Anion Disorder Research Articles

FROM STRUCTURE TOPOLOGY TO CHEMICAL COMPOSITION. XVIII. TITANIUM SILICATES: REVISION OF THE CRYSTAL STRUCTURE AND CHEMICAL FORMULA OF BETALOMONOSOVITE, A GROUP-IV TS-BLOCK MINERAL FROM THE LOVOZERO ALKALINE MASSIF, KOLA PENINSULA, RUSSIA

The crystal structure of betalomonosovite, ideally Na 6 □ 4 Ti 4 (Si 2 O 7 ) 2 [PO 3 (OH)][PO 2 (OH) 2 ]O 2 (OF), a 5.3331(7), b 14.172(2), c 14.509(2) A, α 103.174(2), β 96.320(2), γ 90.278(2)°, V 1060.7(4) A 3 , from the Lovozero alkaline massif, Kola peninsula, Russia, has been refined in the space group P 1 ¯ to R = 6.64% using 3379 observed ( F o > 4σ F ) reflections collected with a single-crystal APEX II ULTRA three-circle diffractometer with a rotating-anode generator (Mo K α), multilayer optics, and an APEX-II 4K CCD detector. Electron-microprobe analysis gave the empirical formula (Na 5.39 Ca 0.36 Mn 0.04 Mg 0.01 ) Σ5.80 (Ti 2.77 Nb 0.48 Mg 0.29 Fe 3+ 0.23 Mn 0.20 Zr 0.02 Ta 0.01 ) Σ4 (Si 2.06 O 7 ) 2 [P 1.98 O 5 (OH) 3 ]O 2 [O 0.82 F 0.65 (OH) 0.53 ] Σ2 , D calc. = 2.969 g cm −3 , Z = 2, calculated on the basis of 26 (O + F) apfu , with H 2 O determined from structure refinement. The crystal structure of betalomonosovite is characterized by extensive cation and anion disorder: more than 50% of cation sites are partly occupied. The crystal structure of betalomonosovite is a combination of a titanium silicate (TS) block and an intermediate ( I ) block. The TS block consists of HOH sheets (H-heteropolyhedral, O-octahedral) and exhibits linkage and stereochemistry typical for Group IV (Ti + Mg + Mn = 4 apfu ) of the TS-block minerals. The I block is a framework of Na polyhedra and P tetrahedra which ideally gives {Na 2 □ 4 [PO 3 (OH)][PO 2 (OH) 2 ]} pfu . Betalomonosovite is an Na-poor OH-bearing analogue of lomonosovite, Na 10 Ti 4 (Si 2 O 7 ) 2 (PO 4 ) 2 O 4 . In the betalomonosovite structure, there is less Na in the I block and in the TS block when compared to the lomonosovite structure. The OH groups occur mainly in the I block where they coordinate P and Na atoms and in the O sheet of the TS block (minor). The presence of OH groups in the I block and in the TS block is supported by IR spectroscopy and bond-valence calculations on anions. High-resolution TEM of lomonosovite shows the presence of pervasive microstructural intergrowths, accounting for the presence of signals from H 2 O in the infrared spectrum of anhydrous lomonosovite. More extensive lamellae in betalomonosovite suggest a topotactic reaction from lomonosovite to betalomonosovite.

Uniaxial Chemical Pressure and Disorder Effects on Magnetic and Dielectric Properties of β′-(BEDT-TTF)2(ICl2)1−x(AuCl2)x

We have studied the structural, magnetic, and dielectric properties of the dimer–Mott insulator β′-(BEDT-TTF)2(ICl2)1−x(AuCl2)x. The AuCl2 substitution for ICl2 decreases the lattice length in the interlayer direction, leading to uniaxial chemical pressure with a negligible effect on the intralayer donor stacking. The variation of the Neel temperature as a function of x is explained on the basis of chemical pressure accompanied by anion disorder, while the dielectric response does not indicate the effect of chemical pressure. This suggests that the charge degrees of freedom are attributed to the intralayer origin such as the charge fluctuation of donor molecules induced by an electric field.

Electronic excitation induced amorphization in titanate pyrochlores: an ab initio molecular dynamics study.

