High-Pressure Polymorphism in Silver Ferrite Delafossite, AgFeO2.

The delafossites are a class of layered metal oxides that are notable for being able to exhibit optical transparency alongside an in-plane electrical conductivity, making them promising platforms for the development of transparent conductive oxides. Pressure-induced polymorphism offers a direct method for altering the electrical and optical properties in this class, and although the copper delafossites have been studied extensively under pressure, the silver delafossites remain only partially studied. We report two new high-pressure polymorphs of silver ferrite delafossite, AgFeO2, that are stabilized above ∼6 and ∼14 GPa. In situ X-ray diffraction and vibrational spectroscopy measurements are used to examine the structural changes across the two phase transitions. The high-pressure structure between 6 and 14 GPa is assigned as a monoclinic C2/c structure that is analogous to the high-pressure phase reported for AgGaO2. Nuclear resonant forward scattering reveals no change in the spin state or valence state at the Fe3+ site up to 15.3(5) GPa.

Computationally Directed Discovery of MoBi2.

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.

Pressure-Induced Collapse of Magnetic Order in Jarosite.

We report a pressure-induced phase transition in the frustrated kagomé material jarosite at ∼45 GPa, which leads to the disappearance of magnetic order. Using a suite of experimental techniques, we characterize the structural, electronic, and magnetic changes in jarosite through this phase transition. Synchrotron powder x-ray diffraction and Fourier transform infrared spectroscopy experiments, analyzed in aggregate with the results from density functional theory calculations, indicate that the material changes from a R3[over ¯]m structure to a structure with a R3[over ¯]c space group. The resulting phase features a rare twisted kagomé lattice in which the integrity of the equilateral Fe^{3+} triangles persists. Based on symmetry arguments we hypothesize that the resulting structural changes alter the magnetic interactions to favor a possible quantum paramagnetic phase at high pressure.

Insights into Single-Molecule-Magnet Behavior from the Experimental Electron Density of Linear Two-Coordinate Iron Complexes.

A breakthrough in the study of single-molecule magnets occurred with the discovery of zero-field slow magnetic relaxation and hysteresis for the linear iron(I) complex [Fe(C(SiMe3)3)2]- (1), which has one of the largest spin-reversal barriers among mononuclear transition-metal single-molecule magnets. Theoretical studies have suggested that the magnetic anisotropy in 1 is made possible by pronounced stabilization of the iron d z2 orbital due to 3d z2-4s mixing, an effect which is predicted to be less pronounced in the neutral iron(II) complex Fe(C(SiMe3)3)2 (2). However, experimental support for this interpretation has remained lacking. Here, we use high-resolution single-crystal X-ray diffraction data to generate multipole models of the electron density in these two complexes, which clearly show that the iron d z2 orbital is more populated in 1 than in 2. This result can be interpreted as arising from greater stabilization of the d z2 orbital in 1, thus offering an unprecedented experimental rationale for the origin of magnetic anisotropy in 1.

Discovery of Cu3Pb.

Materials discovery enables both realization and understanding of new, exotic, physical phenomena. An emerging approach to the discovery of novel phases is high-pressure synthesis within diamond anvil cells, thereby enabling in situ monitoring of phase formation. Now, the discovery via high-pressure synthesis of the first intermetallic compound in the Cu-Pb system, Cu3Pb is reported. Cu3Pb is notably the first structurally characterized mid- to late-first-row transition-metal plumbide. The structure of Cu3Pb can be envisioned as a direct mixture of the two elemental lattices. From this new framework, we gain insight into the structure as a function of pressure and hypothesize that the high-pressure polymorph of lead is a possible prerequisite for the formation of Cu3Pb. Crucially, electronic structure computations reveal band crossings near the Fermi level, suggesting that chemically doped Cu3Pb could be a topologically nontrivial material.

Impact of Pressure on Magnetic Order in Jarosite.

