Rh Substitution Research Articles

Thermoelectric transport in the layered Ca3Co4–xRhxO9 single crystals

We have examined an isovalent Rh substitution effect on the transport properties of the thermoelectric oxide Ca3Co4O9 using single-crystalline form. With increasing Rh content x, both the electrical resistivity and the Seebeck coefficient change systematically up to x = 0.6 for Ca3Co4–xRhxO9 samples. In the Fermi-liquid regime where the resistivity behaves as ρ=ρ0+AT2 around 120 K, the A value decreases with increasing Rh content, indicating that the correlation effect is weakened by Rh 4d electrons with extended orbitals. We find that, in contrast to such a weak correlation effect observed in the resistivity of Rh-substituted samples, the low-temperature Seebeck coefficient is increased with increasing Rh content, which is explained with a possible enhancement of a pseudogap associated with the short-range order of spin density wave. In high-temperature range above room temperature, we show that the resistivity is largely suppressed by Rh substitution while the Seebeck coefficient becomes almost temperature-independent, leading to a significant improvement of the power factor in Rh-substituted samples. This result is also discussed in terms of the differences in the orbital size and the associated spin state between Co 3d and Rh 4d electrons.

Low-temperature specific heat of magnetic superconductors Dy0.6Y0.4Rh3.85Ru0.15B4 and Dy0.6Y0.4Rh4B4

Specific heat CM(T) of polycrystalline Dy0.6Y0.4Rh4B4 and Dy0.6Y0.4Rh3.85Ru0.15B4 was studied in the temperature range of 0.5–9 K and magnetic fields 0–10 kOe for the first time. It was found that the λ-anomaly in the specific heat exists at Tc ≈ 6 K for Dy0.6Y0.4Rh4B4 and at Tc ≈ 6.6 K for Dy0.6Y0.4Rh3.85Ru0.15B4. It is suppressed in a magnetic field and shifted to lower temperatures. Partial substitution of Rh by Ru enhances superconductivity, presumably, due to stronger inner magnetism of the dysprosium sublattice in Dy0.6Y0.4Rh4B4 as compared with Dy0.6Y0.4Rh3.85Ru0.15B4. Furthermore, it was observed that the molar heat capacity CM(T) of Dy0.6Y0.4Rh3.85Ru0.15B4 increases with decreasing temperature for T < 4 K. In Dy0.6Y0.4Rh4B4, an increase in CM(T) with decreasing temperature is accompanied by the appearance of a maximum at Tmax = 1.5 K, which might be a manifestation of the magnetic phase transition in the dysprosium subsystem at this temperature.

A Crucial Role of Rh Substituent Ion in Photoinduced Internal Electron Transfer and Enhanced Photocatalytic Activity of CdS-Ti(5.2-x)/6 Rhx /2 O2 Nanohybrids.

The photocatalytic activity and photostability of CdS quantum dot (QD) can be remarkably enhanced by hybridization with Rh-substituted layered titanate nanosheet even at very low Rh substitution rate (<1%). Mesoporous CdS-Ti(5.2-x)/6 Rhx/2O2 nanohybrids are synthesized by a self-assembly of exfoliated Ti(5.2-x)/6 Rhx/2O2 nanosheets with CdS QDs. The partial substitution of Rh(3+)/Rh(4+) ions for Ti(4+) ions in layered titanate is quite effective in enhancing an electronic coupling between hybridized CdS and titanate components via the formation of interband Rh 4d states. A crucial role of Rh substituent ion in the internal electron transfer is obviously evidenced from in situ X-ray absorption spectroscopy showing the elongation of (RhO) bond under visible light irradiation. This is the first spectroscopic evidence for the important role of substituent ion in the photoinduced electron transfer of hybrid-type photocatalyst. The CdS-Ti(5.2-x)/6 Rhx/2O2 nanohybrids show much higher photocatalytic activity for H2 production and better photostability than do CdS and unsubstituted CdS-TiO2 nanohybrid. This result is ascribable to the enhancement of visible light absorptivity, the depression of electron-hole recombination, and the enhanced hole curing of CdS upon Rh substitution. The present study underscores that the hybridization with composition-controlled inorganic nanosheet provides a novel efficient methodology to optimize the photo-related functionalities of semiconductor nanocrystal.

