Konstantin V. Pokholok

Unprecedented robust antiferromagnetism in fluorinated hexagonal perovskites.

The diversification of antiferromagnetic (AFM) oxides with high Néel temperature is of fundamental as well as technical interest if one considers the need for robust AFM in the field of spin-tronics (exchange bias, multiferroics, etc.). Within the broad series of so-called hexagonal perovskites (HP), the existence of face-sharing octahedral units drastically lowers the strength of magnetic exchanges as compared to corner-sharing octahedral edifices. Here, we show that the partial introduction of F(-) in several Fe-based HP types leads to a drastic increase of the AFM ordering close to the highest values reported in iron oxides (T(N) ≈ 700 K). Our experimental results are supported by ab initio calculations. The T(N) increase is explained by the structural effect of the aliovalent F(-) for O(2-) substitution occurring in preferred anionic positions: it leads to local changes of the Fe-O-Fe connectivity and to chemical reduction into predominant Fe(3+), both responsible for drastic magnetic changes.

Slicing the Perovskite Structure with Crystallographic Shear Planes: The AnBnO3n-2 Homologous Series

A new A(n)B(n)O(3n-2) homologous series of anion-deficient perovskites has been evidenced by preparation of the members with n = 5 (Pb(2.9)Ba(2.1)Fe(4)TiO(13)) and n = 6 (Pb(3.8)Bi(0.2)Ba(2)Fe(4.2)Ti(1.8)O(16)) in a single phase form. The crystal structures of these compounds were determined using a combination of transmission electron microscopy and X-ray and neutron powder diffraction (S.G. Ammm, a = 5.74313(7), b = 3.98402(4), c = 26.8378(4) Å, R(I) = 0.035, R(P) = 0.042 for Pb(2.9)Ba(2.1)Fe(4)TiO(13) and S.G. Imma, a = 5.7199(1), b = 3.97066(7), c = 32.5245(8) Å, R(I) = 0.032, R(P) = 0.037 for Pb(3.8)Bi(0.2)Ba(2)Fe(4.2)Ti(1.8)O(16)). The crystal structures of the A(n)B(n)O(3n-2) homologues are formed by slicing the perovskite structure with (101)(p) crystallographic shear (CS) planes. The shear planes remove a layer of oxygen atoms and displace the perovskite blocks with respect to each other by the 1/2[110](p) vector. The CS planes introduce edge-sharing connections of the transition metal-oxygen polyhedra at the interface between the perovskite blocks. This results in intrinsically frustrated magnetic couplings between the perovskite blocks due to a competition of the exchange interactions between the edge- and the corner-sharing metal-oxygen polyhedra. Despite the magnetic frustration, neutron powder diffraction and Mössbauer spectroscopy reveal that Pb(2.9)Ba(2.1)Fe(4)TiO(13) and Pb(3.8)Bi(0.2)Ba(2)Fe(4.2)Ti(1.8)O(16) are antiferromagnetically ordered below T(N) = 407 and 343 K, respectively. The Pb(2.9)Ba(2.1)Fe(4)TiO(13) and Pb(3.8)Bi(0.2)Ba(2)Fe(4.2)Ti(1.8)O(16) compounds are in a paraelectric state in the 5-300 K temperature range.

A new iron oxophosphate SrFe3(PO4)3O with chain-like structure

A new strontium iron oxophosphate SrFe3(PO4)3O was synthesized by the solid state method and its structure was studied by single-crystal X-ray and electron diffraction, high-resolution electron microscopy, Mossbauer and IR spectroscopy. The compound crystallizes in a monoclinic system (space group P21/m) with unit-cell parameters: a = 7.5395(7), b = 6.3476(7) c = 10.3161(13) A ˚ , b = 99.740(9)1. The structure of SrFe3(PO4)3O represents a new structural type and is made up of isolated PO4 tetrahedra and FeOn polyhedra connected via common vertices and edges to form a 3D framework. Iron cations occupy three crystallographically independent sites with different oxygen environment: Fe1 and Fe2 occupy two octahedral sites, and Fe3 is five-coordinated. Two particularities of this structure are remarkably mentioned: the isolated {FeO6}n octahedral chains along the b direction and the five coordinated environment for the Fe3 position. Mossbauer spectroscopy confirmed the presence of only high-spin Fe 3+ cations in two types of coordination environment. The IR-data show the presence of only PO 3� 4 groups.