The response of titanate pyrochlores (A2Ti2O7, A = Y, Gd and Sm) to electronic excitation is investigated utilizing an ab initio molecular dynamics method. All the titanate pyrochlores are found to undergo a crystalline-to-amorphous structural transition under a low concentration of electronic excitations. The transition temperature at which structural amorphization starts to occur depends on the concentration of electronic excitations. During the structural transition, O2-like molecules are formed, and this anion disorder further drives cation disorder that leads to an amorphous state. This study provides new insights into the mechanisms of amorphization in titanate pyrochlores under laser, electron and ion irradiations.

Compression behavior of Sm2Ti2O7-pyrochlore up to 50 GPa: single-crystal X-ray diffraction and density functional theory calculations

Single-crystal X-ray diffraction at pressures up to 50 GPa has been employed to study the compression behavior of Sm2Ti2O7-pyrochlore. In contrast to earlier reports, we observed no pressure-induced amorphization or pressure-induced anion disorder up to 50 GPa. The experimental study has been complemented by density functional theory-based calculations. A combination of the theoretical and experimental data yields a bulk modulus of $$\approx $$ 185 GPa, significantly higher than a value which had been reported earlier. In comparison to earlier work, the current study provides more reliable data due to the use of neon as a pressure medium, which provides a more hydrostatic pressure than the aluminum, which had been employed as a pressure medium in the earlier studies. An analysis of the compressibility of Al $$_2$$ B $$_2$$ O $$_7$$ pyrochlores shows an approximately linear dependence of the bulk modulus on the unit cell volume.

The new disordered triplite polymorph of Co2[PO4

Abstract The new polymorph of Co2[PO4]F has been synthesized by a hydrothermal route. Its crystal structure was determined from single-crystal X-ray diffraction data. Co2[PO4]F crystallizes with the triplite-type structure, space group C2/c, a=12.856(2), b=6.4369(10), c=9.6742(16) Å, β=117.40(1)°, V=710.7(2) Å3 and Z=8. The structure consists of a 3D-framework buildup of condensed PO4 tetrahedra, and cobalt(II) polyhedra which form chains running along the [101] and [010] directions. The coordination of the cobalt cations and the connectivity between the cobalt polyhedra are not well defined due to the disorder of the fluoride anions which form zigzag chains along [001]. The theoretical ordering of the fluoride anions led to two different structural models, very similar to the triplite Cc-CoFe[PO4]F and the triploidite P21/c-Co2[PO4]F, and in which the cobalt atoms are five and six coordinated.

The Li–Si–(O)–N system revisited: Structural characterization of Li21Si3N11 and Li7SiN3O

A systematic study of the Li–Si–(O)–N system is presented. The synthetic conditions to prepare Li2SiN2, Li5SiN3, Li18Si3N10, Li21Si3N11 and Li7SiN3O are described and the structure of the last two compounds has been solved for the first time. While Li21Si3N11 crystallizes as a superstructure of the anti-fluorite structure with Li and Si ordering, Li7SiN3O exhibits the anti-fluorite structure with both anion and cation disorder.

High pressure Raman spectroscopic study of phase transformation in TaO2F

High pressure Raman spectroscopic measurements on nearly zero thermal expansion material TaO2F are carried out up to 19GPa. Earlier report of high pressure X-ray diffraction studies shows two phase transitions, one at 0.7 and the other at 4GPa with rhombohedral (R-3c) structure above 4GPa, but the structure between 0.7GPa and 4GPa remained unclear. In high pressure Raman measurements, a reversible, cubic to rhombohedral phase transformation onsets around 0.8GPa and gets completed at 4.4GPa with all four predicted normal modes corresponding to R-3c phase and retaining the structure up to 19GPa. A mixture of cubic and rhombohedral phases is observed between 0.8 and 4.4GPa. Optically silent modes in the ambient cubic structure exhibit strong, broad Raman bands due to anionic (O/F) disorder in TaO2F altering the local symmetry and allowing for first order Raman scattering. On compression, these disorder induced first order Raman bands gradually decrease in intensity and disappear around 4.4GPa due to inhibition of local distortion caused by anions, and the modes corresponding to the rhombohedral phase appear. This is a clear evidence of disorder-free rhombohedral single phase exists above 4.4GPa in agreement with the reported HPXRD results. Temperature dependent Raman measurements reveal that the intensities of Raman bands remain almost unchanged with rise in temperature indicating static disorder in TaO2F. Disorder-induced first order Raman modes at 176, 212, 381 and 485cm−1 soften with increase in pressure whereas the other modes show low positive Gruneisen parameter. The thermal expansion coefficient calculated using these Gruneisen parameters (−2.91ppmK−1) is in fair agreement with the reported values (−1 to +1ppmK−1). On the other hand, all four modes of disorder-free rhombohedral phase show the usual hardening behavior with increase in pressure contributing to positive thermal expansion.