Carbon formation on Rh-substituted pyrochlore catalysts during partial oxidation of liquid hydrocarbons

Rh-substituted pyrochlore catalysts were prepared and tested for the partial oxidation (POX) of n-tetradecane (TD), a diesel fuel surrogate. The catalysts were substituted lanthanum zirconates with the chemical formula La1.89Ca0.11Zr1.75−xRhxY0.25O7−y (with either 0, 1, 2, or 3wt% of Rh loading). The catalysts underwent a variety of pre- and post-reaction characterization with a focus on deposited carbon, the primary cause of catalyst deactivation. To assess the nature of potential catalytic sites (typically attributed to Rh), each substituted catalyst produced three temperature programmed reduction peaks, with an increase in the most reducible sites at higher Rh loadings. In additions, the catalysts with 2 and 3wt% Rh substitution displayed a secondary crystal phase in their XRD spectra; however, this phase disappeared when the sample was reduced up to 900°C, representative of reaction conditions. To assess carbon deposition after reaction testing, a temperature programmed oxidation (TPO) was conducted on each sample from 250 to 750°C. Each sample produced three CO2 peaks between 250 and 550°C, referred to as low temperature carbon (LTC) peaks, and two peaks at higher temperatures (>600°C). The 2wt% catalyst was run for different times under POX of TD, and the TPO results showed that the amount of LTC reached a steady state within 2h and did not increase even up to 18h under reaction conditions. This indicates that the LTC is not likely responsible for catalyst deactivation. In contrast, the quantity of carbon associated with the higher temperature peaks under TPO increased with reaction time. The carbon associated with the high temperature peaks was highly graphitic, and is typically attributed to deposits on the catalyst inert surfaces and/or inert bed material. After a partial burn-off of the LTC, Raman analysis showed no significant change in the surface carbon. These results suggest that improvement in catalyst performance may be accomplished by modification of the catalyst formulation to increase the resistance to carbon formation on the oxide surface. Such modifications include surface promoters, B-site dopant level, and addition of an oxygen-conducting support. Future work will explore these approaches.

ChemInform Abstract: Photocatalytic Water Oxidation by a Pyrochlore Oxide upon Irradiation with Visible Light: Rhodium Substitution into Yttrium Titanate.

AbstractA stable visible‐light driven photocatalyst (λ ≥ 450 nm) for H2O oxidation is prepared by Rh substitution into the pyrochlore Y2Ti2O7 to a maximum level of 2% by stoichiometric solid state reactions of Y2O3, TiO2, and Rh2O3 under air (Pt foil lined alumina boat, 1300 °C, 2 d).

Influence of Electron Doping on Magnetic Order in CeRu2Al10

The effect of electron doping by the substitution of Rh for Ru on unconventional magnetic order in CeRu2Al10 was investigated via neutron powder diffraction. In Ce(Ru1−xRhx)2Al10 with x = 0.05, 0.12, and 0.2, reorientation of the ordered moment from the c-axis as in pure CeRu2Al10 to the a-axis takes place in all samples, while the ordering vector \(\boldsymbol{{q}} = (0,1,0)\) remains unchanged within this concentration range. The moment reorientation is accompanied by an increase in its size by a factor of ∼2.4, from μ = 0.43 μB at x = 0 to μ = 1.06, 1.04, and 1.02 μB for x = 0.05, 0.12, and 0.2, respectively. The continuous decrease in the Neel temperature T0(TN), despite an abrupt increase in μ, underlines the strong anisotropy in the exchange interaction in CeRu2Al10 and the fact that this anisotropy is easily suppressed by electron doping.

