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We report ambient pressure stabilization of a previously synthesized high-pressure (6.5 GPa) phase, GdB12, in a Zr1–xGdxB12 solid solution (with ∼54 at. % Gd solubility, as determined by both powder X-ray diffraction and energy-dispersive spectroscopy). Limited solubilities of Sm (∼15 at. % Sm), Nd (∼7 at. % Nd), and Pr (∼4 at. % Pr), in ZrB12 were also achieved. Previous attempts at preparing these rare-earth borides were unsuccessful even under high pressure. On the basis of insights provided from the unit cell sizes observed via solid solutions, at least 6.5 GPa of pressure would be needed to synthesize these rare-earth borides since Sm, Nd, and Pr atomic radii are larger than that of Gd. The solid-solution formation for Zr1–xGdxB12 and Zr1–xSmxB12 can be seen in the change of the unit cell of each of the solid solutions relative to their pure parent compounds as well as in the change of color of the respective alloys. For Zr0.45Gd0.55B12 and Zr0.70Sm0.30B12, the cubic unit cell parameter (a) reached a...

Alloys of metal dodecaborides-YB12 with HfB12-were prepared via arc-melting in order to stabilize the metastable HfB12 high-pressure phase under ambient pressure. Previously, HfB12 had been synthesized only under high-pressure (6.5 GPa). Powder X-ray diffraction (PXRD) and energy-dispersive X-ray spectroscopy (EDS) were used to confirm the purity and phase composition of the prepared samples. The solubility limit for HfB12 in Y1-xHfxB12 (cubic UB12 structure type) was determined to be ∼35 at. % Hf by PXRD and EDS analysis. The value of the cubic unit cell parameter (a) changed from 7.505 Å (pure YB12) to 7.454 Å across the solid solution range. Vickers hardness increased from 40.9 ± 1.6 GPa for pure YB12 to 45.0 ± 1.9 GPa under an applied load of 0.49 N for the Y1-xHfxB12 solid solution composition with ∼28 at. % Hf, suggesting both solid solution hardening and extrinsic hardening due to the formation of secondary phases of hafnium.

Solid solutions of tungsten diboride (WB2) with increasing substitution of tungsten (W) by tantalum (Ta) and niobium (Nb)—ranging from 0 to 50 at. % on a metals basis—were synthesized through resis...

We report ambient pressure stabilization of a previously synthesized high-pressure (6.5 GPa) phase, GdB

Metathesis (exchange) reactions offer the possibility of controlling temperature through a judicious choice of precursors. Here, a reaction between AlCl(3) and Ca(3)N(2) is found to produce phase-pure aluminum nitride (AlN) in seconds. The CaCl(2) byproduct salt, whose formation drives this highly exothermic reaction, is simply washed away after reaction completion. SEM images demonstrate that the AlN product is a micron-sized powder, while TEM shows well-formed crystallites. Thermodynamic calculations indicate that a reaction temperature of 2208 K could be reached under adiabatic conditions. Using an in situ thermocouple and a stainless steel reactor vessel to hold the precursors, a reaction temperature of 1673 K is measured 0.8 s after initiation. Switching to a thermally insulating ceramic vessel produces a maximum reaction temperature of 2010 K because of the more nearly adiabatic conditions. The high reaction temperature appears to be critical to forming phase-pure AlN. Experiments with Li(3)N, instead of Ca(3)N(2), produce lower temperatures (1513 K), resulting in both Al and Al(2)O(3) impurities.

Group 4 phosphides, which are typically prepared at high temperatures (> 800 degrees C) over several days, are synthesized in self-propagating metathesis (exchange) reactions in seconds. These reactions produce cubic forms of zirconium phosphide (ZrP) and hafnium phosphide (HfP) which are normally made at temperatures greater than 1425 degrees C and 1600 degrees C, respectively. To test whether the high temperatures reached in the metathesis reactions are responsible for the formation of the cubic phases, inert salts are added to lower the maximum reaction temperatures. The lower temperature reactions still result in cubic phosphides, although smaller crystallites form. Further experiments with phosphorus addition indicate that the phosphorus content is not responsible for cubic phase formation. Templating is ruled out using lattice mismatched KCl and hexagonal ZnS as additives. Therefore, the direct synthesis of the high-temperature cubic phase in metathesis reactions appears to be caused by nucleation of the metastable cubic form that is then trapped by rapid cooling. Heating the cubic phase of either ZrP or HfP to 1000 degrees C for 18 h, or carrying out metathesis reactions in sealed ampules at 1000 degrees C, results only in the hexagonal phase.

Since the late 1960s, the exact structure of cubic calcium silicon nitride has been a source of debate. This paper offers evidence that the cubic phase CaSiN(2) described in the literature is actually Ca(16)Si(17)N(34). Presented here is a method for synthesizing single crystals of cubic-calcium silicon nitride from calcium nitride and elemental silicon under flowing nitrogen at 1500 °C. The colorless millimeter-sized crystals of Ca(16)Si(17)N(34) with a refractive index (n(25)) = 1.590 were found to be cubic (a = 14.8882 A) and belong to the space group F43m (216). The synthesis of bulk, powdered cubic-Ca(16)Si(17)N(34) from calcium cyanamide and silicon is also discussed. Ca(16)Si(17)N(34) is a relatively air-stable refractory ceramic. In contrast to the orthorhombic phase of CaSiN(2), in which Ca(2+) sits in octahedral sites, this cubic phase has Ca(2+) in cubic sites that makes it an interesting host for new phosphors and gives rise to unique crystal field splitting.

