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Single-phase metal dodecaboride solid solutions, Zr0.5Y0.5B12 and Zr0.5U0.5B12, were prepared by arc melting from pure elements. The phase purity and composition were established by powder X-ray di...

The presence of one-dimensional trigonal arrangements of chromium atoms (Cr-3) in the structure of TiCrIr2B2 leads to a magnetic transition near ambient temperature. Herein we report an investigation of the nature of electronic and magnetic properties of TiCrIr2B2 via ab initio calculations together with B-11 NMR Knight shift (K) and spin-lattice relaxation rate (1/T1) analysis. The presence of a characteristic rectangular powder pattern below 280 K, absence of a Korringa relation, strong enhancement of 1/TI.T at low temperatures and weak temperature dependence of K indicate competition between antiferromagnetic and ferromagnetic spin fluctuations in the itinerant d-band electrons, in agreement with ab initio calculations. One-dimensional trigonal arrangements of magnetically active elements in intermetallic compounds, as is found in TiCrIr2B2, are quite rare and could lead to exotic phenomena such as spin-chirality and quantum criticality in low-dimensional frustrated lattices.

The structure of tungsten diboride, WB2, differs from those of metal diborides with AlB2- or ReB2-type structures that have been previously studied by 11B NMR spectroscopy. As opposed to the single 11B resonance reported for the metal diborides having those AlB2- or ReB2-type structures, four groups of 11B resonances are found for WB2. There is also 11B NMR spectral evidence to support the proposal by Kiesling in 1947 of an additional boron atom at the center of some of the six-membered boron rings. The 11B quadrupolar frequencies of all four groups of resonances in WB2 are all quite small (

Non-oxide perovskites exhibit unusual properties such as negative thermal expansion, negative thermal coefficient of resistance, positive and negative giant magnetoresistance as well as superconductivity. These uncommon properties appear to originate from the basic structure only, in strong contrast to the oxides. Ordering in nonstoichiometric compounds may not only lead to different chemical compositions but also to the promotion of these physical properties. We present a combined NMR and first-principles study of the cubic Zr 2 Ir 6 B perovskite to investigate the boron ordering with boron deficiency leading to the formation of superstructure. Competing ionic and metallic interactions reflect the semimetallic character of this boride and result in the formation of a pseudogap, as predicted by our first principles calculations and verified experimentally by 11 B solid state NMR. Several avoided crossing scenarios were also found for the bands from the conducting states at +1 eV to the Fermi level along specific directions. This observation is of paramount importance for understanding the structure-property relationships in metal boride perovskites and the search for new cubic perovskites.

Crystal orbital Hamilton population (COHP) bonding analysis has predicted that ScPd3 B0.5 is the least stable compound of the entire series Sc2 Ir6-x Pdx B. Here, we report a systematic study of Sc2 Ir6-x Pdx B (x=3, 5 and 6) by means of 11 B nuclear magnetic resonance (NMR), Knight shift (K) and nuclear spin-lattice relaxation rate (1/T1 ). NMR results combined with theoretical band structure calculations provide a measure of s- and non-s-character Fermi-level density of states. We present direct evidence that the enhanced s-state character of the Fermi level density of states (DOS) in ScPd3 B0.5 reduces the strength of the B 2p and Pd 4d hybridized states across the entire Sc2 Ir6-x Pdx B series. This hybridization strength relates to the opening of a deep pseudogap in the density of states of Sc2 IrPd5 B and the chemical bonding instability of ScPd3 B0.5 . This study is an experimental realization of a chemical fine-tuning of the electronic properties in intermetallic perovskites.

