Anion-excess Fluorite Research Articles

Te3O5F2: a new metastable tellurium IV oxyfluoride with an acentric complex structure

The crystal structure of a new metastable tellurium (IV) oxyfluoride Te3O5F2 [non-centrosymmetric P21 space group] is unusually complex with 15 different Te polyhedra. It is built of giant cylindrical columns of oxide units connected via weak Te-F bonds. A strict O/F order is evidenced and a structural comparison with Sb3O2F5 shows that these two phases derive, by a different mechanism, from the anion-excess fluorite superstructure type. Surprisingly, in these two structures is observed a full gradation of various polyhedra of tellurium (IV) or antimony (III) with the oxygen atom ensuring a strong framework at the centre of these structures (cylinders for Te phase and layers for Sb one), and more and more F anions moved away at the periphery of these strong units, connecting these units via weak Te-F or Sb-F bonds. This well illustrates the different roles played by oxygen and fluorine atoms in the oxyfluorides of cations presenting an electronic lone pair E (5s2 in the case of Te4+ cation), in relation with the non-linear properties of these phases.

Sr3ZrF10, a new type of anion-excess fluorite superstructure. Comparison with Pb3ZrF10 and Pb3HfF10. Influence on defect structure models of M1-xZrxF2+2x fluorite solid solutions

The triclinic P1¯ structure of Sr3ZrF10 derives from the fluorite MF2 type by accommodation of the anionic excess (MX2.50) in ZrF8 polyhedra, independent one from another but organized in double columns aligned along [01-1] axis. Contrary to the orthorhombic Cmcm Pb3ZrF10 which also comprises double columns of square antiprisms which are aligned along [001] axis, the successive ZrF8 polyhedra in a column of Sr3ZrF10 take two different orientations in order to decrease the steric constraints resulting from the lower size of Sr2+ compared to Pb2+.In complement, the structure of Pb3ZrF10 is redetermined and the structure of isostructural Pb3HfF10 is refined. A small defect, corresponding to the statistical reorientation of a small proportion of Zr(Hf)F8 polyhedra, in a way similar to that described in Sr3ZrF10, is observed for the first time in both Pb phases but is absent in the isotypic structure of Ba3HfF10 with a higher size Ba2+ cation.The possibility of similar reorientations of the ZrF8 polyhedra in the homologous Pb1-xZrxF2+2x anion excess fluorite solid solution is discussed in relation with the previous models of columnar clusters proposed for this solid solution from neutron diffraction experiments supported by ionic conductivity and 19F NMR measurements of the Pb1-xZrxF2+2x solid solution and of the Pb3ZrF10 ordered superstructure.

Stabilization Factor of Anion-Excess Fluorite Phase for Fast Anion Conduction

Anion-excess fluorite is a unique structure type of inorganic crystals and is well known as an appropriate crystal structure for fast anion conduction. In particular, the introduction of excess ani...

Fluorides containing lanthanides and yttrium at extreme conditions

We are interested in crystal structures and stabilities of fluoride materials containing lanthanides and yttrium that are related to the CaF2 structure. These compounds are laser hosts and luminescent materials, oxygen sensors as well as components of solar cells. They exhibit various schemes of (dis)ordering of cations and anions in fluorite superstructures and anion-excess fluorites. In the last few years, we have performed a series of studies on the bulk AMF4 and MF3 materials (A = Li, Na, K; M = Y, lanthanide) at different pressure-temperature conditions. Among them, ordered LiYF4 is a commercial host for solid state lasers, while partially ordered NaYF4 doped with lanthanides is the most efficient material for green and blue up-conversion known to date. In the system KF–YF3, we have studied not only KYF4 [1] but also KY3F10, which is an anion-excess 2×2×2 superstructure of fluorite at atmospheric conditions. At high temperatures and high pressures, it converts to another fluorite superstructure with disordered fluorine atoms. The pressure-induced LaF3 post-tysonite structure is another example of the anion-excess fluorite [2]. Our work on the fluorite-related materials at extreme conditions provides information on their structural instabilities that could further be used to better understand and control their materials properties. For instance, we demonstrated that the NaMF4 up-converters are unstable and that the ordering of the cations and vacancies in their structure is a slow process [3]. Consequently, the order–disorder transformations have a profound influence over the luminescent properties of these materials when doped.

