Pavel Karen

The crystal structure of magnesium dicarbide

Abstract The crystal structure of MgC 2 is solved from powder X-ray and neutron diffraction data. Magnesium dicarbide is synthesized from Mg powder and ethyne, with yields of up to 40 wt.% (balance: unreacted Mg and MgO). The X-ray pattern is indexed on a tetragonal unit cell, with space group P 4 2 / mnm suggested from systematic extinctions. The unit cell refined from the powder neutron diffraction data has a =3.9342(7) and c =5.021(1) A and Z =2. The crystal structure contains C 2 groups, with a triple-bond length of 1.215(6) A, aligned in …MgCCMgCCMg… chains having Mg–C bond lengths of 2.174(4) A and a weak interaction [2.510(1) A] between Mg and the triple bond of the crossing chains above and below.

Synthesis and structural investigations of the double perovskites REBaFe2O5+w (RE=Nd, Sm)

By selection of appropriately sized rare earth elements and suitable reaction atmosphere, a new double-perovskite-type iron oxide REBaFe 2 O 5+ w (RE=Nd and Sm) has been synthesized, with, ideally, all Fe atoms in square-pyramidal coordinations when w=0. Like in the related triple-perovskite-type YBa 2 Fe 3 O 8+ w ′ , the added oxygen atoms w are accommodated in the RE layer. The homogeneity range in w is very wide, extending from 0.02(1) for RE=Sm and 0.050(6) for RE=Nd to w=0.65 and w=0.80, respectively, seen in O 2 at 985°C without the upper homogeneity limit being crossed. The most reduced REBaFe 2 O 5+ w phases oxidize very easily, even at room temperature. The crystal structure, as seen at room temperature after quenching from ca. 1000°C, is tetragonal, except for the most reduced compositions for RE=Sm (w<0.045) and most oxidized compositions for RE=Nd (w=0.69 as an example) which are orthorhombic. For samples with w=0.5 low temperature (ca. 500°C) annealing leads to ordering of the added oxygens within the rare earth layer. This ordering produces equal concentrations of square pyramidal and octahedral Fe 3+ . The ordered structure, a type which has not previously been observed, belongs to space group Pmna (no. 53) with a≈4a p , b≈a p and c≈2a p , where a p is the primitive cubic perovskite cell edge (a p ≈3.9 A). Structural refinements were obtained by applying the Rietveld method to synchrotron X-ray powder diffraction data.

Oxycarbonates in the Y(OCO3)Ba(OCO3)Cu(OCO3) system

Abstract The Ba-rich oxides in the Y(O)Ba(O)Cu(O) system have a high affinity toward CO2, which is manifested in the formation of oxycarbonates. The pseudoternary system Y( O CO 3 )Ba( O CO 3 )Cu( O CO 3 ) is investigated at temperatures between 780 and 1000°C in atmospheres containing oxygen and ∼5, ∼40, and ∼350 ppm CO2. Three oxycarbonates are identified: (1) Tetragonal Y2Ba3(CO3)uO6−u, u ≈ 1, with a = 438.63(4) and c = 1185.9(2) pm. (2) Y1+xBa8Cu4+z(CO3)uO11+w, u ≈ 2, x ∈ (0, 0.3), z ∈ (0, 0.4), and w ∈ [0.05(2), 1.08(2)] for x = z = 0. Its structure, which accommodates vacancies on both cationic and anionic sites, is closely related to (the hypothetic perovskite) BaCuO3 and represents the actual composition for the so-called “other perovskite phase” claimed for the Y(O)Ba(O)Cu(O) system. (3) Tetragonal YBa2Cu3(CO3)uO7−u−v with a = 387.38(3) and c = 1161.2(4) pm for u ≈ 0.2, v ≈ 0.1, when carbonate-saturated at 800°C and oxygen-saturated at 320°C in an ∼40 ppm CO2 containing oxygen atmosphere. Structurally, this oxycarbonate is derived from YBa2Cu3O7 by replacement of some oxygens by carbonate groups, and this phase represents the actual composition of the so-called “high-oxygen tetragonal 123 phase,” which is nonsuperconducting down to 4 K. The thermal stability of the oxycarbonates in oxygen with pCO2 ≈ 4 Pa decreases with decreasing Ba content from 960°C for (1) to 940°C for (2) and to 830°C for (3).

Oxidation State, A Long-Standing Issue!

