Computationally Directed Discovery of MoBi\u3csub\u3e2\u3c/sub\u3e

Incorporating bismuth, the heaviest element stable to radioactive decay, into new materials enables the creation of emergent properties such as permanent magnetism, superconductivity, and nontrivial topology. Understanding the factors that drive Bi reactivity is critical for the realization of these properties. Using pressure as a tunable synthetic vector, we can access unexplored regions of phase space to foster reactivity between elements that do not react under ambient conditions. Furthermore, combining computational and experimental methods for materials discovery at high-pressures provides broader insight into the thermodynamic landscape than can be achieved through experiment alone, informing our understanding of the dominant chemical factors governing structure formation. Herein, we report our combined computational and experimental exploration of the Mo–Bi system, for which no binary intermetallic structures were previously known. Using the ab initio random structure searching (AIRSS) approach, we identified multiple synthetic targets between 0–50 GPa. High-pressure in situ powder X-ray diffraction experiments performed in diamond anvil cells confirmed that Mo–Bi mixtures exhibit rich chemistry upon the application of pressure, including experimental realization of the computationally predicted CuAl2-type MoBi2 structure at 35.8(5) GPa. Electronic structure and phonon dispersion calculations on MoBi2 revealed a correlation between valence electron count and bonding in high-pressure transition metal–Bi structures as well as identified two dynamically stable ambient pressure polymorphs. Our study demonstrates the power of the combined computational–experimental approach in capturing high-pressure reactivity for efficient materials discovery

A Cu2+ (S = 1/2) Kagom\'e Antiferromagnet: MgxCu4-x(OH)6Cl2

Spin-frustrated systems are one avenue for inducing macroscopic quantum states in materials. However, experimental realization of this goal has been difficult because of the lack of simple materials and, if available, the separation of the unusual magnetic properties arising from exotic magnetic states from behavior associated with chemical disorder, such as site mixing. Here we report the synthesis and magnetic properties of a new series of magnetically frustrated materials, MgxCu4-x(OH)6Cl2. Because of the substantially different ligand-field chemistry of Mg2+ and Cu2+, site disorder within the kagom\'e layers is minimized, as directly measured by X-ray diffraction. Our results reveal that many of the properties of these materials and related systems are not due to disorder of the magnetic lattice but rather reflect an unusual ground state.Comment: Accepted for publication in J. Am. Chem. Soc

Qubit Control Limited by Spin–Lattice Relaxation in a Nuclear Spin-Free Iron(III) Complex

High-spin transition metal complexes are of interest as candidates for quantum information processing owing to the tunability of the pairs of <i>M</i>_<i>S</i> levels for use as quantum bits (qubits). Thus, the design of high-spin systems that afford qubits with stable superposition states is of primary importance. Nuclear spins are a potent instigator of superposition instability; thus, we probed the Ph₄P⁺ salt of the nuclear spin-free complex [Fe­(C₅O₅)₃]^3– (<b>1</b>) to see if long-lived superpositions were possible in such a system. Continuous-wave and pulsed electron paramagnetic resonance (EPR) spectroscopic measurements reveal a strong EPR transition at X-band that can be utilized as a qubit. However, at 5 K the coherent lifetime, <i>T</i>₂, for this resonance is 721(3) ns and decreases rapidly with increasing temperature. Simultaneously, the spin–lattice relaxation time is extremely short, 11.33(1) μs, at 5 K, and also rapidly decreases with increasing temperature. The coincidence of these two temperature-dependent data sets suggests that <i>T</i>₂ in <b>1</b> is strongly limited by the short <i>T</i>₁. Importantly, these results highlight the need for new design parameters in pursuit of high-spin species with appreciable coherence times

(BiSe)_1.23CrSe₂ and (BiSe)_1.22(Cr_1.2Se₂)₂: Magnetic Anisotropy in the First Structurally Characterized Bi–Se–Cr Ternary Compounds