A process for producing a refractory material having the form TB o , e.g. zirconium nitride (ZrN), includes a first step of mixing a first salt having the form TX n , e.g. zirconium tetrachloride (ZrCl 4 ) and a second salt having the form A m B, e.g. lithium nitride (Li 3 N) in a ratio of n/m in a container. The process also includes a second step of igniting the mixture of the first and second salts, e.g. ZrCl 4 and Li 3 N, whereby the refractory material, e.g. ZrN, is produced along with byproducts having forms nAX and (n/m-o)B, e.g. 4LiCl and (1/6)N 2 , respectively. The process further includes a third step of separating the refractory material from the byproducts by solvent extraction. The stoichiometric ratio of the second salt to the first salt is n/m, e.g. 4/3. T is selected from the group consisting of transition metals, e.g. zirconium, and tetrelides, i.e. carbon, silicon, germanium, tin and lead. X is selected from the halide group consisting of fluorine, chlorine, bromine and iodine. A is selected from the group consisting of alkali metals, i.e. lithium, sodium, potassium, rubidium and cesium, and alkaline earth metals, i.e. beryllium, magnesium, calcium, strontium and barium. B is a base selected from the group consisting of pnictides, i.e. nitrogen, hosphorus, arsenic, antimony and bismuth, and tetrelides, i.e. carbon, silicon, germanium, tin and lead, m and n are integers and o is a fraction, e.g. 3/3=1.

High-quality molybdenum-tungsten disulfide solid solutions, (Mo,W)S 2 , are rapidly synthesized in self-propagating, solid-state precursor reactions between (MoCl 5 -WCl 6 ) and Na 2 S. The mixed-metal dichalcogenide products can be readily reduced in a dilute hydrogen atmosphere at approximately 900 o C to the corresponding Mo-W alloys. The metal distribution in the Mo-W alloys is examined using the high-angle 123 powder X-ray reflection and can be correlated to the metal distribution in the metal dichalcogenide product

Solid-state precursor reactions have been investigated as a general synthetic route to binary III-V (13-15) compounds. The generic reaction scheme MX[sub 3] + Na[sub 3]Pn [yields] MPn + 3 NaX (M = Al, Ga, In; X = F, Cl, I; Pn = pnictogen = P, As, Sb) has been used to prepare crystalline powders of the III-V semiconductors. The reaction mixtures can be either heated in sealed tubes or ignited with a hot filament, and the byproduct salts are simply removed by washing with an appropriate solvent. The ignited reactions are self-propagating and highly exothermic, owing to the formation of 3 mol of sodium halide. Products from both types of reactions have been characterized by powder X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and solid-state NMR. In some cases, the products of the ignited solid-state metathesis (SSM) reactions differ from those of the sustained heating reactions. These differences provide clues as to reaction pathways in the solid-state precursor reactions.

Copper acetate and related metal salt films react directly with hydrogen sulfide at room temperature to form metal sulfides, resulting in conductivity changes as large as 108. The observed changes in conductivity are related to the solubility product constant (Ksp) and the difference in conductivity between the metal salt and the resulting metal sulfide. A smaller Ksp indicates a more stable metal sulfide and, therefore, greater metal salt reactivity. Polyaniline nanofiber/metal salt composites were also examined and show metal sulfide conversion with changes in resistance up to 106. The direct electrical measurement of the conversion of metal salt to metal sulfide has the potential to be the basis of a new type of sensitive, thin-film chemical sensor.

Solid-state metathesis (exchange) reactions can be used to synthesize many different transition-metal nitrides under ambient conditions including TiN, ZrN, and NbN. Typical metathesis reactions reach temperatures of greater than 1300 degrees C in a fraction of a second to produce these refractory materials in highly crystalline form. Likely due to the large amount of heat produced in these solid-state reactions, some transition-metal nitrides such as TaN, CrN, and gamma-Mo(2)N cannot easily be synthesized under ambient conditions. Here metathesis reactions are demonstrated to produce the cubic nitrides TaN, CrN, and gamma-Mo(2)N when sufficient pressure is applied before the reaction is initiated. By pressing a pellet of TaCl(5) and Li(3)N with an embedded iron wire, crystalline cubic TaN forms under 45 kbar of pressure after a small current is used to initiate the chemical reaction. Crystalline cubic CrN is synthesized from CrCl(3) and Li(3)N initiated under 49 kbar of pressure. Crystalline gamma-Mo(2)N is produced from MoCl(5) and Ca(3)N(2) (since MoCl(5) and Li(3)N self-detonate) initiated under 57 kbar of pressure. The addition of ammonium chloride to these metathesis reactions drastically lowers the pressure requirements for the synthesis of these cubic nitrides. For example, when 3 mol of NH(4)Cl is added to CrCl(3) and Li(3)N, crystalline CrN forms when the reaction is initiated with a resistively heated wire under ambient conditions. Cubic gamma-Mo(2)N also forms at ambient pressure when 3 mol of NH(4)Cl is added to the reactants MoCl(5) and Ca(3)N(2) and ignited with a resistively heated wire. A potential advantage of synthesizing gamma-Mo(2)N under ambient conditions is the possibility of forming high-surface-area materials, which could prove useful for catalysis. Nitrogen adsorption (BET) indicates a surface area of up to 30 m(2)/g using a Langmuir model for gamma-Mo(2)N produced by a metathesis reaction at ambient pressure. The enhanced surface area is confirmed using scanning electron microscopy.