A. Boireau

Bio: A. Boireau is an academic researcher from University of Bordeaux. The author has contributed to research in topics: Crystal structure & Rietveld refinement. The author has an hindex of 5, co-authored 6 publications receiving 51 citations.

A new linear chain compound: crystal and magnetic structures of thallium manganese fluoride hydrate, Tl2MnF5.H2O

Abstract: The crystal structure of a new liner chain compound Tl 2 MnF 5 .H 2 O has been determined by X-ray diffraction. The structure has been refined down to R=0.034 [Cmcm space group (Z=4); unit cell constants a=9.688 (2) A, b=8.002 (1) A, and c=8.339 (1) A]. Manganese is located within elongated fluorine octahedra sharing two opposite corners to form infinite (MnF 5 ) n 2n- chains along the c-axis separated by Tl atoms and water molecules

Crystal and magnetic structures of the layer Compound TlMnF4

Abstract: The crystal structure and the magnetic properties of the fluoromanganate(III) TlMnF4 have been investigated. The structure has been refined down to R/wR of 0.057/0.043 in the monoclinic I2/a space group with the unit cell constants a = 539.7(2) pm; b = 544.1(2) pm; c = 1248.4(5) pm; β = 90.19(3)° (Z = 4). TlMnF4 is characterized by a layer structure formed of MnF6 octahedra sharing their four equatorial corners. Within each octahedron the MnF distances range from 178 pm to 215 pm. The intralayer magnetic interaction (J/K) has been evaluated to be approximately −0.45 K by fitting the experimental susceptibility in the 10–300 K range using the quadratic layer Heisenberg model. A 3 D-antiferromagnetic ordering occurs at Tn = 4.2(5) K. The magnetic cell corresponding to the nuclear one but with a primitive symmetry. The magnetic structure has been refined down to R = 0.0528 in the P2′/a′ magnetic group. The MnIII moments are colinear to the b-axis and show antiparallel ordering within the layers. Kristall- und magnetische Struktur von TlMnF4, einer Verbindung mit Schichtstruktur Die Kristallstruktur und die magnetischen Eigenschaften des Fluoromanganates(III) TlMnF4 wurden untersucht. Die Struktur wurde in der monoklinen Raumgruppe I2/a, Elementarzelle mit a = 539,7(2); b = 544,1(2); c = 1248,4(5) pm, β = 90,19(3)° (Z = 4) auf R/wR 0,057/0,043 verfeinert. TlMnF4 zeigt eine Schichtstruktur, die durch Eckenverknupfung von MnF6-Oktaedern uber ihre vier aquatorialen Ecken gebildet wird. Die MnF-Abstande innerhalb des Oktaeders liegen im Bereich von 178 bis 215 pm. Die magnetische Austauschenergie (J/K) innerhalb der Schicht wurde durch Anpassung der experimentellen Suszeptibilitatsdaten im Temperaturbereich von 10–300 K auf der Basis des Heisenbergmodells fur quadratische Schichten zu −0,45 K bestimmt. Dreidimensionale antiferromagnetische Ordnung tritt bei 4,2(5) K ein. Die magnetische Zelle entspricht der kristallographischen, jedoch mit primitivem Translationsgitter. Die magnetische Struktur wurde auf R = 0,058 in der magnetischen Raumgruppen P2′/a′ verfeinert. Die magnetischen Momente an MnIII sind colinear zur b-Achse und zeigen antiparallele Ordnung innerhalb der Schichten.

