Mössbauer spectra,magnetic and electrical behavior of Ba1+xFe2S4 phases

The Mössbauer spectra, and magnetic and electrical properties of Ba_1+xFe₂S₄ infinitely adaptive phases with 0.074 ≤ x ≤ 0.142 and of BaFe₂S₄ were studied. The properties are highly anisotropic because of the presence in the structure of one-dimensional infinite chains of edge sharing FeS₄ tetrahedra. BaFe₂S₄ is a semiconductor, E_g = 0.66 eV; magnetic susceptibility can be fit by a one-dimensional Heisenberg model with spin $\text{5}\text{2}$ and $\text{J}\text{k}\text{= ?30°}\text{K}$. The Ba_1+xFe₂S₄ phases have Curie-Weiss behavior with an effective moment of about 2 B.M. The moment increases with x. These phases are metallic. The Mössbauer isomer shift varies linearly with valence, increasing with increasing x. The single quadrupole split absorption line characteristic of these compounds disappears at about 270°K and a complex spectrum consisting of overlapping hyperfine patterns appears at lower temperatures. Magnetic short-range ordering is responsible for this behavior although the susceptibility in this temperature range does not reflect this effect.     

Phases in the Ba3Fe1+xS5 series: The structure of β-Ba9Fe4S15 and its low-temperature α polymorph

The crystal structure of β-Ba₉Fe₄S₁₅ shows that it is a phase in the infinitely adaptive series of compounds Ba₃Fe_1+xS₅, 0 ? x ? 1. The material is synthesized by reacting a slightly sulfur-rich mixture at 900°C in a sealed quartz ampoule. Lattice constants are a = 25.212(3), Å, b = 9.594(1), Å, c = 12.575(1), Å, Pnma, z = 4. Three thousand thirty-three structure amplitudes were refined to R = 0.049. BaS₆ trigonal prisms share triangular faces to form infinite columns; the columns in turn share edges and create nearly hexagonal enclosures. Within these rings are additional Ba and S and tetrahedral interstices are created which can be occupied by Fe. The variation of the Fe occupancy from ring to ring gives rise to phases in which one dimension is an integral multiple of the 8.5-Å repeat observed in one end member of the series, Ba₃FeS₅. The other end member is Ba₃Fe₂S₅. At temperatures below 900°C a polymorphic phase is formed. Its lattice constants are a = b = 9.634(1), Å, c = 34.311(3)Å, $\text{I4}{}_1\text{a}\text{, z = 4}$. One thousand five hundred eighty-three structure amplitudes were refined to R = 0.0483. Trigonal prisms and bisdisphenoids articulate to form a complex three-dimensional structure. Two of the S atoms in the structure have statistical site occupancies.     

Studies on the compounds in BaFeS system. III. Phase relation of Ba1+xFe2S4 with infinitely adaptive structure

Samples of the infinitely adaptive phase Ba_1+xFe₂S₄ (or Ba_p(Fe₂S₄)_q; p, q: integer) were carefully prepared by changing the nominal composition and annealing temperature T_a. The single-phase materials, defined in this paper as a member of the infinitely adaptive series Ba_p(Fe₂S₄)_q, were obtained by the addition of excess sulfur in the nominal composition range 0.05 ≤ x ≤ 0.20 at T_a ranging from 650 to 880°C. X-Ray powder diffraction showed the existence of many members of the Ba_p(Fe₂S₄)_q series. The supercell periodicity was markedly dependent on T_a. The composition of reaction products estimated from X-ray diffraction, the method proposed by Grey based on crystallographic considerations, deviated in practice from the nominal composition. This fact suggests random distribution of Ba and Fe vacancies.     

Preparation and Mo¨ssbauer studies of (FeyNb1−y)1+xS2 compounds

The phase relations for iron niobium sulfides (Fe_yNb_1?y)_1+xS₂ have been examined by varying the partial pressure of sulfur at 950°C. While niobium is difficult to dissolve in iron sulfide, iron dissolves in niobium sulfide up to about 35% of the total metal sites. Iron niobium sulfide has the layered hexagonal type structure (2s-Nb_1+xS₂) with change in the lattice parameters depending on both the value of x and the amount of the iron dissolved. The Mo¨ssbauer spectra of sulfides with three different Fe/Nb ratios, 1/9(y =1/10), 1/4(y =1/5), and 1/2(y =1/3) were taken at 77 and 295 K. Each spectrum is composed of a quadrupole doublet which can be attributed to the Fe²⁺ ions in high spin state. The quadrupole splitting at 295 K decreases markedly with decrease in x which is related to change of the lattice parameters. Fe atoms cannot enter at random into all metal sites, and prefer to intercalate in the sites of partially filled layers. Possible models for the cation distribution in each metal layer are discussed.     

