Rare-earth Silicides Research Articles

Mn 5Si 3-type host-interstitial boron rare-earth metal silicide compounds RE5Si 3: Crystal structures, physical properties and theoretical considerations

A series of binary rare-earth metal silicides RE 5Si 3 and ternary boron-interstitial phases RE 5Si 3B x ( RE=Gd, Dy, Ho, Lu, and Y) adopting the Mn 5Si 3-type structure, have been prepared from the elemental components by arc melting. Boron “stuffed” phases were subsequently heated at 1750 K within a high-frequency furnace. Crystal structures were determined for both binary and ternary series of compounds from single-crystal X-ray data: hexagonal symmetry, space group P6 3/ mcm, Z=2. Boron insertion in the host binary silicides results in a very small decrease of the unit cell parameters with respect to those of the binaries. According to X-ray data, partial or nearly full boron occupancy of the interstitial octahedral sites in the range 0.6–1 is found. The magnetic properties of these compounds were characterized by the onset of magnetic ordering below 100 K. Boron insertion induces a modification of the transition temperature and θ p values in most of the antiferromagnetic binary silicides, with the exception of the ternary phase Er 5Si 3B x which was found to undergo a ferromagnetic transition at 14 K. The electrical resistivities for all binary silicides and ternary boron-interstitial phases resemble the temperature dependence of metals, with characteristic changes of slope in the resistivity curves due to the reduced electron scattering in the magnetically ordered states. Zintl–Klemm concept would predict a limiting composition RE 5Si 3B 0.6 for a valence compound and should then preclude the stoichiometric formula RE 5Si 3B. Density functional theory calculations carried out on some RE 5Si 3Z x systems for different interstitial heteroatoms Z and different x contents from 0 to 1 give some support to this statement.

Portraits of some representatives of metal boride carbide and boride silicide compounds

Different ternary alkaline-earth and rare-earth metal boron carbide and silicide compounds are examined using the solid-state language of Zintl–Klemm concept, band structures, and density of states, in order to show that the topology of the non-metal sub-lattice is highly dependent on the electron count. It is also shown that the chemistry of rare-earth metal–boron–silicon does not parallel that of rare-earth metal–boron–carbon. B–C bonds are easily formed in the latter, leading to a large variety of different structural arrangements, whereas Si–B bonds are hardly observed in the former, except in insertion compounds.

Reaction of Terfenol-D melts with SiO 2 crucibles

The interaction between Terfenol-D (Tb x Dy 1− x Fe y (0.27 ≤ x ≤ 0.30, y ≤ 2)) melts and quartz crucibles has been studied. A rare earth silicide was formed next to a rare earth oxide on SiO 2 substrate and a surface layer of rare earth oxide appeared on top of the silicide layer. The reaction was controlled by diffusion with an activation energy E a = 1.59 eV. The effect of Si contamination on the magneto-elastic property was evaluated. Maximum mangnetostriction of Terfenol-D decreased by 300 ppm while Si addition increased up to 1.0 at.%.

Trends and strain in the structures of two-dimensional rare-earth silicides studied using medium-energy ion scattering

The surface structures of the two-dimensional (2D) rare-earth (RE) silicides formed by Gd and Tm on Si(111) have been determined using medium-energy ion scattering (MEIS). These data have been taken together with those from earlier MEIS studies of the 2D silicides formed by Dy, Ho, Er, and Y to investigate the existence of trends in the structural parameters of this class of surfaces. It was found that the Si-RE bond length associated with the surface bilayer in the 2D silicides followed the same trend as the bond length in the bulk silicide compounds which is simply related to the size of the RE atom. In contrast, changes in the Si-RE layer spacing in the 2D silicides were found to be too large to be accounted for by a size effect alone. It was found that the strain that results from the expansion or compression of the overlayer to fit the Si(111) surface is compensated by changes in the Si-RE layer spacing. Finally, there is a systematic variation in the rumple of the surface bilayer in the 2D RE silicides which appears to increase with the magnitude of the lattice mismatch suggesting that strain in the overlayer results in a weakening of the Si-Si bond in the surface bilayer.