Jarosite, a mineral with a kagomé lattice, displays magnetic frustration yet orders magnetically below 65 K. As magnetic frustration can engender exotic physical properties, understanding the complex magnetism of jarosite comprises a multidecade interdisciplinary challenge. Unraveling the nature of the disparate magnetic coupling interactions that lead to magnetic order in jarosite remains an open question. Specifically, there is no observed trend in the interlayer spacing with magnetic order. Similarly, the relationship between metal-ligand bond distance and magnetic order remains uninvestigated. Here, we use applied pressure to smoothly vary jarosite's structure without manipulating the chemical composition, enabling a chemically invariant structure-function study. Using single-crystal and powder X-ray diffraction, we show that high applied pressures alter both the interlayer spacing and the metal-ligand bond distances. By harnessing a suite of magnetic techniques under pressure, including SQUID-based magnetometry, time-resolved synchrotron Mössbauer spectroscopy, and X-ray magnetic circular dichroism, we construct the magnetic phase diagram for jarosite up to 40 GPa. Notably, we demonstrate that the magnetic ordering temperature increases dramatically to 240 K at the highest pressures. Additionally, we conduct X-ray emission spectroscopy, Mössbauer spectroscopy, and UV-visible absorption spectroscopy experiments to comprehensively map the magnetic and electronic structures of jarosite at high pressure. We use these maps to construct chemically pure magnetostructural correlations which fully explain the nature and role of the disparate magnetic coupling interactions in jarosite.

High-Pressure Synthesis: A New Frontier in the Search for Next-Generation Intermetallic Compounds.

The application of high pressure adds an additional dimension to chemical phase space, opening up an unexplored expanse bearing tremendous potential for discovery. Our continuing mission is to explore this new frontier, to seek out new intermetallic compounds and new solid-state bonding. Simple binary elemental systems, in particular those composed of pairs of elements that do not form compounds under ambient pressures, can yield novel crystalline phases under compression. Thus, high-pressure synthesis can provide access to solid-state compounds that cannot be formed with traditional thermodynamic methods. An emerging approach for the rapid exploration of composition-pressure-temperature phase space is the use of hand-held high-pressure devices known as diamond anvil cells (DACs). These devices were originally developed by geologists as a way to study minerals under conditions relevant to the earth's interior, but they possess a host of capabilities that make them ideal for high-pressure solid-state synthesis. Of particular importance, they offer the capability for in situ spectroscopic and diffraction measurements, thereby enabling continuous reaction monitoring-a powerful capability for solid-state synthesis. In this Account, we provide an overview of this approach in the context of research we have performed in the pursuit of new intermetallic compounds. We start with a discussion of pressure as a fundamental experimental variable that enables the formation of intermetallic compounds that cannot be isolated under ambient conditions. We then introduce the DAC apparatus and explain how it can be repurposed for use as a synthetic vessel with which to explore this phase space, going to extremes of pressure where no chemist has gone before. The remainder of the Account is devoted to discussions of recent experiments we have performed with this approach that have led to the discovery of novel intermetallic compounds in the Fe-Bi, Cu-Bi, and Ni-Bi systems, with a focus on the cutting-edge methods that made these experiments possible. We review the use of in situ laser heating at high pressure, which led to the discovery of FeBi2, the first binary intermetallic compound in the Fe-Bi system. Our work in the Cu-Bi system is described in the context of in situ experiments carried out in the DAC to map its high-pressure phase space, which revealed two intermetallic phases (Cu11Bi7 and CuBi). Finally, we review the discovery of ß-NiBi, a novel high-pressure phase in the Ni-Bi system. We hope that this Account will inspire the next generation of solid-state chemists to boldly explore high-pressure phase space.

Discovery of FeBi2.

Recent advances in high-pressure techniques offer chemists access to vast regions of uncharted synthetic phase space, expanding our experimental reach to pressures comparable to the core of the Earth. These newfound capabilities enable us to revisit simple binary systems in search of compounds that for decades have remained elusive. The most tantalizing of these targets are systems in which the two elements in question do not interact even as molten liquids-so-called immiscible systems. As a prominent example, immiscibility between iron and bismuth is so severe that no material containing Fe-Bi bonds is known to exist. The elusiveness of Fe-Bi bonds has a myriad of consequences; crucially, it precludes completing the iron pnictide superconductor series. Herein we report the first iron-bismuth binary compound, FeBi2, featuring the first Fe-Bi bond in the solid state. We employed geologically relevant pressures, similar to the core of Mars, to access FeBi2, which we synthesized at 30 GPa and 1500 K. The compound crystallizes in the Al2Cu structure type (space group I4/mcm) with a = 6.3121(3) Å and c = 5.4211(4) Å. The new binary intermetallic phase persists from its formation pressure of 30 GPa down to 3 GPa. The existence of this phase at low pressures suggests that it might be quenchable to ambient pressure at low temperatures. These results offer a pathway toward the realization of new exotic materials.

