I.J. McColm

Preparation of yttrium silicides and oxide-silicides

Abstract 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.

Cubic to tetragonal transformations in dicarbides III. enthalpies and strain energies of HoC2-NdC2 and GdC2-LaC2 solid solutions

Abstract 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.

Phases in rapidly cooled scandium-silicon samples

Abstract 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.

Reaction of carbon with lanthanide silicides II: Superstructures and order-disorder phenomena in the Er5Si3-C system

Abstract 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.

The reaction of carbon with rare earth silicides I: the system Er5Si3-C

Abstract 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.

Hydrogen sorption properties of D88-type systems: I. Hydrides of Y5Si3

Abstract 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.

Reaction of carbon with lanthanide silicides III: The Gd5Si3C and Ho5Si3C systems

Abstract 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.

Hydrogen sorption properties of D88-type systems III. The effect of germanium substitution in Ys−aScaSi3 phases☆

Abstract 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.

Reaction of carbon with lanthanide silicides IV: The Y5Si3-C system

Abstract 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 cubic-tetragonal transformation in metal dicarbides IV: f-orbital participation in bonding and its effect on hardness and transition temperatures of rare earth dicarbides and their solid solutions

Abstract 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.