Mössbauer parameters have been reported for high spin Fe2+ in almandine at ambient pressure. However, our data do not rule out the existence of low‐spin iron, which may exist with a smaller fraction and explain the observation of lower spin moments in the X‐ray emission spectroscopy of perovskite at high pressure. From these considerations, our preferred interpretation is that iron in perovskite is mixed or high spin to at least 2000 km depths in the mantle, consistent with computational results. Our study also reveals that reducing conditions do not inhibit the formation of Fe3+ in perovskite at deep‐mantle pressures.

CO and NO adsorption for the IR characterization of Fe 2+ cations in ferrierite: An efficient catalyst for NOx SCR with NH 3 as studied by operando IR spectroscopy

Iron was introduced by ionic exchange inside the FER structure in order to yield a Fe-FER series with increasing metal loading. Characterization of the Fe 2+ cations by adsorption of CO at liquid nitrogen temperature followed by infrared spectroscopy allowed to identify three distinct sites for iron. The most abundant iron species are located on easily accessible sites of the FER structure, whereas high metal loading is required to observe more confined Fe 2+ species. According to the CO adsorption results, the main iron species appears to be coordinatively unsaturated whereas isotopic labelling upon NO adsorption indicates that two distinct iron sites almost give rise to the same mononitrosyl infrared signature. Studying the catalyst upon interaction with NO and O 2 in operando conditions leads to the observation of these mononitrosyl species who behave as reaction intermediates for the NO oxidation into NO 2. All our Fe-FER samples presenting these mononitrosyl complexes are active not only in NO-to-NO 2 reaction but also in the NOx selective catalytic reduction with ammonia. The effects of both NH 3 and SO 2 as adsorption competitor during the low temperature NH 3-SCR are also discussed.

Density functional theory calculations on Mössbauer parameters of nonheme iron nitrosyls.

Density Functional Theory (DFT) calculations on transition metal nitrosyls often reveal unusual spin density profiles, involving substantial spatial separation of majority and minority spin densities. Against this context, there is a significant lack of studies where DFT calculations have been quantitatively calibrated against experimental spectroscopic properties. Reported herein are DFT calculations of Mössbauer isomer shifts and quadrupole splittings for 21 nonheme iron complexes (26 distinct iron sites) including 9 iron nitrosyls. Low- (S = 1/2) and high-spin (S = 3/2) {FeNO}(7) complexes, S = 1/2 {Fe(NO)(2)}(9) species, and polynuclear iron nitrosyls are all represented within the set of compounds examined. The general conclusion with respect to isomer shifts is that DFT (OLYP/STO-TZP) performs comparably well for iron nitrosyls and for iron complexes in general. However, quadrupole splittings are less accurately reproduced for nitrosyl complexes.

Nuclear resonant X-ray spectroscopy of (Mg,Fe)SiO3 orthoenstatites

We present nuclear resonant inelastic X-ray scattering (NRIXS) and synchrotron Mossbauer spectroscopy (SMS) measurements, both nuclear resonant X-ray spectroscopic methods, on synthetic samples of orthoenstatite-structured (Mg,^(57)Fe)SiO_3, a representative component in Earth's upper mantle. All measurements were performed at ambient conditions. NRIXS spectra were measured for three samples of orthoenstatite containing 20, 13, and 7 mol% FeSiO_3. The Debye sound velocities were determined from the low-energy region of the partial phonon density of states (PDOS). With known density and bulk modulus, the shear modulus, compressional and shear wave velocities have been computed. The sound velocities obtained from NRIXS are in good agreement with sound velocities obtained using Brillouin spectroscopy and ultrasonic methods for similar compositions. An important advantage of NRIXS is access to additional thermodynamic information, such as the average force constant, mean-square displacement, obtained from the PDOS. We discuss the contribution of the vibrational spectra to these quantities. In addition to the PDOS, the electronic environment of the iron sites in (Mg_(0.87)^(57)Fe_(0.13))SiO_3 orthoenstatite was determined using ^(57)Fe SMS and conventional Mossbauer spectroscopy. Evaluation of the Mossbauer spectra reveals two distinct iron sites, which are well distinguished by their hyperfine fields. The minority and majority sites are consistent with high-spin Fe^(2+) in the M1 and M2 sites, respectively.