A novel intermediate in the LiAlH4–LiNH2hydrogen storage system

The decomposition pathways for the composite LiAlH4-LiNH2 in different ratios of (1 : 1), (1 : 1.5), (1 : 2) and (1 : 2.5) have been systematically studied using in situ synchrotron radiation powder X-ray diffraction (SR-PXD) as well as simultaneous thermogravimetric analysis and differential scanning calorimetry coupled with mass spectroscopy. The study reveals that LiAlH4 decomposes in two steps to LiH, Al and H2 and, subsequently, the produced LiH reacts with LiNH2 forming Li2NH and H2. A new intermediate, Li(4-x)Al(x)(NH)(2-2x)N(2x), is observed during the decomposition of LiAlH4-LiNH2 (1 : 1.5), (1 : 2) and (1 : 2.5), formed from Li2NH and Al prior to the formation of Li3AlN2. Li(4-x)Al(x)(NH)(2-2x)N(2x) is characterized by Rietveld refinement of SR-PXD data and solid-state (27)Al MAS NMR spectroscopy (chemical shift, δ(Al) = 125 ppm) and both techniques reveal a maximum value for x of ~0.10, i.e., Li(3.90)Al(0.10)(NH)(1.80)N(0.20). The solid solution Li(4-x)Al(x)(NH)(2-2x)N(2x) crystallizes in a cubic unit cell, a = 4.9854(7) Å with space group Fm3m, similar to the crystal structure for Li2NH and is a rare type with both cation and anion disorder. For LiAlH4-LiNH2 (1 : 1) 8.7 wt% of H2 is released during heating from RT to 500 °C, while for LiAlH4-LiNH2 composites with molar ratios of LiNH2 higher than 0.5 the release of both H2 and NH3 is observed.

Effect of chloride substitution on the order–disorder transition in NaBH4 and Na11BD4

Phase transition associated with anion disordering over two orientations in Na11BD4 (NaBH4) and its solid solutions with NaCl, Na(11BD4)1−xClx, is investigated with powder diffraction (neutron and synchrotron radiation), differential scanning calorimetry and Raman spectroscopy. Upon heating, the transition temperature extrapolated to zero rate of heating is 192.2 K for Na11BD4, ΔS = 4.41 J/mol K, hysteresis 1.7 K and the volume increase 0.43%. Thermal parameters of the transition in Na(11BD4)1−xClx follow a colligative-property model of an ideal solution, with x = 0.158(1) as the critical concentration at which the ordering interactions and the transition itself are eliminated. On approaching this limit, the tetragonal distortion of the ordered structure decreases somewhat towards the cubic average, and this is associated with a partial disorder of the tetrahedral anions seen by diffraction methods. In fact, a 3% disorder is already present in the pure solvent of the solid solution (Na11BD4) at 8 K.