Discovery of a Superconducting Cu-Bi Intermetallic Compound by High-Pressure Synthesis.

A new intermetallic compound, the first to be structurally identified in the Cu-Bi binary system, is reported. This compound is accessed by high-pressure reaction of the elements. Its detailed characterization, physical property measurements, and ab initio calculations are described. The commensurate crystal structure of Cu11 Bi7 is a unique variation of the NiAs structure type. Temperature-dependent electrical resistivity and heat capacity measurements reveal a bulk superconducting transition at Tc =1.36 K. Density functional theory calculations further demonstrate that Cu11 Bi7 can be stabilized (relative to decomposition into the elements) at high pressure and temperature. These results highlight the ability of high-pressure syntheses to allow for inroads into heretofore-undiscovered intermetallic systems for which no thermodynamically stable binaries are known.

Magnetism and variable temperature and pressure crystal structures of a linear oligonuclear cobalt bis-semiquinonate.

The crystal structure of the first oligomeric cobalt dioxolene complex, Co3(3,5-DBSQ)2((t)BuCOO)4(NEt3)2, 1, where DBSQ is 3,5-di-tert-butyl-semiquinonate, has been studied at various temperatures between 20 and 200 K. Despite cobalt-dioxolene complexes being generally known for their extensive ability to exhibit valence tautomerism (VT), we show here that the molecular geometry of compound 1 is essentially unchanged over the full temperature range, indicating the complete absence of electron transfer between ligand and metal. Magnetic susceptibility measurements clearly support the lack of VT between 8 and 300 K. The crystal structure is also determined at elevated pressures in the range from 0 to 2.5 GPa. The response of the crystal structure is surprisingly dependent on the dynamics of pressurisation: following rapid pressurization to 2 GPa, a structural phase transition occurs; yet, this is absent when the pressure is increased incrementally to 2.6 GPa. In the new high pressure phase, Z' is 2 and one of the two molecules displays changes in the coordination of one bridging carboxylate from µ2:κO:κO' to µ2:κ(2)O,O':κO', while the other molecule remains unchanged. Despite the significant changes to the molecular connectivity, analysis of the crystal structures shows that the phase transition leaves the spin and oxidation states of the molecules unaltered. Intermolecular interactions in the high pressure crystal structures have been analysed using Hirshfeld surfaces but they cannot explain the origin of the phase transition. The lack of VT in this first oligomeric Co-dioxolene complex is speculated to be due to the coordination geometry of the terminal Co-atoms, which are trigonal bipyramidally coordinated, different from the more common octahedral coordination. The energy that is gained by a hs-to-ls change in Oh is equal to Δ, while in the case of the trigonal bipyramidal (C3v), the energy gain is equal to the splitting between d(z(2)) and degenerate d(x(2) - y(2))/d(xy), which is significantly less.

Electronic Structure of a Mixed-Metal Fluoride-Centered Triangle Complex: A Potential Qubit Component.

A novel fluoride-centered triangular-bridged carboxylate complex, [Ni2Cr(µ3-F)(O2C(t)Bu)6(HO2C(t)Bu)3] (1), is reported. Simple postsynthetic substitution of the terminal pivalic acids in 1 with pyridine and 4-methylpyridine led to the isolation of [Ni2Cr(µ3-F)(O2C(t)Bu)6(C5H5N)3] (2) and [Ni2Cr(µ3-F)(O2C(t)Bu)6((4-CH3)C5H4N)3] (3). Structural and magnetic characterizations carried out on the series reveal a dominating antiferromagnetic interaction between the nickel and chromium centers leading to an S = (1)/2 ground state with a very unusual value of geff = 2.48.

