Anion Disorder Research Articles

B(3,5‐C6H3Cl2)4]– as a Useful Anion for Organometallic Chemistry

AbstractWe highlight the use of the robust [B(3,5‐C6H3Cl2)4]– anion in organometallic chemistry. When partnered with organometallic cations, compared with [BArF4]– it presents desirable solubility properties, a lack of anion disorder in the solid‐state, different coordinating properties with metal fragments and convenient metathetical routes for utilisation in synthesis.

ChemInform Abstract: Synthesis of Barium Imide from the Elements and Orientational Disorder of Anions in BaND Studied by Neutron Diffraction from 8 to 294 K.

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Anion disorder in LnF3 (Ln = La, Ce, Pr) crystals

The mechanism of lattice “melting” in tysonite-structure superionic trifluorides is analyzed in relation to the structure of LaF3 crystals. The ion displacements within the unit cell are shown to correlate with the dynamics of the Raman scattering parameters of the crystals. The frequency and intensity of the lines at 370, 392 (LaF3), 376, and 404 cm−1 (PrF3) show anomalous behavior in three temperature ranges: 160–180, 240–280, and 420–460 K. The anomalies in the two higher temperature ranges are due to lattice disordering in the superionic transition region. The scattering in the range 160–180 K is tentatively attributed to fluoride ion displacements in split sites with a few potential wells. It is shown based on a split-site model that the thermally activated formation of interstices comparable in size to the mobile fluoride ion is an inherent feature of the LnF3 structure.

Comparison of short-range ion–ion correlations in the α, β and δ phases of Bi 2O 3

Neutron total scattering data for α-Bi 2O 3 and δ-Bi 2O 3, considering both the Bragg and diffuse components, have been analysed by the reverse Monte Carlo (RMC) method to extract experimental information concerning the short-range correlations between pairs of ions. These results complement the average structural information provided by conventional Rietveld refinement of the Bragg scattering alone. The radial and angular distribution functions in the stable ambient temperature α phase and superionic δ phase of Bi 2O 3 are presented and compared with the corresponding data for the metastable β phase reported previously. These data provide an important insight into the nature of the anion disorder within the highly conducting δ phase and the relationship between the δ phase and its ordered α and β counterparts.

Radiation tolerance of ceramics—insights from atomistic simulation of damage accumulation in pyrochlores

We have used molecular dynamics simulations to investigate the effects of radiation damage accumulation in two pyrochlore-structured ceramics, namely Gd2Ti2O7 and Gd2Zr2O7. It is well known from experiment that the titanate is susceptible to radiation-induced amorphization, while the zirconate does not go amorphous under prolonged irradiation. Our simulations show that cation Frenkel pair accumulation eventually leads to amorphization of Gd2Ti2O7, and both anion disorder and cation disorder occur during damage accumulation. Amorphization in Gd2Ti2O7 is accompanied by a density decrease of about 12.7% and a decrease of about 50% in the elastic modulus. In Gd2Zr2O7, amorphization does not occur, because the residual damage introduced by radiation is not sufficiently energetic to destabilize the crystal structure and drive the material amorphous. Subtle differences in damage accumulation and annealing between the two pyrochlores lead to drastically different radiation response as the damage accumulates.

An unusual discontinuity in the thermal spin transition in [Co(terpy)2][BF4

Single crystal and powder samples of [Co(terpy)(2)][BF(4)](2) are low spin below 100 K but show a typically gradual thermal spin-transition on warming, centred near 270 K. However, the spin-crossover exhibits an unusual and pronounced discontinuity above room temperature, when the material is ca. 87% high-spin. A crystallographic study at nine temperatures between 30-375 K showed that the discontinuity is not caused by a phase transition or by changes in anion disorder. Rather, it may reflect the steric consequence of small changes in the nearest neighbour interactions between the cations as the spin transition progresses.

Structural and compositional tuning of layered subnitrides; new complex nitride halides

New quaternary bimetallic nitride halides and the first examples of quintinary nitride mixed halides have been synthesised in the phase system Ca - Sr - N - Cl - Br. The variation in structure with composition has been investigated by powder X-ray and powder neutron diffraction techniques. All compounds crystallise in the R-3m spacegroup, with the anti-alpha-NaFeO(2) structure. The layered compounds exhibit unprecedented coexistent disorder of both cations and anions in which only nitride solely occupies a discrete crystallographic position. [N(Ca,Sr)(2)](+) layers alternate with halide ions (Cl, Br)(-), which fill the octahedral voids between the positively charged sheets. A dual cationic-anionic substitution approach allows all aspects of the anisotropic structure to be controlled, including the thickness of the ionic layers and the [N(Ca,Sr)(2)](+) interlayer spacing.

