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Fine (about 2 mm) particle size aluminum powders were produced in a plasma reactor, in a flow of argon gas, and the particle size distribution analyzed. A uniformly thick nickel film was deposited on the aluminum powder particles by electroless nickel plating, thus providing more efficient surface contact than can be obtained using conventional methods such as mechanical mixing of nickel and aluminum powder. Nickel-plated aluminum compacts were subsequently sintered at various temperatures in argon and in vacuum, and Ni3Al intermetallic alloys produced. Differential thermal analyses, scanning electron microscopy, and X-ray diffraction analyses of the sintered samples were carried out. Satisfactory densities of 99.8% were achieved by sintering in vacuum at temperatures as low as 680� C. D 2001 Elsevier Science Inc. All rights reserved.

Carbides of Y 5 Si 3 , of general formula Y 5 Si 3 C x , where x =0.05–0.7, have been prepared by arc melting and their reaction with hydrogen studied. Single-phase carbides can only be made melting pre-formed Y 5 Si 3 with carbon. An activation process, which earlier work did not find, has been developed to induce reaction of the carbide with hydrogen. It involves temperature cycling in gas pressures above 2 atm to higher than 550°C with the severity of these conditions depending on the carbon content. Higher T and P are needed to condition samples of increased carbon content. Conditioned samples, where x ≥0.3 in Y 5 Si 3 C x , will repeatedly cycle 2.5 H (fu) −1 between room temperature and 500°C with between 0.9 and 1.2 H (fu) −1 being desorbed or absorbed with fast kinetics within a very small temperature range. This behaviour is believed to arise from a polymorphic change between an α-form of the D8 8 structure and a supercell β-form when x x >0.3. The first phase change is identified from the rapid expulsion of hydrogen to be close to 438°C and the second at 446°C. Hydrogen saturation of the samples is close to 7 H (fu) −1 and is related to the carbon content. Y 5 Si 3 C 0.5 H 7.33 appears to be an upper limit. The large amount of hydrogen in the crystal structure does not result in loss of good crystallinity or loss of hydrogen reactivity that is always observed for samples without carbon, such as Y 5 Si 3 H ≈5 . The significantly enhanced crystal stability and the greatly improved cycle lifetime of the carbide is analysed in terms of a structural disorder caused by the presence of carbon in at least two crystallographic sites. An attempt to quantify the enhanced stability, compared to Y 5 Si 3 , is made by measuring reaction enthalpies using Van’t Hoff isopleths. To achieve this, rigorous conditions were imposed on the reaction of the binary alloy with hydrogen in order to obtain reliable and repeatable values for Δ H abs and Δ H des .

A six-stage model is proposed to describe the overall process of sorption of water vapor on and into anhydrous nedocromil sodium. The way in which temperature, pressure, and time affect the rate of reaction for each of the stages has been analyzed. Experimental data for the measured rates, where temperature, pressure, and time are variables, are compared with the predictions obtained from each of the six stages. The most useful comparator is a graphical representation of reduced time versus hydration rate. The theoretical equations presented as a shape analysis of the experimental curves show the process to have different controlling mechanisms in three temperature regions: up to 27 degrees C, hydration is controlled by a nucleation and growth mechanism; between 27 and 31 degrees C, the process is dominated by diffusion of water molecules into the crystal; and >31 degrees C, neither nucleation nor diffusion are controlling but some, as yet, undetermined physical processes.

Hardness and fracture toughness measurements are reported for individual phases and interfacial composites present in fired specimens of an Ecuadorian clay mineral. Each investigation used a half disc from strength tests previously done and reported elsewhere. The aim was to rationalise trends in the macroscopic strength and density to quantify contributions from the important components of the microstructure. This was achieved through application of an etching and gold coating procedure prior to microindentation testing. The coating technique improved the clarity of the indents and made the diagonal and radial crack measurements more accurate. Two peaks in the strength versus firing temperature curve are shown to arise from opposite trends in the toughness of the quartz grains and the matrix phase, together with a no-linear build up of the volume of interfacial composite material which confines any cracks initiated in the residual quartz crystals at high firing temperatures.

Three different powder preparations of the drug disodium 9‐ethyl‐4,6‐dioxo‐10‐propyl‐4H,6H pyrano[3,2‐g]quinoline‐2,8‐dicarbox‐ylic trihydrate, Nedocromil sodium (trade name Tilade), have been fully dehydrated in a vacuum and their water vapor adsorption characteristics quantitatively assessed at different water vapor pressures over a temperature range 20 to 40 °C. At saturated vapor pressures, 100% RH, rates of adsorption are around 0.1 s‐1/2. Graphs of square root of time against reduced mass during uptake of water vapor at vapor pressures in the range 20 to 47 mm of Hg, all equivalent to 100% RH, indicate control by a diffusion mechanism with activation energies in the range 8 to 24 kJ mol−1, dependent on the powder preparation method. In two of the powders nonlinear Arrhenius‐type plots are interpreted as showing that control of the process is dependent on the surface's ability to hold water molecules at the experimental temperature. The variation in activation energies and the calculated values for diffusivities, around 1 × 10−13 m2 s−1, are used to explore structural involvement in the overall water adsorption process. The measured values of water vapor diffusivity into the structure have been used to predict the water solubility of nedocromil sodium trihydrate, and the results show good agreement to reported solubilities. This approach to solubility prediction is an alternative to the Noyes and Whitney method where ions leaving the surface are monitored.