Structural and Magnetic Phase Transitions in the AnBnO3n-2 Anion-Deficient Perovskites Pb2Ba2BiFe5O13 and Pb1.5Ba2.5Bi2Fe6O16

Novel anion-deficient perovskite-based ferrites Pb2Ba2BiFe5O13 and Pb(1.5)Ba(2.5)Bi2Fe6O16 were synthesized by solid-state reaction in air. Pb2Ba2BiFe5O13 and Pb(1.5)Ba(2.5)Bi2Fe6O16 belong to the perovskite-based A(n)B(n)O(3n-2) homologous series with n = 5 and 6, respectively, with a unit cell related to the perovskite subcell a(p) as a(p)√2 × a(p) × na(p)√2. Their structures are derived from the perovskite one by slicing it with 1/2[110]p(101)p crystallographic shear (CS) planes. The CS operation results in (101)p-shaped perovskite blocks with a thickness of (n - 2) FeO6 octahedra connected to each other through double chains of edge-sharing FeO5 distorted tetragonal pyramids which can adopt two distinct mirror-related configurations. Ordering of chains with a different configuration provides an extra level of structure complexity. Above T ≈ 750 K for Pb2Ba2BiFe5O13 and T ≈ 400 K for Pb(1.5)Ba(2.5)Bi2Fe6O16 the chains have a disordered arrangement. On cooling, a second-order structural phase transition to the ordered state occurs in both compounds. Symmetry changes upon phase transition are analyzed using a combination of superspace crystallography and group theory approach. Correlations between the chain ordering pattern and octahedral tilting in the perovskite blocks are discussed. Pb2Ba2BiFe5O13 and Pb(1.5)Ba(2.5)Bi2Fe6O16 undergo a transition into an antiferromagnetically (AFM) ordered state, which is characterized by a G-type AFM ordering of the Fe magnetic moments within the perovskite blocks. The AFM perovskite blocks are stacked along the CS planes producing alternating FM and AFM-aligned Fe-Fe pairs. In spite of the apparent frustration of the magnetic coupling between the perovskite blocks, all n = 4, 5, 6 A(n)Fe(n)O(3n-2) (A = Pb, Bi, Ba) feature robust antiferromagnetism with similar Néel temperatures of 623-632 K.

Evidence through Mössbauer spectroscopy of two different states for 57Fe probe atoms in RNiO3 perovskites with intermediate-size rare earths, R = Sm, Eu, Gd, Dy

In the present work, 57Fe probe Mossbauer spectroscopy was developed to study the nickelates RNi0.98Fe0.02O3 (R = Sm,Eu,Gd,Dy) with the perovskite-like structure. The restoration method for a distribution function P(v) of the positions (v) involving individual Lorentzian lines has been used for processing and analysing the Mossbauer spectra. The P(v) profile for the nickelates, at T<TIM (TIM corresponding to the transition temperature from insulator to metal), can be described as a superposition of two symmetric peaks with different intensities. The distance between these peaks monotonically increases with the decreasing ionic radius value of the R3+ cations . The observed asymmetry of the P(v) profile indicates that 57Fe atoms used as a Mossbauer probe are simultaneously stabilized in two non-equivalent crystallographic positions. This result is an indirect evidence for the existence of two types of nickel position in the insulating state of nickelate RNiO3 lattices with intermediate R3+ size, which remained questionable from diffraction methods.

