Oxygen Positional Parameter Research Articles

Mössbauer, infrared, and X‐ray studies of Ti‐doped CoCr1.2Fe0.8O4 ferrites

AbstractThe spinel samples Co1+xTixCr1.2–2xFe0.8O4 (0 ≤ x ≤ 0.6) were investigated at room temperature using Mössbauer, X‐ray, and IR spectra. The Mössbauer spectra for x ≤ 0.1 showed only broad six‐lines patterns and for 0.2 ≤ x ≤ 0.5 showed, in addition to the broad six‐lines patterns, a central paramagnetic phase C. The spectrum for x = 0.6 showed only paramagnetic phase C. The six‐lines patterns were analysed to two magnetic patterns attributed to Fe3+ ions on the tetrahedral A‐sites and to Fe2+ and Fe3+ ions on the octahedral B‐sites. The broader B‐site pattern appeared to be composite for x ≤ 0.3, thus it has been analysed to its multicomponents. The paramagnetic phase C was analysed to two quadrupole doublets due to Fe3+ ions at the A‐ and B‐sites. The obtained hyperfine interaction parameters have been studied and discussed as functions of the composition factor x. The cation distributions have been estimated and supported by X‐ray studies. Six absorption bands were observed in the IR spectra and attributed to Fe3+ ions on the A‐ and B‐sites, to Fe2+ and Co2+ ions, and to the energy of the sublattices. The IR bands revealed the existence of Fe2+ ions in the samples. The true and theoretical lattice parameters, the oxygen positional parameter, and the ionic radii, bonds, and edges of the A‐ and B‐sites have been determined from the X‐ray analysis. They were studied and discussed in dependence of x.

Cation distribution and cooling rates of Cr-substituted Mg-Al spinel from the Olkhon metamorphic complex, Russia

A crystal chemical study was carried out on seven Mg-Al rich and Cr-bearing spinels from a calciphyre of the Olkhon metamorphic complex (western Lake Baikal, Russia) in order to: a) verify the structural effects related to the progressive substitution of Cr 3+ , the main substituent cation in this suite pertaining to the MgAl 2 O 4 - MgCr 2 O 4 binary join; b) estimate the closure temperature of the exchange reaction involving Mg and Al in tetrahedral (T) and octahedral (M) sites. This will be a first step towards evaluation of the cooling rate of the host rock. Chemical composition is Mg (Al 2-p , Cr p ) O 4 with 0.03 ≤ p ≤ 0.23 afu. With increasing p, the cell edge a increases from 8.090 to 8.117 A, whereas the oxygen positional parameter u is almost constant. As u is related to the (M-O)/(T-O) ratio, Cr substituting for Al in the M site causes an increase not only of the M-O bond distance, but also that of the tetrahedral T-O. The observed T-O increase is due to Mg ordering into the T site, with increasing Cr content. Cation distributions where used to estimate the closure temperature of the exchange reaction applying two available models. Both estimates are rather consistent for very low Cr content, but substantial disagreement is observed for Cr-rich samples. The inversion trend of the studied spinel suite extrapolated to Cr= 0, enables us to calculate the closure temperature of the MgAl 2 O 4 end-member with the same cooling rate. This suggests that kinetic results from cooling experiments on MgAl 2 O 4 may be applied to cation distributions measured in samples along the MgAl 2 O 4 - MgCr 2 O 4 binary join.

Oxygen Position, Octahedral Distortion, and Bond‐Valence Parameter from Bond Lengths in Ti1−xSnxO2 (0 ≤x≤ 1)

Based on the virtual crystal approximation (or Vegard's law), the bond lengths of Ti1−xSnxO2 were deduced from those of TiO2 and SnO2, to allow the oxygen position and octahedral distortion to be determined as a function of x. The oxygen positional parameter (u) increased linearly when the Sn4+ cation (which has a larger ionic radius) was substituted for the Ti4+ cation, whereas the octahedral distortion exhibited a nonlinear decay with increasing x in Ti1−xSnxO2. At the same time, the bond‐valence parameter, which relates bond valence to bond length, so that the central atom in the octahedron can retain a constant valence of +4.0, exhibited a correlation with u for Ti1−xSnxO2. The present results indicate that the different phonon/physical properties of TiO2 and SnO2 and/or their dependence on x in Ti1−xSnxO2 can be associated with different octahedral distortions.