Small, 0.5 g, samples of high purity ScSi compositions have been melted in an argon arc furnace and cooled at 150–200 °C s−1 while their surface temperatures have been monitored. From the cooling curves and microscopic, hardness, X-ray powder and differential thermal analyses, a phase diagram is suggested which has many similarities to the most recently proposed phase diagram, but does contain evidence for a peritectically formed Sc5Si4 phase. The highest silicide is non-stoichiometric within the range ScSi1.22-ScSi1.67.

A clay mineral from Ecuador has been analysed mineralogically by X-ray, thermal and infrared analyses and shown to contain approximately 35% of a disordered kaolin phase. Following a dilatometric analysis the vitrification range is established as 1025–1350°C and an isothermal vitrification rate study is reported. The vitrification follows Arrhenius behaviour with Ea=147 kJ mol−1 suggesting a viscous flow control process. Further analysis of the vitrification rate process leads to an equation enabling optimum firing conditions to be calculated: V=∫hohe0.0085rh+e13.23e17,171Ths where V is the total relative degree of vitrification, r is the heating rate, h is the heating time above 1298 K, T is the maximum firing temperature and hs is the soaking period. Mechanical strengths of fired samples have maximum value for the optimum conditions. A detailed athermal DTA analysis of the kaolin dehydroxylation and metakaolin to spinel/mullite transformation is presented and indicates that the shift in peak maximum temperature occurring with changing heating rate can be used to determine activation energies and provide insight into the transformation mechanisms.

Two series of solid solutions, Ho 1- x La x C 2 and Nd 1- x LaC 2 , have been made and their X-ray unit cell parameters measured. The Ho 1- x La x C 2 series contains two tetragonal phases and a cubic solid solution series which has enabled the reaction rate constants for the water hydrolysis reaction of a cubic dicarbide phase to be determined for the first time. By comparing the linear rate constants and the activation energies across the two series the nature of bonding in general and the structure of the cubic phase are elucidated. A comparison with microhardness data is made and the change in M -C 2 bonding as a function of composition is considered.

Two series of solid solution dicarbides made from a light and a heavy dicarbide, Er 1- x Ce x C 2 and Lu 1- x La x C 2 , have been made and the change in X-ray lattice parameters across the series of tetragonal and cubic phases determined. Deviations from Vegard's law behaviour are discussed. Both series have been hydrolysed in water vapour at 11.0 × 10 2 Nm −2 pressure and 14.5 °C and the rate constants for the linear reaction determined. The reaction rates for the Lu 1- x La x C 2 samples with water have been measured together with four cubic phases, Lu 0.5 La 0.5 C 2 , Lu 0.5 Ce 0.5 C 2 , Lu 0.5 Pr 0.5 C 2 and Lu 0.5 Nd 0.5 C 2 . The changes in linear reaction rate constants for these samples, together with those of CeC 2 and LuC 2 , are discussed in terms of CC bond lengths and f character of the metal band electrons which contribute to the MC 2 covalent bond. The reactivity sequence CeC 2 > Ce 0.5 Lu 0.5 C 2 ⪢ LuC 2 is explained.

Three series of tetragonal substitutional solid solutions have been prepared for the systems Ho1−xYxC2, Lu1−xYxC2 and Er1−xGdxC2 and the kinetics of their reaction with water have been studied. A linear reaction rate constant was observed for all samples. The reaction rate for YC2 at 1.93 cm cm−2 min−1 is anomalously fast from ionic size considerations; YC2 is shown to be closer to GdC2 in its hydrolysis and hardness behaviour than to HoC2 with which it is closely related in size. The results are discussed in terms of CC bond length distances and f character of the bonding electrons in the covalent component of the MC2 bond.

Rapid cooling of small samples from the series Y 5 Si 3 Zr 5 Si 3 has enabled a complete range of hexagonal D8 8 phases to be made. The hydrogen reactivity of these metastable phases shows that increasing zirconium in Y 5− x Zr x Si 3 decreases the stability of the hydrides formed. Beyond the composition Y 4.25 Zr 0.75 Si 3 , the ability of the phase to desorb rapidly about 0.9H mol −1 is lost. Zr 5 Si 3 is shown to be stable only when quenched from the melt, while Zr 5 Ge 3 anneals to a well-defined D8 8 phase. A 1250 °C annealing programme carried out on metal-rich arc casts has enabled a section of the YZrSi phase diagram to be presented. Limiting compositions in the annealed samples are Y 4.78 Zr 0.23 Si 3 , Zr 4.55 Y 0.45 Si 3 , Zr 4.0 Y 1.1 Si 3 and Zr 1.83 Y 0.21 Si. The hydrogen reactivity and X-ray parameters of these phases are reported.

Arc-cast samples of Y 5 Ge 3 and Y 5 Ge 3 − x Si x have been made and their reactivity with hydrogen examined. Like Y 5 Si 3 , the germanide has been shown to absorb and desorb hydrogen from the hydride according to Y 5 X 3 H 2.4 ⇋ Y 5 X 3 H 1.6 + 0.4H 2 . The value of ΔH for this reaction has been found for Y 5 Si 3 , Y 5 Ge 3 and five solid solutions. Substitution of germanium into the silicide reduces ΔH but increasing the germanium content leads to materials which are more easily oxidized. Samples in the composition range Y 5 Ge 2.6 Si 0.4 to Y 5 Ge 1.75 Si 1.25 form hydrides showing stepwise desorption of H 2 up to 0.6H 2 (f.u.) −1 . Overall unit cell volumes suggest normal solid-solution behaviour until compositions around Y 5 Si 2 Ge are reached, then multi-plateau hydrogen desorption behaviour, nonlinear X-ray a and c parameters, and lower than expected ΔH values for the above reaction, combine to suggest abnormal behaviour. Some inelastic incoherent neutron scattering results support the observed behaviour.

