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Formation, crystal structure, polymorphism, and transition between polymorphs are reported for M(thd)3, (M = Al, Cr, Mn, Fe, Co, Ga, and In) [(thd)– = anion of H(thd) = C11H20O2 = 2, 2, 6, 6-tetramethylheptane-3, 5-dione]. Fresh crystal-structure data are provided for monoclinic polymorphs of Al(thd)3, Ga(thd)3, and In(thd)3. Apart from adjustment of the M–Ok bond length, the structural characteristics of M(thd)3 complexes remain essentially unaffected by change of M. Analysis of the M–Ok, Ok–Ck, and Ck–Ck distances support the notion that the M–Ok–Ck–Ck–Ck–Ok– ring forms a heterocyclic unit with σ and π contributions to the bonds. Tentative assessments according to the bond-valence or bond-order scheme suggest that the strengths of the σ bonds are approximately equal for the M–Ok, Ok–Ck, and Ck–Ck bonds, whereas the π component of the M–Ok bonds is small compared with those for the Ok–Ck, and Ck–Ck bonds. The contours of a pattern for the occurrence of M(thd)3 polymorphs suggest that polymorphs with structures of orthorhombic or higher symmetry are favored on crystallization from the vapor phase (viz. sublimation). Monoclinic polymorphs prefer crystallization from solution at temperatures closer to ambient. Each of the M(thd)3 complexes subject to this study exhibits three or more polymorphs (further variants are likely to emerge consequent on systematic exploration of the crystallization conditions). High-temperature powder X-ray diffraction shows that the monoclinic polymorphs convert irreversibly to the corresponding rotational disordered orthorhombic variant above some 100–150 °C (depending on M). The orthorhombic variant is in turn transformed into polymorphs of tetragonal and cubic symmetry before entering the molten state. These findings are discussed in light of the current conceptions of rotational disorder in molecular crystals.

The choice of solvent (alcohol or hydrocarbon), oxidising agent (air, pure oxygen or hydrogen peroxide), and temperature (ambient to boiling solution) is significant in the oxidation of Co(thd)2 [(thd)– = anion of H(thd) = C11H20O2 = 2,2,6,6-tetramethylheptane-3,5-dione]. With EtOH as solvent, refluxing conditions, and oxygen atmosphere the solid reaction product was established as a 2:1 stoichiometric mixture of Co(thd)3 and Co3(thd)3(EtO)4(tert-BuCOO). The relative amount of Co(thd)3 increases somewhat with decreasing reaction temperature. One third of the cobalt atoms in Co3(thd)3(EtO)4(tert-BuCOO) retain their original CoII state (the rest being oxidised to CoIII). With MeOH as solvent negligible amounts of cobalt are available for oxidation since the CoII state instead becomes tied up in Co4(thd)4(MeO)4(MeOH)4 which is virtually insoluble in MeOH. Complete oxidation of CoII to CoIII occurs in 1- and 2-propanol, but apart from Co(thd)3 the composition of the poorly crystalline products could not be identified. The crystal and molecular structures of Co3(thd)3(EtO)4(tert-BuCOO) have been determined by single-crystal X-ray diffraction [a = 9.811(4), b = 22.528(8), c = 50.516(5) A, β = 99.178(7)° at 150 K; space group P21/c]. The molecular structure comprises a triangular Co3 central cluster linked together by Co–O–Co bridges. The arrangement in the central cluster can be viewed as a cuboid-resembling Co3O4 configuration (viz. a Co4O4 cuboid with one cobalt vertex vacant). The coordination of the CoIII atoms is approximately octahedral whereas that of the penta-coordinated CoII atoms is distorted square pyramidal.

The mixed-ligand complex [Cr(thd)2(OEt)]2 [(thd)– = anion of H(thd) = C11H20O2 = 2,2,6,6-tetramethylheptane-3,5-dione] appears as by-product when EtOH/H2O is used as solvent during preparation of Cr(thd)3. [Cr(thd)2(OEt)]2 can be difficult to separate from Cr(thd)3 by sublimation, but separation is easily accomplished by extracting Cr(thd)3 with acetone. A detailed account for the sublimation behavior of [Cr(thd)2(OEt)]2/Cr(thd)3 mixtures is advanced. Good yields of [Cr(thd)2(OEt)]2 are obtained when CrCl3, H(thd), and Na(EtO) react in absolute EtOH. [Cr(thd)2(OEt)]2 is obtained in the form of green needle-shaped crystals by recrystallization from toluene. The crystal structure is triclinic [a = 10.2919(15), b = 10.6686(16), c = 14.194(3) A, α = 106.559(2), β = 107.869(2), and γ = 98.326(2)° at 295 K; space group P. The complex contains two crystallographic equivalent chromium atoms, which are bridged by two cis-configured ethoxy groups, the four remaining sites in the octahedral coordination around each chromium atom being occupied by oxygen atoms from two thd ligands. The bond lengths and angles concur with the findings for related molecular complexes. The temperature dependence of the magnetic susceptibility of [Cr(thd)2(OEt)]2 follows Curie–Weiss law with Weiss constant θ ≈ –65 K and μp = 3.87 μB.