Crystalline Cation Conductors with Rotational Anion Disorder: Results of Quasielastic Neutron Scattering Experiments on Orthophosphates

Abstract Results and interpretations of several quasielastic neutron scattering measurements on samples of the isostructural high-temperature phases of Ag3PO4 and Na3PO4, prominent members of the group of fast cation conducting plastic phases are reported.Time-of-flight experiments reveal details on the phosphate reoriention in Ag3PO4: We find isotropic rotational diffusion. A consistent interpretation of the data suggests that silver ions are involved in the fast anion dynamics, a behaviour which is in line with the displacement of phosphorus detected in earlier neutron diffraction experiments. The reorientation activation energy of 0.17 eV is in the typical range found also in other ion conducting rotor phases.Na3PO4 was examined in time-of-flight and backscattering experiments, differing considerably in the time scale of the detected molecular dynamics. Anion reorientation, observed on the time scale of picoseconds and activated with 0.18 eV, appears as a predominantly circular diffusion with one P–O axis fixed. Our results suggest the involvement of sodium ions in the anion reorientation, i.e., a correlated motion of both types of ions in the picosecond regime.The much slower sodium jump diffusion in Na3PO4 was found to be thermally activated with 0.74 eV. Moreover, we find clear evidence for the predominance of jumps between tetrahedrally co-ordinated cation sites.Quasielastic neutron scattering reveals a wealth of dynamic details on the two high-temperature phases of Na3PO4 and Ag3PO4 which exhibit considerable differences (e.g., in the geometric details of anion dynamics) in spite of their structural similarities. Both plastic phases, however, give evidence of the involvement of cations in the anion dynamics, i.e., in a correlated reorientational motion of cations and anions on the picosecond time scale.

Role of anion size, magnetic moment, and disorder on the properties of the organic conductor κ-(BETS) 2Ga 1- xFe xCl 4- yBr y

Shubnikov–de Haas and angular dependent magnetoresistance oscillations have been used to explore the role of anion size, magnetic moment, and disorder in the organic conductors κ ‐ ( BETS ) 2 GaBr 4 and κ ‐ ( BETS ) 2 FeCl 2 Br 2 in the isomorphic class κ ‐ ( BETS ) 2 Ga 1 ‐ x Fe x Cl 4 ‐ y Br y . The results, combined with previous work, show correlations between the anion composition ( Ga 1 ‐ x Fe x Cl 4 ‐ y Br y ) and the superconducting transition temperature, effective mass, Fermi surface topology, and the mean free path.

Thermal Transport Properties of Bechgaard Salts (TMTSF)2PF6 and (TMTSF)2ClO4: Implication of Spin–Charge Separation

We report thermal transport measurements performed on the quasi-one-dimensional Bechgaard salts (TMTSF) 2 ClO 4 and (TMTSF) 2 PF 6 along the a -direction. For both salts, magnon-drag effects are found to contribute considerably to thermopower above 80 K. These results imply spin–charge separation in the metallic state for both salts. Moreover, a linear temperature-dependent thermal conductivity is found to be unaffected by anion disorder below an anion ordering transition temperature T AO ∼25 K in (TMTSF) 2 ClO 4 .

Oxide-Ion Disorder Within the High TemperatureδPhase ofBi2O3

The delta phase of Bi(2)O(3), which adopts an anion-deficient fluorite structure, has the highest known oxide-ion conductivity. Using a combination of neutron powder diffraction and Born-Oppenheimer molecular dynamics, the preferred local anion environment around the Bi(3+) within delta-Bi(2)O(3) is shown to be highly irregular, resembling the asymmetric "lone-pair" coordination found within many (fully ordered) oxides of Bi(3+) under ambient conditions. The asymmetric electron density around the Bi(3+) plays a central role in promoting the extreme anion disorder within delta-Bi(2)O(3), with the ion diffusion facilitated by extensive relaxations of both the surrounding anions and a "soft" cation sublattice. The validity of previously proposed structural models based on a cubic environment in which O(2-) vacancies are aligned in pairs in 100, 110, and 111 directions is discussed in light of these conclusions.