Discrete and polymeric cobalt organophosphates: isolation of a 3-D cobalt phosphate framework exhibiting selective CO2 capture.

Structurally diverse mononuclear, dinuclear, and tetranuclear cobalt organophosphates and a three-dimensional framework based on a D4R cobalt phosphate are reported. The role of auxiliary ligands in determining the nuclearity of the phosphate clusters has further been established. Reaction of cobalt acetate tetrahydrate with 2,6-di-iso-propylphenylphosphate (dippH2) in methanol or DMSO in the presence of ancillary N-donor ligands leads to the formation of mononuclear octahedral cobalt phosphate [Co(dippH)2(py)4] (1), dinuclear octahedral cobalt phosphates [Co(dipp)(NN)(MeOH)2]2·2MeOH (NN = bpy 2; phen 3), tetrahedral cobalt phosphates [Co(dipp)(L)2]2·2(MeOH) (L = imz 4; dmpz 5) and symmetric and asymmetric tetranuclear tetrahedral cobalt phosphates [Co(dipp)(2-apy)]4 (6) and [Co4(dipp)4(2-apy)3(DMSO)]·(DMSO)·(H2O) (7), in nearly quantitative yields. The use of a linear N-donor ditopic linker, 3,6-di(pyridin-4-yl)-1,2,4,5-tetrazine (dptz), as the ancillary ligand leads to the formation of a robust three dimensional, four-fold interpenetrated network based on the D4R platform, {[Co(dipp)(dptz)0.5]4}n (8), under ambient conditions. The new compounds have been characterized by analytical, thermo-analytical and spectroscopic techniques. Further, the molecular structures of compounds 1-8 have been established using single crystal X-ray diffraction studies. Compound 1 is a mononuclear complex in which the dippH ligands occupy trans-positions around the octahedral cobalt ion. The core structure of compounds 2-5, a Co2P2O4 ring, resembles the S4R (single-4-ring) SBU of zeolites, whereas the Co4P4O12 inorganic core found in compounds 6 and 7 resembles the D4R (double-4-ring) SBU. Cobalt organophosphate framework 8 shows significant CO2 adsorption at 273 K (7.73 wt% at 1 bar and 18.21 wt% at 15.5 bar) with high selectivity to CO2 uptake over N2 and H2 at 273 K. Magnetic studies of these symmetric complexes indicate the presence of weak antiferromagnetic interactions between the metal ions via the phosphate bridging moiety.

A synthetic strategy for switching the single ion anisotropy in tetrahedral Co(II) complexes.

Four novel mononuclear tetrahedral cobalt(II) complexes containing exocyclic mesoionic ligands of molecular formulae [Co(II)(L1)(X)2(MeCN)] X = Cl (1) or Br (2) and [Co(II)(L2)(X)2(MeCN)], X = Cl (3) or Br (4) have been reported. It is found that simple substitution of L1 (O donor in 1 and 2) by L2 (S donor in 3 and 4) results in switching of the single ion magnetic anisotropy parameter (D) from positive to negative, with a significant change in magnitude.

Synthesis, structure, and magnetism of non-planar heptanuclear lanthanide(III) complexes.

The reaction of the hydrazone, 2-methoxy-6-(pyridin-2-yl-hydrazonomethyl) phenol (LH) with lanthanide(iii) nitrate salts in the presence of excess triethylamine afforded the heptanuclear Ln(iii) complexes: [Gd7(L)6(µ3-OH)8(NO3)4(H2O)]·(NO3)3·8CH3CN·H2O (), [Tb7(L)6(µ3-OH)8(NO3)4]·(NO3)3·9CH3CN·2CH3OH·3H2O (), [Dy7(L)6(µ3-OH)8(NO3)4(H2O)]·(NO3)3·7CH3CN·3H2O (), [Ho7(L)6(µ3-OH)8(NO3)4]·(NO3)3·11CH3CN·2CH3OH·2H2O () and [Er7(L)6(µ3-OH)8(NO3)4]·(NO3)3·8CH3CN·2CH3OH (). Single crystal X-ray diffraction studies reveal that these complexes are tri-cationic, possessing three nitrate counter anions. The heptanuclear ensemble is non-planar and consists of six [L](-) and eight µ3-OH ligands. These compounds show an interesting structural motif with two incomplete cubes fused to each other through a common Ln(iii) ion. Compound exhibits a magnetocaloric effect, with (-ΔSm(T) = 27.7 J kg(-1) K(-1) at 3 K and under a field change of 0-7 T), while compound shows slow magnetic relaxation at very low temperatures.