A series of ceramic silicide-hydrides was prepared and incoherent inelastic neutron scattering spectra were obtained. The family of hydrides was based on Y5Si3Hx. By increasing x in stages in the range 0.32–7.0, by partial substitution of yttrium by scandium and silicon by germanium, and by solid solution forming to make Y5Si3(X)yHx where X ≡ C, N, O, it was possible to obtain comparative spectra which showed that three hydrogen sites are available in the D88 structure. One site is the 6 metal coordinated octahedra (site 2), one is large and elongated in the c-direction (site 1), whilst the third is tetrahedral with 3:1 or 2:2 M:Si coordination (site 3). Hydrogen enters site 3 via the first two and the fast adsorption-desorption of one hydrogen per formula unit is linked to the movement from sites 1 and 2 to 3. The lack of dispersion shown by the modes suggests that hydrogen may not be bonded in a wholly metallic way despite its high mobility detected in this and earlier work.

The binary dicarbides LaC 2 and ErC 2 (tetragonal, space group I 4/ mmm , Z = 2) and the ternary dicarbides Ce 0.50 Er 0.50 C 2 , Ce 0.75 Lu 0.25 C 2 , Ce 0.50 Lu 0.50 C 2 , Ce 0.25 Lu 0.75 C 2 , U 0.75 La 0.25 C 2 , U 0.50 La 0.50 C 2 , U 0.25 La 0.75 C 2 , U 0.75 Ce 0.25 C 2 , U 0.50 Ce 0.50 C 2 , and U 0.25 Ce 0.75 C 2 (cubic, space group Fm 3 m , Z = 4) have been prepared by arc melting, and their metal compositions confirmed by point energy dispersion X-ray analysis by scanning electron microscopy. Their neutron powder diffraction patterns have been recorded at room temperature and, for the three Ce-Lu compounds, also at 4.2 K (at which temperature the cubic structure is retained). For LaC 2 and ErC 2 , mild hydrolysis indicates the presence of acetylide units, and Rietveld profile analysis of the neutron data has enabled the unit-cell dimensions ( a , c = 3.937(1), 6.579(2) A and 3.622(1), 6.106(1) A, respectively) and the C-C bond lengths (1.284(6) and 1.275(3)A, respectively) to be refined. For the ternary compounds (some of which contain two face-centered cubic (f.c.c.) phases), the distinctive martensitic microstructure is absent from micrographs, and the limited neutron integrated intensity data appear to be consistent with f.c.c. structures having either static or rotating C-C groups.

The kinetics of the reaction between water vapour and nine of the lanthanide dicarbides, YC2 and UC2 have been investigated by gravimetric and gas evolution techniques. Mild conditions were used with water vapour pressures in the range 5.8–16.6 × 102 N m−2 and temperatures from 4.0–48 °C. Over 90% of the reactions studied showed linear kinetics with the remainder following parabolic rates. Linear reaction rate constants lie in the range0.34–18.1 × 10−2 mg cm−2 min−1 depending on the carbide, temperature and pressure. For any given temperature and water vapour pressure the rate constant increased in the series from LaC2 to DyC2 and then decreased in the series DyC2 to LuC2, suggesting that the light lanthanides are least reactive. YC2 had rate constants close to those of GdC2, whilst UC2 reacted some 50 times more slowly. In general, at any given water vapour pressure, in the range studied, the rate constant decreased with increasing temperature — a behaviour leading to negative values for the activation energy. From the activation energies a mechanism controlled by water adsorption on the product layers is postulated which suggests that the data do not give a true picture of LaC2 + H2O reactions. Increasing the water vapour pressure always increases the reaction rate constants. The results are discussed in the light of data from UC2 and CaC2.

Structures and phase compositions in the YSi system are reviewed. Thermal analyses, metallography, X-ray diffraction and hardness measurements have been used to examine the system. Y 5 Si 3 is confirmed as the most stable congruently melting silicide, metal vacancies may be present in the structure of Y 5 Si 3+ x materials when it is made as part of a two-phase system with Y 5 Si 4 . It has proved impossible to prepare Y 5 Si 4 as a pure phase. YSi is confirmed as a peritectic phase with an 1836 °C peritectic line terminating at 51.3 at.% Si. Two eutectic points have been confirmed and established. It is concluded that Y 3 Si 5 is the highest siliconcontaining silicide arising from high temperature arc preparation. Tin has been used as an auxiliary metal bath in low temperature preparations which yield fine crystals of a higher silicide than Y 3 Si 5 approaching more closely to YSi 2 for all the Y:Si ratios used in the solvent. An orthorhombic unstable phase with composition close to Y 3 Si 2 , but probably containing some tin, is the only other silicide prepared from the auxiliary bath. High temperature preparations of Y 5 Si 3 O x , Y 3 Si 2 O x and Y 2 Si 3 O x compositions show the effect oxygen can have on the silicide preparations. Y 5 Si 3 dissolves oxygen to Y 5 Si 3 O 0.6 and the effect is a steadily decreasing lattice parameter. Higher oxygen contents lead to decomposition and an oxygenstabilized Y 3 Si 2 phase which has a Y 5 Si 4 -type structure. YSi dissolves little oxygen and appears unaffected whilst oxygen in Y 3 Si 5 leads to lower Si/Y stoichiometry, around Y 2 Si 3 , for the highest silicon content phase.