The syntheses, crystal structures, and thermal stabilities of six polymorphs of Cr(thd)3 [(thd)– = anion of H(thd) = C11H20O2 = 2,2,6,6-tetramethylheptane-3,5-dione] were studied using X-ray diffraction and differential scanning calorimetry. Compound 1 is thermodynamically stable below ca. 110 °C and forms plate-shaped crystals [a = 9.927(5), b = 18.010(5), c = 21.427(5) A, and β = 95.461(5)° at 295 K; space group C2/c]. Its crystal structure is isotypic with Mn(thd)3. Polymorph 4 is metastable to 1, and the only way to prepare 4 appears to be by precipitation from solution. Crystals of 4 are needle shaped [a = 28.54(3), b = 19.14(2), c = 21.92(2) A, and β = 97.31(2)° at 295 K; space group C2/c] and this structure is isotypic with monoclinic Co(thd)3. Modifications 2, 2*, and 5 also appear as needle-shaped crystals [orthorhombic: a = 18.97(7), b = 18.69(7), and c = 10.62(5) A for 2 and tetragonal: a = b = 18.93(5) and c = 10.57(4) A for 5, both at 110 K]. Crystals of 2 and 5 have limited lifetime, which depends on storage temperature and exposure to X-rays. The conversion sequence is: 5 2 1, where the first step takes 3 to 4 h and the second some 20 h. Attempts to establish detailed accounts of the orthorhombic Co(thd)3-type structure of 2 and 2* by SXD were unsuccessful due to disorder. The structural distinction between 2 and 2* appears to be associated with rotational disorder among the Cr(thd)3 molecules. 2* and 3 are high-temperature modifications with stability range ca. 155–200 and ca. 200–235 °C, respectively [Cr(thd)3 melts at ca. 235 °C]. Quenched 2* can be retained as metastable at room temp. for some weeks, whereas 3 is not quenchable. High-temperature PXD data show that 3 is cubic [a = 13.357(4) A at 225 °C; with strong rotational disorder of the molecules]. There are only minor variations in bond lengths and angles between 1 and 4 or from 100 to 295 K, and disorder is limited.

Thin films of La1-xSrxFeO3 have been prepared by the ALD (atomic layer deposition) technique using La(thd)3 (Hthd = 2,2,6,6-tetramethylheptane-3,5-dione), Sr(thd)2, Fe(thd)3, and ozone as precursors. A so-called ALD window is found in the temperature range 200 to 360 degrees C for LaFeO3. The effect of the pulsing procedure for the precursors on the composition of the films is examined. The results are discussed in relation to a model which ascribes differences between pulsed and obtained stoichiometries to individually different surface-area demands of the precursors. The La1-xSrxFeO3 films turned out to contain only small amounts of carbonate impurities despite the fact that films prepared from Sr(thd)2 and ozone under the same conditions contains virtually pure SrCO3. Films of La1-xSrxFeO3 have been deposited on substrates of (amorphous) soda-lime glass and single crystals of Si(100), SrTiO3(100), and LaAlO3(012). Annealed films on soda-lime glass and Si(100) substrates turned out to be polycrystalline with virtually random orientation of the crystallites. Those on MgO(100) and SrTiO3(100) substrates showed some degree of crystal orientation, whereas the annealed films on LaAlO3(012) proved to contain distinctly oriented crystallites with mosaic features.

Oxidation of Co(thd)2 dissolved in different solvents has been investigated in air and oxygen atmosphere. In oxygen atmosphere and at the boiling point of the solvents this treatment leads to oxidation of CoII to CoIII, but also to degradation of some of the thd ligands and formation of a new mixed-ligand complex. Three pure-cultivated crystalline Co(thd)3 phases are reported: 1 (room-temperature phase), 2 (low-temperature phase), and 3 (metastable phase) and in addition there exists an amorphous Co(thd)3 phase (4) with approximate composition Co(thd)3·xH(thd); x = 0.06. Reaction of metal(II) oxides (MO, M = Mn, Fe, and Co) with H(thd) under air or O2 atmosphere is an easy direct route to M(thd)3 complexes. Structure determinations are reported for Co(thd)3 (1–3) based on single-crystal X-ray diffraction data. Modification 1 crystallizes in space group with a = b = 18.8100(10), c = 18.815(2) A at 295 K; R(wR2) = 0.180, modification 2 in space group C2/c with a = 28.007(12), b = 18.482(8), c = 21.356(9) A, β = 97.999(5)° at 100 K; R(wR2) =0.211, and modification 3 in space group Pnma with a = 19.2394(15), b = 18.8795(15), c = 10.7808(8) A at 100 K; R(wR2) = 0.193. The molecular structures of 1–3 all comprise a central Co atom octahedrally co-ordinated by the ketonato O atoms of three thd ligands. The transformation between modifications 1 and 2 is of a fully reversible second-order character. Modifications 1 and 3 are, on the other hand, related by a quasi-reversible cycle. Heat treatment (specifically sublimation) of 1 leads to 3 whereas re-crystallization or prolonged storage at room temperature is required to regenerate 1. Co(thd)3 has sufficient thermal stability to permit sublimation without degradation. The various forms of Co(thd)3 are all diamagnetic, viz. a confirmation of the CoIII valence state.

Needles of manganese(IV) oxide in the nanometer range have been synthesised using the atomic layer deposition technique. Traditionally the atomic layer deposition technique is used for the fabrication of thin films, however, we find that needles of β-MnO2 are formed when manganese(IV) oxide is deposited as relatively thick (ca. 800 nm) thin films on substrates of α-Al2O3 [(001) and (012) oriented]. There is no formation of needles when the film is deposited on substrates such as Si(100) or soda lime glass. The film is formed using Mn(thd)3 (Hthd = 2,2,6,6-tetramethylheptane-3,5-dione) and ozone as precursors. While thin films (ca. 100 nm) consist of ε′-MnO2,22, 23 the same process applied to thicker films results in the formation of nano-needles of β-MnO2. These needles of β-MnO2 have dimensions ranging from approximately 1.5 μm at the base down to very sharp tips. The nano-needles and the bulk of the films have been analysed by atomic force microscopy, scanning electron microscopy, X-ray diffraction, and transmission electron microscopy.