A new ID‐antiferromagnet: Crystal and magnetic structures of TlMnF4 · H2O

Abstract: The synthesis, crystal structure, and magnetic properties of a new fluoromanganate (III) TlMnF4 · H2O are investigated. Single crystals are obtained from 40% hydrofluoric acid at low temperature (5 °C). The crystal structure is refined down to R/wR = 0.034/0.030 in the monoclinic C 2/c space group with the following unit cell constants: a = 1.3784(1) nm, b = 0.6631(1) nm, c = 1.0537 nm, β = 103.66(1)° (Z = 8). The hydrogen positions are determined from powder neutron diffraction data at 20 K. As isostructural RbMnF4 · H2O, the compound is characterized by infinite zigzag chains of alternating [MnF6] and [Mn(H2O)2F4] octahedra sharing trans corners. The Jahn-Teller elongation is directed along the chain direction in the first type of octahedra, whereas it corresponds to the perpendicular Mn–O equatorial bond in [Mn(H2O)2F4] species. The magnetic properties can be fitted using the isotropic Heisenberg model for chains of finite S = 2 spins and are compared to those of RbMnF4 · H2O. The corresponding exchange constants: J/k = −3.9 K [−4 K] for Tl[Rb]MnF4 · H2O are weaker than those generally observed in chain-type fluoromanganates (III) because of a low Mn–F–Mn bridging angle of about 138°. A 3D magnetic ordering occurs below TN = 8.1(3) K [8.3(3) K] for Tl[Rb]MnF4 · H2O. Along the chain an antiferromagnetic arrangement is observed with components on a- and c-axes. The total magnetic moments are M = 3.39(6) μB for TlMnF4 · H2O at 1.3 K and M = 3.46(6) μB for RbMnF4 · H2O at 4 K. Synthese, Kristallstruktur und magnetischen Eigenschaften eines neuen Fluoromanganats (III), TlMnF4 · H2O werden untersucht. Aus 40%iger Hydrofluorsaure werden bei niedrigen Temperaturen (5 °C) Einkristalle erhalten. Die Kristallstruktur wird bis herab zu R/wR = 0,034/0,030 in der monoklinen C2/c-Raumgruppe mit folgenden Konstanten der Einheitszelle erhalten: a = 1,3784(1) nm, b = 0,6631(1) nm, c = 1,0537 nm, β = 103,66(1)° (Z = 8). Die Wasserstoffpositionen werden aus Neutronenbeugungsdaten bei 20 K bestimmt. Wie fur isostrukturelles RbMnF4 · H2O ist die Verbindung charakterisiert durch unendliche Zick-Zack-Ketten von alternierenden [MnF6]- und [Mn(H2O)2F4]-Oktaedern mit gemeinsamen Ecken. Die Jahn-Teller-Verzerrung liegt in Richtung des ersteren Oktaedertyps entsprechend der senkrechten Mn–O-Aquatorialbindung von [Mn(H2O)2F4]. Die magnetischen Eigenschaften konnen durch ein isotropes Heisenbergmodell fur Ketten mit endlichen S = 2-Spins angepast werden und werden mit denen von RbMnF4 · H2O verglichen. Die entsprechenden Austauschkonstanten: J/k = −3,9 K [−4 K] fur Tl[Rb]MnF4 · H2O sind wegen des niedrigen Mn–F–M-Bruckenwinkels von etwa 138° kleiner als die im allgemeinen in Ketten-Fluoromanganate (III) beobachtet werden. Eine dreidimensionale (3D) magnetische Ordnung tritt unterhalb TN = 8,1(3) K [8,3(3) K] fur Tl[Rb]MnF4 · H2O auf. In Richtung der Kette wird eine antiferromagnetische Ordnung beobachtet mit Komponenten auf der a- und c-Achse. Die magnetischen Gesamtmomente betragen M = 3,39(6) μB fur TlMnF4 · H2O bei 1,3 K und M = 3,46(6) μB fur RbMnF4 · H2O bei 4 K.

Real and Hypothetical Intermediate-Valence AgII/AgIII and AgII/AgI Fluoride Systems as Potential Superconductors