Two methods to make Y 5 Si 3 N x nitrides in an arc furnace are described. When YN is reacted with Y+Si single phase nitrides are produced as long as x ≤0.3. When Si 3 N 4 is used multiphase products containing Y 5 Si 3 (N) x and Y 5 Si 4 (N) y with x + y ≤0.3 result. In reactions with hydrogen the single phase material is shown to quickly saturate with H≥4.5 (fu) −1 and become amorphous. In contrast the multiphase products only slowly reach saturation and can be recycled many times until eventually their reactivity increases and they saturate. This behaviour is related to a loss of nitrogen as ammonia from the nitrides. Evidence for this reaction is given. Reaction enthalpies from Van’t Hoff isopleths have been measured. Curved isopleths similar to those found for Y 5 Si 3 C x samples are observed in most cases.

Two independent athermal methods of analysis have been used to determine the activation energies associated with the dehydration of nedocromil sodium hydrates. For the highest temperature reaction, monohydrate to the anhydrate, the differences in the measured activation energies indicate a three-dimensional nucleation mechanism in the bulk of the crystal with subsequent three-dimensional anhydrate crystal growth. The number of critical nuclei varies inversely with heating rate. Measured enthalpy values for successive removal of water molecules at 31.7 ± 1.0, 91.3 ± 0.8, and 193 ± 0.6°C are the same, within experimental error, at 21.6 ± 2.6 kJ (mol H 2 O) −1 , as determined from differential thermal analysis traces. This result implies that an earlier concept of “strong” and “weak” water binding is not relevant and temperatures at which H 2 O molecules are removed is related to nucleation effects and not bond energies. The low temperature shoulder on the 91.3°C peak is identified as an effect arising from open pan analysis conditions. The appearance of “transient” peaks in the conditioning stage of nedocromil sodium trihydrate thermal analysis experiments have been investigated and an explanation based on the presence of alcoholates [(NS) 4 • 5CH 3 OH, (NS) 5 • 9C 2 H 5 OH, and (NS) 2 • C 3 H 7 OH] in the preparations is proposed. © 2002 Wiley-Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 91:1101–1116, 2002

Carbides of Y 5 Si 3 , of general formula Y 5 Si 3 C x , where x =0.05–0.7, have been prepared by arc melting and their reaction with hydrogen studied. Single-phase carbides can only be made melting pre-formed Y 5 Si 3 with carbon. An activation process, which earlier work did not find, has been developed to induce reaction of the carbide with hydrogen. It involves temperature cycling in gas pressures above 2 atm to higher than 550°C with the severity of these conditions depending on the carbon content. Higher T and P are needed to condition samples of increased carbon content. Conditioned samples, where x ≥0.3 in Y 5 Si 3 C x , will repeatedly cycle 2.5 H (fu) −1 between room temperature and 500°C with between 0.9 and 1.2 H (fu) −1 being desorbed or absorbed with fast kinetics within a very small temperature range. This behaviour is believed to arise from a polymorphic change between an α-form of the D8 8 structure and a supercell β-form when x x >0.3. The first phase change is identified from the rapid expulsion of hydrogen to be close to 438°C and the second at 446°C. Hydrogen saturation of the samples is close to 7 H (fu) −1 and is related to the carbon content. Y 5 Si 3 C 0.5 H 7.33 appears to be an upper limit. The large amount of hydrogen in the crystal structure does not result in loss of good crystallinity or loss of hydrogen reactivity that is always observed for samples without carbon, such as Y 5 Si 3 H ≈5 . The significantly enhanced crystal stability and the greatly improved cycle lifetime of the carbide is analysed in terms of a structural disorder caused by the presence of carbon in at least two crystallographic sites. An attempt to quantify the enhanced stability, compared to Y 5 Si 3 , is made by measuring reaction enthalpies using Van’t Hoff isopleths. To achieve this, rigorous conditions were imposed on the reaction of the binary alloy with hydrogen in order to obtain reliable and repeatable values for Δ H abs and Δ H des .

The solubility of carbon and its effect on the D88 structure of Er5Si3 were investigated by X-ray examination, metallography and hardness measurements. Corrosion products arising from attack by water vapour and dilute nitric acid on the carbides were analysed. The addition of carbon to Er5Si3Cx. in the range from x = 0 to x = 2.0 produced complex changes. Solutions with x up to 0.2 expanded the lattice, but between x = 0.2 and x = 0.8 the expansion was accompanied by the appearance of a superlattice unit cell. At x = 0.8 the superstructure became disordered prior to changing to a new superstructure at x = 1.0. Two new orthorhombic phases in which there appeared to be some C2n−dipoles were identified at Er5Si3C1.8 and Er5Si3C2.0.