Tuning the Curie temperature of L1 ordered FePt thin films through site-specific substitution of Rh

In structurally ordered magnetic thin films, the Curie temperature (TC) of ferromagnetic films depends on the exchange integral of the short range ordered neighboring atoms. The exchange integral may be adjusted by controlling the elemental substitutional concentration at the lattice site of interest. We show how to control the TC in high anisotropy L10 Fe50Pt50 magnetic thin films by substituting Rh into the Pt site. Rh substitution in L10 FePt modified the local atomic environment and the corresponding electronic properties, while retaining the ordered L10 phase. The analysis of extended x-ray Absorption Fine Structure spectra shows that Rh uniformly substitutes for Pt in L10 FePt. A model of antiferromagnetic defects caused by controlled Rh substitution of the Pt site, reducing the TC, is proposed to interpret this phenomenon and its validity is further examined by ab initio density functional calculations.

Combined effects of transition metal (Ni and Rh) substitution and annealing/quenching on the physical properties ofCaFe2As2

We performed systematic studies of the combined effects of annealing/quenching temperature ({\itshape T}$_{A/Q}$) and T = Ni, Rh substitution ({\itshape x}) on the physical properties of Ca(Fe$_{1-x}$T$_{x}$)$_{2}$As$_{2}$. We constructed two-dimensional, {\itshape T}$_{A/Q}$-{\itshape x} phase diagrams for the low-temperature states for both substitutions to map out the relations between ground states and compared them with that of Co-substitution. Ni-substitution, which brings one more extra electron per substituted atom and suppresses the {\itshape c}-lattice parameter at roughly the same rate as Co-substitution, leads to a similar parameter range of antiferromagnetic/orthorhombic in the {\itshape T}$_{A/Q}$-{\itshape x} space as that found for Co-substitution, but has the parameter range for superconductivity shrunk (roughly by a factor of two). This result is similar to what is found when Co- and Ni-substituted BaFe$_{2}$As$_{2}$ are compared. On the other hand, Rh-substitution, which brings the same amount of extra electrons as does Co-substitution, but suppresses the {\itshape c}-lattice parameter more rapidly, has a different phase diagram. The collapsed tetragonal phase exists much more pervasively, to the exclusion of the normal, paramagnetic, tetragonal phase. The range of antiferromagnetic/orthorhombic phase space is noticeably reduced, and the superconducting region is substantially suppressed, essentially truncated by the collapsed tetragonal phase. In addition, we found that whereas for Co-substitution there was no difference between phase diagrams for samples annealed for one or seven days, for Ni- and Rh- substitutions a second, reversible, effect of annealing was revealed by seven-day anneals.

Kinetic and mechanistic study of dry (CO2) reforming of methane over Rh-substituted La2Zr2O7 pyrochlores

The active sites and kinetics of dry reforming of methane (DRM) on lanthanum zirconate (LZ) pyrochlore catalysts are studied as a function of Rh substitution, temperature, and partial pressures of CH4 and CO2. In this work, we focus specifically on determining the catalytic active sites for CH4 and CO2 activation and their role in the DRM mechanism over the two Rh-substituted pyrochlores, i.e., 2wt% Rh, designated L2RhZ and 5wt% Rh, designated L5RhZ. Kinetic rate modeling suggests dual-site mechanism, where CH4 and CO2 are activated on different sites (dual-site mechanism). Eleven different rate models were tested against the kinetic rate data obtained over these two Rh pyrochlores. Statistical analysis shows valid and similar fits for only two of eleven models: one in which activation of CH4 is rate-determining and one in which CO2 activation is rate-determining. This dual-site mechanism is studied further in detail to test the validity of the intermediate steps predicted by the kinetic model. CH4/CD4 isotope switching shows a strong deuterium kinetic isotope effect on CH4 and CO2 conversion, suggesting that CH4 dissociation is the rate-determining step. Higher apparent activation energies of CH4 (Ea, CH4) versus CO2 (Ea, CO2) on both catalysts also confirm that CH4 activation is rate-determining. Basic nature of La–O sites activates mildly acidic CO2 to form La2O2CO3 complexes as confirmed by FTIR. From different polymorphs of La2O2CO3, the spectator and reactive species were distinguished by pulsing CH4 over La2O2CO3. Alternating pulses (CH4/Ar→CO2/Ar→CH4/Ar) at 550°C showed simultaneous formation of CO and H2, suggesting that surface carbon, formed by CH4 decomposition, is oxidized by H2O; a crucial step in understanding the catalytic behavior of pyrochlores in the reaction mechanism. These experiments were used to identify the two types of sites taking part in the dual-site DRM mechanism. The work reported here helps in determining a single set of kinetically significant steps that most closely represent the mechanistic scheme of DRM over L2RhZ and L5RhZ.

