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Six high-entropy rare earth tetraborides of the tetragonal UB4-prototyped structure have been successfully synthesized for the first time. The specimens are prepared from elemental precursors via high-energy ball mill and in-situ reactive spark plasma sintering. The sintered specimens are >98% in relative densities without detectable oxide impurities (albeit the presence of minor hexaborides in some compositions). No detectable secondary phase is observed in the composition (Y$_{0.2}$Nd$_{0.2}$Sm$_{0.2}$Gd$_{0.2}$Tb$_{0.2}$)B$_{4}$, which is proven homogeneous at both microscale and nanoscale. The Vickers microhardness are determined to be ~13-15 GPa at a standard indentation load of 9.8 N. A scientifically interesting observation is represented by the anisotropic lattice distortion from the rule-of-mixture averages. This work expands the family of high-entropy ceramics via fabricating a new class of high-entropy borides with a unique tetragonal quasi-layered crystal structure.
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In the presence of a magnetic field frustrated spin systems may exhibit plateaus at fractional values of saturation magnetization. Such plateau states are stabilized by classical and quantum mechanisms including order-by-disorder, triplon crystallization, and various competing order effects. In the case of electrically conducting systems, free electrons represent an incisive probe for the plateau states. Here we study the electrical transport of Ising-type rare earth tetraborides $R$B$_4$ ($R=$Er, Tm), a metallic Shastry-Sutherland lattice showing magnetization plateaus. We find that the longitudinal and transverse resistivities reflect scattering with both the static and dynamic plateau structure. We model these results consistently with the expected strong uniaxial anisotropy in a quantitative level, providing a framework for the study of plateau states in metallic frustrated systems.
Crystal-field (CF) effects on the rare-earth (RE) ions in ferrimagnetic intermetallics NdCo$_5$ and TbCo$_5$ are evaluated using an ab initio density functional + dynamical mean-field theory approach in conjunction with a quasi-atomic approximation for on-site electronic correlations on the localized 4$f$ shell. The study reveals an important role of the high-order sectoral harmonic component of the CF in the magnetism of RECo$_5$ intermetallics. An unexpectedly large value is computed in the both systems for the corresponding crystal-field parameter (CFP) $A_6^6 langle r^6 rangle$, far beyond what one would expect from only electrostatic contributions. It allows solving the enigma of the non-saturation of zero-temperature Nd magnetic moments in NdCo$_5$ along its easy axis in the Co exchange field. This unsaturated state had been previously found out from magnetization distribution probed by polarised neutron elastic scattering but had so far remained theoretically unexplained. The easy plane magnetic anisotropy of Nd in NdCo$_5$ is strongly enhanced by the large value of $A_6^6langle r^6 rangle$. Counter-intuitively, the polar dependence of anisotropy energy within the easy plane remains rather small. The easy plane magnetic anisotropy of Nd is reinforced up to high temperatures, which is explained through $J$-mixing effects. The calculated ab initio anisotropy constants of NdCo$_5$ and their temperature dependence are in quantitative agreement with experiment. Unlike NdCo$_5$, the $A_6^6 langle r^6 rangle$ CFP has negligible effects on the Tb magnetism in TbCo$_5$ suggesting that its impact on the RE magnetism is ion-specific across the RECo$_5$ series. The origin of its large value is the hybridization of RE and Co states in a hexagonally coordinated local environment of the RE ion in RECo$_5$ intermetallics.
We report a high-pressure study of orthorhombic rare-earth manganites AMnO3 using Raman scattering (for A = Pr, Nd, Sm, Eu, Tb and Dy) and synchrotron X-ray diffraction (for A = Pr, Sm, Eu, and Dy). In all cases, a structural and insulator-to-metal transition was evidenced, with a critical pressure that depends on the A-cation size. We analyze the compression mechanisms at work in the different manganites via the pressure dependence of the lattice parameters, the shear strain in the a-c plane, and the Raman bands associated with out-of-phase MnO6 rotations and in-plane O2 symmetric stretching modes. Our data show a crossover across the rare-earth series between two different kinds of behavior. For the smallest A-cations, the compression is nearly isotropic in the ac plane, with presumably only very slight changes of tilt angles and Jahn-Teller distortion. As the radius of the A-cation increases, the pressure-induced reduction of Jahn-Teller distortion becomes more pronounced and increasingly significant as a compression mechanism, while the pressure-induced bending of octahedra chains becomes conversely less pronounced. We finally discuss our results in the light of the notion of chemical pressure, and show that the analogy with hydrostatic pressure works quite well for manganites with small A-cations but can be misleading with large A-cations.
Strong electron correlations in rare earth hexaborides can give rise to a variety of interesting phenomena like ferromagnetism, Kondo hybridization, mixed valence, superconductivity and possibly topological characteristics. The theoretical prediction of topological properties in SmB$_{6}$ and YbB$_{6}$, has rekindled the scientific interest in the rare earth hexaborides, and high-resolution ARPES has been playing a major role in the debate. The electronic band structure of the hexaborides contains the key to understand the origin of the different phenomena observed, and much can be learned by comparing the experimental data from different rare earth hexaborides. We have performed high-resolution ARPES on the (001) surfaces of YbB$_{6}$, CeB$_{6}$ and SmB$_{6}$. On the most basic level, the data show that the differences in the valence of the rare earth element are reflected in the experimental electronic band structure primarily as a rigid shift of the energy position of the metal 5$textit{d}$ states with respect to the Fermi level. Although the overall shape of the $textit{d}$-derived Fermi surface contours remains the same, we report differences in the dimensionality of these states between the compounds studied. Moreover, the spectroscopic fingerprint of the 4$textit{f}$ states also reveals considerable differences that are related to their coherence and the strength of the $textit{d}$-$textit{f}$ hybridization. For the SmB$_6$ case, we use ARPES in combination with STM imaging and electron diffraction to reveal time dependent changes in the structural symmetry of the highly debated SmB$_{6}$(001) surface. All in all, our study highlights the suitability of electron spectroscopies like high-resolution ARPES to provide links between electronic structure and function in complex and correlated materials such as the rare earth hexaborides.
Based on the electronic band structure obtained from first principles DFT calculations, the opticalspectra of yttrium and neodymium nickelates are computed. We show that the results are in fairagreement with available experimental data. We clarify the electronic transitions at the origin of thefirst two peaks, highlighting the important role of transitions from t2g states neglected in previousmodels. We discuss the evolution of the optical spectra from small to large rare-earth cations andrelate the changes to the electronic band structure.
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