The RCuZn, RAgZn and RAgAl intermetallic compounds (R = rare earth metals) were synthesized and structurally studied by powder and single crystal X-ray diffraction. Among the 28 examined phases, YbAgAl crystallizes in the MgZn2 structure type, while all the others belong to the CeCu2 type. The usual decrease of the cell parameters from lanthanum to erbium is observed, and the clear positive deviation of the three ytterbium compounds can be imputed to Yb divalency. The application of the structural map method to the RMX compounds, where M and X are elements of the 10 to 15 groups, allows some predictions about unknown phases to be proposed.

Ternary phases with compositions RPdMg, RAgMg, RPdPb and RPdHg (R  rare earth) have been prepared and their crystal structure has been investigated. The majority of them adopt the structure of the ZrNiAl type. Numerical data are reported and briefly discussed.

The phases RCuGe of the rare earths were studied structurally in their low-temperature form. For RLaGd (excluding Eu) the structure is of the AlB2 type; for RTLu the CaIn2 type structure was found.

The phase diagram of the La—Rh system (0–70 at.% Rh) was investigated using differential thermal analysis, micrographic analysis, X-ray diffraction and electron microscopy. Eight compounds are formed in this system, one of which was not previously known (La3Rh2). The existence of most of the compounds reported earlier has been confirmed, except two compounds which have to be deleted (La3Rh and La5Rh3). The shape of the diagram is markedly different from that reported in the current literature and shows in the 30–70 at.% Rh range melting points lower by about 100–150 °C. The rhodium-rich side of the diagram has not been investigated owing to the high melting points of the corresponding alloys.

Ternary phases of the composition RNiZn, RPdZn, RPdCd and RCuCd (R = rare earth) have been prepared and their structures investigated. We found that the majority of these compounds adopt the structure of the Fe2P or the ZrNiAl type. Structural data are reported and briefly discussed.

The structures of ScAuSi, YAuSi and LuAuSi were determined by single-crystal diffractometry. ScAuSi, hP6, hexagonal P6m2 (no. 187), a=4.212(1) Å, c=7.546(3) Å, Z=2, R=0.039 using 66 reflections, represents a new ternary ordered type. YAuSi, hP6, hexagonal P63mc (no. 186),a=4.288(2) Å, c=7.546(3) Å, Z=2, R=0.039 using 66 reflections, is isotypic with LiGaGe (ternary ordered derivative of the CaIn2 structure). LuAuSi, isotypic with ScAuSi, a=4.267(2) Å, c=7.151(3) Å, was refined to R=0.052 using 90 reflections. In both ScAuSi and YAuSi structures trigonal prisms of scandium or yttrium sharing their lateral faces are stacked along the c axis. The gold and silicon atoms forming rumpled layers centre the prisms in an ordered way. The difference between the two structures consists in the different stacking of these layers along the c axis. In YAuSi atoms of gold in a layer alternate with atoms of silicon in the subsequent layer, and vice versa; in ScAuSi the layers become equally oriented so that Au-Au and Si-Si pairs are formed.

The pseudobinary systems RZn2-RAl2, RZn2-RGa2, with R ≡ Ce, Sm, Gd, Er, RCu2-RGa2 with R ≡ Ce, Sm, andErNiGa-ErGa2 were studied to determine the structure of the intermediate phases and the extension of their homogeneity ranges.

We report electron-excited Si L2,3 VV Auger line shapes from the whole series of stoichiometric Gd silicides. The evolution of the Auger profiles, as the metal content is varied, is found to be strictly similar to that reported from the family of Ca silicides. This circumstance, along with the well-assessed understanding of the electronic structure of these latter compounds, allows us to ascribe the chemical bond in Gd silicides to the interaction between Si 3(sp) and Gd 5d states, with the Si 3s states progressively demixing from the other symmetry contributions while moving to the Gd-rich silicide and eventually attaining a quasiatomic character in Gd5Si3.