Les conducteurs ioniques NaxYxZr1−xS2

Results about new ionic conductors Na_xY_xZr_1?xS₂ are presented. The electronic conductivity normally induced by sodium intercalation layered chalcogenides has been suppressed by performing at the same time a substitution of zirconium by yttrium in the slabs of the host structure. Electrical measurements using the complex impedance technique lead to the activation energy of the ionic conduction. This energy was found to present a marked minimum around x = 0.50. A NMR study showed quadrupole coupling to prevail in relaxation process. Electrical field gradient is maximum for x = 0.50. The activation energy of the local motion remains practically constant throughout the composition range. The above results are compared to others obtained from compounds having the same structure.     

Structure and magnetic properties of sulfides of the type CdRE2S4 and Mg(GdxYb1−x)2S4

CdRE₂S₄ (RE = Gd, Tb, Dy, Ho, Er, Tm, and Yb) and Mg(Gd_xYb_1?x)₂S₄ were prepared by solid-state reactions. All the cadmium-containing compounds are cubic, i.e., the Th₃P₄ structure for Gd, Tb, and Dy and the spinel type for all the others. The first three compounds were deficient in CdS. In the case of the Mg system, for x = 1 the system is cubic Th₃P₄, for x = 0 cubic spinel, and for 0 < x < 1 orthorhombic MnY₂S₄ (Cmc2₁). All the materials studied are paramagnetic above 77 K. Below 77 K in the magnesium family both cubic materials are paramagnetic down to 4.2 K and the orthorhombic materials show magnetic ordering. In the cadmium family all but CdTm₂S₄ show exchange coupling.     

The crystal structure of “Ba2Fe4S5”: A two-dimensional array of FeS4 tetrahedra

The crystal structure of a compound with a nominal composition Ba₂Fe₄S₅ was determined from 432 observed structure amplitudes. The lattice constants are a = 4.016(2), Å, b = 9.616(4), Å, c = 6.514(4), Å, Pmmn, Z = 1. The structure consists of BaS₆ trigonal prisms that share triangular faces to form infinite columns parallel to a. Zig-zag chains of Fe are formed parallel to b by filling the tetrahedral interstices. The structure can also be viewed as an infinite two-dimensional plane of FeS₄ tetrahedra formed by the sharing of two edges of a face in the ±a directions and connecting by corner sharing in the ±b directions. The Ba are in the trigonal prismatic interstices. The apical sulfur atom shows an anomalously large temperature factor and has a 50% site occupancy. On the basis of full occupancy of that site the composition corresponds to BaFe₂S₃. This structure is closely related to the previously reported BaFe₂S₃ and may be an averaged structure due to twinning of a polymorph of BaFe₂S₃.     

Nonstoichiometric oxides with a layer structure: The compounds A1−x(Ti1−xM1+x)O5

New nonstoichiometric oxides A_1?x(Ti_1?xNb_1+x)O₅ and tantalates ATiTaO₅ with a layer structure of the KTiNbO₅ type have been isolated, with A = K, Rb, Tl, Cs. These oxides, which are orthorhombic, space group Pnma, are characterized by a preferential occupation of one type of site 4c by the titanium atoms. The structural evolution as a function of composition and the stability of these compounds are discussed.     

Luminescent and structural properties of the series Ba6−xEuxTi2+xTa8−xO30 and Ba4−yKyEu2Ti4−yTa6+yO30

The series Ba_6−xEu_xTi_2+xTa_8−xO₃₀ and Ba_4−yK_yEu₂Ti_4−yTa_6+yO₃₀ have been synthesized at 1400°C in air. They exhibit efficient excitation at about 400 nm and typical emission of Eu³⁺ at about 580-620 nm, form solid solutions within 0.0?x?2.0 and 0?y?4 respectively, and crystallized in P4/mbm at room temperature with Eu atoms occupied at centrosymmetric site (0, 0, 0). Their conductivity is very low (2.8×10⁻⁶ Ω⁻¹ cm⁻¹ at 740°C for Ba₆Ti₂Ta₈O₃₀).     

