Rare-earth Hexaborides Research Articles

Selective Area Laser Induced Deposition of Metal Boride Thin Films

ABSTRACTWe have developed a novel technique for the selective area deposition of rare earth hexaborides: laser-induced solution deposition (LISD). This technique is both simple and efficient and combines many advantages of both chemical vapor deposition and electrolytic deposition. The results of LISD deposition show that the polycrystalline thin films of rare earth hexaborides and sub-borides such as MB6, MB4, and MB2 (M = Gd, La) are formed through the light initiated chemical reaction of nido-decaborane (B10H14) and rare earth chloride in solution. These films grow with a strong texture growth axis and morphology that is dependent both on the selection of solvents and laser wavelengths and power used in LISD.

Selective area laser-induced deposition of rare earth hexaborides from solution

We have developed a novel technique for the selective area deposition of rare earth hexaborides: laser-induced solution deposition (LISD). This technique is both simple and efficient and combines many advantages of both chemical vapor deposition and electrolyses deposition. We show that the deposition of polycrystalline thin films of rare earth hexaborides and sub-borides such as GdB 6, GdB 4, and GdB 2 is possible through the light initiated chemical reaction of nido-decaborane (B 10H 14) and gadolinium chloride in solution. These films grow with a strong texture axis and a morphology that is dependent both on the choice of solvents and wavelengths used in LISD.

Optical conductivity of the Kondo insulatorYbB12: Gap formation and low-energy excitations

Optical reflectivity experiments have been conducted on single crystals of the Kondo insulator ${\mathrm{YbB}}_{12}$ in order to obtain its optical conductivity, \ensuremath{\sigma}(\ensuremath{\omega}). Upon cooling below 70 K, a strong suppression of \ensuremath{\sigma}(\ensuremath{\omega}) is seen in the far-infrared region, indicating the opening of an energy gap of $\ensuremath{\sim}25\mathrm{meV}.$ This gap development is coincident with a rapid decrease in the magnetic susceptibility, which shows that the gap opening has significant influence on magnetic properties. A narrow, asymmetric peak is observed at $\ensuremath{\sim}40\mathrm{meV}$ in \ensuremath{\sigma}(\ensuremath{\omega}), which is attributed to optical transitions between the Yb $4f$-derived states across the gap. In addition, a broad peak is observed at $\ensuremath{\sim}0.25\mathrm{eV}.$ This peak is attributed to transitions between Yb $4f$-derived states and the $p\ensuremath{-}d$ band, and is reminiscent of similar peaks previously observed for rare-earth hexaborides.

Neutron-scattering study of DyB 6

We report the first neutron-diffraction experiment on heavy rare-earth hexaboride DyB 6. The results indicate that the structural phase transition occurs at 30 K and the antiferromagnetic order at T N = 26 K. Magnetic structure below T N is found to be triple- k spin structure with the propagation vector k = < 1 4 − 1 4 1 2 > .

Spectroscopy of the rare-earth hexaborides: From correlation gaps to colossal magnetoresistance

We present a temperature- and field-dependent Raman scattering study of the “metal-semiconductor” transitions in SmB 6 and EuB 6. Below a characteristic temperature T ∗ ∼ 50 K , the Raman scattering spectrum of SmB 6 is characterized by (i) an abrupt redistribution of electronic scattering intensity across a temperature-independent energy, Δ c, reflecting the opening of a pseudo-gap, and (ii) the appearance of several in-gap resonances which appear to be crystal-electric-field transitions of the Sm 3+ ion which are split by local lattice distortions. By contrast, the metal-semiconductor (MS) transition in EuB 6 manifests itself as a change from a diffusive electronic scattering response in the high-temperature paramagnetic phase, to a flat continuum scattering response in the low-temperature ferromagnetic metal phase which is characteristic of a strongly-correlated metal. Most interesting is evidence that the MS transition in EuB 6 is precipitated by the formation of bound magnetic polarons.