Crystal structure and Ce valence variation in the solid solution CeRh3 − xPdxB0.5

Crystal structure and physical properties have been studied in the solid solution CeRh3 − xPdxB0.5 (x = 0, 0.5, 1.2, 1.5, 1.7, 2.5, 3) in as-cast state by x-ray powder diffraction and scanning and high-resolution electron microscopy, as well as by low-temperature measurements of the magnetic susceptibility and specific heat. X-ray powder patterns of the alloys were indexed on the basis of a cubic primitive AuCu3 lattice throughout the entire solution showing a monotonous increase in three regimes. For 0 ≦̸ x ≦̸ 1.5, the volume increase corresponds merely to the substitution of Rh by Pd atoms, the sizes of which are rather close. In this regime the tetravalent character of cerium is unaffected. For x > 1.5, the lattice expansion becomes enhanced and is thus attributed to a gradual valence change of the Ce atoms (an intermediate valence regime). Finally, for x > 2.4, the system shows a predominant trivalent behavior. In this region, the specific heat coefficient reaches a value of 4.4 J mol−1 K2, two orders of magnitude larger than that in the Rh-rich border. Selected area electron diffraction patterns of the CeRh3 − xPdxB0.5 compounds revealed the appearance of ½ ½ ½-type superstructures and satellite reflections with respect to the parent AuCu3 structure. Whereas the ½ ½ ½-type superstructure is confined to the Rh-rich part of the solid solution, satellite reflections are observed throughout the solid solution and hint towards the existence of a domain structure enclosed by anti-phase boundaries.

Characterization and activity study of the Rh-substituted pyrochlores for CO2 (dry) reforming of CH4

Isomorphic substitution of Rh at varying levels on the B site of lanthanum zirconate pyrochlore (La2Zr2O7; designated LZ) resulted in the formation of thermally stable catalysts suitable for fuel reforming reactions operating at 900 °C. Three specific catalysts are reported here: (a) unsubstituted lanthanum zirconate (LZ), (b) LZ with 2 wt% substituted Rh (L2RhZ), and (c) LZ with 5 wt% substituted Rh (L5RhZ). These catalysts were characterized by XRD, XPS, and H2-TPR. XRD of the fresh, calcined catalysts showed the formation of the pyrochlore phase (La2Zr2O7) in all three materials. In L5RhZ, the relatively high level of Rh substitution led to the formation of LaRhO3 perovskite phase which was not observed in the L2RhZ and LZ pyrochlores. TPR results show that the L5RhZ consumed 1.57 mg H2/gcat, which is much greater than the 0.508 H2/gcat and 0.155 mg H2/gcat for L2RhZ and LZ, respectively, suggesting that the reducibility of the pyrochlore structure increases with increasing Rh-substitution. DRM was studied on these three catalysts at three different temperatures of 550, 575, and 600 °C. The results showed that CH4 and CO2 conversion was significantly greater for L5RhZ compared to L2RhZ and no activity was observed for LZ, suggesting that the surface Rh sites are required for the DRM reaction. Temperature programmed surface reaction showed that L5RhZ had light-off temperature 80 °C lower than L2RhZ. The spent catalysts after runs at each temperature were characterized by temperature programmed oxidation (TPO) followed by temperature programmed reduction and XRD. The TPO results showed that the amount of carbon formed over L5RhZ is almost half of that formed on L2RhZ.