The present contribution reports on refined and constrained analyses of the complex proton-NMR free-induction-decay (FID) signal of hydrogen-containing amorphous silicon (a-Si:H), produced by thermal decomposition of pressurized monosilane. The spectra comprise clustered and diluted protons and the mole fractions of these are described by an algebraic sum of two exponential terms of the type I · exp(−kP) where I and k are constants and P the initial monosilane pressure in the decomposition reactor. The NMR spin–spin relaxation time of the clustered protons is found to be approximately constant (T2G ≈ 15 μs) and independent of P. For the diluted protons, the spin–spin relaxation time are found to increase monotonically with increasing P (T2L ranging from ∼85 to ∼140 μs) and could be approximated by a second-order polynomial in P. The number of diluted protons and their average nearest-neighbor separation are found to increase with increasing P from ∼7 A at 0.28 MPa to ∼8.5 A at 6.61 MPa. These findings show that the amorphous raw-silicon exhibits an inhomogeneous distribution of diluted protons. The distribution becomes more homogeneous with increasing P. The simplest description of the clustered configurations is that the number of protons and their average nearest-neighbor distance remain constant and independent of P, and that only the number of the clusters increases with increasing P. The amount of diluted and clustered protons, complies with the total amount of hydrogen found by chemical analysis and by FT-IR spectroscopy.

This first article in a series on simulation of growth of thin films by the ALD (= atomic layer deposition) technique deals with the theoretical background for growth and preparation of amorphous films. The growth dynamics of amorphous thin films have been simulated on the basis of growth on randomly positioned hemisphere-shaped seed objects. The objects were positioned in the simulation space according to rules which intend to create entirely random or guided random patterns. The simulations are able to reproduce the non-linear growth in the initial stages of ALD growth according to type-2 substrate-inhibited growth, and provide a mechanism for this based on purely geometrical concepts; also a self-driving mechanism toward flat surfaces is demonstrated. An initial stage of roughening of the films occurs before smoothening takes over.

Materials with one- and two-dimensional crystal structures often form crystals with the shape of needles and platelets, respectively. This should be expected to have great influence on the properties of thin films grown from such materials, and the effect on texture and topography is shown in this paper by a series of simulations. The topic of this paper is the effect of different aspect ratios of crystals with tetragonal symmetry on the resulting thin films. However the major results should also apply to crystals of other types of symmetry having aspect ratios deviating from one. The growth dynamics in atomic layer deposition of polycrystalline thin films have been simulated for randomly oriented and randomly positioned crystallites. Crystalline seed objects adapting shapes of tetragonal boxes with aspect ratios from 0.1 (platelets) to 10 (needles) has been used as examples for growth of films from tetragonal crystals. The dependence of roughness, surface crystal density, and texture on the film thickness is shown. Topography and cross sections of simulated films with different aspect ratios are discussed. Non-linear growth regimes are consistently found in the initial stage of the film formation. A conversion from type-2 to type-1 substrate inhibited growth is observed as the aspect ratio deviates significantly from one.

The homogeneity range of the Ca3Co2–vMnvO6 solid-solution phase covers the entire composition interval from v=0 to 1. A systematic powder X-ray and neutron diffraction, magnetic susceptibility, and magnetization study has been carried out to investigate effects of the Mn-for-Co substitution on structural and magnetic properties. The Mn substitution concerns primarily only the octahedral Co1 site of the Ca3Co1Co2O6 crystal structure, whereas the trigonal-prismatic Co2 site structurally is left essentially unaffected. The Ca3Co2–vMnvO6 crystal structure belongs to space group R 3 ¯ c with unit-cell dimensions (in hexagonal setting) 9.084⩽a⩽9.134 A and 10.448⩽c⩽10.583 A. A cut through the magnetic phase diagram at 10 K shows a ferrimagnetic domain for 0⩽v

Kinetic aspects of the thermal decomposition of monosilane at 690–830 K and initial pressures of 0.1–3.7 MPa in a free-space reactor are considered. Neglecting the preparatory initiation period for the reaction (which is difficult to evaluate under the present dynamic conditions), the onset temperature for the decomposition is stipulated to some 700–710 K, independent of the initial monosilane pressure. The overall reaction appears to be of first order throughout the progressing decomposition process. We observe considerably lower reaction rates under the high-pressure conditions than existing models in the literature suggest. A modified model is proposed that simulates the observed reaction rates within ±1% and moreover predicts credible concentrations of the involved gaseous species. A key feature of the modified model is incorporation of two third-body assisted surface reactions, which generate monosilane from disilane and disilane from trisilane. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 309–321, 2006

Morphological changes introduced by chemical etching of Pt, Pt/10–30 wt.% Rh, and Rh wires in different oxygen-containing atmospheres at 900–1200 °C are studied by scanning electron microscopy and powder X-ray diffraction. The chemical etching ability of different sources for molecular oxygen is ranked as: wet O2 > N2O > dry O2 > NO. Treatment in these atmospheres gave similar surface patterns with numerous pits and small deposited crystals. Powder X-ray diffraction showed that the surfaces of the tested Rh-containing specimens are more or less covered with Rh2O3. In wet oxygen and N2O atmospheres the formation of Rh2O3 is somewhat suppressed toward lower temperatures and/or higher Rh content. None of the alloys tested at 900 °C showed any significant loss of material. Some Rh-containing specimens showed a slight weight gain due to Rh2O3 surface coverage. The occurrence of PtO2 and RhO2 in the vapor phase during the etching process was verified by in situ mass spectrometry. Factors of importance for chemical and catalytic etching are discussed.

