Rare-earth Carbides Research Articles

La3Cl3BC – Structure, Bonding and Electrical Conductivity

A new rare earth carbide boride halide, La3Cl3BC, has been prepared by heating a mixture of stoichiometric quantities of LaCl3, La, B and C at 1050 °C for 10 days. La3Cl3BC (La3Br3BC type) crystallizes in the monoclinic system with space group P21/m (No. 11), a = 8.2040(16), b = 3.8824(8), c=11.328(2)Å , β =100.82(3)°. In the structure, monocapped trigonal prisms containing B-C units are condensed into chains along the b direction, and the chains are further linked by Cl atoms in the a and c directions. The condensation results in a polymeric anion 1 ∞[BC] with a spine of B atoms in a trigonal prismatic coordination by La, and the C atoms attached in a square pyramidal coordination. The B-B and B-C distances are 2.16 and 1.63 Å , respectively. La3Cl3BC is metallic. The EH calculation shows that the distribution of valence electrons can be formulated as (La3+)3(Cl−)3(BC)5− · e−.

Encapsulation of radioactive metals inside multilayered polyhedral shells of carbon as a barrier to radionuclide release

A carbon nanoencapsulate has a polyhedral outer shell of nested, concentric layers of carbon. The shell defines an internal cavity where a metal is encapsulated. Although the rare-earth carbides readily hydrolyze in moist air, the carbides in these carbon shells did not degrade after exposure to air for considerable lengths of time. This means that the carbide particle is physically enclosed within the carbon cavity completely, and the cavity protects it perfectly against attack of water molecules. Considering intrinsic chemical stability of carbon under oxygen free condition, this structure may be a perfect barrier to extremely long-term release of radionuclides. Because encapsulation of LaC 2 within carbon nanoparticles increased drastically from by-product to major product, it would be possible to find the optimized condition that complete encapsulation is achieved. Intrinsic stability of carbon and carbon coated waste nanoparticles may provide an improved barrier to radionuclide release by groundwater.

Superconductivity and magnetoresistance in unusual layered rare earth halides and rare earth carbides

Abstract We have been exploring routes and systems alternative to the oxocuprates and the oxomanganates and searched for superconductivity and large magnetoresistance in rare earth compounds, particularly, in the family of layered metallic rare earth halides and rare earth carbides. Here we review the superconducting properties of YC2, La2C3, and RE2C2X2 (RE=Y, La; X=Cl, Br, I) as well as some recent results of the large magnetoresistance effect in the ferromagnet GdI2.

Phase evolution, magnetic properties, and coercivity mechanism of melt-spun Pr 2Fe 14(C, B)/α-Fe-type ribbons

High-performance Pr 2Fe 14(C, B)/α-Fe-type nanocomposites have been prepared by a melt spinning technique. The phase transformations, magnetic properties, and coercivity mechanism of Pr 11− x DyFe 72+ x Co 10C 4B 2 ( x=0–3) ribbons have been investigated. All ribbons are comprised of the 2:14:1, 2:17, α-Fe phases and a small amount of 1:2 paramagnetic rare-earth carbides. The volume fraction of the α-Fe phase increases with increasing x, which leads to a fast decrease of coercivity of the ribbons. Intergranular exchange coupling effect is pronounced at low x ( x⩽2), and diminishes with further increase in x. Consequently, the B r of the ribbons exhibits the same trend. A sample with the optimum values of B r=9.4 kG, i H c=10.8 kOe and ( BH) max=16 MGOe, respectively, has been obtained in Pr 10DyFe 73Co 10C 4B 2 (i.e. x=1) ribbons. Moreover, it has been found that the magnetization reversal of Pr 2Fe 14(C, B)/α-Fe-type nanocomposites varies with the increase of x.

Pressure dependence of TC of the layered superconductor Y2C2I2: lattice versus electronic effects

The response to application of pressure of the crystal structure of the 10K layered rare-earth carbide halide superconductor Y2C2I2 is studied. We find an anisotropic response with largest compression perpendicular to the metal atom layers. From the decrease of the cell volume with pressure the equation of state is established using a bulk modulus of 37(1)GPa. It is concluded that the large increase of TC with pressure (1/TCdTC/dP=0.13(1)GPa−1) is due to a growth of the electronic density of states with pressure for which a value of 1/N(EF)dN(EF)/dP to ≈0.12 states/eVGPa is estimated.