New Supramolecular Structural Motif Coupling a Ruthenium(II) Polyazine Light Absorber to a Rhodium(I) Center

Two new complexes, [(bpy)2Ru(dpp)Rh(I)(COD)](PF6)3 and [(Me2bpy)2Ru(dpp)Rh(I)(COD)](PF6)2(BF4) (bpy = 2,2'-bipyridine, Me2bpy = 4,4'-dimethyl-2,2'-bipyridine, dpp = 2,3-bis(2-pyridyl)pyrazine, and COD = 1,5-cyclooctadiene), representing a new Ru(II),Rh(I) structural motif, have been prepared and characterized by mass spectrometry, (1)H NMR spectroscopy, electrochemistry, electronic absorption spectroscopy, and emission spectroscopy. These two complexes represent a new type of supramolecular complex with a [(TL)2Ru(dpp)](2+) (TL = terminal ligand) light absorber (LA) coupled to a Rh(I) center and are models for Ru(II),Rh(I) intermediates in the photochemical reduction of water using dpp-bridged Ru(II),Rh(III) photocatalysts. Electrochemical study reveals overlapping reversible Ru(II/III) and irreversible Rh(I/II/III) oxidations and a quasi-reversible dpp(0/-) reduction, demonstrating that the lowest unoccupied molecular orbital (LUMO) is dpp(π*) based. The COD ligand is sterically bulky, displaying steric repulsions between hydrogen atoms on the alkene of COD and dpp about the square planar Rh(I) center. An interesting reactivity occurs in coordinating solvents such as CH3CN, where Rh(I) substitution leads to an equilibrium between the Ru(II),Rh(I) bimetallic and [(TL)2Ru(dpp)](2+) and [Rh(I)(COD)(solvent)2](+) monometallic species. The electronic absorption spectra of both complexes feature transitions at ca. 500 nm attributed to a Ru(dπ) → dpp(π*) metal-to-ligand charge transfer (MLCT) transition that is slightly red-shifted from the Ru synthon upon Rh(I) complexation. The methylation of TL on the Ru impacts the electrochemical and optical properties in a minor but predictable manner. The photophysical studies, by comparison with the model complex [{Ru(bpy)2}2(dpp)](PF6)4 and related Rh(III) complex [(bpy)2Ru(dpp)Rh(III)Cl2(phen)](PF6)3, reveal the expected absence of a Ru(dπ) → Rh(dσ*) (3)MMCT state (metal-to-metal charge transfer) in the title complexes, which is present in Rh(III) systems. The absence of this (3)MMCT state in Ru(II),Rh(I) complexes results in a longer lifetime and higher emission quantum yield for the Ru(dπ) → dpp(π*) (3)MLCT state than [(bpy)2Ru(dpp)Rh(III)Cl2(phen)](PF6)3. Both complexes display photocatalytic hydrogen production activity in the presence of water and a sacrificial electron donor, with the [(bpy)2Ru(dpp)Rh(I)(COD)](PF6)3 possessing a higher catalytic activity than the methyl analogue. Both display low activities, hypothesized to occur due to steric crowding about the Rh(I) site.

Effect of Pd for Rh substitution on thermoelectric power in the semimetallic compound URhGa5

Abstract Single crystals of URhGa 5 and URh 0.97 Pd 0.03 Ga 5 grown by the self-flux method were examined by X-ray diffraction, thermoelectric power, thermal and electrical conductivity, magnetization, and specific heat measurements. The giant thermoelectric peak exceeding 250 μV/K at 15 K observed for just grown URh 0.97 Pd 0.03 Ga 5 crystals decreased drastically with ageing. Three percent palladium for Ru substitution in URhGa 5 weakly changes the thermoelectric power beyond the peak area but substantially increases the electronic specific heat and magnetic susceptibility. The results of the diffusion thermoelectric power calculation, based on the electron structure and the Fermi surface literature data, are consistent with the determined data for URhGa 5 crystal and its URh 0.97 Pd 0.03 Ga 5 derivative.