Magnetic and structural phase transition in heavy rare-earth compound DyB 6

We report the neutron diffraction experiment on heavy rare-earth hexaboride DyB 6. The results indicate that the structural phase transition occurs at 32 K, and the antiferromagnetic order at T N = 26 K. The propagation vector of the spin structure is found to be k = 〈 1 4 1 4 1 2〉 and triple- k spin structure is proposed.

Borides in Thin Film Technology

The borides of transition and rare-earth metals are considered for application as wear- and corrosion-resistant, decorative or thermionic coatings. After a review of physical vapor deposition (PVD) techniques used for the deposition of these coatings, a survey of investigations to apply these coatings is given. As a result of the strong directionality of covalent boron–boron bonds, boride coatings show an increasing tendency to amorphous film growth with increasing B/Meatomic ratio and, for rare-earth hexaborides, with decreasing metallic radius of the rare-earth metal. Mechanical and optical properties are strongly influenced by the crystallographic structure of the boride phase. Because of their high hardness combined with good adhesion, crystalline films based on the diborides of transition metals seem to be promising candidates for wear resistant coatings on cutting tools. Alloying of these films with nitrogen by reactive PVD processes results in the formation of extremely fine-grained multiphase hard coatings with excellent tribological and corrosion behavior, thus offering new applications in the coating of engineering components. Because of their distinct colorations, some of the hexaborides of rare-earth elements may be used as decorative coatings on consumer products like wristwatch casings or eyeglass frames. Another promising field is the development of thermionic coatings based on rare-earth hexaborides, which may offer the possibility of the production of inexpensive and simple high emission filaments.

Mean-square displacements of atoms in hexaborides

High-resolution neutron powder and single-crystal X-ray diffraction studies has been used to investigate the temperature dependencies of atomic mean-square displacements in rare-earth hexaborides. The experimental data on elastic constants and phonon frequencies have been used to improve the valence force field parameters for SmB 6 and the temperature dependencies of atomic mean-square displacements in SmB 6 is compared with theoretical values. The correlation between ionic radii and mean-square displacements of rare-earth atoms are shown.

Effect of pressure on the Néel temperature of DyB 6

Electrical resistivity ϱ( T) of the rare-earth hexaboride DyB 6, which shows an antiferromagnetism below 25 K (= T N) and an another phase transition at 30 K ( = T ∗ ), has been measured under high pressure up to 2.2 GPa and in high magnetic fields to 5 T. It is found that T N increases with increasing pressure with the rate d T N/d P = 2.6 K/GPa. A small anomaly in ϱ( T) near T ∗ disappears by applying pressure. The metamagnetic transition field H M (=4.13 T at 4.2 K) increases with pressure. These facts indicate that the antiferromagnetic interaction of 4f spin in DyB 6 is enhanced by pressure.

Structure and properties of decorative rare-earth hexaboride coatings

Hexaborides of the rare-earth elements are considered for applications as wear- and corrosion-resistant hard coatings for decoration of consumer products such as eye-glass frames and wristwatch casings. After a review of physical vapour deposition (PVD) methods used for the deposition of rare-earth hexaborides, a survey of studies is given that extends the range of suitable decorative coating materials. Within the scope of the search for new colours, the interdependence between deposition conditions, microstructure and mechanical and optical properties of boride coatings based on LaB 6, CeB 6, SmB 6 and YB 6 is presented and discussed. As a result of the strong directionally of covalent boron—boron bonds, hexaboride coatings show an increasing tendency to amorphous film growth with decreasing metallic radius of the rare-earth metal. Mechanical as well as optical properties are strongly influenced by the crystallographic structure of the hexaboride phase. The most pronounced visual colour impression is observed for LaB 6-based coatings corresponding to a dark violet colour. Proper selection of process parameters for these coatings may widen the conventional spectrum of decorative coatings.

Dense Kondo behavior and many other effects in metallic point-contacts of rare-earth hexaborides

Point-contact spectra of rare-earth (La, Ce, Gd, Tb, and Dy) hexaborides are reported and compared with their electrical resistivity. Two important points are discussed as an experimental guide of point-contact spectroscopy (PCS). One is the difference between the PCS and a tunneling experiment. The other is the problem of reproducibility, i.e., differences in the observed spectra between different contacts. Examples of these problems are discussed for the PCS of rare-earth hexaboride homocontacts. Based on the above discussions the dense Kondo behavior of CeB 6 is lastly discussed.