Behavior of iron in (Mg,Fe)SiO3 post‐perovskite assemblages at Mbar pressures

The electronic environment of the iron sites in post‐perovskite (PPv) structured (57Fe,Mg)SiO3 has been measured in‐situ at 1.12 and 1.19 Mbar at room temperature using 57Fe synchrotron Mössbauer spectroscopy. Evaluation of the time spectra reveals two distinct iron sites, which are well distinguished by their hyperfine fields. The dominant site is consistent with an Fe3+‐like site in a high spin state. The second site is characterized by a small negative isomer shift with respect to α‐iron and no quadrupole splitting, consistent with a metallic iron phase. Combined with SEM/EDS analyses of the quenched assemblage, our results are consistent with the presence of a metallic iron phase co‐existing with a ferric‐rich PPv. Such a reaction pathway may aid in our understanding of the chemical evolution of Earth's core‐mantle‐boundary region.

Polynuclear Spin Crossover Complexes: Synthesis, Structure, and Magnetic Behavior of [Fe4(μ-CN)4(phen)4(L)2)]4+Squares

Three new tetranuclear compounds of formula [Fe(4)(mu-CN)(4)(phen)(4)(L)(2))](PF(6))(4) x G where L = tris(pyridin-2-ylmethyl)amine (TPMA) [G = 0] (1), (6-methylpyrid-2-ylmethyl)-bis(pyrid-2-ylmethyl)amine (MeTPMA) [G = 0] (2), or bis(6-methylpyrid-2-ylmethyl)-(pyrid-2-ylmethyl)amine (Me(2)TPMA) [G = NH(4)PF(6)] (3) and phen = 1,10-phenanthroline) have been synthesized and characterized. The three compounds crystallize in the C2/c space group and consist of [Fe(4)(mu-CN)(4)(phen)(4)(L)(2))](4+) square shaped cations with two distinct iron(II) sites. The sites, associated with [Fe(phen)(2)(CN)(2)] and [Fe(L)(NC)(2)] moieties, are connected by cyanide bridging ligands and reside in different [FeN(4)C(2)] and [FeN(6)] ligand field strength environments. For 1, the structural features of both sites at 100 and 293 K are those of an iron(II) atom in the low-spin state, according to the magnetic properties. At 370 K the structure of the [FeN(6)] site is consistent with a quite complete change of spin state from the low-spin state to the high-spin state, a behavior confirmed by the magnetic study. Introduction of a methyl substituent in the sixth position of one or two pyridine groups to get MeTPMA or Me(2)TPMA derivatives, respectively, induce in 2 and 3 notable steric constraint in the [FeN(6)] site making longer the average Fe-N bond distances thereby weakening the ligand field strength and stabilizing the high-spin state. The [FeN(4)C(2)] site remains in the low-spin state in the three compounds.

Hydrogen bonding effects on the electronic configuration of five-coordinate high-spin iron(II) porphyrinates.

The characterization of a new five-coordinate derivative of (2-methylimidazole)(tetraphenylporphinato)iron(II) provides new and unique information about the effects of forming a hydrogen bond to the coordinated imidazole on the geometric and electronic structure of iron in these species. The complex studied has two crystallographically distinct iron sites; one site has an axial imidazole ligand modified by an external hydrogen bond, and the other site has an axial imidazole ligand with no external interactions. The iron atoms at the two sites have distinct geometric features, as revealed in their molecular structures, and distinct electronic structures, as shown by Mössbauer spectroscopy, although both are high spin (S = 2). The molecule with the external hydrogen bond has longer equatorial Fe-N(p) bonds, a larger displacement of the iron atom out of the porphyrin plane, and a shorter axial bond compared to its counterpart with no hydrogen bonding. The Mössbauer features are distinct for the two sites, with differing quadrupole splitting and isomer shift values and probably differing signs for the quadrupole splitting as shown by variable-temperature measurements in applied magnetic field. These features are consistent with a significant change in the nature of the doubly populated d orbital and are all in the direction of the dichotomy displayed by related imidazole and imidazolate species where deprotonation leads to major differences. The results points out the possible effects of strong hydrogen bonding in heme proteins.

A magnetic and Mössbauer spectral study of SmFe 11Ti, LuFe 11Ti, and their respective hydrides

ac Magnetic susceptibility measurements have been carried out on LuFe 11Ti and LuFe 11TiH and reveal the absence of any spin-reorientation between 4.5 and 293 K. Iron-57 Mössbauer spectral measurements between 4.2 and 295 K have been carried out on SmFe 11Ti and LuFe 11Ti and their respective hydrides, SmFe 11TiH and LuFe 11TiH. The Mössbauer spectra have been analyzed with a model that considers both the orientation of the iron magnetic moments and the distribution of titanium atoms in the near neighbor environment of the three crystallographically distinct iron sites. The assignment and the temperature dependence of the hyperfine fields are in complete agreement with the unit cell volume and its expansion upon hydrogenation and with those observed in the related RFe 11Ti and RFe 11TiH compounds. The site assignments and their temperature dependencies of the isomer shifts are in complete agreement both with the Wigner–Seitz cell volumes of the inequivalent iron sites and the crystallographic changes upon hydrogen insertion.