Crystal structures, physical properties and NMR experiments on the ternary rare-earth metal silicide boride compounds RE5Si 2B 8 ( RE=Y, Sm, Gd, Tb, Dy, Ho)

The ternary rare-earth metal silicide borides RE 5Si 2B 8 ( RE=Y, Sm, Gd, Tb, Dy Ho) were prepared by arc melting the elemental components and subsequent annealing up to T = 1850 K . The crystal structure was determined for each term of the series from single-crystal X-ray data: tetragonal symmetry, space group P4/ mbm, Z = 2 ; unit cell parameters a = 7.2616 ( 3 ) , c = 8.2260 ( 3 ) Å and a = 7.1830 ( 2 ) , c = 7.9900 ( 3 ) Å for Sm 5Si 2B 8 and Ho 5Si 2B 8, respectively. The structure is a new type and can be structurally described as an intergrowth of ThB 4-like and U 3Si 2-like slabs of composition REB 4 and RE 3Si 2, respectively, alternating along the c direction. The boron and silicon substructures are wholly independent and well ordered. The magnetic properties are as follows: Y 5Si 2B 8 is a Pauli-type paramagnet above 1.8 K, Gd 5Si 2B 8 undergoes a weak (canted) ferromagnetic-like order at 70 K followed by a colinear antiferromagnetic spin alignment at 44 K. Tb 5Si 2B 8 and Dy 5Si 2B 8 order antiferromagnetically at a Néel temperature of T N = 45 and 28 K, respectively. In the paramagnetic regime, the effective moments are in good accord with the theoretical RE 3+ free ion moments. The temperature dependence of the electrical resistivities for the Y, Gd, Tb, and Dy containing samples corroborates with the metallic state of the nonmagnetic (Y) and the magnetically ordered compounds. 11B, 29Si and 89Y nuclear magnetic resonance (NMR) spectroscopy on nonmagnetic Y 5Si 2B 8 shows different signals, which correspond to the expected number of distinct crystallographic sites in the structure. 11B NMR on Y 5Si 2B 8 indicates that the local magnetic susceptibilities are substantially different from the ones observed in the related compound YB 4.

Growth and electronic structure of holmium silicides by STM and STS

The growth of holmium on the Si(1 1 1) surface in the sub-monolayer region was investigated using STM. Several metastable reconstructions, including 5 × 2 and ( 2 3 × 2 3 ) R30° were observed during the nucleation of the 1 × 1 2D rare earth (RE) silicide surface. The authors attempt to combine this data into a detailed picture of the development of the surface, and identify future directions for improving interface quality. Scanning tunnelling spectroscopy data were taken from the 1 × 1 surface, and compared with published theoretical and photoemission results from other 2D RE silicides.

Structural chemistry and physical properties of the rare earth silicide Dy 3Si 4

Structural chemistry, magnetic and electrical transport properties of the new binary silicide Dy 3Si 4 were studied. The title compound is a low-temperature phase, formed peritectoidally below 1600 K between DySi and DySi 2− x . Its crystal structure, which belongs to the Ho 3Si 4 type, was determined at room temperature and 100 K from X-ray single crystal intensity data: orthorhombic symmetry, space group Cmcm, unit cell parameters (293 K) a = 4.2107(2) Å, b = 23.892(2) Å, and c = 3.8144(2) Å. The Dy 3Si 4 structure shows a partial disorder on the silicon substructure, even at low temperature. Its salient characteristics result from the stacking of CrB-like and AlB 2-like slabs alternating along the b-axis. Magnetic measurements reveal that Dy 3Si 4 orders antiferromagnetically at T N = 16 K with an effective moment μ eff = 10.8(2) μ B in the paramagnetic regime, in good accord with the theoretical Dy 3+ free ion moment. The temperature dependence of the electrical resistivity confirms the metallic behaviour of this compound.