An experimental survey of preparative work in the WO 3 Fe system in which temperatures up to 1100°C and pressures up to 55 kbar have been employed is presented. Two structures not usually found in the WO 3 or FeWO 3 system are encountered. Under the influence of high pressures the concentration of iron in an iron-tungsten bronze phase has been extended beyond the previously known limit of Fe 0.02 WO 3 to approximately Fe 0.1 WO 3 , which has a hexagonal structure with a = 7.422 ± 0.003A, c = 3.766 ± 0.002 A. In the absence of pressure but after long heating times at 1100°C and in the presence of small concentrations of iron, compositions close to W 5 O 14 yield a tetragonal phase a = 23.33 ± 0.01 A, c = 3.797 ± 0.001 A. The details of a single-crystal X-ray structural analysis of this phase are presented which show it to contain pentagonal and hexagonal tunnels and to be isostructural with Mo 5 O 14 as seen in |001| projection. The space group is identified as P 42 1 m which is different from that of Mo 5 O 14 . The difference between W 5 O 14 and Mo 5 O 14 is seen in the presence of less severe puckering and a simpler arrangement of the metal atoms about z = 1 2 which does not give rise to a superlattice cell hence leading to the observed difference in space group.

The three ternary cerium-neodymium dicarbides Ce x Nd 1− x C 2 ( x = 0.25, 0.50, 0.75) and the two binary dicarbides CeC 2 and NdC 2 have been prepared and their room temperature crystal structures examined by neutron powder diffraction by the Rietveld profile analysis method. The five compounds are isostructural, with tetragonal unit cell dimensions changing monotonically over the range a = 3.881(2), c = 6.487(3) A to a = 3.827(2), c = 6.407(2) A and they contain discrete acetylenic CC groups with carbon-carbon bond lengths close to 1.29(1) A.

Crystal-structure line profile refinements have been made for neutron powder diffraction data collected at room temperature from the ternary uranium-thorium dicarbide U 0.1 Th 0.9 C 2 and the two binary dicarbides UC 2 and ThC 2 . U 0.1 Th 0.9 C 2 is monoclinic, with unit cell dimensionsa = 6.630(2), b = 4.183(1), c = 6.690(2)A˚, β = 103.86(1)°, isostructural with ThC 2 , a = 6.684(2), b = 4.220(1), c = 6.735(2)A˚, β = 103.91(1)°. The tetragonal structure for UC 2 is confirmed, witha = 3.522(1), c = 5.988(1)A˚. U 0.1 Th 0.9 C 2 contains discrete C-C groups 1.297(8)A˚long; more precise values for C-C lengths have been determined for UC 2 (1.322(4)A˚) and ThC 2 (1.304(6)A˚).

Sc5Si3Cx carbides (0.01 < x < 1.0) have been made by arc melting Sc5Si3 with carbon. X-ray powder methods have failed to reveal any D88 superstructure phases and this is used as the basis of a discussion on hydride formation for Sc5Si3. Elevated pressures and temperatures do not result in an Sc5Si3Hx phase. Limited solid solubility in the Y5Si3-Sc5Si3 system has been established. The reaction of samples from the two single-phase regions Y5−aScaSi3 and Sc5−aYaSi3 for a in the range 0.05 < a < 1.0 has been studied. The substitution of scandium into Y5Si3 raises the equilibrium pressure and reduces the temperature at which a half mole of hydrogen per Y5−aScaSi3H2.5 can be reversibly stored; ΔH and ΔS values have been established from pressure-composition isotherms. Substitution of scandium by yttrium in Sc5−aYaSi3 results in hydride formation when a ⪢ 0.5; the pressure dependence of this hydride composition has been established and reversing temperatures and enthalpies estimated.

The technique of microhardness measurements using diamond indenters is outlined and assessed for its potential use in quantifying bonding changes and studying reactions in nonstoichiometric crystals. Results are presented for both Vickers and Knoop hardness values on {001} and {011} crystal planes of cubic sodium tungsten bronzes, NaxWO3, with x in the range 0.5 to 0.75. The Knoop data show that in only one direction, 〈110〉 on {001}, is the hardness sensitive to changes in composition. Hardness in the 〈110〉 directions and the degree of anisotropy increase as the sodium content of the bronze increases. All the crystal faces examined showed marked anisotropic behavior, with 〈110〉 being about 50% harder than 〈100〉 on {001} faces, while on {011} planes hardness increases in the sequence 〈100〉:〈211〉:〈111〉 ≈ 〈011〉. Hardness results from isomorphous and isoelectronic ReO3 are considered with the NaxWO3 data to show the dominant role played by Na+WO3 matrix interactions in determining the properties of these materials. The results are discussed in terms of current bonding theories for bronzes.

The electronic expansion factor for bronze phases with the perovskite structure are derived from a wide variety of literature sources and shown to fit a curve when plotted against the Ahrens ionic radius of the non-network-forming cations. Examples are given which show how the curve can be used to characterize and compare bronze-like phases.

Two series of f.c.c. nitride-carbide phases of praseodymium have been identified in the products of the reactions between PrC 2 + PrN, PrN + C and Pr + HCN. The highest yields were found in the case of reactive arc melting of the metal with hydrogen cyanide. From a combination of X-ray, metallographic and hydrolysis techniques the composition ranges of these nitride-carbides have been determined. One series is found in the presence of small amounts of Pr metal and has a total non-metal content x + y in PrN x C y close to unity while the second series is found with Pr 2 C 3 and has x + y 1 carbon units even when y > 0·8. The presence of nitrogen is shown to be essential to the formation of methanide carbides in this system. The results from these experiments are compared to those from the CeNC system where greater difficulty is encountered in stabilising methanide phases.