Compounds containing both heavy main group elements and paramagnetic transition metals form a fertile area for the study of magnetic anisotropy. We pursued the synthesis, characterization, and magnetic measurements of Bi–Se–Cr compounds: a ternary system with no structurally characterized materials. Those efforts led to the isolation of two novel misfit layer compounds, namely, (BiSe)_1.23CrSe₂ (<b>1</b>) and (BiSe)_1.22(Cr_1.2Se₂)₂ (<b>2</b>). The crystal structure of <b>1</b> consists of alternating BiSe and CrSe₂ layers along the <i>c</i>-axis, and <b>2</b> is composed of alternating BiSe and (Cr_1.2Se₂)₂ layers along the <i>c</i>-axis. Lattice mismatch occurs in both compounds along the <i>b</i>-axis and leads to positional modulation of the atoms. Field- and temperature-dependent measurements were performed to assess the degree of magnetic anisotropy. Temperature-dependent susceptibility measurements on aligned crystals of <b>1</b> display increased bifurcation of zero-field cooled and field cooled data when crystals are oriented with <i>H</i> perpendicular to <i>c</i> than when the crystals are oriented with <i>H</i> parallel to <i>c</i>. Magnetic anisotropy is less pronounced in <b>2</b> where both crystallographic orientations exhibit bifurcation at 26 K. The complexity of the magnetic behavior in both compounds likely signifies a competition between CrSe₂ intralayer ferromagnetic coupling and interlayer antiferromagnetic coupling. These results highlight the exciting magnetic properties that can arise from the exploration of new ternary phases

Magnetic Anisotropy from Main-Group Elements: Halides versus Group 14 Elements

Precise modulation of the magnetic anisotropy of a transition-metal center would affect physical properties ranging from photoluminescence to magnetism. Over the past decade, exerting nuanced control over ligand fields enabled the incorporation of significant magnetic anisotropy in a number of mononuclear transition-metal complexes. An alternate approach to increasing spin–orbit coupling relies upon using heavy diamagnetic main-group elements as sources of magnetic anisotropy. Interacting first-row transition metals with main-group elements enables the transfer of magnetic anisotropy to the paramagnetic metal center without restricting coordination geometry. We sought to study the effect of covalency on this anisotropy transfer by probing the effect of halides in comparison to early main-group elements. Toward that end, we synthesized a series of four isostructural heterobimetallic complexes, with germanium or tin covalently bound to a triplet spin Fe­(II) center. These complexes are ligated by a halide (Br^– or I^–) in the apical position to yield a series of complexes with variation in the mass of the main-group elements. This series enabled us to interrogate which electronic structure factors influence the heavy-atom effect. Using a suite of approaches including magnetometry, computation, and Mössbauer spectroscopy, we probed the electronic structure and the spin–orbit coupling, as parametrized by axial zero-field splitting across the series of complexes, and found an increase in zero-field splitting from −11.8 to −17.9 cm^–1 by increasing the axial ligand mass. Through direct comparison between halides and group 14 elements, we observe a greater impact on magnetic anisotropy from the halide interaction. We attribute this counterintuitive effect to a larger spin population on the halide elements, despite greater covalency in the group 14 interactions. These results recommend modification of the intuitive design principle of increasing covalency toward a deeper focus on the interactions of the spin-bearing orbitals

Millisecond Coherence Time in a Tunable Molecular Electronic Spin Qubit

Quantum information processing (QIP) could revolutionize areas ranging from chemical modeling to cryptography. One key figure of merit for the smallest unit for QIP, the qubit, is the coherence time (<i>T</i>₂), which establishes the lifetime for the qubit. Transition metal complexes offer tremendous potential as tunable qubits, yet their development is hampered by the absence of synthetic design principles to achieve a long <i>T</i>₂. We harnessed molecular design to create a series of qubits, (Ph₄P)₂[V­(C₈S₈)₃] (<b>1</b>), (Ph₄P)₂[V­(β-C₃S₅)₃] (<b>2</b>), (Ph₄P)₂[V­(α-C₃S₅)₃] (<b>3</b>), and (Ph₄P)₂[V­(C₃S₄O)₃] (<b>4</b>), with <i>T</i>₂s of 1–4 μs at 80 K in protiated and deuterated environments. Crucially, through chemical tuning of nuclear spin content in the vanadium­(IV) environment we realized a <i>T</i>₂ of ∼1 ms for the species (<i>d</i>₂₀-Ph₄P)₂[V­(C₈S₈)₃] (<b>1</b>′) in CS₂, a value that surpasses the coordination complex record by an order of magnitude. This value even eclipses some prominent solid-state qubits. Electrochemical and continuous wave electron paramagnetic resonance (EPR) data reveal variation in the electronic influence of the ligands on the metal ion across <b>1</b>–<b>4</b>. However, pulsed measurements indicate that the most important influence on decoherence is nuclear spins in the protiated and deuterated solvents utilized herein. Our results illuminate a path forward in synthetic design principles, which should unite CS₂ solubility with nuclear spin free ligand fields to develop a new generation of molecular qubits