Dynamical response and instability in ceria under lattice expansion

We present results of density functional theory calculations on the phonon dispersion and elastic constants of bulk ceria (CeO${}_{2}$) as a function of positive and negative isotropic strain, which could be induced thermally or by cationic doping. We find that, as the lattice is expanded, there is a significant softening of the ${B}_{1u}$ mode at the $X$ point. This mode consists of motions of oxygens in the $[001]$ direction. At a strain of $1.6$$%$, corresponding to a temperature of 1600 K, the ${B}_{1u}$ and ${E}_{u}$ modes at the $X$ point cross, with an associated high, narrow peak in the phonon density of states appearing. We infer that this crossing indicates a coupling of the modes, leading to a transition to a superionic phase, where conductivity occurs in the $[001]$ direction, mediated by anion interstitial site occupation. As the lattice is expanded further, the ${B}_{1u}$ mode continues to soften, becoming imaginary at a strain of $3.4$$%$, corresponding to a temperature of 2500 K. Following the imaginary mode would result in a cubic to tetragonal phase transition, similar to those known to occur with reducing temperature in zirconia (ZrO${}_{2}$) and hafnia (HfO${}_{2}$). Our calculated elastic constants, however, indicate that the structure remains mechanically stable, even at this level of expansion. As confirmed by our semiclassical free energy calculations, the cubic phase of ceria remains the most stable, while the imaginary mode indicates a change to a thermally disordered cubic phase, with the majority of disorder occurring on the anion sublattice. Our results explain the high temperature ionic conductivity in ceria and other fluorite-structured materials in terms of the intrinsic lattice dynamics, and give insight to the stability and anionic disorder at elevated temperatures.

Ab initiostudy of the stabilities of and mechanism of superionic transport in lithium-rich antiperovskites

Recently, a family of halogen-based Li-rich antiperovskites were synthesized [J. Am. Chem. Soc. 134, 15042 (2012)], and the measured superionic conductivity makes these materials promising candidates as solid electrolytes for applications in Li-ion and Li-air batteries. This discovery raises several pressing issues on the fundamental physics concerning the thermodynamic and electrochemical stability of the synthesized materials and the mechanism of the observed superionic Li${}^{+}$ transport. Here, we study the reported antiperovskites Li${}_{3}$OCl, Li${}_{3}$OBr, and their mixed compounds using first-principles density functional theory and molecular dynamics simulations. Our calculations show that these materials are thermodynamically metastable. Their large electronic band gaps and chemical stability against electrodes suggest the excellent electrochemical performance, which bodes well for the use in potentially harsh working conditions in practical battery applications. The calculated low activation enthalpy for Li-ion migration well below the crystal melting temperature and superionic transport near the Li sublattice melting state explain the experimentally observed phenomena. Our study identifies mobile Li vacancies and anion disorder as the primary driving mechanisms for superionic Li${}^{+}$ conductivity in the antiperovskites. This work unveils essential working principles of the Li-rich antiperovskites, which are crucial to further exploration, development, and application of these and other charge-inverted materials with tailored properties.

Synthesis and characterization of perovskite-type SrxY1−xFeO3−δ (0.63≤x

Oxygen-deficient ferrates with the cubic perovskite structure SrxY1−xFeO3−δ were prepared in air (0.71≤x≤0.91) as well as in N2 (x=0.75 and 0.79) at 1573K. The oxygen content of the compounds prepared in air increases with increasing strontium content from 3−δ=2.79(2) for x=0.75 to 3−δ=2.83(2) for x=0.91. Refinement of the crystal structure of Sr0.75Y0.25FeO2.79 using TOF neutron powder diffraction (NPD) data shows high anisotropic atomic displacement parameter (ADP) for the oxygen atom resulting from a substantial cation and anion disorder. Electron diffraction (ED) and high-resolution electron microscopy (HREM) studies of Sr0.75Y0.25FeO2.79 reveal a modulation along 〈100〉p with G± ∼0.4〈100〉p indicating a local ordering of oxygen vacancies. Magnetic susceptibility measurements at 5–390K show spin-glass behaviour with dominating antiferromagnetic coupling between the magnetic moments of Fe cations. Among the studied compositions, Sr0.75Y0.25FeO2.79 shows the lowest thermal expansion coefficient (TEC) of 10.5ppm/K in air at 298–673K. At 773–1173K TEC increases up to 17.2ppm/K due to substantial reduction of oxygen content. The latter also results in a dramatic decrease of the electrical conductivity in air above 673K. Partial substitution of Fe by Cr, Mn and Ni according to the formula Sr0.75Y0.25Fe1−yMyO3−δ (y=0.2, 0.33, 0.5) leads to cubic perovskites for all substituents with y=0.2. Their TECs are higher in comparison with un-doped Sr0.75Y0.25FeO2.79. Only M=Ni has increased electrical conductivity compared to un-doped Sr0.75Y0.25FeO2.79.