Cation ordering and structural variations with temperature in MgAl2O4spinel: An X-ray single-crystal study

The equilibrium distribution of Mg and Al between the tetrahedral and octahedral sites of a flux grown, stoichiometric MgAl2O4 spinel was investigated between 600 and 1100 °C by single-crystal X-ray diffraction of quenched samples. The cation distribution for both ordering and disordering runs was obtained by minimizing accurate crystallographic parameters and effective ionic radii. Along with the variation of the degree of inversion from 0.18 to 0.29 between 600 and 1100 °C, both unit cell and oxygen positional parameter decreased linearly. Multiple non-linear least-squares fit of our data with the thermodynamic model of O’Neill and Navrotsky (1983) gave α = 23 ± 2 KJ/mol and β = 13 ± 4 KJ/mol. The influence of both cation inversion and thermal expansion on T-O and M-O bond length variation was determined by comparison of our data with previous in situ studies. In the thermal interval investigated, the inversion accounts for change of −0.014 A for T-O and +0.007 A for M-O. Mean linear polyhedral thermal expansion coefficients of 6.5 × 10−6 °C−1 and 8.9 × 10−6 °C−1 were calculated for T and M sites, respectively.

Cation distribution and magnetic interactions in Zn-substituted CuFe2O4 ferrites

Abstract A series of Cu 1− x Zn x Fe 2 O 4 , spinel ferrites with x =0, 0.25, 0.50, 0.75 and 1.0, respectively, were prepared by standard ceramic method to study the relationship between structural parameters and concentration of the substituted non-magnetic (Zn) ion. Determination of crystal structure oxygen positional parameter and cation distribution using X-ray diffraction (XRD) and R -factor method revealed that these ferrites belong to the family of mixed or partially inverse spinels. The magnetic interaction has been calculated from ac susceptibility measurements using mutual inductance technique as a function of concentration of the substituted ions in these mixed ferrites. The dominant interaction in all ferrite samples is A–B interaction which is due to the negative values of paramagnetic Curie temperature θ (K) and the distribution of cations among A and B sites showing that the magnetic ordering is antiferromagnetic. The Curie temperature ( T c ) rises upto x =0.75 possibly due to increase in exchange interaction and magnetic moments. For x >0.75, T c decreases due to cation distribution (normal) on A and B-sites resulting in the weakening of A–B interaction due to the presence of triangular spin arrangement of Y–K type on B-sublattice.

Cation distribution in Cu-substituted manganese ferrites

A series of spinel ferrites, i.e., Mn 1− x Cu x Fe 2O 4, with x=0, 0.25, 0.50, 0.75 and 1.0 were prepared by the standard ceramic method, aiming to study the relationship between structural parameters and concentration of the substituted copper ions. By determining the crystal structure, oxygen positional parameter and cation distribution through X-ray diffraction (XRD) and the R-factor method, it is revealed that these ferrites belong to the family of mixed or partially inverse spinels.

Bond Valence Analysis of Tetragonal Zirconias

In tetragonal zirconia, the cation is coordinated by two interpenetrating tetrahedra of oxygen ions, implying two different cation–oxygen bond lengths. On substituting the different tetravalent ions Ge, Ti, Sn, and Ce into tetragonal ZrO2–2 mol% Y2O3, the mean value of the shorter cation–anion bond length varies linearly with the concentration of the substituent ion where the bond length increases or decreases depending on whether the substituted ion is larger or smaller than the zirconium ion it replaces. It is argued in this paper that the length of the longer bond is determined by the requirement that the bond valence sum remains constant. In each case the length of the longer bond calculated on this basis is in good agreement with the measured bond length (from neutron diffraction), and following small adjustments of the bond valence constants, excellent agreement is obtained. The requirement for the bond valence sum evidently accounts for the physics of the situation, and at the same time the available bond length data allow very precise determination of the bond valence constants of the different ions in the tetragonal zirconia environment. It is shown how these bond length considerations provide an explanation for the variation with composition of oxygen position and lattice parameters in all of the materials considered. Among the interesting features accounted for by this analysis are the increase in cell volume occurring when Zr is replaced by the smaller Sn ion, and slight departures from Vegard's law observed in the substitution of Zr by Ti.