Thermal decomposition of pure monosilane at initial pressures between 0.1 and 6.6 MPa produces a black/gray powder of loosely agglomerated spherical silicon particles (diameter ca. 3 μm), which contain ∼0.07–0.19 wt.% (ca. 2–5 at.%) hydrogen. Hydrogen is residing as diluted and clustered mono- (SiH), di- (SiH 2 ), and trihydride (SiH 3 ) units. Except for the trihydrides these Si–H configurations were observed by FT-IR spectroscopy whereas qualitative 29 Si-NMR analyses additionally revealed SiH 3 configurations. Desorption of hydrogen from the different kinds of hydride groupings is studied by FT-IR, 29 Si-NMR, and hydrogen element analyses. The results show that on increasing temperature the desorption, broadly speaking, follows the hydrogen content of the hydride groupings, viz. (clustered) trihydrides being first and diluted monohydrid last stripped off from a-Si:H materials. Hydrogen desorption from diluted monohydride locations show second-order kinetics with E a = 160 ± 20 kJ mol −1 , while hydrogen desorption from clustered monohydride units appears to exhibit a first-order behavior with an activation energy of 110 ± 25 kJ mol −1 . Hydrogen residing in clustered polyhydride regions appears to desorb according to a second-order mechanism below ∼873 K ( E a = 90 ± 25 kJ mol −1 ; hydrogen content exceeding ca. 36% saturation) and first-order behavior above 873 K. The desorption of hydrogen from the trihydride units is more difficult to evaluate, but seems to be of first order with an activation energy of 10 ± 5 kJ mol −1 . The present a-Si:H materials show clear signs of crystallinity after 5 h of heating at 873 K.

Chemical vapor transport of platinum (Pt) and rhodium (Rh) has been studied in closed ampoules using a 900–800 °C over 100 mm gradient for periods of 123–365 days. The ability of O 2 , air, and H 2 O to act as transport agents has been evaluated, control experiments have been performed with vacuum, N 2 , and Ar, and surveying experiments have been made with NH 3 , NO, and H 2 . Transport of Pt and Rh occurs in the presence of oxygen (regardless of source), manifested as single-crystal growth at the colder end of the ampoules. PtO 2 and RhO 2 were identified as vapor molecules by in situ mass spectrometric monitoring. A small transport of Rh is indicated for H 2 O as agent, whereas no transport is observed in N 2 , H 2 , NH 3 , and Ar atmospheres.

Surface reconstructions on Pd, Pd/5 wt.% Ni, Ag, Ir, Pt/10 wt.% Ir, Pt/20 wt.% Ir, and Au wire-shaped specimens have been studied after ammonia oxidation at 900 °C and atmospheric pressure. The tested specimens were examined by powder X-ray diffraction, electron microprobe analysis, and scanning electron microscopy. Extensive surface reconstructions, with numerous pits, well developed facets, and “cauliflower”-like excrescences were, except for Au, observed on all tested specimens. Neglecting Au, the tested catalyst materials were active for oxidation of NH 3 to NO, however, the selectivity and effective lifetime of these catalysts are strongly composition dependent. Au did not catalyze ammonia oxidation and its surface reconstructions are negligible. Solid oxides of NiO, PdO and IrO 2 were identified on the surfaces of Pd–Ni, Pd, and Ir specimens, respectively. The results are discussed in relation to findings for Pt–Rh alloy catalysts, for which surface reconstruction is attributed to chemical-vapor-transport reactions driven by local temperature gradients between hotspots and colder regions on the catalyst surface.

Surface coating of α-Fe 2 O 3 has been shown to affect the reconstruction of platinum–rhodium catalysts during oxidation of ammonia at 900 °C and atmospheric pressure. Surfaces of wire-formed Pt–Rh specimens with 0–30 and 100 wt.% Rh were coated with thin layers of α-Fe 2 O 3 , deposited by thermal decomposition of iron(III) nitrate or by atomic layer chemical vapor deposition. Scanning electron microscopy, electron microprobe analysis, and powder X-ray diffraction were used to examine the catalyst wires before and after use in ammonia oxidation. The reconstruction on Pt–Pt/10 wt.% Rh and Rh catalysts with α-Fe 2 O 3 -coated surfaces involves “cauliflower”-like excrescences similar to those observed on corresponding materials without coatings. The reconstructions on α-Fe 2 O 3 -coated catalysts of Pt/20 wt.% Rh and Pt/30 wt.% Rh carry the same characteristics. However, for these alloys the reconstruction process becomes much faster and the resulting patterns more extensive than for corresponding materials without α-Fe 2 O 3 coating. The boundary zones between the α-Fe 2 O 3 cover and the liberated Pt–Rh surfaces appear to stand out as spots (hotspots in the thermal sense) with enhanced catalytic activity. A certain decrease in activity and selectivity with time is observed for all tested specimens. This is attributed to gradual degradation of α-Fe 2 O 3 to more inactive Fe 3 O 4 . The progressing degradation of α-Fe 2 O 3 to Fe 3 O 4 shows that the temperature in the hotspots must exceed some 1400 °C or that reducing conditions prevail locally at the surface.