Sc5Ge3 has been made but cannot be hydrided. It can be alloyed with Y5−aScaSi3 to form quaternary phases with the D88 hexagonal structure that do form hydrides. The effect of replacing silicon by germanium is to lower the hydride formation temperature and to lower substantially the temperature at which M5X3H2.4 ⇋ 0.45H2 + M5X3H1.5 is in equilibrium at one atmosphere pressure of hydrogen. Enthalpy changes for the above reaction show that a composition Y4.28Sc0.72Si2.57Ge0.43 is optimum for fast reversibility with ΔH = −39.4 kJ (mol H2)−1. Increasing the complexity of composition by substitution in the non-metal sublattice does not increase the width of the hysteresis as markedly as substitution in the metal sublattice. There is an increase in the slope of the plateau region in the pressurecomposition isotherms. Germanium substitution greatly improves the tolerance to impurities and makes it difficult to form the γ-hydride at H > 4.5 atom f.u.−1. These factors make the quaternary phases stable to numerous hydriding cycles.

X-ray single-crystal investigations of two superstructure variants of the hexagonal D8 8 structure of Er 5 Si 3 which arise when carbon is present are described. The first superstructure (type 1) occurs at compositions near Er 5 Si 3 C 0.5 ; a crystal of composition Er 5 Si 2.86 C 0.71 has a unit cell in which the hexagonal a axis is a factor of √3 larger than the parent Er 5 Si 3 axis but has only a slightly expanded c axis. A crystal of composition Er 5 Si 3 C 1.0 has the a √3 parameter together with a triple Er 5 Si 3 c axis (type 2 superstructure). A disordered transition structure between these two superstructure variants exists over a small range of composition from Er 5 Si 3 C 0.72 to Er 5 Si 5 C 0.78 . The disorder is manifested by streaking of the reciprocal lattice points along [0001]. It is proposed that partial occupancy of the octahedral sites by carbon together with defects in the erbium octahedra account for the ordering in Er 5 Si 2.86 C 0.71 . The relationship of the disordered structure to the two ordered superstructures and to the parent Er 5 Si 3 structure is discussed.

High temperature reactions between LaC 2 , La 2 C 3 and nitrogen; La 2 C 3 , LaC 2 and LaN; LaN and C; and La metal and carbonaceous gases in an arc furnace are described. It was not possible to achieve conditions leading to an equilibrium nitride-carbide phase. In all the reactions studied, after short melting times, large amounts of an f.c.c. phase are present which contains methanide carbon units and nitrogen. Chemical, metallographic, hydrolytic and X-ray examinations of the nitride-carbide fix the composition as LaC x N y with x + y in the range 0.3–0.4 and y ≥ 0.04 in all cases. This phase is present in the conditions pertaining to an arc furnace, and its existence to low temperature on relatively fast cooling, while slowly decreasing in amount on annealing at all temperatures up to 1700°, indicate that it is a high temperature phase that can be quenched. The X-ray lattice parameter of the nitride-carbide is an increasing linear function of nitrogen content but decreases as the carbon content increases. The appearance of this phase in the lanthanum system and its limited composition range is compared to the other light lanthanide systems cerium and praeseodymium. In the absence of nitrogen no f.c.c. phase could be found.

Crystal chemistry and phase relations of the bronze forming region of the SnWO system have been investigated. Above 780°C the tin bronzes SnxWO3 are shown to be thermally unstable and an equilibrium diagram is established at 700°C which shows that the composition limits of the tetragonal phase are 0.21 ⩽ x ⩽ 0.29. Below x = 0.21 a series of single and two phase regions containing orthorhombic bronzes exists for which the composition limits have been established. In the range 0.29 ⩽ x ⩽ 0.76 the system comprises the tetragonal bronze, Sn2W3O8 and SnWO4, while above 0.76 there is no bronze, only Sn2W3O8, SnWO4 and free Sn. The phase Sn2W3O8 has been isolated and shown to have a hexagonal unit cell, a = 7.696 A, c = 18.654 A. The evidence of differential thermal analysis and X-ray studies suggests that this hexagonal phase arises from the decomposition of the tungsten bronze phase and is itself decomposed to cubic SnWO4 above 700°C. Small thermal effects observed in the DTA scans of tin-containing tetragonal bronzes are interpreted in terms of an order-disorder phenomenon arising from asymmetric tunnel occupancy by Sn2+ ions caused by the presence of the lone pair of electrons. Hydrogen reduction of SnxWO3 has been shown to result in complete removal of oxygen, producing Sn + α-W in the range 600–850°C. Some activation energy data are given for the reduction process.

Crystal structure line profile refinements have been made for neutron powder diffraction data collected at room temperature from the ternary yttrium-holmium dicarbides Y x Ho 1− x C 2 , with x = 0.75, 0.50, and 0.25, and the binary dicarbides YC 2 and HoC 2 . The five compounds are isostructural, with tetragonal unit-cell dimensions in the range a = 3.663(1), c = 6.171(1) A (for YC 2 ) to a = 3.646(1), c = 6.142(1) A (for HoC 2 ), and they contain discrete acetylenic C-C groups with carbon-carbon bond lengths close to 1.28 A.