Heat capacity measurements of MnxFe3−xO4

Heat capacities of Mn_xFe_3?xO₄ with the composition x = 1.0, 1.5, and 2.0 were measured from 200 to 740 K. λ-type heat capacity anomalies due to the ferri-paramagnetic transition were observed for all the compositions. The transition temperatures were 577, 471, and 385 K for the composition x = 1.0, 1.5 and 2.0, respectively, which are in good agreement with the results of magnetic measurements. The difference in heat capacities between the different samples was small except for the temperature range of the transition. The magnetic contribution to the observed heat capacity was obtained by assuming that the heat capacity can be expressed by the sum of the lattice heat capacity C_v (l), the dilation contribution C(d), and the magnetic contribution C(m). Entropy changes due to the transition were obtained from C(m) as 55.5, 50.7 and 49.2 J K^?1 mole^?1 for the composition x = 1.0, 1.5, and 2.0, respectively. The entropy changes were also calculated by assuming the randomization of unpaired electron spins on each ion, but they were from 6 to 10 J K^?1 mole^?1 smaller than the observed ones. The difference between the experimental and the calculated values is roughly explained by taking into account the cation exchange reaction between the tetrahedral and the octahedral sites in the spinel structure.     

Crystal structure,Mössbauer,and magnetic behavior of mixed valence compounds in the BaFeS system: Ba3(Ba1−xAlx)Fe2S6[S1−y(S2)y]

The compounds $\text{Ba}{}_4\text{Fe}{}_2\text{S}{}_6\text{[S}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}\text{(}\text{S}{}_2\text{)}{}_{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{]}$ and Ba_3.6Al_0.4Fe₂S₆[S_0.6(S₂)_0.4], designated I and II, were prepared by reacting BaS, Fe, and S powders and Al foils in graphite containers sealed in evacuated quartz ampoules at approximately 1100°C. The crystal structure of I was determined using 1682 independent, nonzero X-ray reflections, while 3589 were used for II. They are triclinic, $\text{A}\text{l}$:

$\text{a=9.002(2)}\text{A}\text{?}\text{,b=6.7086(8)}\text{A}\text{?}\text{,c=24.658(4)}\text{A}\text{?}\text{α91.49(2)°,}$

$\text{β=105.10(2)°y=90.74(2)°,ψ}{}_{\mathrm{calc}}\text{=4.15g/cm}{}^3\text{,for I:}$

$\text{a=8.993(6)}\text{A}\text{?}\text{,b=6.708(7)}\text{A}\text{?}\text{,c=24.70(1)}\text{A}\text{?}\text{α91.11(6)°,}$

$\text{β=105.04(6)°y=90.90(9)°,ψ}{}_{\mathrm{calc}}\text{=3.90g/cm}{}^3\text{,for II:}$

BaS₆ trigonal prisms share edges to form distorted hexagonal rings which form one-dimensional chains leaving two free lateral edges. The chains link in a stairstep manner with the rings offset along the [301] direction. These stairsteps join in a complicated manner to form a three-dimensional network. Fe ions are in two sites forming isolated FeS₄ tetrahedra and isolated Fe₂S₆ dimers by edge-sharing tetrahedra. The Al substitution occurs in the trigonal prisms which have free edges with Al replacing Ba. Room-temperature Mössbauer isomer shifts are 0.20 mm/sec. for I and 0.30 mm/sec for II. These data indicate that upon Al substitution charge compensation occurs by reducing Fe³⁺. Valence calculations indicate that Fe in edge-sharing tetrahedra are reduced while the Fe in the isolated tetrahedron remains unchanged. The effective charge distribution in the Al substituted compound is approximately Fe³⁺, Fe^2.5+ with electron delocalization across the shared edge. Room temperature electrical resistivity is 10⁵ ohm/cm. The compositions of the crystals are best represented by the formulas $\text{[}\text{Ba}{}_4\text{Fe}{}_2\text{S}{}_7\text{]}{}_{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}\text{·[}\text{Ba}{}_4\text{Fe}{}_2\text{S}{}_6\text{(S}{}_2\text{)]}{}_{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}$ and [Ba₃AlFe₂S₇]_0.4·[Ba₄Fe₂S₇]_0.2·[Ba₄Fe₂S₆(S₂)]_0.4. The replacement of a sulfide by a disulfide ion is thought to be strongly dependent on the sulfur activity during the preparation.     