Photoemission spectra of pseudobinary Yb(1-x)M(x)Al2 (M = Sc, Ca) compounds derived from YbAl2 are given (hv = 40.8, 65, 181.5 and 1253.6 eV) at various stoichiometries corresponding to compressed (with Sc) and decompressed compounds (with Ca). The surface divalent Yb 4f signal depends on the chemical substitution and is stronger in the compressed compounds (2.9 times with respect to YbAl2 in the case of 80% Sc measured at 40.8 eV). Auger results are also reported. The effect is discussed in connection with the release of compression energy in the surface region.

We present photoemission results with uv and with x rays from a family of pseudobinary compounds obtained by partial substitution of Yb, with Ca or Sc, in the reference compound YbAl2. The substitution with Ca gives rise to a decompression and the one with Sc a compression of the lattice. Ultraviolet photoemission shows the perturbation of the electron states due to chemical substitution; Sc introduces a d character extending up to about 3 eV below the Fermi level. The weights of the Yb3+ and Yb2+ multiplets given by x-ray photoemission are compared with the results from x-ray absorption; the two spectroscopies agree in YbAl2 while the relative weights Yb3+ versus Yb2+ from x-ray photoemission are definitely below the x-ray-absorption values in the compounds heavily substituted with Sc; this is interpreted as a photoemission final-state effect connected with the d character introduced by the substitution.

We present Yb LIII X-ray absorption spectra (XAS) from a series of valence fluctuating Yb1-xScxAl2 (x = 0; 0.2; 0.4; 0.6; 0.8) isostructural pseudobinary alloys. Yb mean valences, as derived via a conventional intensity analysis of the Yb LIII double-peak singularities, result to monotonically increase upon decresing the lattice spacing, i.e. as the crystal is progressively shrinked by increasing the Sc-content. The Yb valence vs lattice spacing trend is found to be strictly similar to that already reported on the YbAl2 binary compound while spanning different conditions of external pressure. Discrepancies in the Yb valence evaluation outcome when comparing Yb LIII XAS versus Yb 4ƒ XPS results, XAS valences being significantly higher than XPS values in the case of Sc-rich alloys (x = 0.6 and 0.8).

Empty electronic states of gadolinium silicides (${\mathrm{Gd}}_{3}$${\mathrm{Si}}_{5}$ and ${\mathrm{Gd}}_{5}$${\mathrm{Si}}_{3}$) are investigated by means of inverse photoemission in the ultraviolet photon range. Isochromat spectra taken at different photon energies (11--25 eV) are presented. The results are discussed in terms of bond formation between metal and silicon orbitals, in analogy with silicides of other low-d-occupancy metals (Ca silicides).

${\mathrm{Gd}}_{3}$${\mathrm{Si}}_{5}$, GdSi, and ${\mathrm{Gd}}_{5}$${\mathrm{Si}}_{3}$ were investigated with photoemission spectroscopy in the photon-energy range 40.8--149 eV by exploiting the energy dependence of the photoemission cross sections and the valence resonance at the crossing of the Gd 4d-4f threshold. The modification of the spectra versus photon energy, along with their stoichiometry dependence, show the relevance of covalent mixed Gd 5d--Si 3sp states in the formation of the chemical bond. In the region close to the Fermi level an increase of the d contribution is observed. These points are discussed in connection with the existing models of the silicide bond.

We present a Si ${\mathit{L}}_{2,3}$VV Auger line-shape analysis of the whole set of Ca silicides, ${\mathrm{Ca}}_{2}$Si, CaSi, and ${\mathrm{CaSi}}_{2}$. Measured spectra are compared, after data processing, to theoretical ones, computed within the framework of a one-electron picture. The overall good agreement found in this comparison allows us to understand the observed Si ${\mathit{L}}_{2,3}$VV spectral changes in terms of the Si valence states' evolution across the whole family of Ca silicides. The narrowing of the pp spectral region and the appearance of a distinct sp shoulder observed on going from ${\mathrm{CaSi}}_{2}$ to ${\mathrm{Ca}}_{2}$Si are related, respectively, to the decreased width of the Si 3p band and to the narrowing of the Si 3s states together with their increased energy separation from the Si 3p states. A breakdown of the one-electron picture is observed in ${\mathrm{Ca}}_{2}$Si. We find that this effect is due to the quasiatomic configuration of the Si 3s states in this compound. The introduction of a correlation energy between two sp-like--final-state holes improves the comparison between theory and experiment, but does not appear to fully account for the Si ${\mathit{L}}_{2,3}$VV spectrum from ${\mathrm{Ca}}_{2}$Si. New theoretical effort is therefore needed to understand this spectrum.