Crystal chemistry and phase relations for the bronze-forming region of the EuWO system have been investigated. A bronze EuxWO3 is stable up to 1000°C when x ⩽ 0.125 and in the region 0.085 ⩽ x ⩽ 0.125 the symmetry is cubic. A tetragonal bronze exists at x = 0.05, and an orthorhombic bronze with a structure closely related to the orthorhombic form of WO3 exists below x = 0.01. Mossbauer spectra at room temperature and at 80 K indicate that in all these phases the europium is highly ionized as Eu(III) with no electron localization to give (EuII) even at low values for x. The decomposition products of the bronzes have been established, and the Mossbauer parameters for the highly nonstoichiometric tungstates EuxWO4 were determined. Both Eu(II) and Eu(III) resonances were obtained, and a cation vacancy model for EuxWO4 was found to fit the data best. In conformity with the foregoing data, a sample of composition “Eu2W2O7” was found not be be a pyrochlore but to comprise a mixture of Eu6WO12, EuxWO4, and W. The phase relationships for the europium bronze system EuxWO3 are compared with those of other ionic bronzes NaxWO3, LixWO3, and AlxWO3.

The reaction of the cubic sodium bronzes, NaxWO3, with powdered iron metal has been studied by heating samples in vacuo and also at high pressure. Evidence for reaction is found at unexpectedly low temperatures. The reaction is an overall reduction which proceeds via an increase in the sodium content of the bronze phase up to some temperature-dependent limiting composition for which x < 1. The existence of this limit, its temperature dependence, and the identity of the other products of reduction have been explained in terms of the partial oxygen pressure of the system. The course of the reduction has been followed through the evolution of the bronze lattice parameter and a reaction mechanism is postulated. No evidence of significant incorporation of iron into a stable cubic sodium bronze phase has been found.

The reaction of carbon with Gd5Si3 and Ho5Si3 was studied by arc melting the alloys with carbon and comparing the resultant phases with those identified previously in the Er5Si3 system. Ordering in the structure detected at x = 0.5 and x = 0.95 in Ln5Si3Cx in both these systems is identical with that detected in the erbium system. Lower metal volatility in the gadolinium preparations produces single-phase systems more readily, but above x = 0.5 the carbide Gd15C19 is present in small amounts up to x = 0.95. Results for hardness, hydrolysis product distribution and X-ray and metallographic examination are presented.

The hydrogen absorption behaviour of Ln5Si3, (Ln ≡ Y, La, Nd, Ho) and related compounds is similar to that of the parent metal. This behaviour has been studied by gravimetric and volumetric methods. Particular attention was paid to the question of how strain in the host compound shifted the α → β phase transition temperature in Ho5Si3Hx. Only a minor affect was observed. XPS measurements showed that hydrogen changes the oxidation state of silicon from slightly negative to strongly positive.

The solubility of carbon and its effect on the D8 8 structure of Y 5 Si 3 were investigated and were shown to be analogous with the previously investigated Gd 5 Si 3 system. A new carbosilicide corresponding to the composition Y 5 Si 3 C 1.8 was identified. The role of electron transfer effects in the solubility of carbon in the D8 8 structure is discussed and is shown to lend support to the proposal that a defect structure is responsible for the superstructure rearrangements.

The Vickers microhardness and the pendulum hardness of arc-melted dicarbide specimens LnC 2 ( Ln ≡ Sc , Y , La , Ce , Pr , Nd , Sm , Gd , Tb , Dy , Ho , Er , Lu ) were obtained and were compared with some earlier Knoop data. It is possible to divide these compounds into a soft group (LaC 2 , CeC 2 , PrC 2 and NdC 2 ) and a hard group (from SmC 2 to LuC 2 ); YC 2 and ScC 2 are also in the hard group. This division into two groups is rationalized in terms of a model for lanthanide compounds which describes two kinds of 4f electrons, atomic 4f and band 4f. The 4f band electrons contribute to the bonding in such a way that the hardness is diminished. Two series of solid solutions were examined, LaC 2 -HoC 2 and LaC 2 -NdC 2 . In the LaC 2 -HoC 2 series the f-electron structure effect leads to solid solution softening which is in competition with the expected solid solution hardening mechanism. The small difference in f band electron concentration in the LaC 2 -NdC 2 series results in only a solid solution hardening effect. The Vickers hardness HV of ten lanthanide 50:50 solid solutions was obtained and a direct relationship was established between the depression of the transition temperature T t and HV when dicarbides of larger cell volume are dissolved in dicarbides of smaller cell volume.

The hevagonal D88-type structure suicide Y5Si3 is described and shown to react with hydrogen under a variety of conditions. At 1 atm reaction begins at 250 °C without any preconditioning; hydrogen is absorbed with no detectable change in the hevagonal lattice parameters until the composition reaches Y5Si3H1.0 when there is a structural change to a supercell variant: ass = √3abasic, css = 3cbasic. This change is accompanied by a very rapid absorption to a composition around Y5Si3H2.0. The second hydrogen is then rapidly desorbed or absorbed as the temperature is cycled through 420 °C. Ln Pversus 1/T plots give ΔH = 53.3 kJ (mol H2)−1ΔS = 69.7 J K−1(mol H2)−1 for the process. It is shown that this reversibility can be lost by two mechanisms. The first, due to ovygen diffusion into the D88 structure, occurs at lower temperatures and at low ovygen potentials; this type of contamination becomes critical when y in Y5Si3HvOy reaches 0.15. Carbon is also shown to cause loss of reversibility as it stabilives the supercell structure. When ovygen is absorbed at higher temperatures it occurs as Y2O3 which has little effect on the reversible behaviour. The second mechanism leading to the loss of rapid reversibility is associated with hydrogen compositions v in evcess of 3.0 brought about by hydriding at high pressure and temperature. Hydrides with v > 4.0 are very strained and disordered producing evtremely poor or amorphous v-ray patterns. There is some indication that the highest hydride is v ≈ 6 with an orthorhombic structure like Y5Si3C1.8. Carbon monovide is shown to be able to remove hydrogen from these phases.

