Nitride coatings are increasingly demanded in the cutting- and machining-tool industry owing to their hardness, thermal stability and resistance to corrosion. These properties derive from strongly covalent bonds; understanding the bonding is a requirement for the design of superhard materials with improved capabilities. Here, we report a pressure-induced cubic-to-orthorhombic transition at approximately 1 GPa in CrN. High-pressure X-ray diffraction and ab initio calculations show an unexpected reduction of the bulk modulus, K0, of about 25% in the high-pressure (lower volume) phase. Our combined theoretical and experimental approach shows that this effect is the result of a large exchange striction due to the approach of the localized Cr:t3 electrons to becoming molecular-orbital electrons in Cr-Cr bonds. The softening of CrN under pressure is a manifestation of a strong competition between different types of chemical bond that are found at a crossover from a localized to a molecular-orbital electronic transition.

The interconversion of charge and spin currents via spin-Hall effect is essential for spintronics. Energy-efficient and deterministic switching of magnetization can be achieved when spin polarizations of these spin currents are collinear with the magnetization. However, symmetry conditions generally restrict spin polarizations to be orthogonal to both the charge and spin flows. Spin polarizations can deviate from such direction in nonmagnetic materials only when the crystalline symmetry is reduced. Here, we show control of the spin polarization direction by using a non-collinear antiferromagnet Mn3GaN, in which the triangular spin structure creates a low magnetic symmetry while maintaining a high crystalline symmetry. We demonstrate that epitaxial Mn3GaN/permalloy heterostructures can generate unconventional spin-orbit torques at room temperature corresponding to out-of-plane and Dresselhaus-like spin polarizations which are forbidden in any sample with two-fold rotational symmetry. Our results demonstrate an approach based on spin-structure design for controlling spin-orbit torque, enabling high-efficient antiferromagnetic spintronics.

We report the thermoelectric figure of merit of chromium nitride, CrN, and its optimization through hole-doping. CrN is a degenerate semiconductor with large thermoelectric power, reaching −185 μV/K at 420 K. The resistivity can be reduced through hole-doping in the series Cr1−xVxN, keeping a large thermopower. The thermal conductivity of CrN is rather low compared to other transition-metal nitrides, reaching its minimum value of 1.0 W/m K at 267 K. The largest ZT=0.04 was measured for Cr0.9V0.1N at room temperature. Our results suggest that CrN could be a good starting point for the design of a thermoelectric material with optimal mechanical properties.

The design of efficient thermoelectric (TE) devices for energy harvesting and advanced cooling applications is one of the current challenges in materials science. [1] So far, the most common materials used in commercial TE devices are rock-salt IV-VI (PbTe, PbSe) and distorted rock-salt V2-VI3 (Bi2Te3, Bi2Se3) semiconductors. [2] One of the key factors behind the high TE performance of these materials is their abnormally low lattice thermal conductivity (κl), 2 which is one of the fundamental parameters that define the dimensionless TE figure of merit zT = S 2 σT/(κl+κe), in which S is the Seebeck coefficient, σ the electrical conductivity, T the absolute temperature, and κe the electronic thermal conductivity.In a recent paper, Lee et al. [3] suggested that the main reason for the low lattice thermal conductivity in rock-salt IV-VI compounds is the resonant bonding (RB) effect: the p-orbitals with 3 electrons per atom cannot form the six saturated bonds of the rock-salt lattice, and therefore an RB structure is established. [ 4 ] Using first-principles calculations, they demonstrated that the large electronic polarizability of the resonant bonds introduces long-range interactions and a softening of the transverse optical phonon mode. This ultimately causes acoustic phonon scattering and is responsible for the low lattice thermal conductivity in IV-VI and V2-VI3 compounds. An interesting question is whether or not the concept of RB can be extrapolated to transition-metal (TM) compounds with a rock-salt structure. The versatility of the oxidation states and ionic sizes shown by TM ions would offer enormous possibilities for tuning their TE properties, which would allow for new approaches regarding the design of TE materials with improved capabilities.In this communication we demonstrate that rock-salt CrN shows intrinsic lattice instabilities that suppress its thermal conductivity. Using ab-initio calculations, we determined that the origin of these instabilities is similar to that observed in IV-VI compounds with RB states. [3,5] Through the fabrication of high quality epitaxial (001) CrN thin films we report a 250% increase in the zT at room temperature compared to bulk CrN. [6] These results along with its high thermal stability, resistance to corrosion, and exceptional mechanical properties, make CrN a promising n-type material for high-temperature TE applications.The presence of extrinsic factors, such as N-vacancies or epitaxial constrains, are likely behind the large variety of structural and transport properties previously reported for CrN films. [7] In the case of polycrystalline bulk CrN, the intergrain contribution to the electrical and 3 thermal conductivities can be significant enough to mask its intrinsic transport properties and, ultimately, its thermoelectric performance. Therefore, in order to access the intrinsic thermoelectric properties of CrN, it is necessary to develop the fabrication of epitaxial, stoichiometric, and fully relaxed CrN films. The results discussed in this pap...

CrN was doped with Mo and W to study the effect of heavy elements alloying on its thermoelectric properties. An spontaneous phase segregation into Mo-and W-rich regions was observed even at the lowest concentrations probed at this work ð' 1%Þ. In the particular case of W, this segregation creates nanoinclusions into the Cr 1-x W x N matrix, which results in a substantial reduction of the thermal conductivity in the whole temperature range compared to undoped CrN. In addition, an increased hybridization of N:2p and 4d/5d orbitals with respect to Cr:3d decreases the electrical resistivity in lightly doped samples. This improves substantially the thermoelectric figure of merit with respect to the undoped compound, providing a pathway for further improvement of the thermoelectric performance of CrN. V

We report the structural, magnetic, and electronic phase diagram of Cr 1−x V x N. Stoichiometric CrN is a narrow gap, correlation-induced, semiconductor that orders antiferromagnetically below 286 K. The changes in the chemical bond associated to the magnetic order result in a nonactivated behavior of the resistivity in the antiferromagnetic state. Introducing holes into this system produces a series of inhomogeneous magnetic/ electronic states, as identified through electronic and thermal conductivity, and magnetic susceptibility. The magnetic/electronic phase diagram of Cr 1−x V x N is an example of electronic complexity in a simple system from the chemical and structural point of view.

Engineered heterostructures formed by complex oxide materials are a rich source of emergent phenomena and technological applications. In the quest for new functionality, a vastly unexplored avenue is interfacing oxide perovskites with materials having dissimilar crystallochemical properties. Here, we propose a unique class of heterointerfaces based on nitride antiperovskite and oxide perovskite materials as a previously unidentified direction for materials design. We demonstrate the fabrication of atomically sharp interfaces between nitride antiperovskite Mn3GaN and oxide perovskites (La0.3Sr0.7)(Al0.65Ta0.35)O3 and SrTiO3. Using atomic-resolution imaging/spectroscopic techniques and first-principles calculations, we determine the atomic-scale structure, composition, and bonding at the interface. The epitaxial antiperovskite/perovskite heterointerface is mediated by a coherent interfacial monolayer that interpolates between the two antistructures. We anticipate our results to be an important step for the development of functional antiperovskite/perovskite heterostructures, combining their unique characteristics such as topological properties for ultralow-power applications.

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