Composition dependence of the nucleation energy of iron antimonides from modulated elemental reactants.

Modulated elemental reactants containing iron and antimony were found to react at low temperature (T < 200 degrees C) forming either FeSb(2) or FeSb(3) depending on both the layer thicknesses and the overall composition of the initial reactant. For films containing 75% antimony and 25% iron, the metastable compound FeSb(3) was observed to nucleate and grow if the layer thickness was below approximately 35 A. Above this critical thickness for the modulation, annealing led to the formation of FeSb(2). This, combined with low-angle diffraction data, suggests that the initial interdiffusion between iron and antimony layers in an elementally modulated reactant results in the formation of an amorphous reaction intermediate if the layering thickness is less than 35 A. For modulated reactants with composition between 70 and 90 atomic % antimony and below this critical layer thickness, the metastable compound FeSb(3) formed. In more iron-rich modulated reactants FeSb(2) nucleates. The nucleation temperature and the nucleation energy of FeSb(3) were found to be a function of the composition of the amorphous intermediate, while those of FeSb(2) were found to be relatively independent of composition.     

Synthesis of aluminium borate-boron oxide and binary titanium-boron and zirconium-boron oxides from metal alkoxides and (MeO)3B3O3 in non-aqueous solvents

The reaction of metal alkoxides M(OR)4 (M = Ti, Zr; R = organyl) with (MeO)3B3O3 (1 : 0.67) in dry propan-2-one at room temperature led to gels which when dried and calcined in air for 24 h at 500-1000 degrees C afforded bi-phased mixed-oxide materials formulated as 4TiO2 x 3B2O3 and ZrO2 x B2O3 in high ceramic yields and purity; the B2O3 phases of these materials were amorphous. The materials remained amorphous upon calcination at lower temperatures. The TiO2 phase of the 4TiO2 x 3B2O3 was crystalline when calcined at higher temperatures with either anatase (600 degrees C) or rutile (>800 degrees C) being obtained. The ZrO2 phase of the ZrO2 x B2O3 was crystalline when calcined at higher temperatures and was obtained as a metastable tetragonal phase (<700 degrees C) or baddeleylite (>800 degrees C). In a similar reaction, Al(O(i)Pr)3 (2 : 1) gave a bi-phased aluminium borate-boron oxide (Al18B4O(33).7B2O3) after calcination at >700 degrees C. The dried gels and oxide materials were all characterized by elemental analysis, TGA-DSC, and powder XRD.     

Coupling sol-gel synthesis and microwave-assisted techniques: a new route from amorphous to crystalline high-surface-area aluminium fluoride

A non-aqueous sol-gel Al-based fluoride has been subjected to the microwave solvothermal process. The final material depends on the temperature heat treatment used. Three types of material have been prepared: 1) for low temperature heat treatment (90 degrees C) X-ray amorphous alkoxy fluoride was obtained; 2) for the highest temperature used (200 degrees C) the metastable form beta-AlF3 was obtained with a very large surface area of 125 m2 g(-1). The mechanism of the amorphous=crystalline transformation has been rationalised by the occurrence of a decomposition reaction of the gel fluoride induced by the microwave irradiation. 3) Finally, at intermediate temperature (180 degrees C) a multi-component material mixture exhibiting a huge surface area of 525 m2 g(-1) has been obtained and further investigated after mild post-treatment fluorination using F2 gas. The resulting aluminium-based fluoride still possesses a high-surface-area of 330 m2 g(-1). HRTEM revealed that the solid is built from large particles (50 nm) identified as alpha-AlF3, and small ones (10 nm), relative to an unidentified phase. This new high-surface-area material exhibits strong Lewis acidity as revealed by pyridine adsorption and catalytic tests. By comparison with other materials, it has been shown that whatever the composition/structure of the Al-based fluoride materials, the number of strong Lewis acid sites is related to the surface area, highlighting the role of surface reconstruction occurring on a nanoscopic scale on the formation of the strongest Lewis acid sites.     

Synthesis of thin Cr3Se4 films from modulated elemental reactants via two amorphous intermediates: a detailed examination of the reaction mechanism

The reaction of Cr/Se multilayers when they are annealed occurs in two steps: interdiffusion of the single layers to an amorphous Cr-Se alloy and crystallization of Cr3Se4. Both reaction steps were characterized using various techniques. At approximately 300 degrees C the layers have interdiffused completely to form a homogeneous amorphous Cr-Se alloy. Short-range order in the alloy was probed with X-ray absorption spectroscopy (XAS) and, according to the results of this, is already very similar to Cr3Se4, which crystallizes around 500 degrees C. Crystallization occurs at a well-defined temperature, whereas crystallite growth proceeds in the whole temperature interval above the crystallization temperature and is not finished at 660 degrees C. The reaction yields a polycrystalline thin film of Cr3Se4 in a preferred orientation exhibiting a (00l) texture. In Cr-rich samples amorphous Cr is present as a by-product. A Cr-Se/Se multilayer was observed as an intermediate in the interdiffusion of some Cr-rich samples which is stable between 200 and 250 degrees C.     