Reversible pressure-induced first-order phase transitions in anion-excess fluorite KY3F10 studied with single-crystal X-ray diffraction

KY3F10 (Fm3¯mZ = 8) has been studied with single-crystal X-ray diffraction to 9.5 GPa in a diamond anvil cell under hydrostatic conditions. At atmospheric pressure, it is an anion-excess 2 × 2 × 2 superstructure of fluorite with two building blocks Y6F36 and Y6F32. In the Y6F36 block, six square YF8 antiprisms share their corners to form an empty F12 cubooctahedron. In the Y6F32 block, six square YF8 antiprisms share their edges to form an empty F8 cube. Upon compression, there occur phase transitions between 5 and 7 GPa due to an instability of the empty F12 cubooctahedra in the 2 × 2 × 2 superstructure. At about 7 GPa, the cubooctahedra collapse completely and all the cations in the new structure (Pm3¯m, Z = 1) are surrounded by cubes of the F atoms. The excessive fluorine is located in the F8 cubes. At hydrostatic conditions, all the transformations are reversible. These observations imply that at high pressures the anion-excess fluorite superstructures in general tend to transform to the CaF2 structure.

The God Delusion. R. Dawkins. Black Swan, London (2007). 464 pp., Paperback with new foreword, €8.99, ISBN: 978-0-552-77331-7

The crystal structure of a new metastable tellurium (IV) oxyfluoride Te3O5F2 [non-centrosymmetric P21 space group] is unusually complex with 15 different Te polyhedra. It is built of giant cylindrical columns of oxide units connected via weak Te-F bonds. A strict O/F order is evidenced and a structural comparison with Sb3O2F5 shows that these two phases derive, by a different mechanism, from the anion-excess fluorite superstructure type. Surprisingly, in these two structures is observed a full gradation of various polyhedra of tellurium (IV) or antimony (III) with the oxygen atom ensuring a strong framework at the centre of these structures (cylinders for Te phase and layers for Sb one), and more and more F anions moved away at the periphery of these strong units, connecting these units via weak Te-F or Sb-F bonds. This well illustrates the different roles played by oxygen and fluorine atoms in the oxyfluorides of cations presenting an electronic lone pair E (5s2 in the case of Te4+ cation), in relation with the non-linear properties of these phases.

Point defects and clustering in uranium dioxide byLSDA+Ucalculations

A comprehensive investigation on point defects and their clustering behavior in nonstoichiometric uranium dioxide $\mathrm{U}{\mathrm{O}}_{2\ifmmode\pm\else\textpm\fi{}x}$ is carried out using the $\mathrm{LSDA}+\mathrm{U}$ method based on density functional theory. Accurate energetic information and charge transfers available so far are obtained. With these energies that have improved more than 50% over that of pure generalized gradient approximation and local density approximation, we show that the density functional theory predicts the predominance of oxygen defects over uranium ones at any compositions, which is possible only after properly treating the localized $5f$ electrons. Calculations also suggest an upper bound of $x\ensuremath{\sim}0.03$ for oxygen clusters to start off. The volume change induced by point uranium defects is monotonic but nonlinear, whereas for oxygen defects, increasing $x$ always reduces the system volume linearly, except dimers that require extra space for accommodation, which has been identified as a metastable ionic molecule. Though oxygen dimers usually occupy Willis ${\mathrm{O}}^{\ensuremath{''}}$ sites and mimic a single oxygen in energetics and charge state, they are rare at ambient conditions. Its decomposition process and vibrational properties have been studied carefully. To a general clustering mechanism in anion-excess fluorites systematically obtain, we also analyze the local stabilities of possible basic clustering modes of oxygen defects. The result shows a unified way to understand the structure of Willis-type and cuboctahedral clusters in $\mathrm{U}{\mathrm{O}}_{2+x}$ and $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{U}}_{4}{\mathrm{O}}_{9}$. Finally, we generalize the point defect model to the independent cluster approximation to include clustering effects; the impact on defect populations is discussed.

Preparation and structural elucidation of the new anion-excess fluorite variant Ba4Er3F17

Abstract Pink coloured crystals of the new complex fluoride Ba4Er3F17 were prepared via solid state reactions from mixed powders of BaF2 and ErF3 (4:3) in platinum crucibles under Ar (1973 K, 48 h). Ba4Er3F17 crystallizes in the space group R 3 m with a=11.059(1) and c=20.346(2) A. The structural parameters have been refined using single crystal data. The structure of Ba4Er3F17 can be described as an ordered anion-rich fluorite type structure. The excess fluorine content is directly correlated with the occurence of Er6F36 clusters. Within these units the central F12 cuboctahedra are centered by one additional F which lies on a sixfold split position. The trivalent Er are square antiprismatically surrounded by 8 F with distances Er F ranging from 2.24 to 2.44 A and an additional capping F at 2.59 A. The two crystallographically different Ba are coordinated by 10 and 11 F, respectively, at distances of 2.59–3.15 A. Calculations of the Madelung part of the lattice energy, MAPLE, were performed for the analysis of the chemical bonding.