Low-temperature studies of the magnetic and superconducting properties of theR2Ir3Si5(R=Y,La,Ce−Nd,Gd−Tm)system

Polycrystalline samples of the ternary rare-earth silicide compounds R 2 Ir 3 Si 5 (R=Y,La,Ce-Nd,Gd-Tm) have been prepared and characterized using room temperature powder x-ray, magnetic susceptibility, electrical resistivity, and low-temperature (1.6-30 K) zero-field heat capacity measurements. All the compounds crystallize in the tetragonal U 2 Co 3 Si 5 type structure (space group Ibam). The effective moments estimated from Curie-Weiss fits to the high-temperature inverse susceptibility data suggest that Ir may carry an induced moment in these compounds. The systematics of the lattice parameters across the series suggest that Ce is trivalent in Ce 2 Ir 3 Si 5 , whereas the magnetic, transport, and heat capacity measurements reveal a nonmagnetic behavior for this compound, suggesting that this is a strongly hybridized compound similar to Ce 2 Rh 3 Si 5 . Pr 2 Ir 3 Si 5 also does not order down to 1.8 K. Large and prominent peaks at low temperatures in the susceptibility and heat capacity of the other magnetic rare-earth-containing compounds indicate bulk magnetic ordering of trivalent rare-earth moments in these compounds. The heat capacity data for Gd 2 Ir 3 Si 5 show three anomalies, one above and one below the main λ type antiferromagnetic transition peak at 11.8 K, while the data for Tb 2 Ir 3 Si 5 also suggest two transitions. Most of the compounds have transition temperatures which follow the de Gennes scaling. However, Nd 2 Ir 3 Si 5 and Tb 2 Ir 3 Si 5 have anomalously large transition temperatures compared to that expected by the de Gennes scaling. Large crystalline electric field effects are indicated by the reduced value of the magnetic entropy at 30 K, and this could be a possible reason for the deviation of the ordering temperature T N from the de Gennes factor. From the temperature dependence of the magnetic entropy we conclude that Nd 2 Ir 3 Si 5 , Tb 2 Ir 3 Si 5 , Dy 2 Ir 3 Si 5 , and Ho 2 Ir 3 Si 5 are in a doublet ground state. The anomalies in the susceptibility and heat capacity near 2 K for La 2 Ir 3 Si 5 and 2.9 K for Y 2 Ir 3 Si 5 suggest that these compounds undergo a transition into the superconducting state at low temperatures. A reduced jump ΔC/γT C at T C and a large linear term in the temperature dependence of the heat capacity in the superconducting state are observed, and we compare this with similar behavior seen in the nonmagnetic superconducting members of the structurally related series R 2 Fe 3 Si 5 , where it has been suggested that a fraction of the Fermi surface remains normal below T C .

Electronic structure and Fermi surface of two-dimensional rare-earth silicides epitaxially grown on Si(111)

The electronic structure and the Fermi surface of two-dimensional rare-earth silicides epitaxially grown on Si(111), ${\mathrm{YSi}}_{2}$ and ${\mathrm{GdSi}}_{2},$ have been studied by a combination of angle-resolved ultraviolet photoemission spectroscopy and density functional theory calculations. Both silicides present a very similar electronic structure, with two characteristic electronic bands below the Fermi energy. One crosses the Fermi energy near the $\overline{\ensuremath{\Gamma}}$ point of the surface Brillouin zone (hole pocket) and the other one close to the $\overline{M}$ point (electron pocket). These two bands arise from surface (localized) states and are responsible for all the Fermi surface features. The theoretical calculations are in good qualitative agreement with the experimental results, and also allow to examine the nature of the bonding between the rare earth and the neighboring silicon atoms. We have found a combination of sp metallic type bond together with covalent bonds involving the rare-earth d states and Si $3p$ states.

Planar defects and double-domain epitaxy in epitaxial YSi2−x and ErSi2−x thin films on Si substrates

Interfacial reactions of Y and Er thin films on both (111)Si and (001)Si have been studied by transmission electron microscopy (TEM). Epitaxial rare-earth (RE) silicide films were grown on (111)Si. Planar defects, identified to be stacking faults on \(\left\{ {10\bar 10} \right\}\) planes with 1/6\(\left\langle {\bar 12\bar 13} \right\rangle \) displacement vectors, were formed as a result of the coalescence of epitaxial silicide islands. Double-domain epitaxy was found to form in RE silicides on (001)Si samples resulting from a large lattice mismatch along one direction and symmetry conditions at the silicide/(001)Si interfaces. The orientation relationships are [0001]RESi2−x//\(\left[ {1\bar 10} \right]\)Si, \((1\bar 100)\)RESi2−x//(001)Si and [0001]RESi2−x/\([\bar 1\bar 10]\)Si, \((1\bar 100)\)RESi2−x//(001)Si. The density of staking faults in (111) samples and the domain size in (001) samples were found to decrease and increase with annealing temperature, respectively.