The pyrochlore to defect fluorite phase transition in Y2Sn2−xZrxO7

The system Y2Sn2−xZrxO7 (0.0 ≤ x ≤ 2.0) undergoes a phase transformation from ordered pyrochlore (Fdm) to defect fluorite (Fmm) actuated by the substitution of Zr for Sn. X-ray diffraction patterns map the retention of the pyrochlore structure up to x = 1.2. For samples with x = 1.4–2.0 the structure can be described as defect fluorite in broad terms. Electron diffraction patterns are consistent with this interpretation; however, they also demonstrate that the defect fluorite phase exhibits a strain driven compositional/displacive modulation that changes gradually with increasing Zr content. Raman spectra are consistent with gradual anion disorder up to x = 1.0 and highly disordered anion distributions inferred for x > 1.4, but the spectra also suggest the presence of residual order due to the modulated structure. The phase transformation in this system occurs at a higher Zr content than predicted by classical radius ratio models, consistent with the covalent character of Sn–O bonding. An unusual finding of this work comes from 119Sn MAS NMR and Sn L3-edge XANES analyses, indicating that Sn4+ prefers to occupy lattice sites with a 6-fold local coordination environment throughout the series. These results suggest that the incorporation of Sn or other metal cations having significant covalent bonding or a strong preference for octahedral coordination in pyrochlore-based materials may have a detrimental effect on ionic conductivity.

Formation and conductivity studies of lithium argyrodite solid electrolytes using in-situ neutron diffraction

Lithium-ion conducting argyrodites Li6PS5X (X=Cl, Br, I) are a promising class of fast-ion conductors for all-solid state Li-ion batteries. To gain a deeper insight into the phase formation of Li6PS5Cl, in situ neutron diffraction studies are carried out on a stoichiometric ball-milled precursor mixture during thermal treatment. The evolution of the S2−/Cl− anion disorder and its correlation with ionic conductivity are reported here. In contrast to earlier reports, an argyrodite phase is found to form between 80 and 150°C, but the phase shows only moderate conductivity when crystallized at such low temperatures and further thermal treatment is required to access the highly conducting phase. The maximum room-temperature ionic conductivity of 1.1×10−3S/cm is observed for samples annealed at intermediate temperatures (250°C). When ball-milled glass-ceramic precursors for Li6PS5Cl are crystallized with a constant slow heating rate, the initially formed argyrodite phase is found to be Li7PS6, which is then gradually converted into Li6PS5Cl at higher temperatures. The industrial requirements for minimizing cost by using lower annealing temperatures thus need to be balanced with the requirements of obtaining the highest conducting composition of the phase for performance in all-solid state batteries.

Hydrogen bis[tris(4-fluorophenyl)phosphane oxide] triiodide

In the title compound, C(36)H(25)F(6)O(2)P(2)(+)·I(3)(-), hydrogen-bonded [{(p-FC(6)H(4))(3)PO}(2)H](+) dimers assemble along the crystallographic c axis to form channels that house extended chains of triiodide anions. Although the I-I bond lengths of 2.9452 (14) and 2.9023 (15) Å are typical, the inter-ion I...I distance of 3.5774 (10) Å is unusually short. A posteriori modelling of nonmerohedral twinning about (100) has been only partially successful, achieving a reduction in the maximum residual electron density from 5.28 to 3.24 e Å(-3). The inclusion of two low-occupancy I-atom sites (total 1.7%), which can be interpreted as translational disorder of the triiodide anions along the channels, reduced the maximum residual electron density to 2.03 e Å(-3). The minor fractional contribution volume of the nonmerohedral twin domains refined to 0.24 and simultaneous refinement of the inversion twin domains showed the crystal to be a 0.5:0.5 inversion twin.

Structural and spectroscopic studies of La2Ce2O7: Disordered fluorite versus pyrochlore structure

We present the neutron and synchrotron x-ray diffraction and Ce ${L}_{3}$-edge x-ray absorption near-edge structure results for polycrystalline La${}_{2}$Ce${}_{2}$O${}_{7}$. We have found that this compound adopts a disordered defect fluorite structure with diffuse features evident in the neutron-diffraction pattern indicative of anion disorder. Our experimental results are discussed in the context of a recent theoretical paper on this material.