The oxidation of ammonia to nitrogen monoxide is catalyzed by platinum–rhodium alloys. During the first few hours of operation the originally smooth surfaces on Pt–Rh catalysts with Rh contents of 0–13 and 100 wt.% are heavily reconstructed. The process starts on the grain boundaries, but soon spreads to cover the entire surface with “cauliflower”-like excrescences. Only slight reconstructions on the grain boundaries are observed for Pt/20 wt.% Rh and Pt/30 wt.% Rh catalysts. The present paper describes studies of oxidation of ammonia along Pt–Rh catalyst wires, including mass spectrometric surveys of the gaseous species formed. Scanning electron microscopy, electron microprobe, and X-ray powder diffraction characterizations were made of the catalyst wires before and after reaction. The amount and appearance of the reconstructions depend strongly on the Rh content of the catalyst. Marked concentration gradients in platinum-to-rhodium content are observed between wires and “cauliflowers” as well as within the “cauliflowers”. A qualitative account for the formation of the “cauliflowers” and related features is advanced using concepts from chemical vapor-transport reactions. PtO 2 and/or RhO 2 act as transporting molecules and the processes are driven by local temperature gradients between hotspots and colder regions on the catalyst surface. The ultimate net effect of the involved atomic transfer processes is to establish an approximately equiatomic (Pt 50 Rh 50 ) composition at all catalyst surfaces.

Thermal decomposition of undiluted monosilane at high pressures produces amorphous spherical silicon particles, with a mean diameter of 3 μm, clustered together. The size of the particles remains unchanged regardless of monosilane initial pressures between 0.13 and 6.61 MPa. This may reflect that the particle nucleation process is completed at an early stage of the decomposition reaction and/or demonstrate growth inhibition caused by the high hydrogen pressure resulting from the decomposition. The hydrogen content of the particles goes through a maximum of ∼5 at% at ∼2 MPa initial pressure and seems to flatten at some 3.5 at% hydrogen at the highest monosilane initial pressures studied. After dehydrogenation by heat treatment, the present raw-silicon powder is easily melted and crystalline silicon obtained on cooling.

The ground-state crystal structures of the boron hydride series ABH4 (A = Li, Na, K, Rb, Cs) have been established from first principle projected-augmented plane-wave calculations by considering different possible structural arrangements and subjecting the test structures to structural relaxations. LiBH4 crystallizes with orthorhombic symmetry (KGaH4-type; P n m a ), NaBH4, KBH4, and RbBH4 with tetragonal symmetry ( γ -NaBH4-type; P 4 2 / n m c ), and CsBH4 in another tetragonal space group ( β -NaBH4-type; P 4 ¯ 2 1 c ). A possible reason for the transitions from lower- to higher-symmetric structures along the series has been advanced. The theoretically derived positional parameters for LiBH4 gives an almost ideal tetrahedral configuration for the [BH4]− complex whereas the experimental results show that these units are strongly distorted. The theoretically derived structures for KBH4, RbBH4, and CsBH4 differ appreciably from the experimentally established low-temperature structures. From total-energy calculations it seems that syntheses under appropriate pressure and temperature conditions may be able to generate a number of metastable phases of the ABH4 series. The density-of-states characteristics reveal that all these ABH4 compounds exhibit insulating behavior with band gaps varying between 5.5 and 7.0 eV.

Reactions between CrO3 and 50– 95 wt % H 2 SO 4 are studied at temperatures up to the boiling point of the acid. Depending on the H2SO4 concentration and synthesis temperature, Cr2(SO4)3, CrH(SO4)2, (H3O)[Cr(SO4)2], Cr2(SO4)3·H2SO4·4H2O (gross formula), and (H5O2)[Cr(H2O)2(SO4)2], are obtained as identified reaction products in addition to the incompletely characterized chromic–sulfuric acid. The CrIII-based sulfates are characterized by X-ray powder diffraction, thermogravimetric, and magnetic susceptibility measurements. The nuclear and magnetic structures of Cr2(SO4)3 at 10 K are determined, the structure type of (H3O)[Cr(SO4)2] is established, and the crystal structure of (H5O2)[Cr(H2O)2(SO4)2] is firmly stipulated. Magnetic susceptibility data suggest that the samples of CrH(SO4)2 are in a micro-crystalline rather than in an amorphous state. All CrIII-based sulfates synthesized in this study appear to undergo paramagnetic-to-antiferromagnetic transitions at around 10 K .

Structural aspects of the distorted perovskite AB O 3 phase Pr 1− x Sr x FeO 3− w , x =0.00–0.80, w =0.000–0.332, were studied by powder X-ray diffraction, powder neutron diffraction, 57 Fe Mossbauer spectroscopy, and Fe K -, Sr K -, and Pr L III -edge EXAFS techniques. The diffraction data revealed no indications for ordering of Pr and Sr at the A site, nor for oxygen vacancy ordering at O sites for heavily reduced samples. Mossbauer spectroscopy showed octahedral, square pyramidal, and tetrahedral Fe coordinations with relative amounts closely following the predictions for a binomial distribution of oxygen vacancies. In addition to Fe 3+ and Fe 4+ , also Fe 5+ appears at 77 K for (G-type) antiferromagnetic samples with high average Fe valence. This suggests dynamic 2 Fe 4+ ↔Fe 3+ +Fe 5+ fluctuations. At 296 K, a mixed valence Fe (3+ n )+ component significantly improved the fit of Mossbauer spectra for the most oxidized paramagnetic samples. The qualitative EXAFS study shows that the local environments for Fe, Pr, and Sr strongly depend on x and w . The local Pr- and Sr-site geometries differ significantly from the cubic average structure for Pr 0.50 Sr 0.50 FeO 2.746 .