A magnetic and Mössbauer spectral study ofTbFe11Ti andTbFe11TiH

Magnetic and iron-57 Mössbauer spectral measurements between 4.2 and 640 K have been carried out onTbFe11Ti andTbFe11TiH. The insertion ofhydrogen into TbFe11Ti to form TbFe11TiH increases its ordering temperature, magnetization, magnetic hyperfine fields, and isomershifts as a result of lattice expansion. Further, the insertion of hydrogen reinforces the basalmagnetic anisotropy of the terbium sublattice and, as is shown by ac susceptibilitymeasurements and thermomagnetic analysis, the spin reorientation observed inTbFe11Ti at 338 Kdisappears in TbFe11TiH.The Mössbauer spectra have been analysed with a model that considers both the easymagnetization direction and the distribution of titanium atoms in the near-neighbourenvironment of the three crystallographically distinct iron sites. The assignment andthe temperature dependencies of the hyperfine fields and isomer shifts are incomplete agreement with a Wigner–Seitz cell analysis of the three iron sites inRFe11Ti andRFe11TiH,where R is a rare-earth element. A complete analysis of the quadrupoleinteractions in both magnetic phases and in the paramagnetic phase ofTbFe11Ti supports the Mössbauer spectral analysis, and indicates that in the basal magnetic phase theiron magnetic moments are oriented along the equivalent [100] and [010] directions of theunit cell.

Magnetic and Mössbauer spectral properties of DyFe 11Ti and DyFe 11TiH

The temperature dependence of the AC magnetic susceptibility, between 4.2 and 300 K, and the magnetization, between 300 and 600 K, has been measured for DyFe 11Ti and DyFe 11TiH. Iron-57 Mössbauer spectral measurements between 4.2 and 295 K have also been carried out on DyFe 11Ti and DyFe 11TiH and analyzed with a model which considers both the direction of the magnetization in the different magnetic phases of these compounds and the distribution of titanium atoms in the near-neighbor environment of the three crystallographically distinct iron sites. The magnetic measurements and the Mössbauer spectra of DyFe 11Ti clearly show the influence of the two spin reorientations occurring in this compound and indicate that the iron magnetic moments are oriented along the [1 0 0] direction of the basal plane below 100 K. The magnetic measurements and the Mössbauer spectra of DyFe 11TiH do not show any evidence for spin reorientations and are consistent with a basal magnetic anisotropy between 4.2 and 600 K. Hence, the hydrogen insertion into DyFe 11Ti reinforces the dysprosium magnetic anisotropy. The assignment and the temperature dependencies of the Mössbauer spectral hyperfine fields and isomer shifts are in full agreement with a Wigner–Seitz cell analysis of the three iron sites in DyFe 11Ti and DyFe 11TiH.

A magnetic and Mössbauer spectral study of HoFe 11Ti and HoFe 11TiH

A.c. susceptibility measurements were carried out between 4.2 and 295 K on HoFe 11TiH and reveal a spin-reorientation transition at ∼150 K. Iron-57 Mössbauer spectral measurements between 4.2 and 295 K were carried out on both HoFe 11Ti and HoFe 11TiH. The Mössbauer spectra were analyzed with a model which considers both the orientation of the iron magnetic moments and the distribution of titanium atoms in the near-neighbor environment of the three crystallographically distinct iron sites. The assignment and the temperature dependence of the hyperfine fields is in complete agreement with both the iron magnetic moments measured by neutron diffraction and the changes expected at the spin-reorientation. The assignment and the temperature dependence of the isomer shifts is in complete agreement both with the crystallographic changes occurring at the spin reorientation and upon hydrogen insertion. The changes in hyperfine field and isomer shift with the number of titanium near neighbors for each of the three iron sites are consistent with the values observed for related titanium–iron intermetallic compounds.