Change in Unusual Magnetic Properties by Rh Substitution in CeRu2Al10

We have studied the Rh substitution (electron doping) effect in the Kondo semiconductor CeRu2Al10, which orders antiferromagnetically below (\(T_{0}=27\) K), by specific heat \(C\), magnetic susceptibility χ, electrical resistivity ρ, and Hall resistivity ρ\(_{\text{H}}\) measurements of Ce(Ru1-xRhx)2Al10 (\(x=0.12\), 0.2, 0.23, 0.34) single crystals. \(T_{0}\) decreases monotonically with an increase in \(x\). In addition, new anomalies occur at \(T_{1}\sim 6.5\) and \(T_{2}\sim 3.0\) K in all the Rh substitution samples. However, \(T_{1}\) and \(T_{2}\) are almost independent of \(x\). The shape of the anomaly at \(T_{0}\) in χ\((T)\) changes with Rh substitution, which implies the reorientation of the ordered moment. These phenomena indicate that the magnetic properties of CeRu2Al10 are strongly associated with the number of \(4d\) electrons.

Enhanced Thermoelectric Performance and Room-Temperature Spin-State Transition of Co4+ Ions in the Ca3Co4–xRhxO9 System

The effects of Rh doping on the structural, magnetic, electrical, and thermal transport properties of Ca3Co4–xRhxO9 (0 ≤ x ≤ 0.4) samples have been investigated systematically. XRD and XPS results show that the doped Rh ions are in the form of Rh3+. Only a metal–insulator transition (MIT) and an anomaly of the slope related to the transition from a Fermi liquid to an incoherent metal at low temperatures were observed in the resistivity curve for the undoped sample. As Rh ions were doped into the samples, an additional anomaly and MIT occurred in the resistivity curve near room temperature, which are suggested to originate from the spin-state transition (SST) of Co ions. The low-temperature MIT temperature increased with increasing Rh-doping content, indicating that the spin-density-wave state became stable as a result of the enhanced random Coulomb potential in CoO2 octahedral block layers induced by Rh substitution. Based on an analysis of the thermopower and XPS data, Rh3+ ions are suggested to substitute at the Co3+ sites of CoO2 layers. The substitution induced a partial SST of Co4+ ions from the low-spin to the high-spin state, leading to the formation of a spin-state polaron. The evolution of the electrical and magnetic properties with Rh doping is summarized in a single phase diagram for Ca3Co4–xRhxO9. It should be noted that the thermopower of the system did not change obviously with Rh doping, but the thermal conductivity decreased significantly. As a result, the ZT value increased markedly with increasing Rh-doping content. The ZT value at room temperature for Ca3Co3.6Rh0.4O9 reached 0.014, which is about 2.4 times larger than that of Ca3Co4O9. The results show that Rh doping might be an effective route to improving the thermoelectric performance of the Ca3Co4O9 system.

Marked Change in the Ground State of CeRu2Al10Induced by Small Amount of Rh Substitution

We have investigated the marked change in the magnetic properties of CeRu2Al10 induced by a small amount of Rh substitution [Ce(Ru0.95Rh0.05)2Al10]. The magnetic susceptibility along the \(a\)-axis (χa) shows a large decrease with decreasing temperature after showing a sharp peak at \(T_{0}=24\) K and the high field magnetization for the magnetic field along the \(a\)-axis exhibits a spin-flop transition at \({\sim}13\) T. These indicate that the antiferromagnetic (AFM) order where the AFM moment is parallel to the \(a\)-axis takes place, which is consistent with the anisotropy of χa>χc>χb in the paramagnetic region, being different from the AFM order in CeRu2Al10. These results were analyzed by mean field calculation for the two-sublattice model using the crystalline electric field level scheme by Strigari et al. [Phys. Rev. B 86 (2012) 081105]. On the basis of both experimental and calculated results, we conclude that, by a small amount of Rh substitution with one extra \(4d\) electron, the large \(c\)–...

