Ternary nitride materials hold promise for many optical, electronic, and refractory applications yet their preparation via solid-state synthesis remains challenging. Often, high pressures or reactive gasses are used to manipulate the effective chemic
al potential of nitrogen, yet these strategies require specialized equipment. Here we report on a simple two-step synthesis using ion-exchange reactions that yield rocksalt-derived MgZrN$_2$ and Mg$_2$NbN$_3$, as well as layered MgMoN$_2$. All three compounds show nearly temperature-independent and weak paramagnetic responses to an applied magnetic field at cryogenic temperatures indicating phase pure products. The key to synthesizing these ternary materials is an initial low-temperature step (300-450 $^{circ}$C) to promote Mg-M-N bond formation. Then the products are annealed (800-900 $^{circ}$C) to increase crystalline domains of the ternary product. Calorimetry experiments reveal that initial reaction temperatures are determined by phase transitions of reaction precursors, whereas heating directly to high temperatures results in decomposition. These two-step reactions provide a rational guide to material discovery of other bulk ternary nitrides.
Hybrid halide perovskite semiconductors exhibit complex, dynamical disorder while also harboring properties ideal for optoelectronic applications that include photovoltaics. However, these materials are structurally and compositionally distinct from
traditional compound semiconductors composed of tetrahedrally-coordinated elements with an average valence electron count of silicon. As discussed here, the additional dynamic degrees of freedom of hybrid halide perovskites underlie many of their potentially transformative physical properties. Neutron scattering and spectroscopy studies of the atomic dynamics of these materials have yielded significant insights to the functional properties. Specifically, inelastic neutron scattering has been used to elucidate the phonon band structure, and quasi-elastic neutron scattering (QENS) has revealed the nature of the uncorrelated dynamics pertaining to molecular reorientations. Understanding the dynamics of these complex semiconductors has elucidated the temperature-dependent phase stability and origins of the defect-tolerant electronic transport from the highly polarizable dielectric response. Furthermore, the dynamic degrees of freedom of the hybrid perovskites provides additional opportunities for application engineering and innovation.
We report x-ray total scattering and pair distribution function (PDF) studies of the structural relaxation around interstitial manganese (Mn$_i$) in ferromagnetic Mn$_{1+delta}$Sb ($0.03 le delta le 0.23$) alloys, guided by density functional theory
(DFT). Refinements to the experimental PDF using a crystallographically constrained structural model indicate an expansion in the equatorial plane of the Mn$_i$Sb$_5$ trigonal bipyramidal site, which introduces significant positional disorder in addition to the nominally-random occupation of interstitial voids. Observation of a weak diffuse signal near the symmetry-forbidden (001) reflection position is indicative of correlated disorder from the clustering of Mn$_i$. Density functional relaxation of supercells approximating the $delta = 0.08$, $0.15,$ and $0.23$ compositions provides improved models that accurately describe the short-range structural distortions captured in the PDFs. Such structural relaxation increases the DFT calculated moment on Mn$_i$, which aligns antiparallel to the primary Mn moments, but leads to insubstantial changes in the average Mn and Sb moments and moments of Mn and Sb proximal to interstitials, thus providing a more accurate description of the observed bulk magnetic properties.
With the increased availability of high intensity time-of-flight neutron and synchrotron X-ray scattering sources that can access wide ranges of momentum transfer, the pair distribution function method has become a standard analysis technique for stu
dying disorder of local coordination spheres and at intermediate atomic separations. In some cases, rational modeling of the total scattering data (Bragg and diffuse) becomes intractable with least-squares approaches and necessitates reverse Monte Carlo (RMC) simulations using large supercells. However, the extraction of meaningful information from the resulting atomistic ensembles is challenging, especially at intermediate length scales. We use representational analysis to describe displacements of atoms in RMC ensembles from an ideal crystallographic structure. Rewriting the displacements in terms of a local basis that is descriptive of the ideal crystallographic symmetry provides a robust approach to characterizing medium-range order (and disorder) and symmetry breaking in complex and disordered crystalline materials. This method enables the extraction of statistically relevant displacement modes (orientation, amplitude, and distribution) of the crystalline disorder and provides directly meaningful information in a symmetry-adapted basis set that is most descriptive of the crystal chemistry and physics.
KNi$_2$Se$_2$ exhibits an increase of symmetry on cooling below $Tle50$ K, as observed by Raman spectroscopy and synchrotron x-ray diffraction. X-ray absorption spectroscopy confirms that the symmetry increase is due to changes in nickel-nickel inter
actions and suppression of charge density wave fluctuations. Density functional theory calculations reveal a zone- boundary lattice instability that provides a model of the room-temperature x-ray pair distribution function data, but fails to describe the higher local symmetry observed for $Tle50$K. Together, these results support many- body correlation effects as drivers for the unusual heavy fermion electronic ground state in KNi$_2$Se$_2$.
The emergence of complex electronic behaviour from simple ingredients has resulted in the discovery of numerous states of matter. Many examples are found in systems exhibiting geometric magnetic frustration, which prevents simultaneous satisfaction o
f all magnetic interactions. This frustration gives rise to complex magnetic properties such as chiral spin structures orbitally-driven magnetism, spin-ice behavior exhibiting Dirac strings with magnetic monopoles, valence bond solids, and spin liquids. Here we report the synthesis and characterization of LiZn2Mo3O8, a geometrically frustrated antiferromagnet in which the magnetic moments are localized on small transition metal clusters rather than individual ions. By doing so, first order Jahn-Teller instabilities and orbital ordering are prevented, allowing the strongly interacting magnetic clusters in LiZn2Mo3O8 to probably give rise to an exotic condensed valence-bond ground state reminiscent of the proposed resonating valence bond state. Our results also link magnetism on clusters to geometric magnetic frustration in extended solids, demonstrating a new approach for unparalleled chemical control and tunability in the search for collective, emergent electronic states of matter.
Understanding the complexities of electronic and magnetic ground states in solids is one of the main goals of solid-state physics. Materials with the canonical ThCr$_2$Si$_2$-type structure have proved particularly fruitful in this regards, as they e
xhibit a wide range of technologically advantageous physical properties described by many-body physics, including high-temperature superconductivity and heavy fermion behavior. Here, using high-resolution synchrotron X-ray diffraction and time-of-flight neutron scattering, we show that the isostructural mixed valence compound, KNi$_2$S$_2$, displays a number of highly unusual structural transitions, most notably the presence of charge density wave fluctuations that disappear on cooling. This behavior occurs without magnetic or charge order, in contrast to expectations based on all other known materials. Furthermore, the low-temperature electronic state of KNi$_2$S$_2$ is found to exhibit many characteristics of heavy-fermion behavior, including a heavy electron state ($m^*/m_e sim$ 24), with a negative coefficient of thermal expansion, and superconductivity below $T_c$ = 0.46(2) K. In the potassium nickel sulfide, these behaviors arise in the absence of localized magnetism, and instead appear to originate in proximity to charge order.
In many ostensibly crystalline materials, unit-cell-based descriptions do not always capture the complete physics of the system due to disruption in long-range order. In the series of cobalt hydroxides studied here, Co(OH)$_{2-x}$(Cl)$_x$(H$_2$O)$_{n
}$, magnetic Bragg diffraction reveals a fully compensated Neel state, yet the materials show significant and open magnetization loops. A detailed analysis of the local structure defines the aperiodic arrangement of cobalt coordination polyhedra. Representation of the structure as a combination of distinct polyhedral motifs explains the existence of locally uncompensated moments and provides a quantitative agreement with bulk magnetic measurements and magnetic Bragg diffraction.