Alkali metal insertion and dielectric studies on anion excess fluorite related mixed oxide Bi 2Te 2W 3O 16

Insertion of alkali metals like Li and Na into the tunnels of anion excess fluorite related complex mixed oxide Bi 2Te 2W 3O 16 using n-butyllithium and NaN 3, respectively has been performed; the preliminary structural, Raman spectra, and electrical resistivity of the alkali metal inserted phases and the dielectric properties of Bi 2Te 2W 3O 16 are reported.

Bi(F, O)2.45: An Anion-Excess Fluorite Defect Structure Deriving from Rhombohedral LnFO Type

During a reinvestigation of the Bi2O3–BiF3 system, a nonstoichiometric oxidefluoride, of composition range Bi(F, O)2.50–Bi(F, O)2.43 at 500°C, was characterized by annealing at temperatures higher than 300°C.Its crystal structure was solved by X-ray diffraction on a single crystal of composition Bi(F, O)2.45, in the R-3m space group with cell parameters: a=4.1378(9) Å and c=20.321(3) Å (Shelxl program: wR2=7.2%, R1=3.5%).The structure of BiF1.90O0.55 derives from the well-known rhombohedral LnFO type by formation of 1 :0: 3 (or 1 : 0 : 2) clusters orientated along the [001] axis of the hexagonal associated cell in a partly ordered way. This orientation of the clusters preserves the O/F long range order characteristic of the LnFO type, despite a statistical replacement of almost half the O anions by F ones. As in the tetragonal anion-excess LaF1+2xO1−x and the orthorhombic SmF1+2xO1−x fluorite-related phases, the clustering mainly affects the F site: F anionic vacancies are associated to Fi interstitial anions and to Fr anions relaxed from the normal F site. In the clusters, Bi cations are in 9-fold or 10-fold coordination and the higher limit of anionic insertion (Bi2F4O composition) corresponds to a complete juxtaposition of 1 : 0 : 3 clusters. Near this limit, the excess anions tend to form quasi-continuous irregular 36 sheets perpendicular to the [001] axis of the hexagonal cell.

A neutron diffraction study of the superionic transition in with x = 0.06

We have investigated the high-temperature superionic transition of the anion-excess fluorite with x = 0.06 using both monochromatic and time-of-flight Laue single-crystal neutron diffraction. The measured Bragg intensities indicate that the cuboctahedral defect clusters found at ambient temperature start to break up into smaller fragments even below the superionic transition temperature, . Information concerning the local defect configuration at T = 1173 K has been provided by modelling the measured distribution of the coherent elastic diffuse scattering within the plane of reciprocal space. The high-temperature defects are of the `Willis' type and strongly resemble the short-lived Frenkel clusters found in the pure fluorites such as above .

Composite modulated structures in anion excess fluorites

Composite modulated structures in anion excess fluorites

The defect structure of anion-excess (Ca 1− xY x)F 2+ x with x = 0.06

The defect structure of the anion-excess fluorite (Ca 1− x Y x )F 2+ x with x = 0.06 has been investigated using single-crystal neutron diffraction. Least-squares refinement of the measured Bragg intensities has been performed considering four positions for the disordered anions within the Fm 3m unit cell: in 48 i sites at ( 1 2, u, u ) with u = 0.39; in 32 f sites at ( w, w, w) with w = 0.38; in 32 f sites at ( v, v, v) with v = 0.27, and in the empty cube center 4 b sites at ( 1 2 , 1 2 , 1 2 ) . The results indicate that only the ( 1 2 , u, u) and ( v, v, v) sites are significantly occupied. Information concerning the local defect configuration has been provided by least-squares refinement of the measured distribution of coherent elastic diffuse scattering within the (1 10) plane of reciprocal space. The best agreement with both Bragg diffraction and diffuse scattering data is obtained by adopting a model of randomly distributed cuboctahedral anion polyhedra, of the type observed in ordered anion-excess fluorite superlattices.

Diffuse neutron scattering from anion-excess strontium chloride

The defect structure and diffusional processes have been studied in the anion-excess fluorite ( Sr, Y) Cl 2.03 by diffuse neutron scattering techniques. Static cuboctahedral clusters found at ambient temperature break up at temperatures below 1050 K, where the anion disorder is highly dynamic. The anion diffusion rate is faster than that for pure SrCl 2 at the same temperature.