Supramolecular assemblies prepared from an iron(II) tripodal complex, tetrafluoroborate, and alkali metal cations. The effect of cation size on coordination number, anion disorder and hydrogen bonding

The iron(II) complex cation, 12+, of the 1 : 3 Schiff base condensate of tris(2-aminoethyl)amine (tren) with imidazole-2-carboxaldehyde (H3L1) was reacted with MBF4 (M = Na, K, Rb, Cs and NH4). The products were double salts of the formula {[FeH3L1](BF4)2}·MBF4, 1[M(BF4)3]. The complexes have been characterized by EA, IR, X-ray crystallography, and Mossbauer spectroscopy. The resulting complexes crystallize in P-3 and the cations are located at the origin (0, 0, 0) and c/2 (0, 0, 0.5) on the c axis. Six tetrafluoroborate anions surround the cation sites and each binds to the sodium in a monodentate fashion and to the other cations in a bidentate fashion resulting in distorted octahedral and icosahedral complexes, respectively. Disorder is observed in the tetrafluoroborate anion of both 1[Na(BF4)3] and 1[Cs(BF4)3], resulting in two sets of fluorine atom positions (F set A and F set B), but is absent in the ammonium, potassium, and rubidium double salts. The disorder is attributed to coordination number preference and poor size match, respectively. The tetrafluoroborate anions are involved in extensive hydrogen bonding with the imidazole NH and imine CH of the iron complex. The hydrogen bonding arrangements observed with potassium, rubidium, cesium (F set A) and ammonium are almost identical with one another, but are altered in 1[Na(BF4)3] and 1[Cs(BF4)3] (F set B) complexes. These effects are explained on the basis of cation size. The formation of the double salt results in a shift in the spin equilibrium of the parent iron(II) complex from 62% high spin: 38% low spin at 295 K to pure low spin in the double salts.

On the Characterisatiopn of Order-Disorder in Ion-Irradiated Pyrochlore Compounds by Electron Scattering Methods

AbstractSelected area electron diffraction patterns are routinely used to determine the effects of irradiation damage in nuclear materials. Using zone axis orientations, the intensities of Bragg beams change from a dynamical to kinematic-like state due to the presence of amorphous domains in the material. Such changes in beam intensities, together with the increased diffuse scattering from the increasing amorphous fraction, present a major obstacle to the determination of cation or anion disorder in the crystalline fraction.

Layered double hydroxide intercalated with p-methylbenzoate and p-bromobenzoate: Molecular simulations and XRD analysis

Samples of Mg 4Al 2 layered double hydroxide (LDH) intercalated with p-methylbenzoate and p-bromobenzoate anions were prepared by reconstruction of calcined LDH. The interlayer arrangement of guests was investigated by molecular modeling combined with X-ray powder diffraction and thermogravimetry. Molecular modeling was carried out in a Cerius 2 modeling environment. In both structures the guest anions adopt a nearly perpendicular arrangement of their long axis with respect to the host layers and they are anchored to the OH groups of the layers through COO − groups via electrostatic interactions. Molecular modeling revealed that both structures of the intercalates exhibit a certain disorder of guest anions in the interlayer space. In the case of LDH– p-methylbenzoate intercalate the anions tend to be situated in disordered rows, and the LDH– p-bromobenzoate intercalate exhibits a total disorientation of guest anions. A good agreement between calculated and measured X-ray diffraction patterns and between experimental and calculated basal spacings was obtained. In the LDH– p-methylbenzoate intercalate d exp = 16.96 Å and d calc = 16.97 Å , and in the case of LDH– p-bromobenzoate intercalate d exp = 17.19 Å and d calc = 17.40 Å .

Nature of the chemical bond and prediction of radiation tolerance in pyrochlore and defect fluorite compounds

The radiation tolerance of synthetic pyrochlore and defect fluorite compounds has been studied using ion irradiation. We show that the results can be quantified in terms of the critical temperature for amorphization, structural parameters, classical Pauling electronegativity difference, and disorder energies. Our results demonstrate that radiation tolerance is correlated with a change in the structure from pyrochlore to defect fluorite, a smaller unit cell dimension, and lower cation–anion disorder energy. Radiation tolerance is promoted by an increase in the Pauling cation–anion electronegativity difference or, in other words, an increase in the ionicity of the chemical bonds. A further analysis of the data indicates that, of the two possible cation sites in ideal pyrochlore, the smaller B-site cation appears to play the major role in bonding. This result is supported by ab initio calculations of the structure and bonding, showing a correlation between the Mulliken overlap populations of the B-site cation and the critical temperature.