Phase evolution of lead titanate from its amorphous precursor synthesized by the OPM wet-chemical route

Lead titanate was synthesized by the OPM wet-chemical route by the dissolution of Ti metal in H₂O₂ followed by the addition of Pb²⁺ at high pH, resulting in a reactive and amorphous precipitate with (Pb:Ti=1:1) mole ratio, which was heat treated between 400°C and 700°C. The amorphous precipitate was characterized by DSC, and all of the powders were characterized by X-ray diffraction, Raman and XAS (EXAFS and XANES) spectroscopy at the Ti K edge. A metastable, stoichiometric and cubic pyrochlore phase (Pb₂Ti₂O₆, Fd3m) was identified by XRD and Raman spectroscopy up to approx. 450°C. Only tetragonal PbTiO₃ was identified at higher temperatures. XAS spectra showed that the local structure around the absorbing Ti atom of the intermediate pyrochlore phase is similar to that observed in the amorphous precursor. This fact indicates that the metastable intermediate pyrochlore (Pb₂Ti₂O₆) is kinetically favored to be formed because of its similarity to the amorphous precipitate, instead of the slightly different and thermodynamically favored tetragonal (PbTiO₃, P4/mmm) perovskite structure that is only formed at higher temperatures, after the crystallization of the metastable intermediate pyrochlore.     

Nanoscale zinc antimonides: synthesis and phase stability

Highly crystalline single-phase nanoparticles of the important thermoelectric materials Zn4Sb3 and ZnSb were prepared from solvochemically activated powders of elemental zinc and elemental antimony. Low-temperature reactions with reaction temperatures of 275-300 degrees C were applied using an excess of elemental zinc. The nanoscale thermoelectrics obtained were characterized by X-ray powder diffraction, transmission electron microscopy, and thermal analysis. nc-Zn4Sb3 showed particle sizes of 50-70 nm, whereas particle sizes of 15-20 nm were observed for nc-ZnSb. Calorimetric investigations showed an increased heat capacity, Cp, for nc-Zn4Sb3 with respect to the bulk material which could be reduced to the bulk value by annealing nc-Zn4Sb3 at 190 degrees C. Interestingly, nc-Zn4Sb3 showed exothermic decomposition into zinc-poorer ZnSn at 196 degrees C in an open system, indicating that Zn4Sb3 is metastable in nanocrystalline form at room temperature.     

化学镀镍-高磷合金晶化行为的现场XRD研究

从柠檬酸-酒石酸-乳酸-EDTA混合体系中得到含P 12％(质量比)的化学镀高磷Ni-P合金，其差热曲线显示，在350和420 ℃出现两个放热过程.现场XRD分析结果显示，镀层在300 ℃以下保持非晶态结构，在320 ℃之后开始晶化，首先析出介稳的Ni5P2和Ni12P5相，在360 ℃后开始有稳定的Ni3P和Ni相的衍射峰出现， 400 ℃以上只有Ni3P和Ni相. 325 ℃恒温时，镀层在4 min内保持非晶态的衍射特征，随即析出Ni5P2和Ni12P5相, 并在2 h内基本保持不变. 350 ℃恒温时，析出的Ni5P2和Ni12P5介稳相只存在40 min.实验结果表明, DTA曲线上350 ℃的放热峰不仅有非晶相转变为介稳相的过程，也包含部分介稳相转化为稳定相的过程.     

Suppression of binary nucleation in amorphous La-Fe-Sb mixtures

The nucleation energy of a series of La(x)Fe(y)Sb(z) modulated elemental reactants was measured as a function of the Fe/Sb ratio over a large composition range while holding the La content constant. The nucleation energy of the ternary compound La(0.5)Fe(4)Sb(12) with the skutterudite crystal structure was found to depend very strongly on the Fe/Sb ratio in the modulated elemental reactant, with a higher nucleation energy as the Fe/Sb ratio is moved away from the 1:3 stoichiometric value. When the results of this study are compared with those from Fe(y)Sb(z) modulated reactants, the addition of lanthanum was found to suppress the nucleation of FeSb(2), thereby broadening the Fe/Sb composition range in which the ternary skutterudite compound La(x)Fe(4)Sb(12) nucleates. This suppression of nucleation of a binary phase on addition of a ternary component to an amorphous intermediate is in agreement with theoretical arguments. The observed suppression of nucleation also provides rational for the observed nucleation of metastable ternary and higher-order compounds from homogeneous amorphous reactants.     