Analyses of the one-dimensional 11B NMR spectra from static and MAS experiments of a polycrystalline ReB2 sample have extracted parameters characterizing both the electric field gradient and the shielding at the boron site. In contrast to that previously assumed for AlB2, the principal axis systems for these two interactions are not coincident in ReB2. A brief discussion of the literature regarding spin–lattice relaxation for half-integer quadrupolar nuclei and the mathematical functions used for the extraction of relaxation time constants is presented. With radiofrequency conditions chosen to excite the central and satellite transitions, the saturation recovery spin–lattice relaxation rate of ReB2 is better described by a single exponential rather than the Andrew–Tunstall multiexponential model for I = 3/2. In addition, relaxation measurements for 11B in ReB2 have been extended to temperatures above ambient and show a deviation from the Korringa relationship.

Understanding different bonding environments in various metal borides provides insight into their structures and physical properties. Polycrystalline aluminum diboride (AlB2) samples have been synthesized and compared both with a commercial sample and with the literature. One issue that arose is the relative ease with which boron-rich and aluminum deficient phases of aluminum borides can be presented in AlB2. Here, we report 27Al, 11B nuclear magnetic resonance (NMR) spectroscopy and first-principles calculations on AlB2 in order to shed light on these different bonding environments at the atomic level and compare the structural and electronic properties of the products of different preparations. Along with the aforementioned, the present study also takes an in-depth look at the nature of the 11B and 27Al nuclear spin–lattice relaxation recovery data for the AlB2 and other superhard materials. The nuclear spin–lattice relaxation has been measured for a static sample and with magic-angle spinning. The combination of NMR and band structure calculations highlights the synthetic challenges with superhard materials.

Cd3As2 is a Dirac semimetal that is a 3D analog of graphene. We investigated the local structure and nuclear-spin dynamics in Cd3As2 via Cd-113 NMR. The wideline spectrum of the static sample at 295K is asymmetric and its features are well described by a two-site model with the shielding parameters extracted via Herzfeld-Berger analysis of the magic-angle spinning spectrum. Surprisingly, the Cd-113 spin-lattice relaxation time (T-1) is extremely long (T-1 = 95 s at 295 K), in stark contrast to conductors and the effects of native defects upon semiconductors; but it is similar to that of C-13 in graphene (T-1 = 110 s). The temperature dependence of 1/T-1 revealed a complex bipartite mechanism that included a T-2 power-law behavior below 330 K and a thermally activated process above 330 K. In the high-temperature regime, the Arrhenius behavior is consistent with a field-dependent Cd atomic hopping relaxation process. At low temperatures, a T-2 behavior consistent with a spin-1/2 Raman-like process provides evidence of a time-dependent spin-rotation magnetic field caused by angular oscillations of internuclear vectors due to lattice vibrations. The observed mechanism does not conform to the conventional two-band model of semimetals, but is instead closer to a mechanism observed in high-Z element ionic solids with large magnetorotation constant [A.J. Vega et al., Phys. Rev. B 74, 214420 (2006)/. Published by AIP Publishing.

We report the NMR Knight shift and spin-lattice relaxation of $^{8}\mathrm{Li}{}^{+}$ implanted $\ensuremath{\sim}100$ nm into single crystals of semimetallic Sb, Bi, and topologically insulating ${\mathrm{Bi}}_{0.9}{\mathrm{Sb}}_{0.1}$. We find small negative shifts (of order 100 ppm) in all three. In the insulator, the shift is nearly temperature independent, while in Bi and Sb it becomes more negative at low temperature without following the bulk susceptibility, suggesting two distinct temperature dependent contributions, possibly from the orbital and spin response. However, a simple model is unable to account for the observed shift. The spin-lattice relaxation differs in both scale and temperature dependence in all three. It is Korringa-like in Bi and remarkably is fastest in the insulating alloy and slowest in Sb with the highest bulk carrier density. These surprising results call for detailed calculations, but phenomenologically demonstrate that $\ensuremath{\beta}$-detected NMR of implanted $^{8}\mathrm{Li}{}^{+}$ is sensitive to the magnetic response of low-density carriers. The prospects for depth-resolved studies of conventional and topological surface states at lower implantation energies are good.