Formation of ytterbium silicide nanowires on Si(0 0 1)

We demonstrate ability to form self-assembled nanowires of Yb silicide on Si(001) by means of solid phase epitaxy. Such nanowires, in contrast to those of other rare earth silicides, are shown to exhibit an unexpectedly wide variety of orientations with respect to the Si substrate. Their spontaneous formation is explained in terms of the hexagonal AlB2 structure of silicon-rich Yb disilicide.

Ordered arrays of rare-earth silicide nanowires on Si(0 0 1)

Ordered arrays of self-assembled ErSi 2− x , SmSi 2− x , and DySi 2− x nanowires aligned along Si [1 1 0] have been grown at 600°C on Si(0 0 1) vicinal substrates. The nanowires grow perpendicular to the Si dimer rows and are 1.5–5 nm wide, approximately 2 monolayers high and up to 1 μm long. These nanowires were characterized in situ with scanning tunneling microscopy.

Structural determination of two-dimensionalYSi2epitaxially grown on Si(111)

We have used a combination of dynamical low-energy-electron diffraction and density functional formalism calculations to find a structural model for two-dimensional (2D) ${\mathrm{YSi}}_{2}$ layers epitaxially grown on Si(111). Both techniques show that the geometric structure of the yttrium silicide is quite similar to other 2D rare-earth silicides. The surface termination consists of a relaxed Si-bilayer and underlying Y atoms on ${T}_{4}$ sites [with respect to the Si(111) interface]. The low-energy electron diffraction study shows several occurrences of minima in the R factor. The analysis of diffracted beams measured at non-normal incidence allows us to discriminate the spurious minima.

Evolution of vacancy ordering structures in epitaxial YbSi2−x thin films on (111) and (001)Si

The evolution of vacancy ordering structures in epitaxial YbSi2−x thin films on both (111) and (001)Si has been studied by transmission electron microscopy (TEM). Epitaxial YbSi2−x thin films were grown on Si by either room temperature deposition with subsequent thermal annealing or deposition at elevated temperature. Epitaxial YbSi2−x was found to form in samples annealed at 300 °C for 30 min and the appearance of additional diffraction spots is attributed to the formation of an ordered vacancy superstructure in the epitaxial YbSi2−x thin films. In other samples, the split or streaking of extra diffraction spots is attributed to the formation of out-of-step structures. The variation of out-of-step structures with the annealing temperature is correlated to a change in vacancy concentration that makes compressive stress in the Si sublattice relax. From studying plan-view and cross-sectional TEM samples in conjunction with simulated diffraction patterns, the three-dimensional structures of vacancy ordering were determined. The vacancy ordering structures are expected to exert a strong influence on the physical properties of epitaxial rare-earth silicide nanowires grown on (001)Si.

Epitaxial Growth of Strained Nanocrystals

The growth of strained nanocrystals producing self-assembled quantum dots (QDs) and wires (QWRs) has produced convincing examples of low-dimensional behavior. Moreover, the growth of InAs on GaAs (001) in particular has been shown to produce not only interesting examples of fundamental science but also of technologically relevant applications, mainly in the area of laser research. It is then of utmost importance the investigation of the growth processes in strained epitaxy, involving both thermodynamic and kinetic pathways to the production of such systems. Here the focus is on the thermodynamic aspects of nanocrystal growth, choosing as a model system the epitaxial growth of Ge:Si (001). This is an interesting system to build the knowledge of nanocrystal growth, giving the reduced number of experimental parameters one has. With the lessons learned from this system, InAs:GaAs (001) QDs were grown near thermal equilibrium, generating a reproducible and uniform ensemble of nanocrystals. In a different regime, the growth of self-organized rare-earth silicide wires can be achieved upon proper choice of growth temperature and growth rate. These few examples show that the subtleties of strained nanocrystal growth can be explored in such a way that by knowing the thermodynamics and kinetics of a particular material system many interesting low-dimensional structures can be created.