High pressure single crystal X-ray diffraction study on ruby up to 31 GPa

Abstract The high pressure behavior of ruby (α-Al2O3 doped with Cr3+) was investigated to pressures up to 31 GPa. Whereas data on compressibility has been obtained at pressures up to 175 GPa, no structure determination exists above 9 GPa. Results show that the highly incompressible material has a bulk modulus of K = 242(8) GPa and its pressure derivative K′ is calculated to be 6(1). In order to compensate for the applied forces, the MO6 octahedra (M = Al, Cr) volume decreases with increasing pressure. At pressures above 20 GPa, a distinct change in the oxygen positional parameter is observed. The reason might be a short M–O distance of 1.814(2) Å which stays constant throughout the pressure range investigated. A comparable effect of a considerable change of the internal structure under high pressure despite a very low compressibility was recently observed for the mineral spinel [1].

High-density phases of ZnO: Structural and compressive parameters

Dense ZnO phases, obtained by static compression at room temperature, are studied by x-ray diffraction using synchrotron radiation. The initial wurtzite structure of ZnO is observed to transform to the rocksalt structure at $9.1\ifmmode\pm\else\textpm\fi{}0.2\mathrm{GPa}$ with a large volume collapse of 16.7% on increasing pressure. Accurately determined structural and compressive parameters over a large pressure range for both phases are presented. We report the following values for the null compression bulk moduli and their pressure derivatives: 142.6 GPa and 3.6 and 202.5 GPa and 3.54, for the wurtzite and the rocksalt phases, respectively. From a comparison between the present results and calculated ones, it is inferred that Hartree-Fock calculations yield the best estimation of the compressive parameters in dense ZnO. Furthermore, we show that the ideal relationship between the $c/a$ axial ratio and the $u$ oxygen positional parameter is preserved as the wurtzite lattice becomes denser while approaching the structural transition pressure.

Structural characterisation of the [formula omitted]-type, high pressure phase of ruthenium dioxide

Cubic Pa3̄-type RuO 2 was prepared from the ambient pressure, rutile-structured phase at 20 GPa and 1100 °C in a multianvil device. The structure of this Pa3̄ phase was refined by time-of-flight, neutron powder diffraction on the quenched sample yielding a cell constant a = 4.85892(3) A ̊ and an oxygen positional parameter u = 0.35115(8). The ruthenium cation is rhombohedrally coordinated with six anions at 1.9893(4) Å and two more distant anions at 2.9552(4) Å. The minimum interpolyhedral OO distance of 2.5045(5) Å in this structure is the shortest known in any solid and is shorter than the intrapolyhedral OO distances, which are of 2.6208(5) Å. These short distances are the origin of the very low compressibility of this oxide phase, which approaches that of diamond. The Raman spectrum of Pa3̄-type RuO 2 is consistent with group-theoretical calculations.

X-ray diffraction studies of Sn/Nb-substituted Mn-Zn ferrites (*)

X-ray diffraction studies of tin- or niobium-substituted Mn-Zn ferrites have been carried out. The lattice parameter was obtained. Using some theoretical equations, possible cation distribution of these ferrites was proposed theoretically. Experimentally obtained lattice constant values were compared with the theoretically evaluated values. Radius of the tetrahedral, octahedral sites, ratio of the octa-to-tetra bond lengths and oxygen positional parameter values have been calculated and these are interpreted in terms of ionic radius of the cations.

Neutron Diffraction Study of the Ambient-Pressure, Rutile-Type and the High-Pressure, CaCl2-Type Phases of Ruthenium Dioxide

The structures of the ambient-pressure, rutile-type and the high-pressure, CaCl2-type phases of RuO2 were refined by time-of-flight neutron powder diffraction. Refinement of the data obtained from the ambient-pressure phase (P42/mnm, Z = 2) yielded cell constants a = 4.49307 (7) and c = 3.10639 (7) Å and an oxygen positional parameter x = 0.3056 (1). The principal axes of the displacement ellipsoids of the oxygen ions lie along the directions which correspond to the normal coordinate of the B 1g soft mode that drives the transition to the CaCl2-type structure. The CaCl2-type phase (Pnnm, Z = 2) was refined in situ at 5.3 (3) GPa and the following cell constants and oxygen positional parameters were obtained: a = 4.4865 (5), b = 4.4347 (5), c = 3.0934 (3) Å, x = 0.3101 (5) and y = 0.3005 (5). In the CaCl2-type phase at 5.3 (3) GPa the RuO6 octahedra are rotated by 1.1 (2)° with respect to their orientation in the ruffle-type phase. There is little change in the internal angles of the octahedron between 0.1 MPa and 5.3 GPa. Compression of the octahedron is anisotropic with a much greater reduction of the Ru—O contacts in the ab plane.