Platinum and its alloys are widely used as catalysts for several oxidation reactions during which they often experience an extensive reconstruction of the surface and develop excrescences that look like ‘cauliflowers’ of platinum. This work has aimed at finding a possible mechanism for the formation of the ‘cauliflowers’ and related features. The experimental basis for the work is the oxidation of ammonia on Pt and Pt/10%Rh materials and the features observed during these reactions. The experiments that have been conducted are: (1) heat treatment of platinum wires in different gaseous atmospheres, (2) oxidation of ammonia along a platinum wire, (3) microprobe analysis of the variations in the relative content of Pt and Rh in ‘cauliflowers’ and wires, and (4) an in situ mass spectrometric investigation of the oxidation process. Through these experiments it has been shown that Pt, PtO and PtO2 are the only platinum-containing species in the gas phase. Rhodium is also transported via the gas phase by similar, simple species and this results in an increasing Rh concentration toward the tip of the ‘cauliflower curds’. A working hypothesis has been advanced to explain the dynamic growth of the ‘cauliflowers’ through etching of specific planes and depositions on edges and other planes.

Reactions between α-MoO3 or MoO3·0.43H2O and 65–95 wt% H2SO4 were studied from room temperature up to the boiling point of the acid. Four new compounds, MoO2(SO4), (modifications I, II and III) and MoO2(SO4)· H2SO4·H2O, have been isolated depending on the H2SO4 concentration, reaction temperature, reaction time and molybdenum source. The characterization of these phases was performed with powder X-ray diffraction, thermogravimetric analysis, differential thermal analysis, chemical analysis and density measurements. Both starting materials react with sulfuric acid and form MoO2(SO4) II at temperatures above ca. 75 °C. This compound is unstable in the reaction mixture and converts into MoO2(SO4) I or III depending on time and temperature. The three modifications of MoO2(SO4) are hygroscopic and decomposition in moist air is studied for the I and II modifications. On heating, all modifications of MoO2(SO4) decompose into α-MoO3 or a mixture of α-MoO3 and β-MoO3 depending on heating rate and sample size. The three-dimensional, open-framework structures of I and II, have been determined from single-crystal X-ray diffraction data. I crystallizes in the monoclinic space group C2/c and II in the orthorhombic space group Pna21. Both structures are made up of MoO6 octahedra and SO4 tetrahedra and contain layers of eight- and four-membered rings of alternating, corner-sharing octahedra and tetrahedra. These layers are linked (also via Mo–S bridges) to form a three-dimensional framework. The MoO6 octahedra are rather distorted, as demonstrated by large variations in the bonding Mo–O interatomic distances, which reflect the double-bond character of the binding between molybdenum and terminal oxygen atoms pointing into the eight-membered rings. The SO4 tetrahedra are quite regular. The structural relation to MoO2(SO4) III is briefly considered.

The phase relations in the ternary system Au–Cr–Sn have been studied by powder X-ray diffraction, metallography, electron microprobe analysis and thermal analysis. The condensed phases occurring, tie-lines and tie-triangles are presented for an isothermal section of the phase diagram at 300°C. Most of the binary phases show no significant solid solubility in the ternary dimension at 300°C. The present study has not been able to confirm or rule out the existence of the Au4Cr phase. The earlier reported Cr2Sn3 phase is shown to have the composition CrSn2. A new, genuine ternary phase, Au1.15Cr2Sn8.85 (Au0.540CrSn4.460–Au0.690CrSn4.310), has been uncovered. Its unit cell is tetragonal (space group P4/mcc) with a=687.7(1) and c=566.0(2) pm at room temperature, and the crystal structure is determined by direct methods from single crystal data (150(2) K). There is a random distribution of 58% Au and 42% Sn in the 2b position, Cr holds the 2c position and Sn the 8m position with x=0.1774(1) and y=0.3763(1). The Au1.15Cr2Sn8.85 structure is related to the AlCu2- and CoNb4Si-type structures and its atomic arrangement is discussed in relation to these prototypes. Au1.15Cr2Sn8.85 exhibits weak (Pauli-like) paramagnetism with a slight temperature dependence (5–300 K).

Theoretical considerations based on published thermodynamic data show that condensed aluminum should not be formed by direct reaction between hydrogen and alumina. Nevertheless, laboratory experiments by the authors and observations reported by others in the literature have led to the hypothesis that hydrogen dissolved in molten aluminum can possibly reduce alumina to aluminum at high temperature (700–1,700°C).

Reactions between Te, TeO2, TeCl4, TeO3 or H6TeO6 and 30–95 wt% H2SO4 were studied at temperatures up to the boiling point of the acid. Depending on the tellurium-containing reactant, H2SO4 concentration and synthesis temperature, Te2O3(SO4), TeO3 or TeO3·xH2O (x = 1.37–1.58) is obtained as the final product (alone or in admixture). The reaction products as well as thermally decomposed and/or crystallized/recrystallized products of these are characterized by powder X-ray diffraction and thermoanalytical techniques. The findings obtained by syntheses with, and thermal decompositions of, H6TeO6 concern three crystalline [TeO3 (I), TeO3 (II) and TeO3 (III) in the present notations] and one amorphous modification of TeO3, an amorphous modification of H2TeO4, one crystalline and one amorphous modification of TeO3·xH2O (x = 1.37–1.58) and two crystalline (one with somewhat more uncertain composition) and one amorphous modification of Te2O5. The crystal structures of TeO3 (I) and Te2O3(SO4) have been redetermined by Rietveld refinements of powder X-ray and neutron diffraction data, respectively.