Photoemission, Inverse Photoemission and Optical Studies of Rare-Earth Hexaborides

Electronic structures of rare-earth hexaborides (RB 6 ) have been investigated by photoemission and inverse photoemission spectroscopy. The X-ray photoemission (XPS) features of the 3 d and 4 d core states of heavy R elements are interpreted in comparison with the multiplet calculation. The valence band spectra probed by XPS and X-ray bremsstrahlung isochromat spectroscopy (X-BIS) methods are fully utilized to interpret the optical conductivity spectra.

Single-crystal X-ray diffraction study of NdB 6, EuB 6 and YbB 6

The structure of three rare earth hexaborides, NdB 6, EuB 6 and YbB 6, was studied by single-crystal X-ray diffraction. The refinements indicated boron vacancies for EuB 6 and YbB 6, but the site occupancies of NdB 6 were strongly model dependent, showing either boron or metal vacancies. No impurity atoms could be detected in the structures. Refinements of a model allowing anharmonic thermal motion of atoms gave small but significant fourth-order anharmonic thermal parameters for NdB 6. Static displacements of both the rare earth and boron atoms were considered and tested as an alternative representation. It was concluded that the observed features in the difference density maps were due to charge asphericity.

Low-energy optical excitation in rare-earth hexaborides.

Reflectivity spectra of trivalent rare-earth hexaborides (R${\mathrm{B}}_{6}$; R=La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, and Y) and mixed-valent ${\mathrm{SmB}}_{6}$ have been measured in the photon energy region from 1 meV to 40 eV at 300 and 9 K. We discuss the absorption structure due to conduction electrons. Even in ${\mathrm{LaB}}_{6}$ and ${\mathrm{YB}}_{6}$, which have no 4f electrons, the optical conductivity spectra were not fitted by a simple Drude model and a frequency-dependent relaxation time was observed. This was attributed to the electron-phonon and electron-electron scattering. In R${\mathrm{B}}_{6}$ with 4f electrons, a common absorption structure was observed at the energy position of about 0.6 eV. The intensity was found to be almost proportional to the 4f spin angular momentum. From the f-sum rule, the absorption was found to be due to the conduction electrons. The origin was assigned to be due to the transition or to the relevant exciton absorption from the saddle points of ${\mathrm{\ensuremath{\Sigma}}}_{1}$ at the neck point to the saddle points of ${\mathrm{\ensuremath{\Gamma}}}_{12}$ and ${\mathrm{\ensuremath{\Gamma}}}_{25}$ assisted by the scattering of the intra-atomic 5d-4f Coulomb exchange interaction, in particular enhanced in the heavy R${\mathrm{B}}_{6}$ compounds by the lattice instability. In ${\mathrm{SmB}}_{6}$, the absorption that was seen in trivalent R${\mathrm{B}}_{6}$'s was also observed in addition to absorptions from 4f states of ${\mathrm{Sm}}^{2+}$ to 5d states and from 5d to 4f in ${\mathrm{Sm}}^{3+}$. This is thought to mean that the Fermi level of ${\mathrm{SmB}}_{6}$ is located at the same energy position in the band structure as that of the trivalent R${\mathrm{B}}_{6}$.

Quadrupole-Strain Interaction in Rare Earth Hexaborides

Temperature dependence of the elastic constants in a series of rare earth hexaborides has been investigated by means of the ultrasonic method. The remarkable softening of the transverse C 44 mode in PrB 6 , NdB 6 , DyB 6 and HoB 6 has been discussed in connection with the cooperative Jahn-Teller effect due to the CEF ground state degenerated by the quadrupolar moment. The low temperature behavior of C 44 in Ce x La 1- x B 6 compounds is presented to show the Kondo effect on the quadrupolar moment of Γ 8 ground state. The anomaly of the bulk modulus C B in SmB 6 is related to the valence fluctuation state in the system.

4f levels of rare-earth hexaborides: A simple approach based on a modified orthogonalized-plane-wave method and a self-consistent-field atomic-structure calculation.