Structural trends in Bi containing pyrochlores: The structure of Bi 2Rh 2O 7 − δ

The room temperature structure of Bi 2Rh 2O 7 − δ has been refined from neutron powder diffraction data, λ = 1.8857 Å, cubic Fd 3̄m, a = 10.2763(1) Å, V = 1085.23 Å 3, Z = 8, R p = 0.040, R wp = 0.054, R exp = 0.035 for 45 reflections. The structure is of a pyrochlore type with the oxygen position parameter of 0.3317(4) where there are both bismuth and oxygen vacancies. The anion vacancies are in the 8b O′ site. The structure is compared with that of a number of other Bi containing pyrochlores.

Structural and Bonding Trends in Tin Pyrochlore Oxides

The structures of a number of pyrochlore stannates of the typeLn2Sn2O7, withLn=Y, La, Pr, Nd, Tb–Lu andLn=Sm, Eu, and Gd, have been refined by Rietveld analysis from 1.4925 Å neutron powder and 1.0 Å X-ray powder diffraction data, respectively. The single variable oxygen positional parameter is observed to decrease systematically with the increasing lattice parameter, causing periodic shifts in both the Sn–O and Sn–O–Sn interactions. TheLntemperature factors are highly anisotropic, and this is explained in terms of the stereochemistry of theLnatoms. Bond valence sum calculations of the materials have been performed to explore changes in the bonding resulting from the structural changes.

Determination of Cation Distribution in Nickel Zinc Ferrites by X-Ray Diffraction

A series of ferrites of chemical composition Ni_{1-x}Zn_{x}Fe_{2}O_{4} (x=0.0, 0.25, 0.50, 0.75 \& 1.00) prepared by solid-state reaction are found to have cubic spinel structure at 1300^{\circ}C. The cation distribution is studied through structural parameters such as the oxygen positional parameter U and nearest neighbour distances between an oxygen ion and tetrahedral position in these materials. The lattice parameter and the oxygen positional parameter are found to vary with the concentration of Zn. The results are analyzed on the basis of tetrahedral and octahedral positions preference of the cations.

Neutron diffraction study of chromium substituted nickel ferrite

Neutron diffraction study has been carried out to determine lattice parameter ( a), oxygen positional parameter ( u) and cation distribution in NiCr x Fe 2− x O 4 system. The magnetic moment ( n B ) values measured from hysteresis and from neutron diffraction study agree with each other. However, these are lower than the theoretical values computed from Neel's colinear model. The Gilleo's model is adopted to explain the experimental results. The T c s calculated by this model and those measured from a.c. susceptibility agree well up to x < 0.8 and differ for higher content of x ≥ 0.8. The results show that spinel with x < 0.8 have Neel type magnetic order while for x ≥ 0.8 the magnetic order is not of Neel type.

A Powder Neutron Diffraction Study of Semiconducting and Metallic Niobium Dioxide

Rietveld analysis of neutron powder diffraction patterns from niobium dioxide at 295 and 1300 K have been performed. At 1300 K NbO2 adopts a rutile-type structure (P42/mnm, a = 4.8463 Å, c = 3.0315 Å, x = 0.2924), whereas the room temperature form can be described as a superstructure with a subcell of the rutile type (I41/a, a = 13.7020 Å, c = 5.9850 Å). Both the c axis and the oxygen position parameter in the rutile phase are anomalously small compared with other rutile dioxides. This can be rationalized in terms of M-M bonding. Both the niobium and the oxygen thermal displacement are large in the rutile phase.