A mixed-valence state, formally denoted as ${\mathrm{Fe}}^{2.5+},$ is observed in the 300 K M\"ossbauer spectra of the most reduced samples of ${\mathrm{SmBaFe}}_{2}{\mathrm{O}}_{5+w}.$ Upon cooling below the Verwey-type transition temperature ${(T}_{\mathrm{V}}\ensuremath{\approx}200\mathrm{K}),$ the component assigned to ${\mathrm{Fe}}^{2.5+}$ separates into a high-spin ${\mathrm{Fe}}^{3+}$ state and an ${\mathrm{Fe}}^{2+}$ state with an unusually low internal field. The separation of the mixed-valence state at ${T}_{\mathrm{V}}$ is also confirmed by magnetic susceptibility measurements and differential scanning calorimetry. A model is proposed which accounts for the variation of the amount of the mixed-valence state with the oxygen content parameter w.

Deposition of thin films of LaMnO 3 from β-diketonate-type (thd) precursors and ozone in an ALE reactor has been demonstrated. At low temperatures, the Mn-O growth from the Mn(thd) 3 precursor is retarded by the growth of the La-O deposit. By tuning of the pulsing ratio, full control of the stoichiometry of the deposited film is achieved in the temperature interval 300-400 °C. In this temperature range, the composition set by the pulsing ratio is transferred, within a few percent accuracy, to the deposited thin film. Indications for an ‘ALE window’ are found around 250-300 °C. Amorphous LaMnO 3 films could be deposited at temperatures as low as 250 °C, however, the deposition of crystalline films requires temperatures above 350 °C. X-Ray diffraction analyses show that the crystalline LaMnO 3 film was of the rhombohedral type with a=5.46(2) A and α=60.28(10)°.

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.

The crystal structure and magnetic properties of AuMnSn have been studied on single crystals and powders. AuMnSn has a narrow composition range between Au35Mn31Sn34 and Au33Mn35Sn32. The unit cell, as determined by single crystal X-ray diffraction, is cubic (space group F43m) with a=632.33(1) pm at 150(2) K, Au in 4c, Mn in 4b and Sn in 4a. There is a certain degree of statistical distribution of Mn and Sn over the 4a and 4b positions, corresponding to 0.10–0.15 Mn on the Sn site and vice versa (the spread reflects the dependence on the different model constraints imposed during the refinements). The crystal structure of AuMnSn is of the AlLiSi type (AgAsMg type when the origin is shifted). Magnetization measurements show that AuMnSn takes a ferromagnetic state from 5 K to above room temperature. AuMnSn is a typical soft ferromagnetic material with a very narrow hysteresis loop and low saturation field (ca. 0.8 T at 5 K). The saturation moment per Mn atom is 3.8 μB (±0.1 μB according to repeated measurements) at 5 K and the extrapolated Curie temperature is ca. 600 K.

The catalytic activity for CO oxidation has been studied for samples of the solid-solution series LaCo1−tCrtO3, and NdCoO3 for comparison. The catalytic activity over LaCo1−tCrtO3 generally decreases with increasing chromium content. For samples with t = 0.70 and 0.80, distinct activity changes occur during the reaction, for t = 0.80 representing a permanent deactivation. Samples of LaCo1−tCrtO3 were also studied by XRD, TPR, TG and XPS. Variations in catalytic activity were compared with variations in structural, physical and chemical properties. A rough correlation was found between the starting temperature for reduction during TPR and the temperature required for 5% CO conversion. Furthermore, the activity pattern at 600 K follows the atom percentage of surface oxygen species as seen by XPS. Possible surface reactions on LaCo1−tCrtO3 during the catalytic oxidation of CO are discussed.

A synthetic procedure has been developed for the yttrium-barium-copper oxides, in terms of a liquid-mixed citrate precursor and the use of yttrium oxide, barium carbonate, and basic copper carbonate as starting materials. The stability of the citrate gels against phase separation is investigated as a function of the content of metal oxides in the gels, and critical concentrations for the individual metals are derived in terms of the maximum metal-to-citric acid ratio across the Y(O)–Ba(O)–Cu(O) phase diagram. Sintering properties and compositions of the products are investigated as functions of the thermal treatment of the citrate precursor. The present citrate-gel procedure does not make use of internal oxidants like nitrates and polymerization agents like glycol. This approach decreases the incineration temperature and leads to a nanodispersed precursor (except submicrometer Cu), which, e.g., yields virtually phase-pure (< 1% of impurities) YBa2Cu3O7 already after the first firing at 900°C in pure O2.

A synthetic procedure has been developed for the yttrium-barium-copper oxides, in terms of a liquid-mixed citrate precursor and the use of yttrium oxide, barium carbonate, and basic copper carbonate as starting materials. The stability of the citrate gels against phase separation is investigated as a function of the content of metal oxides in the gels, and critical concentrations for the individual metals are derived in terms of the maximum metal-to-citric acid ratio across the Y(O)–Ba(O)–Cu(O) phase diagram. Sintering properties and compositions of the products are investigated as functions of the thermal treatment of the citrate precursor. The present citrate-gel procedure does not make use of internal oxidants like nitrates and polymerization agents like glycol. This approach decreases the incineration temperature and leads to a nanodispersed precursor (except submicrometer Cu), which, e.g., yields virtually phase-pure (< 1% of impurities) YBa2Cu3O7 already after the first firing at 900°C in pure O2.

The crystal structure and magnetic properties of AuMnSn have been studied on single crystals and powders. AuMnSn has a narrow composition range between Au35Mn31Sn34 and Au33Mn35Sn32. The unit cell, as determined by single crystal X-ray diffraction, is cubic (space group F43m) with a=632.33(1) pm at 150(2) K, Au in 4c, Mn in 4b and Sn in 4a. There is a certain degree of statistical distribution of Mn and Sn over the 4a and 4b positions, corresponding to 0.10–0.15 Mn on the Sn site and vice versa (the spread reflects the dependence on the different model constraints imposed during the refinements). The crystal structure of AuMnSn is of the AlLiSi type (AgAsMg type when the origin is shifted). Magnetization measurements show that AuMnSn takes a ferromagnetic state from 5 K to above room temperature. AuMnSn is a typical soft ferromagnetic material with a very narrow hysteresis loop and low saturation field (ca. 0.8 T at 5 K). The saturation moment per Mn atom is 3.8 μB (±0.1 μB according to repeated measurements) at 5 K and the extrapolated Curie temperature is ca. 600 K.