The carbon species present in the ternary uranium-transition-metal carbide phases U MC/sub 2/ (M = Fe, Cr, W, Mo), prepared by arc melting, have been investigated by mild hydrolysis, thermal analysis, and X-ray and (for M = W, Mo) neutron powder diffraction. For M = W and Mo, the Rietveld neutron-diffraction profile analyses confirm that U MC/sub 2/ (M = W, Mo) are isostructural at room temperature with orthorhombic (Pnma) unit cell dimensions a = 5.612(2), b = 3.241(1), c = 10.956(3) A for UMoC/sub 2/ and a = 5.614(2), b = 3.245(1), c = 10.964(4) A for UWC/sub 2/. Accurate determinations of carbon positions establish that carbon-carbon interatomic distances correspond to the presence of C/sub 1/ rather than C/sub 2/ groups in both crystal structures. For M = Fe and Cr, the hydrolysis, thermal, and X-ray results also show the presence of isolated C/sub 1/ carbon units.

Data for the transformation of several pairs of lanthanide dicarbides from cubic to tetragonal structures have been obtained. These are shown to support a model which proposes that a local strain on C22−ions, caused by local variations in ion size and represented by X-ray unit cell volume difference, is responsible for lowering the nucleation temperature of a tetragonal nucleus in a cubic matrix. For the HoC2-NdC2 system, the strain energy as derived from D.T.A. results is shown to be proportional to the depression of the transformation temperature of HoC2. Results published elsewhere for the GdC2-LaC2 system are shown to be in agreement with the model so long as the depression of transformation temperature is consistently defined. Strain energies up to 13.8 kJ mole−1 are present in these systems and it is estimated that when the strain exceeds a value of 16 kJ mole−1, cubic phases are stabilised down to ambient temperatures.

An isothermal section has been established for the system EuxWO3SnyWO3 at 1000°C from X-ray powder and single-crystal data and Mossbauer spectroscopy and chemical analyses. A single-phase region with the (12 × 3) tetragonal structure has been established. Single-crystal X-ray analysis shows that the Sn and Eu occupy pentagonal tunnel sites. Mossbauer spectroscopy shows that both Sn(II) and Eu(II) are present. The extent of the single-phase field is controlled by two factors: First, the ratio EuSn must be

The hydrolysis products of some rare earth carbides, oxycarbides and nitride-carbides have been studied. With hydrolysis under gentle conditions, product distributions are simple and consistently reproducible, and are representative of the carbon species in the lattice. Consideration of the surface chemistry of the hydrolysis reaction is important in interpretation of results. The use of the hydrolysis reaction to study the nature of the carbon species in carbon-containing phases of several binary and ternary rare earth metal systems is outlined.

The reaction between NH 4 CN and liquid ammonia solutions of Ce, Pr, Sm, Ho, Eu and Yb yields simple cyanides of the composition Ce(CN) 3 , Pr(CN) 3 , Ho(CN) 3 , Sm(CN) 3 , Sm(CN) 2 , Eu(CN) 3 , Eu(CN) 3 , Yb(CN) 3 and Yb(CN) 2 . An alternative route to these compounds using Hg(CN) 2 has been investigated. Some chemical properties, infrared and X-ray data are reported.

Preparation and properties of some new tungsten bronzes SnxWO3 are reported with compositions in the range x = 0.4–0.01. Details of the structure types as elucidated by single crystal methods are described which relate some of these bronzes to known alkali bronzes while a number of hitherto unknown orthorhombic and hexagonal types are described. Tin Mossbauer spectral parameters indicate a loss of electrons from every tin atom to produce Sn(II) in several crystallographic sites but do not support the view that inclusion metal atom clusters exist in the tungsten bronze structures.

The preparation of some new indium trithiocyanate adducts with ethers, aminoethers, and amines are described. Changes in the i.r. spectrum of the thiocyanate group are related to changes in the electronegativity of the ligand and the method of preparation. Complex formation through ether group oxygens results in a polymeric solid containing SCN bridging groups.

Mossbauer parameters have been obtained at 80°K for a number of lanthanon-tin and iron-tin phases, including Ce 3 Sn, Ce 2 Sn, Ce 2 Sn 3 , CeSn 3 , Pr 3 Sn and Fe 3 Sn. These data have been used in conjunction with other studies to help clarify the phase relationships in the Ln-Sn systems. Reaction of substoichiometric quantities of carbon with the binary alloys Ce 3 Sn and Pr 3 Sn and the mixed phases of composition La 3 Sn and Ho 3 Sn gave single-phase products Ln 3 SnCPin x ( x ≤ 0·66); with excess carbon, complete decomposition to Ln 2 C 3 and Sn occurred. By contrast Ln 3 Sn reacted with excess of nitrogen to give LnN and LnSn 3 ; with limited nitrogen a cubic ternary phase Ln 2 SnN x appears to be formed. Further information on bonding has come from a comparison of the 57 Fe and 119 Sn Mossbauer spectra of Fe 3 Sn and Fe 3 SnC.