Effect of substituting elements on hydrogen uptake for Pd–Rh–H and Pd–Ag–H systems evaluated by magnetic susceptibility measurement

The magnetic susceptibility and the pressure-composition isotherm were measured simultaneously for Pd–Rh–H and Pd–Ag–H systems in order to clarify the effect of Rh or Ag substitution on the hydrogen uptake from viewpoint of the electronic band structure. The magnetic susceptibility of all Pd binary alloys prepared decreased monotonically with increasing hydrogen content. At high hydrogen contents, the magnetic susceptibility became approximately zero for Pd–Rh–H and Pd–Ag–H system, and the hydrogen content at which the magnetic susceptibility gives zero corresponded with the terminal of the plateau region in the isotherm curve. The results indicated that the magnetic susceptibility of hydride phase was almost zero for all Pd binary alloys. On the basis of the band structure of Pd metal, we concluded that atom substitution only affected shift of the energy at Fermi level, and the amount of the hydrogen uptake was dominated by the number of unoccupied d-band in the alloys.

Increasing Curie temperature in tetragonal Mn2RhSn Heusler compound through substitution of Rh by Co and Mn by Rh

The tetragonal Mn2RhSn Heusler compound shows better lattice match with MgO than do Mn3−xGa and tetragonal Mn3−xCoxGa Heusler compounds and hence is better suited for spin transfer torque applications. Beside the improved lattice match, this compound shows a low saturation magnetic moment reduces the current to switch which makes it more relevant for application in spin transfer torque devices. This compound shows a low Curie temperature; introducing Co into this system and increasing the Rh content leads to an increase in the Curie temperature. Doping with Co retains the tetragonal structure, with improved lattice match with MgO and low magnetic moment, intact up to x = 0.6, although doping with Rh changes the structure from tetragonal to cubic.

Electron spin resonance of the Yb 4f‐moment in Yb(Rh1−xFex)2Si2

AbstractThe electron spin resonance (ESR) signal of the heavy‐fermion metal YbRh2Si2 is investigated in terms of its dependence on the partial substitution of Rh by Fe. This non‐isovalent substitution gives rise to a chemical pressure and also leads to a reduction of the conduction electron density of states (hole doping). The values and anisotropy of the ESR parameters are comparable with those of Yb(Rh1−x Cox)2Si2 for Fe‐contents up to at least 10%. This result provides additional evidence that the Kondo interaction allows narrow ESR spectra in a heavy fermion compound. The absence of an ESR signal for 30% Fe content points towards a distribution of different crystal electric field symmetries at the Yb‐sites, leading to a dominating disorder‐related broadening.

Structure and thermal stability of nanocrystalline Ce1−xRhxO2−y in reducing and oxidizing atmosphere

Nanocrystalline (4–5nm) Ce1−xRhxO2−y mixed oxides (x=0.03–0.21) were synthesized using water in oil microemulsion method. Morphology, microstructure and phase composition of the samples subjected to heat treatment in oxidizing and reducing atmosphere were investigated by TEM, SEM-EDS, XRD, Raman and IR spectroscopy. Samples with x≤0.16 were structurally stable in oxidizing atmosphere up to 850°C. Above this temperature the samples decomposed into Rh deficient, nanosized Ce1−xRhxO2−y and large (few μm) Rh2O3 crystals. In hydrogen atmosphere segregation of small (1–2nm) Rh crystallites in special epitaxial orientation Rh (111)||CeO2 (111) started at 500°C. The epitaxial orientation of small Rh crystallites was preserved up to 1000°C indicating their strong bonding to the ceria surface. Partial substitution of Rh for Ce strongly enhanced reducibility of ceria at low temperatures (below 200°C) and hindered the sintering of ceria at high temperatures. Ce0.89Rh0.11O2−y shows an interesting property of reversible extraction–dissolution of Rh upon reduction–oxidation cycles at 500°C which is important for potential catalytic applications.