Localized moments in the superconducting Li1+xTi2−xO4 spinel system

Electron spin resonance (ESR) and magnetic-susceptibility measurements on the Li_1+xTi_2?xO₄ spinel system ($\text{0 ≤ x ≤}\text{1}\text{3}$) indicate the presence of two types of localized moments in this material. In both cases, an unpaired electron is trapped as a Ti³⁺ ion in a crystal field that is predominantly octahedral, but with a strong tetragonal component. This type of crystal field cannot arise in the stoichiometric spinel. We propose two types of defect in the title spinel system: an oxygen vacancy and a hydroxyl ion. Unpaired electrons are trapped as Ti³⁺ ions adjacent to these defects, and it is argued that the strong tetragonal field is associated with the formation of a static (TiO)⁺ ion by a displacement of the titanium ion from the defect. Spin relaxation occurs via a thermal ionization of the trapped electron that appears to be associated with a static-dynamic transition in the titanium-ion displacement.     

Magnetic properties of FexV3−xS4 (0 ≤ x ≤ 2)

The magnetic susceptibility data of Fe_xV_3?xS₄ (0 ≤ x ≤ 2) are reported in the temperature range between 4.2 and 1300 K. The behavior of the susceptibility at high temperatures changes significantly at the composition boundary x = 1.0. The magnitude of the effective magnetic moment remains unchanged at 3.2 μ_B in the composition range x < 1.0. It decreases with increasing iron content in the range > 1.0, and rapidly decreases for x close to 2.0. The c lattice parameter varies in a manner analogous to the change in magnetic moment. These phenomena suggest that metallic bonding forms between metal layers and that it becomes stronger with increasing in x. The susceptibility measurements at low temperatures show that Fe_xV_3?xS₄ is basically antiferromagnetic, although some of the Fe_xV_3?xS₄ compounds become weakly ferromagnetic after cooling in a magnetic field. The origin of the weak ferromagnetism is briefly discussed.     

Structure de La2Fe2S5 et de La2Fe1.87S5

The compound La₂Fe₂S₅ is orthorhombic. Cell parameters are: a = 3.997(2)Å; b = 16.485(5)Å; c = 11.394(4)Å. Space group is Cmc2₁ (Z = 4. In the cell, chains of polyedra comprised of sulfur atoms tetrahedrally or octahedrally coordinating centrally located iron atoms give a monodimensional character to the structure. This one is refined to R = 0.037. To complete the study of these chains, in the La₂Fe_2?xS₅ system, vacancies are introduced on iron atom sites. The ordered compound, La₂Fe_1.87S₅, having such vacancies, is an orthorhombic superstructure of the stoechiometric compound. Cell parameters are: a = 3.9996(5)Å; b = 49.508(3)Å; c = 11.308(3)Å. Space group is Cmc2₁ and Z = 12. The structure is refined to R = 0.068. Only two iron atom sites have vacancies. One is tetrahedral, the other octahedral. In this last case the chain deformations are the more important. The chain becomes a sort of tunnel made of atoms of sulfur, with in its center the short iron-iron separation of 2.82 Å.     

The Ba1−ySryMnO3−x system

Subsolidus phase relations at ambient atmospheric pressure and elevated temperatures in the Ba_1?ySr_yMnO_3?x system were investigated by quenching, gravimetric, and X-ray diffraction methods. The system is not binary above ～1035°C because of reactions with atmospheric oxygen. The air isolar, P_O2 = 0.2 atm, was characterized at 1225, 1375, 1490, and 1610°C. Seven oxygen-deficient phases including a perovskite phase characterize the system. Their stability depends on the values of y and x in Ba_1?ySr_yMnO_3?x. The cell dimensions of these phases expand as x increases at fixed y. These seven modifications can be retained in stoichiometric form by oxidation at lower temperatures.     

Etude structurale et magnétique de pérovskites de formule Ba3Fe2−xHoxUO9

The compounds crystallize in cubic perovskite-type system. The space group is Fm3m and the cell is entirely ordered. 4a positions (Wyckoff's notation) contain iron and uranium, 4b positions, barium and iron, 8c positions, barium and any holmium, which therefore fills coordinance-12 crystallographic sites. For iron-rich compounds (0 < x ? 0.8), interactions between Fe³⁺ ions from two magnetic sublattices 4a and 4b are antiferromagnetic. The interactions between the third magnetic sublattice, which corresponds to Ho³⁺ ions filling 8c positions, and the resultant of the first two, are slightly ferromagnetic. Such data have been obtained with all the other compounds for which the formula is Ba₃Fe_2?xLn_xUO₉, where Ln is Yb, Tm, Er, Dy, Tb, and Gd.     