Anion diffusion within highly conducting layered fluorides

Neutron diffraction has played an important role in the study of solids which possess high values of ionic conductivity, ranging from ‘model’ superionic compounds such as AgI and β-PbF 2 to more technologically relevant materials for use within lightweight Li + batteries and oxide fuel cells. An understanding of the structural properties of these systems is essential to identify those factors which promote high ionic conductivity and underpin the quest for new materials. In recent years, attention has focussed on the role of cations characterized by lone-pair electrons (such as Bi 3+, Pb 2+, Sn 2+ and Tl +) within highly conducting oxides and halides, with differing views concerning their importance in promoting extensive anion disorder. In this context, the results of powder neutron diffraction studies of two layered compounds within the MSnF 4 (M=Pb, Sr and Ba) and MSn 2F 5 (M=K, Rb, Cs and Tl) families are presented, to illustrate the structural role of Sn 2+ ions within these anion conductors.

High-pressure study of theβ-to-αtransition inGa2O3

The high-pressure behavior of ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ is studied up to $40\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ using synchrotron x-ray diffraction and Raman spectroscopy in diamond anvil cells. A phase transformation from the monoclinic $\ensuremath{\beta}$-phase $(C2∕m)$ is observed at a pressure above 20--$22\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. The high-pressure polymorph is identified as the $\ensuremath{\alpha}$-phase that is isostructural with corundum ($R\overline{3}c$ symmetry) from the x-ray diffraction data. However, considerable anion disorder is indicated by the appearance of broad bands in the Raman spectra. The experimental results are complemented by ab initio theoretical calculations of the energetics of the two structures and the lattice dynamics.

Nanoscopic characterization of Pr2Zr2O7 at Zr sites

AbstractBy using Perturbed Angular Correlation Spectroscopy, a suitable technique to explore internal fields at nanoscopic scale, the electric field gradients at Zr4+ sites in the Pr2Zr2O7 compound have been determined as a function of temperature. Three nonequivalent nanoconfigurations are present, which have been interpreted with the aid of point charge model calculations. Two of them correspond to pyrochlore – oxygen defective and perfect structures –, and the third one to the pyrochlore‐related defect fluorite structure. The most abundant interaction is a disordered and fluctuating electric field gradient, which describes the oxygen defective pyrochlore. As temperature increases, its gradual and reversible transformation towards the perfect form is observed. Below 750 °C the oxygen vacancies movement, which exhibits an activation energy of 0.14 eV, is assumed to be due to vacancies jumping among 48f equivalent sites. At higher temperatures the movement is interpreted as the fast diffusion of oxygen vacancies involving 48f and 8b sites, thus giving place to anionic disorder. The activation energy for this movement has been determined to be of 0.85 eV. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Anion and cation disorder in [CN 3H 6]⋅(TaF 6)

A new guanidinium fluoride tantalate, [CN 3H 6]⋅(TaF 6), crystallises at 50 °C from a solution of Ta 2O 5 in 40% aqueous HF and of guanidinium chloride. The structure is rhombohedral, R 3 ¯ m space group, with the equivalent hexagonal cell a H = 8.647 ( 1 ) Å , c H = 8.507 ( 2 ) Å , Z = 3 and R = 0.029 , R w = 0.077 for 312 reflections. The three-dimensional network is built up from parallel (0001) H layers ∞([CN 3H 6]F 6) 5− between which tantalum atoms are inserted. These tantalum atoms, in 3a sites, adopt an octahedral coordination with 〈 d Ta F 〉 = 1.894 ( 7 ) Å . (TaF 6) − anions are disordered over (0001) H mirror plane related positions while guanidinium cations are disordered over centrosymmetric positions; site occupancy of fluoride and nitrogen sites is fifty per cent.

On structural phase transitions in 4-aminopyridinium fluoroborate, [4-NH 2C 5H 5N][BF 4]: differential scanning calorimetry, dielectric and infrared studies

Crystal structure of the 4-aminopyridinium tetrafluoroborate, [4-NH 2C 5H 5N][BF 4], has been determined at 293 K by means of X-ray diffraction as monoclinic space group, C2. The crystal shows a reach sequence of phase transition, four solid-solid transitions: at 250, 281, 388 and 485 K on heating. Most of the phase transitions, except that at 388 K, are clearly of first order and classified as an ‘order-disorder’ type. The rotational disorder both of cations and anions was studied by differential scanning calorimetry, dielectric and infrared spectroscopy. The low temperature phase transitions are associated with plastic crystal behavior.