Infrared spectroscopy of the solid phases of ammonia

Thin films of solid ammonia (NH(3) and ND(3)) have been characterized using low temperature (25-110 K) Fourier-transform infrared (FTIR) spectroscopy, and the three solid phase (amorphous, metastable, and crystalline) spectra are reported. This work has been motivated by confusion in the literature about the metastable and crystalline phases as a result of an early erroneous report by Staats and Morgan [(J. Chem. Phys. 31, 553 (1959)]. Although the crystalline phase has subsequently been reported correctly, the metastable phase has not been described in the literature in detail. The unique characteristics of the metastable phase, reported here for the first time, include multiple peaks in the nu(2) and nu(3) regions and peak intensities that are dependent on the deposition temperature. This behavior may be the result of (a) preferential molecular orientations in the solid, or (b) exciton splitting due to different crystal shapes in the solid. The amorphous and metastable phases of deuterated ammonia are also reported for the first time.     

In situ TEM studies of metal-carbon reactions.

The reactions which occur between amorphous carbon and a number of first transition metals (Ti, Cr, Fe, Co, Ni, and Cu) have been studied by transmission electron microscopy (TEM). The materials are in thin-film form with the metal layer sandwiched between thicker carbon layers. In four cases, the predominant reaction is the graphitization of the amorphous carbon, at temperatures well below 800 degrees C. This is brought about by the elements themselves in the case of Co and Ni, and by metastable carbides in the case of Fe (Fe3C) and Cr (Cr3C2-x). The Ti-C and Cu-C systems do not exhibit graphitization. For the former, only TiC is produced up to 1000 degrees C, while the carbon does not react at all with copper. In situ TEM studies show the mechanism to be of the dissolution-precipitation type, which is equivalent to the metal-mediated crystallization process for amorphous silicon and germanium. The heat of graphitization is found to be 18-19 kcal/mol-C by differential scanning calorimetry.     

Preparation and topotactical oxidation of ScVO3 with bixbyte structure: a low-temperature route to stabilize the new defect fluorite ScVO(3.5) metastable phase

ScVO3 has been prepared by controlled reduction of a ScVO4 precursor under an H2/N2 flow at 1250 degrees C. The crystal structure of this material has been studied at room temperature by Rietveld refinement of high-resolution neutron powder diffraction (NPD) data. Sc3+ and V3+ are distributed at random over the metal sites of a C-M2O3 bixbyite-type structure, space group Ia3, a = 9.6182(1) Angstroms. The thermal analysis of ScVO3 in an air flow shows two subsequent oxidation processes, with a final reversal to ScVO4 above 600 degrees C. An intermediate phase of composition ScVO(3.5), containing V4+ cations, can be isolated by isothermal annealing at 350 degrees C in air. This metastable phase has been identified by X-ray diffraction (XRD) as a fluorite-type oxide (space group Fm3m, a = 4.947(2) Angstroms), also showing a random distribution of Sc and V cations over the metal positions. The Rietveld refinement of the ScVO(3.5) structure from powder XRD data in a fluorite structural model yields abnormally high thermal factors for the oxygen atoms, suggesting oxygen mobility in this metastable material.     

Synthesis of nickel sulfides in aqueous solutions using sodium dithionite

Reaction of nickel chloride with sodium dithionite in aqueous solutions at ambient temperature has been investigated systematically to obtain nickel sulfides. The products are characterized by X-ray diffraction, thermogravimetric analysis, and electrical resistivity and magnetic susceptibility measurements. It is found that the compositions and structures of the products are controlled by the reaction pH and the amount of the reactants. While reactions under highly acidic (pH < or = 2) and basic (pH > or = 7) conditions yield crystalline sulfur and amorphous or poorly crystalline NiySx, respectively, those at intermediate 3 < or = pH < or = 6 give crystalline NiySx. Both crystalline Ni3S2 (heazlewoodite structure) and Ni3S4 (spinel structure) have been obtained at room temperature. Additionally, NiS (millerite structure) is obtained by carefully heating Ni3S4 at 200 degrees C in a mixture of 90% Ar and 10% H2. Ni3S4 is found to be metastable, and it begins to disproportionate above 100 degrees C. Both Ni3S2 and Ni3S4 show metallic behavior. While Ni3S2 exhibits temperature-independent magnetic susceptibility, Ni3S4 shows ferrimagnetic ordering below 20 K.     