Electronic properties and Raman spectra of rare-earth carbide halides investigated from first principles

${\mathrm{Y}}_{2}{\mathrm{C}}_{2}{X}_{2}$ $(X=\mathrm{Br},\mathrm{I})$ are layered metals, where superconductivity up to 11.6 K has been found. We have investigated these materials by means of first-principles calculations within the full-potential linearized augmented plane wave method based on density functional theory. In particular, we report electronic densities of states (DOS) and electronic band structures for the experimental geometry as well as for the geometry optimized structures. We discuss the peak in the DOS at the Fermi energy that has been suggested to be responsible for the ${T}_{c}$ variation in the ${\mathrm{Y}}_{2}{\mathrm{C}}_{2}(\mathrm{Br},\mathrm{I}{)}_{2}$ system when applying pressure or changing the bromine to iodine ratio. Moreover, we study in detail all Raman-active modes of ${\mathrm{Y}}_{2}{\mathrm{C}}_{2}{\mathrm{Br}}_{2}$ and ${\mathrm{Y}}_{2}{\mathrm{C}}_{2}{\mathrm{I}}_{2},$ where the phonon frequencies are determined within the frozen-phonon approach by carrying out total-energy and atomic-force calculations. The excellent agreement of theoretical and experimental vibrational frequencies together with the calculated Raman intensities allow for a clear assignment of all measured Raman-active modes to ${A}_{g}$ and ${B}_{g}$ symmetry.

Rare earth doped new cemented carbide with Ni-Co binder for mining

A method, doping WC with La(NO3)3, of producing cemented carbide with rare earth was introduced. The effects of lanthanum on the mechanical properties and microstructure of WC-9(Co-75%Ni) cemented carbide were studied and a rock drilling experiment was carried out. The experimental results show that both transverse rupture strength (TRS) and hardness of WC-9(Co-75%Ni) rare earth cemented carbides can match that of WC-9Co cemented carbide, when La2O3/(Co + Ni) ratio is 0.3%, the abnormal growth of WC grain in the cemented carbide can be restrained effectively, the homogeneity of grain size in microstructure and the wear resistance are improved, which can be matchable to that of WC-9Co cemented carbide for mining.

Toward Efficient Synthesis of Endohedral Metallofullerenes by Arc Discharge of Carbon Rods Containing Encapsulated Rare Earth Carbides and Ultrasonic Soxhlet Extraction

We have modified the conventional procedures for endohedral metallofullerene production by (1) arc discharge of carbon rods containing carbon-coated rare-earth carbides and (2) ultrasonic Soxhlet extraction (USE) of the raw soots. The enhanced efficiency of endohedral metallofullerene production due to the modifications above is demonstrated by high-performance liquid chromatography (HPLC) through the synthesis of the rare-earth mono-metallofullerenes Er@C82 and Pr@C82.

Raman Scattering Studies of the Carbon Atom Vibrations in Superconducting Rare-Earth Carbide Halides RE2C2(X,X′)2 (RE = Y, Gd; X, X′ = Br, I)

The Raman spectra of the layered superconductors RE2C2(X,X′)2 (RE = Y, Gd; X, X′ = Br, I) were measured on single-crystal and polycrystalline specimens using a micro-Raman technique. Low energy modes (ω < 300 cm—1) correspond to mixed vibrations of rare-earth and halogen atom sublattices. Two high energy modes at ≈︂ 400 and ≈︂ 1600 cmm1) can be assigned to the tilting and stretching vibrations of the carbon C2 units. These carbon-related phonons were identified by analyzing the isotope shifts of the Raman lines in samples in which the natural carbon was replaced by isotope-enriched 13C. The energy of the stretching vibration is independent of the particular host compound while the energy of the tilting vibration correlates with the size of the rare-earth atom cage that surrounds the carbon atoms.

Raman Scattering Studies of the Carbon Atom Vibrations in Superconducting Rare-Earth Carbide Halides RE2C2(X,X?)2 (RE = Y, Gd; X, X? = Br, I)

Raman Scattering Studies of the Carbon Atom Vibrations in Superconducting Rare-Earth Carbide Halides RE2C2(X,X?)2 (RE = Y, Gd; X, X? = Br, I)

Neutron diffraction and micro-Raman scattering studies on rare-earth carbide halides