The defect structure of anion excess ( Ca, Y) F2+ x

The defect structure of the anion-excess fluorite Ca 1- x Y x F 2+ x with x = 0.06 has been investigated by single crystal neutron diffraction. Analysis of both Bragg and coherent diffuse scattering data indicate that the charge compensating excess anions are incorporated into the lattice in the form of cuboctahedral defect clusters.

Coherent diffuse neutron scattering from the anion-excess fluorite (Sr,Y)Cl2.03

The anion-excess fluorite (Sr,Y)Cl2.03 has been studied by coherent diffuse neutron scattering techniques. At ambient temperature the disorder is found to be static within instrumental resolution. The parameters of defect cluster models have been fitted directly to the data. Excess chlorine ions are found to aggregate into cuboctahedral clusters whose ionic coordinates agree with those calculated from a simple hard sphere model. At elevated temperatures the scattering exhibits quasi-elastic energy broadening indicating the dynamic nature of the disorder. Data obtained at 1050 K show that the cuboctahedral clusters have broken up, the scattering being reminiscent of that obtained from pure fluorites in the fast-ion phase. It is possible to account for the high temperature scattering in terms of 'snapshot' models of the diffusing anions and their associated relaxation fields.

Preparation and characterization of fluorite-related superstructure phases Sr 2RF 7 with R ≡ Sm - Lu and Y

The binary systems SrF 2-RF 3 with R ≡ La — Nd, Sm — Lu, and Y were investigated systematically for the appearance of an intermediate phase with composition Sr 2RF 7. As a result of this study, we have discovered that this type of compound exists only for R ≡ Sm — Lu and Y. For R ≡ La — Nd, either the corresponding disordered solid solution with anion-excess fluorite (cα) was found, or a diphasic mixture of cα and other intermediate phases with compositions richer in RF 3. Guinier X-ray powder diffraction was used for the structural characterization of all phases. Additionally, electron diffraction from single crystals was carried out for Sr 2RF 7 phases with R ≡ Sm, Gd, Tb, Er, Yb, Y. For all Sr 2RF 7 phases, a tetragonal fluorite-related superstructure was established with a s = 0.5(10) 1 2 a F , c s = 3 a F , Z = 30, space group I4/ m. This superstructure type t is the same as in the case of Ca 2RF 7 phases, but different from Ba 2RF 7 (t') where only partial ordering occurs one-dimensionally along the z axis.

Synthèse et caractéristiques cristallographiques des phases solides de type fluorine des systèmes PbF 2 LnF 3

A preliminary study of the PbF 2 LnF 3 systems ( Ln = lanthanides and Y) has allowed the characterisation of three phases: a disordered fluorite-like solid solution Pb 1− x Ln x F 2+ x the domain of which increases with increasing temperature and dopant ion radius, and two anion-excess fluorite related superstructures: Pb 2YF 7 (tetragonal, space group I4 or I4 m , a # a F √2 , c # 3a F ) and Pb 4 Ln 3F 17 with Ln = SmLu (rhombohedral, space group R3, a h # ( a F √2 )√7, c h # 2a F √3 ). The crystallographic characteristics of the two ordered phases have been confirmed by electron diffraction.

Defect aggregation in anion-excess fluorites II. Clusters containing more than two impurity atoms

A theoretical study has been made of defect clustering in fluorite crystals. The emphasis is on the aggregation processes that can occur beyond the dimer stage which was discussed previously (J. Corish, C. R. A. Catlow, P. W. M. Jacobs, and S. H. Ong, Phys. Rev. B 25, 6425 (1982)). The systems studied comprise CaF 2, SrF 2, and BaF 2 doped with MF 3where M =Y, Lu, Yb, Er, Tb, Gd, Eu, or La, and also SrCl 2 doped with GdCl 3, LaCl 3, or PrCl 3. Two tetramers, one containing a single anion vacancy and two 〈111〉 relaxed F − ions and the other with a planar arrangement of the dopant ions, are stable for certain systems while a trimer and hexamers, with an octahedral arrangement of substitutional M 3+, also show considerable stability. A notable feature is the increase in the relative stability of larger complexes as the ratio of the radius of the dopant ion to that of the host lattice cation is decreased. The results are discussed in relation to recent EXAFS and neutron scattering work on these systems and should provide a useful guide in future experimental studies.

The defect structure of anion excess CaF 2

Neutron scattering and computer simulation techniques have been used to investigate the defect cluster structure of CaF 2 doped with 5% La 3−. The results strongly support the formation of small discrete clusters rather than the superstructures that have been suggested in recent studies of anion excess fluorites. The type of cluster that emerges as dominant comprises an interstitial-dopant dimer (of the 2:2:2 type) which has captured an additional F − interstitial. The formation of such clusters is supported by recent ITC studies.