Magnetic and Mössbauer spectral study of ErFe11Ti and ErFe11TiH

X-ray diffraction, isothermal magnetization at 5 and 300 K, ac magnetic susceptibility measurements between 5 and 200 K, and iron-57 Mössbauer spectral measurements between 4.2 and 295 K have been carried out on ErFe11Ti and ErFe11TiH. Hydrogen uptake has been measured by gravimetric analysis and the insertion of hydrogen into ErFe11Ti increases its magnetization, magnetic hyperfine fields, and isomer shifts as a result of the associated lattice expansion. Peaks and steplike changes in both the real and imaginary components of the ac magnetic susceptibility are observed at ∼50 and 40 K for ErFe11Ti and ErFe11TiH, respectively, and are assigned to spin-reorientation transitions resulting from the temperature dependence of the sixth-order Stevens crystal-field term of erbium. The Mössbauer spectra have been analyzed with a model which considers both these spin reorientations and the distribution of titanium atoms in the near-neighbor environment of the three crystallographically distinct iron sites. The assignment and the temperature dependencies of the hyperfine fields and isomer shifts are in complete agreement with the Wigner–Seitz cell analysis of the three iron sites in ErFe11Ti and ErFe11TiH. The changes in the hyperfine field and isomer shift with the number of titanium near neighbors of the three iron sites are in agreement with the values observed for related titanium–iron intermetallic compounds.

A Mössbauer spectral study of GdFe 11Ti and GdFe 11TiD

Iron-57 Mössbauer spectral measurements between 4.2 and 295 K have been carried out on GdFe 11Ti and GdFe 11TiD and analyzed with a model which considers both the uniaxial magnetic anisotropy and the distribution of titanium atoms in the near-neighbor environment of the three crystallographically distinct iron sites. The assignment and the temperature dependencies of the hyperfine fields and isomer shifts are in complete agreement with the Wigner–Seitz cell analysis of the three iron sites in GdFe 11Ti and GdFe 11TiD. The changes in hyperfine field and isomer shift with the number of titanium near neighbors of the three iron sites are in agreement with the values observed for related titanium–iron intermetallic compounds. The hyperfine parameters also agree with those observed in other RFe 11Ti compounds and their hydrides.

Acid–base and spectroscopic properties of a novel supramolecular porphyrin bonded to four pentacyanoferrate(II) groups

The synthesis and characterization of a new supermolecular porphyrin obtained by the coordination of four pentacyanoferrate(II) groups to the pyridyl N-atoms of meso-(3-pyridyl)porphyrin are reported. The redox potential and the 57Fe Mössbauer isomeric shift indicated that the pyridylporphyrin behaves as an electron-withdrawing group. The binding of pentacyanoferrate(II) groups induced a remarkable increase in the basicity of the porphyrin ring, in contrast to the [Ru(bipy) 2Cl] + groups. In the presence of Ni(II) ions, the pentacyanoferrateporphyrin complex precipitated forming a Prussian blue type material. The voltammograms of a carbon paste electrode of this material exhibited two broad redox waves around 0.43 and 0.63 V ascribed to two distinct iron sites, as confirmed by 57Fe Mössbauer spectroscopy.

Magnetic properties of pure deuteronium and diamagnetically-doped hydronium iron jarosite

D3OFe3(SO4)2(OD)6 is a model example of an S = 5/2 Kagome anti-ferromagnet that exhibits unconventional two-dimensional spin-glass properties. We report quasi-elastic neutron-scattering data on D3OFe3(SO4)2(OD)6 derived from three-directional neutron spin-polarization analysis in combination with time-of-flight analysis (Δ E [Formula: see text] 1 meV). At 1.5 K, within the time window of the experiment, only 45% of the total elastic magnetic moment is frozen — a broad magnetic response in Q is still observed at temperatures in excess of ten times the spin-glass freezing temperature. The dc-magnetization measurements on H3OFe3–xAlx(SO4)2(OD)6 show that dilution of the magnetic lattice with Al3+ disrupts the spin-glass state. Mössbauer measurements indicate that the Al replaces the Fe ions on the magnetic sites of the Kagome lattice, giving rise to two distinct iron sites with equal occupation levels but different hyperfine fields. PACS Nos.: 75.10N, 75.30C, 75.30H, 75.50L

Raman study of substituted barium ferrite single crystals, BaFe12−2xMexCoxO19 (Me=Ir, Ti)

This paper reports the first single-crystal Raman spectra of substituted barium hexaferrites, BaFe12−2x MexCoxO19 (Me=Ir, Ti). The spectra were investigated in the 200–900 cm−1 range and are discussed in comparison with unsubstituted barium hexaferrite. Emphasis was placed on the different factors influencing the Raman spectra of substituted barium hexaferrites, namely the mass of the substituting cation, the ionic radius and charge and the ordering on the five distinct iron sites. Copyright © 1999 John Wiley & Sons, Ltd.