The oxidation state is the simplest attribute of an element in a compound. It is taught early in the chemistry curriculum as a convenient electron-counting scheme for redox reactions. Its applications range from descriptive chemistry of elements to nomenclature and electrochemistry, or as an independent variable in plots and databases of bonded-atom properties (such as radius, bond-valence parameter, standard reduction potentials, spectral parameters, or spin). The history of the oxidation state goes back about 200 years when it described the stepwise increase in the amount of oxygen bound by elements that form more than one oxide. In his 1835 textbook Unorganische Chemie,[1] Wohler speaks of such an “oxydationsstufe” (an older German spelling for oxidation grade). This expression remains in use for oxidation state in several languages. The equivalent term oxidation number is also common; in English this refers more to redox balancing than to the chemical systematics of an element.[2] Under the entry for oxidation number, the IUPAC “Gold Book”[3] gives a defining algorithm for the oxidation state of a central atom as the charge it obtains after removal of its ligands along with the shared electron pairs. The entry for oxidation state in Ref. [3] complements this with a set of charge-balance rules and of postulated oxidation states for oxygen and hydrogen with exceptions. Details vary from textbook to textbook. Some list the rules according to decreasing priority to avoid the explicit exceptions; here is an example:[4] Atoms in an element have oxidation state 0. The sum of the oxidation states for atoms in a compound is 0. Fluorine in compounds has the oxidation state −1. Alkaline metals in compounds have the oxidation state +1, alkaline-earth metals +2. Hydrogen in compounds has the oxidation state +1. Oxygen in compounds has the oxidation state −2. In recent debates, Steinborn[5] and Loock[6] advocate Pauling’s[7] approach of assigning shared electron pairs to the more electronegative atom. Jensen[8] elaborates on some of the points considered by Loock. Smith[9] and Parkin[10] address the oxidation state in the context of related terms. Calzaferri[11] as well as Linford and co-workers[12] make suggestions on the oxidation state of organic compounds. Jansen and Wedig[13] point out the heuristic nature of the oxidation state and require that “concepts need to be defined as precisely as possible, and these definitions must always be kept in mind during applications”. IUPAC also realized the need to approach a connotative definition of the oxidation state. In 2009, a project was initiated “Toward Comprehensive Definition of Oxidation State”, led by the author of this Essay, and its results have recently been published in an extensive Technical Report.[14] We started with a generic definition of oxidation state in terms broad enough to ensure validity. Then we refined those terms to obtain typical values by algorithms tailored for Lewis, summary, and bond-graph formulas.

Toward a comprehensive definition of oxidation state (IUPAC Technical Report)

Abstract A generic definition of oxidation state (OS) is formulated: “The OS of a bonded atom equals its charge after ionic approximation”. In the ionic approximation, the atom that contributes more to the bonding molecular orbital (MO) becomes negative. This sign can also be estimated by comparing Allen electronegativities of the two bonded atoms, but this simplification carries an exception when the more electronegative atom is bonded as a Lewis acid. Two principal algorithms are outlined for OS determination of an atom in a compound; one based on composition, the other on topology. Both provide the same generic OS because both the ionic approximation and structural formula obey rules of stable electron configurations. A sufficiently simple empirical formula yields OS via the algorithm of direct ionic approximation (DIA) by these rules. The topological algorithm works on a Lewis formula (for a molecule) or a bond graph (for an extended solid) and has two variants. One assigns bonding electrons to more electronegative bond partners, the other sums an atom’s formal charge with bond orders (or bond valences) of sign defined by the ionic approximation of each particular bond at the atom. A glossary of terms and auxiliary rules needed for determination of OS are provided, illustrated with examples, and the origins of ambiguous OS values are pointed out. An electrochemical OS is suggested with a nominal value equal to the average OS for atoms of the same element in a moiety that is charged or otherwise electrochemically relevant.

YBa2Fe3O8 and the YCu(O)BaCu(O)YFe(O)BaFe(O) phase diagram

Abstract Subsolidus phase relations are given under pseudo-isothermal conditions (910–950°C) for the YCu (O) BaCu(O) YFe(O) BaFe(O) square section of the Y(O) Ba(O) Cu(O) Fe(O) tetrahedral phase diagram. All samples are prepared from liquid-mixed citrate precursors, ensuring a relatively rapid establishment of equilibrium and high compositional resolution. A new phase, tetragonalYBa2Fe3O8+w (w ∼ 0.07 when saturated in 1 atm oxygen), witha = 391.67(3) andc = 1181.78(12)pm, is synthesized and its stability limits toward the alternative formation of a disordered perovskite [a = 399.58(5)pm] are given. A total of 8(1)% of the Fe atoms in YBa2Fe3O8 can be replaced by Cu, which may be comparable to the situation for the YBaCuFeO5 phase [13(5)%]. Up to 17(5)% of the Cu atoms in YBaCuFeO5 can be replaced by Fe, as compared to 22(2)% for YBa2Cu3O7 and some 20% for BaCuO2. The existence of a large homogeneity envelope, originating at a perovskite-type BaFe1−zYzOw phase with az variable around 0.10 andw ∼ 2.6, is found from mole-balance calculations of phase content. The homogeneity envelope comprises up to 20(5)% Cu for Fe substitution or up to 10(5)% Y for Ba substitution depending on the simultaneous presence of Y at the Fe site.