Reaction sintering and microstructural evolution in metakaolin-metastable alumina composites

Fine needles of mullite grains were obtained successfully in a compact and low porous matrix using solid state sintering. We treated high-grade kaolin and sand-rich kaolin at 750 °C to amorphous metakaolins, and bauxite at 1,000 °C to metastable alumina. By designing a stochiometric composition of mullite, each amorphous metakaolin was added to metastable alumina. Fine grains of mullite with almost complete crystallization were obtained from 1,350 °C in a case of amorphous metakaolin from high-grade kaolin and at 1,550 °C in the other case where amorphous metakaolin is from sand-rich kaolin. The difference in the temperatures of mullitization was linked to the late dissolution of silica from the cristobalite and quartz phases which were still present in the sand-rich metakaolin sample at 1,350 °C. The use of metastable alumina and metakaolin instead of kaolin to design the mullite matrix allows the increase in number of mullite nucleation sites. This results to high densification and crystallization, fine grain size, and high mechanical properties of the final matrix.     

Noncrystalline phases in poly(9,9-di-n-octyl-2,7-fluorene)

Selective formation of amorphous, nematic (N), and beta phases in poly(9,9-di-n-octyl-2,7-fluorene) (PFO) films was achieved via judicious choice of process parameters. Phase structure and film morphology were carefully examined by means of X-ray diffraction as well as electron microscopy. "Amorphous" thin films were obtained by quick evaporation of solvent. Slow solvent removal during film formation or extended treatment of the amorphous film with solvent vapor resulted in predominantly the beta phase, which corresponds to a frozen (due to decreased segmental mobility upon solvent removal) and intrinsically metastable state of transformation midway between a solvent-induced clathrate phase and the equilibrium crystalline order in the undiluted state. The frozen transformation process is reactivated upon an increase in temperature beyond 100 degrees C. Compared to the amorphous film, extended backbone conjugation in the beta phase is evidenced from the emergence of a characteristic absorption peak around 430 nm near the absorption edge. For films of frozen nematic order (obtained by quenching from the nematic state), the conjugation length is also greater than the amorphous films as revealed by an absorption shoulder around 420 nm. Well-behaved single-chromophore emission with single-mode phonon coupling was observed for the beta phase; in the case of nematic films, dual-mode phonon coupling must exist if single-chromophore emission is assumed. In comparison, the emission spectrum of the amorphous film of generally shorter conjugation lengths exhibited mixed characteristics of nematic and beta phases, implying the presence of minor populations of extended conjugation similar to those in nematic and beta phases, which are of biased weightings in the emission spectra. All these films consist of nanograins (ca. 10 nm in size) of collapsed chains; the films are therefore inherently inhomogeneous in this length scale. In combination with previous observations on the crystalline (alpha and alpha') forms, the phase behavior of PFO is then generally summarized in terms of relative thermodynamic stability.     

Solution decomposition of the layered double hydroxide of Co with Fe: phase segregation of normal and inverse spinels

The nitrate-intercalated layered double hydroxide of Co with Fe decomposes on hydrothermal treatment to yield an oxide residue at a temperature as low as 180 degrees C. The oxide product is phase segregated into a Co(3)O(4)-type normal spinel and a CoFe(2)O(4)-type inverse spinel. Phase segregation is facilitated as decomposition in a solution medium takes place by dissolution of the precursor hydroxide followed by reprecipitation of the oxide phases. In contrast, thermal decomposition takes place at 400 degrees C. This temperature is inadequate to induce diffusion in the solid state whereby phase segregation into the thermodynamically stable individual spinels is suppressed. The result is a single-phase metastable mixed spinel oxide. This is rather uncommon in that a hydrothermal treatment yields thermodynamically stable products where as thermal decomposition yields a metastable product.     