Neutron-diffraction experiments on powder samples and micro-Raman scattering investigations on single crystals of the layered compounds ${R}_{2}{\mathrm{C}}_{x}{\mathrm{Hal}}_{2},$ $(R=\mathrm{Y},$ Gd, $x=1,2,$ and $Hal=\mathrm{Br},$ I) have been performed in order to study their static and dynamic lattice properties. For the superconductors ${\mathrm{Y}}_{2}{\mathrm{C}}_{2}{\mathrm{I}}_{2}$ ${(T}_{\mathrm{c}}=9.97\mathrm{K})$ and ${\mathrm{Y}}_{2}{\mathrm{C}}_{2}{\mathrm{Br}}_{2}$ ${(T}_{\mathrm{c}}=5.04\mathrm{K}),$ the C-C atomic distances were obtained with high accuracy from neutron-diffraction experiments between $T=1.5$ and 270 K. The expected Raman-active phonons were determined from a factor-group analysis of the crystal structures. In the monocarbide ${\mathrm{Y}}_{2}{\mathrm{CBr}}_{2},$ the Raman-active phonons of the heavy-ion sublattices have been observed. In the dicarbide compounds ${R}_{2}{\mathrm{C}}_{2}{\mathrm{Hal}}_{2},$ additionally, the stretching and tilting modes of the dimeric ${\mathrm{C}}_{2}$ units were clearly identified by analyzing spectra from ${}^{\mathrm{nat}}\mathrm{C}$ and ${}^{13}\mathrm{C}$ substituted samples. The influence of the quasimolecular ${\mathrm{C}}_{2}$ unit on the electronic properties in the ${R}_{2}{\mathrm{C}}_{2}{\mathrm{Hal}}_{2}$ compounds and its interaction with the surrounding metal atom octahedra is discussed.

Electronic state and magnetism of the rare-earth carbide GdCx (x ≅ 0.33)

The electronic state and the magnetism of GdCx (x = 0.33) were investigated by the discrete variational (DV)-Xα calculation method together with those of Gd metal. Magnetic moments on the gadolinium atoms located at the center of the Gd17 and Gd4C2 clusters extracted from Gd metal and GdtC were calculated to be 8.38 and 9.70 μB, in qualitative agreement with the experimental values (7.96 and 8.28 μB respectively). The exchange splitting of the Gd 4f band in Gd3C was found to increase from that of Gd metal and accounts for the higher Curie temperature of GdC1 compounds observed in experiments. The calculated results show that the carbon atoms are charge acceptors in these compounds. The higher Curie temperature values (Tc = 350 K) observed in the GdCx (x ≅ 0.33) compounds compared with Gd metal (Tc = 293 K) are discussed on the basis of the spin-fluctuation theory.

Magnetic phase transition in the metal-rich rare-earth carbide halides Gd2XC (X= Br,I).

The critical behavior of the ferromagnetic-paramagnetic phase transition of metal-rich rare-earth carbide halides ${\mathrm{Gd}}_{2}$BrC and ${\mathrm{Gd}}_{2}$IC is investigated. These compounds exhibit a layered structure of Gd-C-Gd slabs which are sandwiched between layers of halogen atoms (Br,I). The critical exponents \ensuremath{\beta}, \ensuremath{\gamma}', \ensuremath{\gamma}, and \ensuremath{\delta} are determined by several methods: modified Arrott plots, Kouvel-Fisher plots, scaling plots, and lnJ vs ln(${\mathrm{\ensuremath{\mu}}}_{0}$H) plots. The asymptotic critical exponents \ensuremath{\beta} and \ensuremath{\gamma} as well as the critical amplitudes ${\mathit{J}}_{0}$ and ${\mathit{h}}_{0}$/${\mathit{J}}_{0}$ were determined from both the Kouvel-Fisher and the correction to scaling analysis. The derived critical exponents are identical for all methods and in good agreement with the predictions of the three-dimensional Heisenberg model whereas the reduced critical amplitudes ${\mathit{J}}_{0}$/${\mathit{J}}_{\mathit{S}}$(0), \ensuremath{\mu} ${\mathit{h}}_{0}$/${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{c}}$, and ${\mathit{DJ}}_{0}^{\mathrm{\ensuremath{\delta}}}$/${\mathit{h}}_{0}$ cannot be related to a particular model. The temperature dependence of the Kouvel-Fisher exponent \ensuremath{\gamma}(T) is discussed with respect to chemical disorder.

Synthesis and Magnetic Properties of NdFe10Mo2Cx and NdFe10Mo2CxNy Using NdC2 as a Starting Material

Abstract Interstitial rare earth carbide (NdFe10Mo2Cx) and carbonitride (NdFe10Mo2CxNy) have been prepared by a novel preparation method, which comprises the replacement of a partial amount of Nd metal with NdC2 up to the carbon content of x &amp;lt; 1. Temperature dependence of their magnetization characteristics fairly agreed with those of NdFe10Mo2Cx and NdFe10Mo2CxNy obtained by a conventional metallurgical technique and/or a subsequent plasma nitriding. The results demonstrated that these interstitial rare earth compounds are produced in much reduced cost compared with that for the conventional one using Nd metal.