Effects of oxygen nonstoichiometry and of its distribution on Verwey-type transitions and structure of

Abstract Evolutions of the mixed-valence (MV) and charge-ordered (CO) phases of GdBaFe 2 O 5+ w are investigated as a function of w (−0.02 w ) by differential scanning calorimetry (DSC) and synchrotron X-ray powder diffraction. Whereas the oxygen nonstoichiometry level w follows from cerimetric titrations, the oxygen content distribution X(w) is evaluated from deconvolutions of Bragg-peak broadenings. The somewhat tailed X(w) does not change between the CO and MV phases, but increases as a function of the w level. This suggests synthesis as its main origin. Below w =0.23 , the discontinuous CO/MV Verwey transition appears, showing a hysteresis as well as a temperature range where the CO and MV phases coexist, differing in the degree of orthorhombic distortion and molar volume. Convolutions with the w dependence of the transition temperature TV indicate that the coexistence is largely owing to X(w), resulting in different parts of the sample having slightly different TV. Below w =0.04 , a premonitory charge-ordering transition is registered by DSC, having no detectable structural effect. Below w ≈0.02 , a 3D long-range order of charges of the TbBaFe2O5-type appears. For a w =−0.001 composition, 21 weak superstructure Bragg reflections is visually identified. Intensity resolution for several of them depends heavily on fine modeling of the main-peak footings affected by two types of oxygen disorder: X(w) and a residual tetragonal disorder. Both are successfully simulated by a multiple-phase Rietveld refinement. Thermodynamic parameters of the two transitions at the ideal composition with w =0 are evaluated from composition dependences.

X-ray photoelectron spectroscopy study of Y2BaCuO5 and YBa2Cu3O9−δ

Abstract X-ray photoelectron spectroscopy (XPS) data for the green, semiconducting Y2BaCuO5, and the orthorhombic and tetragonal variants of the black, superconducting YBa2Cu3O9−δ(δ≈2) are reported. The recorded Cu 2p 3 2 spectra are accompanied by a satellite shifted ∼ 10eV towards higher binding energy, which is characteristic of Cu2+ (or even higher oxidation states). The Cu 2p 3 2 binding energy for Y2BaCuO5 and YBa2Cu3O9−δ is slightly lower than found for CuO, which for YBa2Cu3O9−δ can be understood in terms of metallic conduction properties. The Ba 3d 5 2 line in YBa2Cu3O9−δ displays an additional, minor feature shifted approximately -2eV relative to the leading line.

Neutron powder diffraction study of nuclear and magnetic structures of oxidized and reduced YBa2Fe3O8+w

Abstract YBa2Fe3O8+w has been investigated by neutron powder diffraction as function of temperature and oxygen nonstoichiometry close to the limits of the homogeneity range, −0.24 0) in the structural layers of Y, or by creating oxygen vacancies (w (1 1 0/110/002 ), having orthorhombic symmetry when the nuclear structure is tetragonal and monoclinic symmetry when the nuclear structure is orthorhombic. The iron moments are coupled antiferromagnetically in all three directions, the Neel temperature is almost constant as a function of w (T N ≈660 K ) , and so is also the low-temperature saturation moment μAF≈4.0μB.

Synthesis and characterization of color variants of nitrogen- and fluorine-substituted TiO2

Reaction pathways to nitrogen- and fluorine-doped TiO2 have been investigated and the compositions analyzed by neutron powder diffraction, nitrogen analyses and titrations of Ti3+. The reported formation of TiNF under pyrolysis of (NH4)2TiF6 in NH3 could not be reproduced when H2O was carefully excluded. If special precautions are not taken to exclude H2O, a product of the previously reported olive-green appearance is obtained; TiN0.05O1.89F0.06, in which 1% of Ti is trivalent. The prolonged hydrolysis and ammonolysis leading to this product proceeds via the intermediates (NH4)1−xTiOF3−x, which adopts the hexagonal tungsten–bronze structure, and/or TiOF2. The latter is therefore suggested as a convenient starting material for doped anatase pigments. Reflectance measurements couple the green color with valence to conduction band excitations (Eg = 2.3 eV) and intraband transitions involving Ti3+. The green phase can be converted to a brilliant yellow in the presence of water vapor at 400–600 °C, which further hydrolyzes the fluoride and oxidizes the titanium, yielding TiN0.04O1.92F0.04 in a particular case. The yellow color and band gap (Eg = 2.4 eV) may be promising for applications as a pigment or photocatalyst. Neutron powder diffraction characterizations and UV–Visible reflectance measurements indicate homogenous doping throughout the bulk. The combined results suggest that the green and yellow phases are part of a homogeneity range adjacent to TiO2, in which the band gap narrowing results largely from nitrogen for oxygen subsitution and the green color is linked to formation of Ti3+ defects.