We present Yb LIII near edge X-ray absorption spectra (XAS) for a series of Yb1-xCaxAl2 (x = 0; 0.4; 0.6; 0.8) isostructural pseudobinary alloys. The Yb mean valence derived by intensity analysis of the Yb2+ and Yb3+ white lines of the XAS spectra results to gradually decrease as the Ca-content is progressively increased and, in turn, the nearest neighbours atomic shell surrounding the Yb sites is increasingly expanded.

We present a combined theoretical and experimental investigation of the electron properties of calcium silicides (${\mathrm{Ca}}_{2}$Si, CaSi, and ${\mathrm{CaSi}}_{2}$). The theoretical study is performed by a self-consistent calculation of the electron states, while the experimental analysis is based on synchrotron-radiation photoemission measurements. The overall agreement between the computed and measured spectra allows us to investigate the main features of the Ca-Si interaction in different compounds. We find that covalent character is present in the Ca-Si bond and that the strength of this interaction increases with Si concentration. Furthermore all the Ca s-p-d states are involved in this coupling with Si. In ${\mathrm{Ca}}_{2}$Si, the Si s states are found in a corelike configuration, while in the other compounds they are promoted to form an s-p valence band. The covalent interaction is not sufficient to interpret the results and some ionic character is present in the Ca-Si bond. ${\mathrm{Ca}}_{2}$Si is found to be a semimetal and many structures in the density of states can be correlated to well-defined interactions between the nonequivalent Ca-Si couples that are found in these complex compounds.

The electronic properties of calcium silicides (CaSi and CaSi2) are investigated through a joint experimental and theoretical study using Bremsstrahlung Isochromat spectroscopy (BIS) and self-consistent calculation of the electronic states by the Linear Muffin-Tin Orbitals (LMTO) method in the Atomic Sphere Approximation (ASA). The peculiar crystal structure of CaSi2 with two inequivalent Si atoms is responsible for a well defined BIS feature. We found that the calculated high energy DOS features are consistently lower in energy compared to the BIS spectra. Inclusion of the cross-sections of different states into the calculations improves the agreement between the experimental spectra and the calculated curve. We also show that considerable covalent character is present in the bond of calcium silicides.

A joint experimental and theoretical analysis of the Si ${L}_{2}$,3VV line shape of ${\mathrm{CaSi}}_{2}$ and ${\mathrm{Ca}}_{2}$Si is presented. While the pp-two-hole contribution to the Auger line shape of the two Ca silicides is well described by a one-particle picture, strong hole-hole repulsive effects (U\ensuremath{\sim}3 eV) are present in the Auger process involving Si 3s states in ${\mathrm{Ca}}_{2}$Si. These correlation effects are consistently related to the quasiatomic nature of the Si 3s states in ${\mathrm{Ca}}_{2}$Si.

We present angle integrated X-ray photoemission studies of Yb0.6Ca0.4Al2 and YbAl2 alloys. Mixed valence behaviour was observed for Yb in both compounds with room temperature valence estimated at 2.28 ± 0.03 for Yb0.6Ca0.4Al2 and 2.33 ± 0.03 for YbAl2. The lower intermediate valence for Yb in the pseudo-binary compound Yb0.6Ca0.4Al2 is related to chemical pressure effects. Results at liquid nitrogen temperature indicate further reduction (about 10%) in the trivalent state contribution.

ChemInformVolume 18, Issue 33 Physical Inorganic Chemistry ChemInform Abstract: The Structure of Some Ternary Phases of Calcium. A. IANDELLI, A. IANDELLI Ist. Chim. fis., Univ. Genova, 16132 Genova, ItaliaSearch for more papers by this author A. IANDELLI, A. IANDELLI Ist. Chim. fis., Univ. Genova, 16132 Genova, ItaliaSearch for more papers by this author First published: August 18, 1987 https://doi.org/10.1002/chin.198733012Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume18, Issue33August 18, 1987 RelatedInformation