Thermal decomposition of monocalcium aluminate decahydrate (CaAl2O4.10H2O) investigated by in-situ synchrotron X-ray powder diffraction, thermal analysis and 27Al, 2H MAS NMR spectroscopy

The stability of monocalcium aluminate decahydrate, with the nominal composition CaAl(2)O(4).10H(2)O (CAH(10)), has a decisive role for the strength development and durability of cementitious materials based on high alumina cements. This has prompted an investigation of the thermal transformation of crystalline monocalcium aluminate decahydrate in air to an amorphous phase by in-situ synchrotron X-ray powder diffraction in the temperature range from 25 to 500 degrees C, by DTA/TGA, and (2)H, (27)Al MAS NMR spectroscopy. The decomposition includes the loss of hydrogen-bonded water molecules in the temperature range up to 175 degrees C, coupled with a reduction of the unit cell volume from 1928 A(3) at 25 degrees C, to 1674 A(3) at 185 degrees C. Furthermore, X-ray diffraction shows that CaAl(2)O(4).10H(2)O starts to transform to an amorphous phase at approximately 65 degrees C. This phase is fully developed at approximately 175 degrees C and it converts to crystalline CaAl(2)O(4) when heated to 1300 degrees C. The thermal decomposition in the temperature range from approximately 65 to approximately 175 degrees C involves both formation of an amorphous phase including AlO(4) tetrahedra and structural changes in the remaining crystalline phase.     

Physiochemical properties of phenobarbital solid dispersion with phosphatidylcholine

We prepared a phenobarbital (PB) solid dispersion (SD) with phosphatidylcholine (PC). PB was present in an amorphous state in SD if its mole fraction was under 0.75. An infrared (IR) spectra study suggested a hydrogen bond between NH in PB and phosphate in PC, with a ratio of about 1:1. When the mole fraction of PB was less than 0.50, differential scanning calorimetry (DSC) curves showed endothermic peaks at 57, 90 and 145 degrees C, and an exothermic peak at 60 degrees C. The IR spectrum and X-ray diffraction pattern changed after holding at 70 degrees C, so at this point it is considered that the metastable state of SD changed into a stable state, and extra energy was released. When the mole fraction of PB was high, PB also arranged near hydrophobic group because an endothermic peak was observed at 46-52 degrees C, which was lower than fully hydrated PC. PB is similar to indomethacin (IM) in molecular shape and to phenytoin (PHT) in chemical structure. Its DSC curve and IR spectra are similar to PHT, and the limit ratio of its amorphous state is the same as IM. It is considered that the chemical structure is an important factor in its interaction to PC, and also, the molecular shape is important to arrange into PC lattice.     

Direct synthesis of mesostructured lamellar molybdenum disulfides using a molten neutral n-alkylamine as the solvent and template

A series of novel mesostructured lamellar molybdenum disulfides with the d spacings from 17 to 30 A can be prepared by the reaction of Mo(CO)6 with elemental sulfur using a molten n-alkylamine as the solvent as well as the template at 140 degrees C. Such intercalated phases can be transformed into mesoporous molybdenum disulfides by slow thermal treatments at 200 degrees C.     

Thermogravimetric study on the decomposition of hydromagnesite 4 MgCO3 · Mg(OH)2 · 4 H2O

Isothermal and non-isothermal decomposition of hydromagnesite 4 MgCO₃ · Mg(OH)₂ · 4 H₂O was studied thermogravimetrically. Decarbonation was strongly influenced by the partial pressure of carbon dioxide. Decarbonation in an argon atmosphere proceeded via an amorphous lower carbonate to MgO. Decarbonation in a carbon dioxide atmosphere was interrupted at ～460–480°C. This interruption was explained by the formation of a metastable intermediate and the subsequent crystallization of MgCO₃, both from the amorphous lower carbonate. This explanation was supported by DTA and power X-ray diffraction analysis of the quenched specimens.     

Study of Ba3M(II)M(IV)WO9 (M(II) = Ca, Zn; M(IV) = Ti, Zr) perovskite oxides: competition between 3C and 6H perovskite structures

We describe an investigation of Ba3MIIMIVWO9 oxides for MII = Ca, Zn, and other divalent metals and MIV = Ti, Zr. In general, a 1:2-ordered 6H (hexagonal, P63/mmc) perovskite structure is stabilized at high temperatures (1300 degrees C) for all of the Ba3MIITiWO9 oxides investigated. An intermediate phase possessing a partially ordered 1:1 double perovskite (3C) structure with the cation distribution, Ba2(Zn2/3Ti1/3)(W2/3Ti1/3)O6, is obtained at 1200 degrees C for Ba3ZnTiWO9. Sr substitution for Ba in the latter stabilizes the cubic 3C structure instead of the 6H structure. A metastable Ba3CaZrWO9 that adopts the 3C (cubic, Fmm) structure has also been synthesized by a low-temperature metathesis route. Besides yielding several new perovskite oxides that may be useful as dielectric ceramics, the present investigation provides new insights into the complex interplay of crystal chemistry (tolerance factor) and chemical bonding (anion polarization and d0-induced distortion of metal-oxygen octahedra) in the stabilization of 6H versus 3C perovskite structures for the Ba3MIIMIVWO9 series.