Structural study of 2D dysprosium germanide and silicide by means of quantitative LEED I–V analysis

Quantitative low energy electron diffraction (LEED) studies have been carried out to determine the structure of the rare-earth germanide Ge(1 1 1)1×1–Dy and the rare-earth silicide Si(1 1 1)1×1–Dy. The analyses reveal structures similar to that of other rare-earth silicides previously studied in which the dysprosium is located sub-surface below a reverse-buckled silicon/germanium bilayer. The LEED study has clearly demonstrated that the dysprosium atom is located above a substrate T4 site and refines earlier MEIS results.

Single-crystal growth of binary and ternary rare earth silicides

Single crystals of the binary CeSi 2 and selected ternary RE 2TMSi 3 (RE=Ce, Tb, Dy; TM=Pd, Co, Ni) compounds were grown by vertical floating zone melting both with RF induction heating and optical heating. The compounds exhibit congruent melting behaviour at a composition different from the stoichiometric one. Accordingly, inferior element segregation along the crystal axis was detected. A silicothermic reaction during the growth process serves for melt refining with respect to the oxygen content and crystals practically free of oxide inclusions. The anisotropy of magnetic ordering and the magnetoresistance of selected single-crystalline ternary compounds is briefly described.

Interaction of copper metallization with rare-earth metals and silicides

Solid-phase reactions of copper films with underlying gadolinium, erbium, and erbium–silicide layers on Si(100) substrates were investigated. For the phase analysis, x-ray diffraction and cross-sectional transmission electron microscopy were used. In the case of Cu/Gd/Si(100), an orthorhombic GdSi2 formed, and, at higher temperatures, copper aggregated into islands. Annealed Cu/Er/Si(100) samples resulted in a hexagonal Er5Si3 phase. In the Cu/ErSi2−x/Si system, the copper catalyzes the transformation of the highly oriented hexagonal ErSi2−x phase into hexagonal Er5Si3. Diverse phase developments of the samples with Gd and Er are based on reactivity differences of the two rare-earth metals.

Self-assembled rare-earth silicide nanowires on Si(001)

This paper presents scanning tunneling microscope images of several rare-earth metal silicides grown on silicon (001). For certain of the metals studied (Dy, Ho), an anisotropy in lattice match with the substrate results in the formation of nanowires. These nanowires have desirable properties such as nanometer lateral dimension, crystalline structure with a low density of defects, and micrometer scale length. Tunneling spectroscopy on the nanowires indicates that they are metallic.

Magnetism ofNd3+ions inNd5Rh4Sn10

In the family of rare-earth transition metal silicides, compounds belonging to the series ${R}_{5}{\mathrm{Ir}}_{4}{\mathrm{Si}}_{10}$ (where R is a rare-earth element from Dy to Lu and Y) exhibits multiple phase transitions. However, there exists a series ${R}_{5}{\mathrm{Rh}}_{4}{\mathrm{Sn}}_{10},$ which can only be formed with light rare-earth elements from Ce to Nd. In this work, we report detailed resistivity, susceptibility, magnetization, heat-capacity, thermal expansion, and magnetoresistance studies, which suggest that the ${\mathrm{Nd}}^{3+}$ moments undergo unusual magnetic orderings in a single crystal of ${\mathrm{Nd}}_{5}{\mathrm{Rh}}_{4}{\mathrm{Sn}}_{10}.$ A second-order magnetic transition at 7.15 K is followed by a first-order transition below 6.15 K that results in a huge heat-capacity peak of 100 J/mol K. The two transitions are strongly suppressed by a magnetic field (4 T) along the c axis while they are only weakly influenced by fields as high as 16 T in the basal plane. Moreover, the magnetoresistance data along the a and c axes from 1.5 to 15 K in fields up to 18 T revealed a giant positive magnetoresistance along the a axis below 6 K and clear evidence of a metamagnetic transition along the c axis. We attribute this unusual magnetic behavior to the unique crystal structure of this ${\mathrm{Nd}}_{5}{\mathrm{Rh}}_{4}{\mathrm{Sn}}_{10}$ compound with its three different Nd sites.