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.

The nuclear and magnetic structures of ${\mathrm{YBa}}_{2}$${\mathrm{Fe}}_{3}$${\mathrm{O}}_{8}$ have been investigated by powder neutron diffraction at room temperature. The nuclear structure of the compound has the symmetry of space group P4/mmm and lattice parameters a=3.9170(1) and c=11.8252(4) \AA{}. The configuration of the atoms in the unit cell is very similar to that of the superconductor ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$, with the exception that the iron ions corresponding to the Cu-chain ions have octahedral coordination, rather than square planar; the octahedra thus are arranged in layers rather than in chains. This significant difference is a consequence of the fact that all possible oxygen sites in these layers are fully occupied, resulting in an oxygen content of eight rather than seven atoms per formula unit. A second consequence of the full occupancy of the oxygen sites is that the Ba ions have a twelvefold cuboctahedral coordination in the iron compound, rather than tenfold. The magnetic structure is based on a unit cell related to that of the nuclear structure by an axis transformation of matrix (11\ifmmode\bar\else\textasciimacron\fi{}0/110/002). The magnetic origin of the extra intensities and the basic spin configuration were determined by polarized-neutron-diffraction measurements. The iron moments are coupled antiferromagnetically within each ${\mathrm{FeO}}_{2}$ layer, as well as along the c axis. The magnetic moments of all the iron ions are the same, with a value of 3.49(2)${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$. This configuration results in the magnetic symmetry ${\mathit{I}}_{\mathit{c}}$mm'm.

© 1991 Acta Chemica Scandinavica

Racemic 2-methyl-1,1-diphenyl-3-(1-piperidyl)-1-propanol (4) and 2-methyl-1,1-diphenyl-3-(1-pyrrolidyl)-1-propanol (5) have been synthesized and optically resolved employing (R)- and (S)-mandelic acid, respectively, as resolving agents. The absolute configurations of the enantiomers of 4 and 5 have been established as (-)-(R) and (+)-(S) by crystal structure analyses of (-)-(R)-2-methyl-1,1-diphenyl-3-(1-piperidinio)-1-propanol (+)-(S)-mandelate and (+)-(S)-2-methyl-1,1-diphenyl-3-(1-pyrrolidinio)-1-propanol (-)-(R)-mandelate. The corresponding methiodides have been prepared.

Yttrium- and rare-earth-substituted derivatives of Ca{sub 3-} {sub v}R{sub v} Co{sub 2}O{sub 6} (R =Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Yb, and Lu) have been synthesized and structurally characterized by powder X-ray and neutron diffraction. All phases adopt the K{sub 4}CdCl{sub 6}-type structure with space group R3-barc), in which the trivalent R {sup 3+} substituents randomly occupy the Ca{sup 2+} site. The homogeneity range of Ca{sub 3-} {sub v}R{sub v} Co{sub 2}O{sub 6} extends to v{approx}0.90 for the substituents concerned. A significant increase in the Co2-O distances within the trigonal-prismatic Co2O{sub 6} co-ordination polyhedra upon introduction of R {sup 3+} confirms that extra electrons from the R {sup 3+}-for-Ca{sup 2+} substitution exclusively enter the Co2 site of the quasi-one-dimensional Ca{sub 3-} {sub v}R{sub v} Co{sub 2}O{sub 6} structure, thereby formally reducing its oxidation state. This is furthermore supported by magnetic susceptibility and low-temperature neutron diffraction measurements. The long-range ferrimagnetic ordering temperature is reduced upon R substitution and appears to vanish for v>{approx}0.30. - Graphical abstract: Yttrium- and rare-earth-substituted derivatives of Ca{sub 3-} {sub v}R{sub v} Co{sub 2}O{sub 6} have been synthesized. A significant increase in the Co2-O distances within the trigonal-prismatic Co{sub 2}O{sub 6} coordination polyhedra confirmsmore » that extra electrons from the R {sup 3+}-for-Ca{sup 2+} substitution enter the Co2 site, thereby formally reducing its valence state. The long-range ferrimagnetic ordering of the parent Ca{sub 3}Co{sub 2}O{sub 6} phase vanishes at v>{approx}0.30 in Ca{sub 3-} {sub v}R{sub v} Co{sub 2}O{sub 6}.« less

The phase relations in the Au–Mn–Sb system have been studied by powder X-ray diffraction, metallography, electron microprobe analysis, and thermal analysis. The condensed phases occurring, tie-lines, and tie-triangles are presented for an isothermal section of the phase diagram at 400 °C. At this temperature the solid solubility of the third component in the binary phases is generally limited to a few percent. The phase relations are relatively complex in regions which involve the numerous binary gold–manganese phases. The only genuine ternary phase, AuMnSb, has a very narrow composition range around Au 35 Mn 30 Sb 35 and is stable up to 575±5 °C, where it decomposes peritectically. The AuMnSb phase is ferromagnetic with a Curie temperature of 135±10 K. The solid-solution phase AuMnSn 1− x Sb x exhibits ferromagnetic properties throughout the entire homogeneity range with the Curie temperature ranging between 625±100 and 135±10 K.