Magnetic properties of carbon-coated rare-earth carbide nanocrystallites produced by a carbon arc method

Carbon-coated gadolinum and holmium carbide nanocrystallites have been generated using a modification of the Huffman-Krätschmer carbon arc process. Bulk amounts of these particles were isolated from the other by-products using a magnetic field gradient. Transmission electron microscopy revealed the presence of 10–50 nm diameter crystallites coated with numerous graphitic layers. The nanocrystallite phases were identified as Gd2C3 and Ho2C3, respectively, by x-ray and electron diffraction. Magnetization measurements were performed using a superconducting quantum interference device magnetometer between ±5 T at temperatures ranging from 4 to 200 K. The magnetization curves were shown to scale as a function of H/T. The RE3+ sites in RE2C3 have C3 site symmetry. For Gd2C3 the universal curve was fit with a Brillouin function consistent with the Gd3+ free-ion ground-state values of J=7/2 and g=2. The 5I8 Ho3+ free-ion ground state is split, presumably due to a C3 symmetry crystal field. Consequently, for Ho2C3 the Ho3+ free-ion parameters could not be used to fit the experimental data. Empirical fits to the Brillouin function yield a reduced moment of 7.5μB, compared to the free-ion value of 10.6μB. A similarly reduced moment was observed in holmium-containing endohedral fullerenes.

Standard enthalpies of formation of some rare earth carbides by direct synthesis calorimetry

The standard enthalpies of formation of carbides in the binary systems Sc-C, Y-C and La-C have been determined by direct synthesis calorimetry at 1473 ± 2 K. The following values are reported: ΔH f 0(Sc 2C) = −45.5 ± 1.6 kJ (g atom) −1; ΔH f 0(Y 2C) = −31.8 ± 1.3 kJ (g atom) −1 and ΔH f 0 (LaC 2) = −27.2 ± 1.4 kJ (g atom) −1. The results are compared with earlier experimental data and with values predicted from Miedema's semi-empirical model. The enthalpies of formation for the carbides considered are also compared with the corresponding data for silicides and germanides.

Synthesis of Sm2Fe17Cx via the Arc Melting of Sm, SmC2, and Fe

Abstract An interstitial rare earth carbide Sm2Fe17Cx (x &amp;lt; 1) has been prepared from Sm metal, SmC2, and Fe by a cast method, and the magnetic property was investigated. The magnetization characteristics fairly agreed with those of the Sm2Fe17Cx compound obtained by the conventional method, and the result demonstrated that Sm metal can be partially replaced by SmC2. However, Sm2Fe17Cx hardly formed without some amounts of Sm metal and the sample with x &amp;gt; 1 was a mixture of α-Fe and an unknown phase.

Structure and magnetic ordering of holmium carbide fluoride, Ho 2CF 2

The preparation of a new rare earth carbide fluoride, Ho 2CF 2, is described. Ho 2CF 2 is trigonal, space group P 3m1 , Z=1, with a=3.6567(2) A ̊ and c=6.3171(5) A ̊ at room temperature. On cooling below the Néel temperature of 3.7 K, Ho 2CF 2 transforms to an antiferromagnet with magnetic space group P a 1′ . The phase transition from the paramagnetic to antiferromagnetic state is accompanied by a distortion of the lattice to P 1 due to magnetoelastic effects. The nuclear and magnetic structures have been fully determined from high resolution powder neutron diffraction measurements at 1.5 and 50 K. Evidence for critical scattering above T N is shown from specific heat, magnetic susceptibility and temperature-dependent neutron diffraction studies. Electrical conductivity measurements show that Ho 2CF 2 is an insulator. The relationship between the low dimensionality of the structure and the magnetic and physical properties of Ho 2CF 2 is discussed.

An odyssey through the rare earths. A quest for knowledge

Research highlights on rare earth materials are described from the earliest days as a graduate student during the mid 1950s, through the six years at Los Alamos, to the last 28 at Iowa State University. The early work was concerned with the rare earth carbides which led to an appreciation of systematics of the properties of rare earth materials. Use of the systematic variation of properties and behaviors is a powerful tool in understanding the nature of certain phenomena - in particular solid solution formation and the role of 4f electrons in bonding (4f hybridization). Some of the anomalous 4f properties of cerium metal and cerium compounds, and the quenching of spin fluctuations in exchanged enhanced materials, are also discussed.

Magnetic properties of the new rare earth carbide fluoride layered compound: Ho 2CF 2

We present magnetic susceptibility, specific heat and neutron diffraction data of the new layered rare earth carbide flouride Ho 2CF 2. The susceptibility displays a rounded maximum centered around 4.6 K and a Curie-Weiss law at higher temperatures with a paramagnetic Curie temperature of -4.9(2) K. The specific heat shows a sharp anomaly at 3.61(5) K indicating the onset of long range ordering but also a considerable high temperature tail pointing to low dimensional magnetic behaviour. Low temperature neutron powder diffraction reveals additional magnetic Bragg peaks with significant critical scattering remaining well above the long range ordering temperature.