Low temperature synthesis of tetragonal nickel manganite spinels: Thermal behaviour and reactivity

A route for the low temperature preparation of nickel manganite spinels is described. This method has allowed to obtain, for the first time, tetragonal nickel manganite spinels NixMn3-xO4, in the composition range 0.6 ≤ x ≤ 1.0. This phase has been prepared by heating at 200°C gels obtained by addition of 1M n-butylamine to mixed Ni2+ and Mn2+ solutions. In this paper, we report about the results obtained from X-ray studies, of the thermal evolution and reactivity for the tetragonal spinel with x = 0.8, between 200° to 1000°C. The formation of a metastable γ-cation deficient cubic spinel is attained at 300°C; it decomposes at ≈ 500°C, giving at 600°C a mixture of NiMnO3 and α-Mn2O3. Above 800°C, the reaction between these two phases leads to the formation of the high temperature cubic spinel, in which (Mn4+Mn3+)B ≈ 1.0. Cell lattice and oxygen positional parameters have been determined from Rietveld refinements. From these data, ionic configuration and cation distribution formulae for cubic spinels have been derived. These results can account for the mechanism of transformations that takes place during the thermal treatments.

Crystal structures and cation distributions in simple spinels from powder XRD structural refinements: MgCr2O4, ZnCr2O4, Fe3O4 and the temperature dependence of the cation distribution in ZnAl2O4

The crystal structure and cation distributions in the spinels MgCr2O4, ZnCr2O4, Fe3O4 and a suite of ZnAl2O4 samples annealed at 900 to 1400° C and then rapidly quenched, have been determined by powder X-ray diffraction, using several different X-ray procedures and both conventional structure-factor refinement and whole-pattern (or Rietveld) refinement methods. The chromite spinels are expected from crystal chemical considerations to have an almost completely normal cation distribution (inversion parameter, x, equal to zero). In agreement with this expectation, three samples of MgCr2O4 annealed at 900, 1100 and 1300° C, and ZnCr2O4 were all found to have x=0 within two estimated standard deviations (esd), suggesting that the accuracy with which cation distributions in spinels may be determined by powder XRD is close to the estimated precision. Slightly better results are obtained assuming neutral-atom scattering curves rather than half-ionized or fully ionized, but the differences are small (within the esd). The results from the Rietveld refinements are similarly in good agreement with those using the conventional structure factor refinement approach (agreement within the combined esd's), although in detail the Rietveld procedure sometimes produces small systematic differences in refined parameters. The suite of ZnAl2O4 spinels show a smooth increase in x from 0.01 at 900° C to 0.05 at 1300° C, and this behaviour is well described by the simple thermodynamic model for disordering in spinels with αZn-Al=89 kJ/mol, assuming β=−20 kJ/mol. The oxygen positional parameters for Fe3O4 are similar to those from published single crystal studies, indicating that the powder method also yields accurate interatomic distances in spinels.

Crystal Structure Refinements of Zircon-Type MVO 4 ( M = Sc, Y, Ce, Pr, Nd, Tb, Ho, Er, Tm, Yb, Lu)

Improved structural parameters for the lanthanide orthovanadates (including also YVO 4 and ScVO 4 and new results for PrVO 4, TmVO 4, and the LuVO 4 end members) have been determined by employing a full-profile Rietveld structure analysis of neutron powder-diffraction data. High-quality powders of the zircon-type MV O 4 vanadates were prepared by a homogeneous coprecipitation in molten urea. The zircon structure has space group symmetry I 4 1/ amd, Z = 4, with 3 atoms occupying the asymmetric unit. In all, 22 parameters were refined with final agreement values of R p = 0.0313-0.0421, R wp = 0.0361-0.0511, and G-of-F = 1.210-1.962. The average V-O bond length for all of the MVO 4 samples is 1.709(2) Å, and the V-O distance exhibits a small systematic shortening with decreasing M atom size. The oxygen positional parameters, the cell dimensions, and the M-O distances vary systematically with the metal-ion atomic number. The anisotropic thermal motion of the oxygen atom has the largest amplitude normal to the shared polyhedral edge between the VO 4 tetrahedron and MO 8 bisdisphenoid. Empirical relations are given for the cell dimensions and oxygen atom position as a function of the size of the lanthanide ion which can be used to estimate structural parameters for other zircon-type lanthanide vanadate end-members and vanadate solid solutions.