Sorption of 237Np(V), 238U(VI), and 137Cs on clays: Role of surface films of Fe(III) compounds

Mineralogical and sorption properties of sandy-argillaceous rocks proposed as a material of protecting barriers in near-surface repositories of nuclear wastes were studied. Different sorption behavior with respect to Np(V), U(VI), and 137Cs is caused by formation of iron-containing films on the particle surface of coarse (>0.25 mm) and fine (<0.01 mm) rock fractions. The presence of Fe in the compositions of different minerals contained in the rock (chlorite, illite, montmorillonite, and hematite) was determined by scanning electron microscopy with X-ray probe microanalysis (SEM-XPMA), transmitting electron microscopy with electron energy loss spectroscopy (TEM-EELS), and Mössbauer spectroscopy. Hematite is present in the form of both separate particles and films on quartz grains. The Fe fraction in these formations was estimated.

Preparation and properties of exfoliated graphite modified with iron compounds

Using the anodic oxidation of graphite in Fe(NO3)3-HNO3 mixed electrolytes, followed by heat treatment, we obtained exfoliated graphite modified with iron oxides. All of the samples obtained were characterized by X-ray diffraction, Mössbauer spectroscopy, and scanning electron microscopy. The results demonstrate that the proposed approach allows one to produce exfoliated graphite containing up to 14% Fe in the form of oxides and hydroxides, whose composition depends on the exfoliation temperature.

Cation ordering in Sr(Fe1 − xCox)0.9Ga0.1O2.5 with brownmillerite structure

Samples of solid solutions of the series Sr(Fe1 − xCox)0.9Ga0.1O2.5 with brownmillerite structure are produced from appropriate amounts of oxides Fe, Co, Ga, and SrCO3 in different gaseous media and temperature conditions using solid-phase synthesis. Features of the distribution of cations over nonequivalent positions of the crystalline structure of the obtained samples are studied using the 57Fe Mössbauer absorption and emission spectroscopy.

Heterovalent substitution of group IV and V diamagnetic ions in the ferromagnetic spinel CuCr2S4

Heterovalent substitution of As5+, Sb5+, V5+, and Sn4+ diamagnetic ions in CuCr2S4 was studied. The results show that, similar to other 3d10 diamagnetic ions, the As5+ ions occupy only tetrahedral interstices in the close packing of sulfur ions, while the Sb5+ and Sn4+ 4d10 ions, as well as the Sc3+, Ti4+, and V5+ 3d0 ions occupy only octahedral sites. A new compound of composition Cu3AsCr8S16 was synthesized and characterized. It crystallizes in an orthorhombic (sp. gr.Pmm2) spinel-derived structure containing ordered Cu and As ions in tetrahedral sites(a = 13.942 ± 0.004 Å,b = 6.878 ± 0.002 Å,c = 19.692 ± 0.006 Å, Z = 4, V = 1888.43 ± 0.92 Å). The CuCr1.5Sb0.5S4 spinel phase is found to crystallize in sp. gr.Fd3m (a = 10.009 ± 0.002 Å,u = 0.3815 ± 0.0004 Å) with partial ordering on octahedral sites.

Preparation of Exfoliated Graphite Modified with Magnesium Ferrite

A magnetic sorbent based on exfoliated graphite modified with magnesium ferrite has been prepared by impregnating oxidized graphite in a mixed solution of FeCl3 and Mg(NO3)2, followed by heat treatment of the impregnated oxidized graphite in air. X-ray diffraction and Mössbauer spectroscopy results demonstrate that the structure of the magnesium ferrite is an inverse spinel with a degree of inversion of 0.59. The saturation magnetization of the magnesium ferrite-containing exfoliated graphite is 16.1 emu/g, whereas its oil sorption capacity is as high as 54 g/g. Compaction of the exfoliated graphite to a density of 0.03 g/cm3 reduced its sorption capacity to 26 g/g. Further increasing the density of the material led to a considerable decrease in its sorption capacity.