Abstract: With the aim of gauging their potential as conducting or superconducting materials, we examine the crystal structures and magnetic properties of the roughly one hundred binary, ternary, and quaternary Ag(II) and Ag(III) fluorides in the solid state reported up to date. The Ag(II) cation appears in these species usually in a distorted octahedral environment, either in an [AgF](+) infinite chain or as [AgF(2)] sheets. Sometimes one finds discrete square-planar [AgF(4)](2-) ions. The Ag(III) cation occurs usually in the form of isolated square-planar [AgF(4)](-) ions. Systems containing Ag(III) (d(8)) centers are typically diamagnetic. On the other hand, the rich spectrum of Ag(II) (d(9)) environments in binary and ternary fluorides leads to most diverse magnetic properties, ranging from paramagnetism, through temperature-independent paramagnetism (characteristic for half-filled band and metallic behavior) and antiferromagnetism, to weak ferromagnetism. Ag(II) and Ag(III) have the same d-electron count as Cu(II) (d(9)) and Cu(III) (d(8)), respectively. F(-) and O(2-) ions are isoelectronic, closed-shell (s(2)p(6)) species; both are weak-field ligands. Led by these similarities, and by some experimental evidence, we examine analogies between the superconducting cuprates (Cu(II)/Cu(III)-O(2-) and Cu(II)/Cu(I)-O(2-) systems) and the formally mixed-valence Ag(II)/Ag(III)-F(-) and Ag(II)/Ag(I)-F(-) phases. For this purpose we perform electronic-structure computations for a number of structurally characterized binary and ternary Ag(I), Ag(II), and Ag(III) fluorides and compare the results with similar calculations for oxocuprate superconductors. Electronic levels in the vicinity of the Fermi level (x(2)-y(2) or z(2)) have usually strongly mixed Ag(d)/F(p) character and are Ag-F antibonding, thus providing the potential of efficient vibronic coupling (typical for d(9) systems with substantially covalent bonds). According to our computations this is the result not only of a coincidence in orbital energies; surprisingly the Ag-F bonding is substantially covalent in Ag(II) and Ag(III) fluorides. The electron density of state at the Fermi level (DOS(F)) for silver fluoride materials and frequencies of the metal-ligand stretching modes have values close to those for copper oxides. The above features suggest that properly hole- or electron-doped Ag(II) fluorides might be good BCS-type superconductors. We analyze a comproportionation/disproportionation equilibrium in the hole-doped Ag(II) fluorides, and the possible appearance of holes in the F(p) band. It seems that there is a chance of generating an Ag(III)-F(-)/Ag(II)-F(0) "ionic/covalent" curve crossing in the hole-doped Ag(II)-F(-) fluorides, significantly increasing vibronic coupling.

Complex ceramic structures. I. Weberites

Abstract: The weberite structure (A2B2X7) is an anion-deficient fluorite-related superstructure. Compared with fluorites, the reduction in the number of anions leads to a decrease in the coordination number of the B cations (VI coordination) with respect to the A cations (VIII coordination), thus allowing the accommodation of diverse cations. As a result, weberite compounds have a broad range of chemical and physical properties and great technological potential. This article summarizes the structural features of weberite and describes the structure in several different ways. This is the first time that the stacking vector and stacking angle are used to represent the weberite structure. This paper also discusses the crystallographic relationship between weberite, fluorite and pyrochlore (another fluorite-related structure). The cation sublattices of weberite and pyrochlore are correlated by an axial transformation. It has been shown that the different coordination environment of anions is due to the alternating layering of the AB3 and A3B close-packed cation layers. A stability field of weberite oxides is proposed in terms of the ratio of ionic radius of cations and relative bond ionicity. In addition, a selection of weberite compounds with interesting properties is discussed.

Pressure effects on NaMnF4: Structural correlations and Jahn-Teller effect from crystal-field spectroscopy

Abstract: This work investigates the optical absorption spectrum of the NaMnF4-layered perovskite and its variation with pressure. The spectrum basically consists of three broadbands located at 1.916, 2.263, and 2.817 eV, which correspond to the crystal-field (CF) transitions (Formula presented) (Γi = A1g, B2g, and Eg) with the Jahn-Teller- (JT-) distorted (Formula presented) complex (Mn3+ d4 configuration). In addition, there are two spin-flip (Formula presented) peaks at 2.397 and 2.890 eV, which are activated by the exchange mechanism. Their variation with pressure reveals that the JT energy does not change significantly with pressure: ∂EJT/∂P = 0.8 meV/GPa. Furthermore, the variation of the JT tetragonal splitting of the parent octahedral eg and t2g, termed Δe and Δt, respectively, clearly indicate that ∂Δe/∂P ≪ ∂Δt/∂P, although Δe ≈ 4Δt. The CF energies and their pressure shift are explained in terms of local structural changes within the (Formula presented) complex induced by pressure. The structural correlation analysis reveals that the reduction of the (Formula presented) JT distortion is smaller than the expected one on the basis of the crystal volume reduction, thus indicating tilt phenomena. This interpretation is supported by the decrease of in-layer Mn-F-Mn superexchange, such as is derived from the optical spectra. We demonstrate that the equatorial and axial distances decrease from 1.839 to 1.808 A and from 2.167 to 2.107 A, respectively, in the 0–10 GPa range. © 2003 The American Physical Society.