Crystal Structural, Magnetic, and Transport Properties of Layered Cobalt Oxyfluorides, Sr2CoO3+xF1–x (0 ≤ x ≤ 0.15)

The crystal structure of the layered cobalt oxyfluoride Sr(2)CoO(3)F synthesized under high-pressure and high-temperature conditions has been determined from neutron powder diffraction and synchrotron powder diffraction data collected at temperatures ranging from 320 to 3 K. This material adopts the tetragonal space group I4/mmm over the measured temperature range and the crystal structure is analogous to n = 1 Ruddlesden-Popper type layered perovskite. In contrast to related oxyhalide compounds, the present material exhibits the unique coordination environment around the Co metal center: coexistence of square pyramidal coordination around Co and anion disorder between O and F at the apical sites. Magnetic susceptibility and electrical resistivity measurements reveal that Sr(2)CoO(3)F is an antiferromagnetic insulator with the Néel temperature T(N) = 323(2) K. The magnetic structure that has been determined by neutron diffraction adopts a G-type antiferromagnetic order with the propagation vector k = (1/2 1/2 0) with an ordered cobalt moment μ = 3.18(5) μ(B) at 3 K, consistent with the high spin electron configuration for the Co(3+) ions. The antiferromagnetic and electrically insulating states remain robust even against 15%-O substation for F at the apical sites. However, applying pressure exhibits the onset of the metallic state, probably coming from change in the electronic state of square-pyramidal coordinated cobalt.

Anion-Induced Solid Solution Electrochemical Behavior in Iron Tavorite Phosphates

A simple hydrothermal method was used to produce a new iron hydroxy-fluoro phosphate tavorite: LiFePO4(OH)0.6F0.4. Electrochemical discharge−charge of this 2.60 V material featured a sloping profile over the entire stoichiometry range, indicative of solid-solution behavior, a result of both anion disorder and high ionic conductivity in the lattice.

Methylammonium tetrafluoridoborate 18-crown-6 clathrate

In the title compound, CH3NH3 +·BF4 −·C12H24O6, the methyl­ammonium cation makes three N—H⋯O hydrogen bonds to the 18-crown-6 mol­ecule. The –NH3 + and –CH3 groups of the cation adopt a staggered conformation. The F atoms of the tetra­fluoridoborate anion are disordered over two sets of sites in a 0.519 (11):0.481 (11) ratio. Weak C—H⋯F inter­actions occur in the crystal, which possibly correlate with the anion disorder.

The 1 : 1 and 1 : 2 salts of 1,4-diazabicyclo[2.2.2]octane and bis(trifluoromethylsulfonyl)amine: thermal behaviour and polymorphism

The salts of 1,4-diazabicyclo[2.2.2]octane (DABCO) and bis(trifluoromethylsulfonyl)amine (in 1 : 1 and 1 : 2 stoichiometry, 1 and 2, respectively) were investigated by DSC, TGA, PXRD and single crystal X-ray diffraction. Thermal analysis of the two compounds revealed reversible phase transitions, indicating enantiotropic relationships between all the polymorphs. Compound 1 can exist as a room-temperature form 1L and as a high-temperature modification 1H. By contrast, 2 forms a low- (2L), an intermediate- (2I) and a high-temperature (2H) polymorph. The structures of 1L and 2L were determined by single crystal X-ray crystallography. N–H⋯O bonded cations of 1L are arranged in a staggered fashion and form infinite chains. The crystal contains two sets of symmetry related H-bonded chains, which are in an inclined orientation relative to one another. In the structure of 2L, the cation is N–H⋯O bonded to two neighbouring triflimide anions. The discrete H-bonded structure fragments resulting from these interactions assemble to a layer structure. The structure determination of the high-temperature forms 1H and 2H from PXRD data was negatively affected by a high degree of disorder of the triflimide anions. All N–H⋯O bonded DABCO chains in 1H adopt a parallel orientation, which is in contrast to the inclined orientation found in 1L.