The electronic structures of trivalent and divalent rare-earth hexaborides R${\mathrm{B}}_{6}$ are calculated by using the modified orthogonalized-plane-wave method within the framework of the muffin-tin-potential approximation based on the self-consistent-field atomic-structure calculations. By regarding the 4f state as a partially filled corelike state and for the transition between 4f and 5d states, and adopting the transition state introduced by Slater which includes the effect of the screening of 4f states by 5d electrons, an attempt is made to understand the x-ray-photoemission spectroscopy (XPS) experiments and bremsstrahlung isochromat spectra (BIS). It is shown that the calculated results of the 4f levels relative to the Fermi level generally agree with the experimental ones obtained from the XPS for ${\mathrm{CeB}}_{6}$, ${\mathrm{PrB}}_{6}$, ${\mathrm{NdB}}_{6}$, ${\mathrm{SmB}}_{6}$, ${\mathrm{GdB}}_{6}$, and ${\mathrm{YbB}}_{6}$ and the BIS for ${\mathrm{GdB}}_{6}$, ${\mathrm{TbB}}_{6}$, and ${\mathrm{DyB}}_{6}$, but there is a considerable difference between the calculations and experiments for the XPS for ${\mathrm{TbB}}_{6}$, ${\mathrm{DyB}}_{6}$, and ${\mathrm{HoB}}_{6}$ and the BIS for ${\mathrm{LaB}}_{6}$, ${\mathrm{CeB}}_{6}$, and ${\mathrm{PrB}}_{6}$. The reason for this disagreement is mainly attributed to the effect of 4f multiplets which is not taken into account in the present study.

Rare-EarthN4,5Emission Spectra of Some Rare-Earth Hexaborides

The rare-earth N 4,5 emission spectra of PrB 6 and NdB 6 were measured to investigate the 4 f states. These spectra show some emission peaks above and below the one-electron 4 d threshold. The emission spectra are compared with existing results of absorption and resonant photoemission. Comparison shows that the emission spectra are composed of two types of 4 f →4 d emission processes, 4 d 9 4 f n →4 d 10 4 f n -1 + h ν and 4 d 9 4 f n +1 →4 d 10 4 f n + h ν ′ , where n is the number of 4 f electrons in the ground state. The intensity of the 4 f →4 d emission is estimated for light rare-earth hexaborides. This result suggests that the intensity of the 4 f →4 d emission is weaker than the 5 p →4 d emission.

Electrical and magnetic properties of heavy rare-earth hexaborides

The magnetic phase diagrams (external magnetic field versus temperature) of heavy rare-earth (Tb, Dy and Ho) hexaborides are determined for the first time on the basis of measurements of electrical resistivity, magnetoresistance, magnetic susceptibility, magnetization and specific heat. These results indicate the complex behavior of heavy rare-earth hexaborides at low temperature.

Magnetoelastic interaction of rare earth hexaborides

We have measured the temperature dependences of the various elastic constants of rare earth hexaborides. The characteristic features of the electron-strain interaction relating to the cooperative Jahn-Teller effect of PrB 6, NdB 6, DyB 6, the Kondo effect of Ce xLa 1−xB 6 and the valence fluctuating state of SmB 6 have been described.

Electronic structure of rare-earth hexaborides.

Reflectivity spectra of all rare-earth hexaboride R${\mathrm{B}}_{6}$ (R=La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Yb, and Y) single crystals have been measured systematically in the energy region from 1 meV to 40 eV at 300 K in order to investigate the electronic state and the contribution of the 4f electron to the band structure. The analysis of the optical conductivity and the loss-function spectra, which were derived from the Kramers-Kronig transformation of the reflectivity spectra, allowed us to make clear the origin of the peak structure in the spectrum due to the various interband transitions. The origins of the main peaks in the spectrum were assigned to the interband transitions from the bonding to the antibonding bands of the boron 2s and 2p states and to the rare-earth 5d state. The intra-atomic transition from the 4f and the 5p to the 5d(${\mathit{t}}_{2\mathit{g}}$) states in the rare-earth ion was also observed.