Study of Pr1−xMn1+xO3 perovskites

The structural and magnetic properties of the Pr_1?xMn_1+xO₃ perovskites were studied. The increase of x (i.e., $\text{Pr}\text{Mn}\text{< 1}$) leads to the decrease of the orthorhombic deformation and of the Néel temperature and, simultaneously, to an increase of the ferromagnetic contribution. The latter effect is explained from the suggested distribution of the cations (Pr³⁺_1?xMn²⁺_x)_A(Mn³⁺_1?xMn⁴⁺_x)O^2?₃ by the double exchange of Mn³⁺Mn⁴⁺ pairs at the B—sublattice.     

Thermodynamic determination of the cation distribution in NixMn1−γ−xFe2+γO4 ferrites

The cation distribution in the spinel ferrite system Ni_xMn_1−γ−xFe_2+γO₄ (x=0, 0.25, 0.5, 0.75 and γ=0.137) has been calculated analytically in complete form as a function of thermodynamic parameters. A generalized theoretical framework based on the O’Neill-Navrotsky model and Newton methods was used to solve a multicomponent system for up to 10 cation species. The relationship between the cation distribution and composition is given. The results are shown to agree with the available experimental results.     

A study of the Li1+xTi2−xO4 spinel system by diffuse-reflectance spectroscopy

The room-temperature diffuse-reflectance spectra of compositions within the Li_1+xTi_2?xO₄ spinel system ($\text{0 ≤ x ≤}\text{1}\text{3}$) show three absorption bands in the range 4000 to 48,000 cm^?1. Two high-energy absorption bands correspond to charge-transfer transitions from the oxygen-2p valence band to the titanium t_2g and σ1 conduction bands, where the σ1 band of e_g character has hybridized titanium-3d and titanium-4s parentage. The absorption band arising from promotion of electrons to the empty σ1 band does not alter with composition whereas the absorption band arising from promotion of electrons to the partially filled t_2g band narrows as the concentration of conduction electrons in the t_2g band decreases. These two high-energy absorption bands fall entirely within the ultraviolet spectral region, and the absorption edge in $\text{Li}{}_{\mathrm{<mtext>4</mtext><mtext>3</mtext>}}\text{Ti}{}_{\mathrm{<mtext>5</mtext><mtext>3</mtext>}}\text{O}{}_4\text{(x =}\text{1}\text{3}\text{)}$ occurs at 24,300 cm^?1 (3.02 eV). A low-energy absorption band is observed in compositions with $\text{x <}\text{1}\text{3}$ and in samples of $\text{Li}{}_{\mathrm{<mtext>4</mtext><mtext>3</mtext>}}\text{Ti}{}_{\mathrm{<mtext>5</mtext><mtext>3</mtext>}}\text{O}{}_4$ reduced in hydrogen at elevated temperatures. This band straddles the boundary between the visible and infrared spectral regions and shifts toward lower energy as the concentration of conduction electrons in the t_2g band decreases. The possible origins of the band are discussed; the argument is in favor of a d-d interband transition from states in the partially filled t_2g band to states in the empty σ1 band.     

Structural Characterization of the Complex Perovskites Ba1−xLaxTi1−xCrxO3

The series Ba_1−xLa_xTi_1−xCr_xO₃ (0≤x≤1) was synthesized at 1400°C for about 60 h. Their structure was carefully analyzed by the use of powder X-ray diffraction and Rietveld analysis software GSAS (General Structure Analysis System). Four solid solutions are found in this series: tetragonal solid solution Ba_1−xLa_xTi_1−xCr_xO₃ (0≤x≤0.029), cubic solid solution Ba_1−xLa_xTi_1−xCr_xO₃ (0.0365≤x≤0.600), rhombohedral solid solution Ba_1−xLa_xTi_1−xCr_xO₃ (0.700≤x≤0.873), and orthorhombic solid solution Ba_1−xLa_xTi_1−xCr_xO₃ (0.956≤x≤1). There are corresponding two-phase regions between the adjacent two solid solutions. The detailed lattice parameters are presented. The relationship between the lattice parameters and the composition of the solid solutions is developed.