Relationships between electron density and magnetic properties in water-bridged dimetal complexes.

The electron densities in two analogous dimetallic transition metal compounds, namely, [M2(µ-OH2)((t)BuCOO)4((t)BuCOOH)2(C5H5N)2] (M = Co(1), Ni(2)), were determined from combined X-ray and neutron single-crystal diffraction at 100 K. Excellent correspondence between the thermal parameters from X- and N-derived atomic displacement parameters is found, indicating high-quality X-ray data and a successful separation of thermal and electronic effects. Topological analysis of electron densities derived from high-resolution X-ray diffraction, as well as density functional theory calculations, shows no direct metal-metal bonding in either compound, while the total energy density at the bond critical points suggests stronger metal-oxygen interactions for the Ni system, in correspondence with its shorter bond distances. The analysis also allows for estimation of the relative strength of binding of terminal and bridging ligands to the metals, showing that the bridging water molecule is more strongly bound than terminal carboxylic acid, but less so than bridging carboxylates. Recently, modeling of magnetic and spectroscopic data in both of these systems has shown weak ferromagnetic interactions between the metal atoms. Factors related to large zero-field splitting effects complicate the magnetic analysis in both compounds, albeit to a much greater degree in 1. The current results support the conclusion drawn from previous magnetic and spectroscopic measurements that there is no appreciable direct communication between metal centers.

Self-assembly of a 3d-5f trinuclear single-molecule magnet from a pentavalent uranyl complex.

Mixed-metal uranium compounds are very attractive candidates in the design of single-molecule magnets (SMMs), but only one 3d-5f hetero-polymetallic SMM containing a uranium center is known. Herein, we report two trimeric heterodimetallic 3d-5f complexes self-assembled by cation-cation interactions between a uranyl(V) complex and a TPA-capped M(II)  complex (M=Mn (1), Cd (2); TPA=tris(2-pyridylmethyl)amine). The metal centers were strategically chosen to promote the formation of discrete molecules rather than extended chains. Compound 1, which contains an almost linear {MnOUOMn} core, exhibits SMM behavior with a relaxation barrier of 81±0.5 K-the highest reported for a mono-uranium system-arising from intramolecular Mn-U exchange interactions combined with the high Ising anisotropy of the uranyl(V) moiety. Compound 1 also exhibits an open magnetic hysteresis loop at temperatures less than 3 K, with a significant coercive field of 1.9 T at 1.8 K.

On the possibility of magneto-structural correlations: detailed studies of dinickel carboxylate complexes.

A series of water-bridged dinickel complexes of the general formula [Ni2(µ2-OH2)(µ2-O2C(t)Bu)2(O2C(t)Bu)2(L)(L')] (L = HO2C(t)Bu, L' = HO2C(t)Bu (1), pyridine (2), 3-methylpyridine (4); L = L' = pyridine (3), 3-methylpyridine (5)) has been synthesized and structurally characterized by X-ray crystallography. The magnetic properties have been probed by magnetometry and EPR spectroscopy, and detailed measurements show that the axial zero-field splitting, D, of the nickel(II) ions is on the same order as the isotropic exchange interaction, J, between the nickel sites. The isotropic exchange interaction can be related to the angle between the nickel centers and the bridging water molecule, while the magnitude of D can be related to the coordination sphere at the nickel sites.

Tetranuclear lanthanide(III) complexes in a seesaw geometry: synthesis, structure, and magnetism.

The reaction of 2-methoxy-6-(pyridin-2-ylhydrazonomethyl)phenol (LH) with Ln(III) (Ln = Gd, Tb, Dy, Ho) salts in the presence of an excess of triethylamine afforded [Gd4(L)4(µ4-OH)(µ3-OH)2(NO3)4]·(NO3)·4CH3CN·CH3OH·2H2O (1), [Tb4(L)4(µ4-OH)(µ3-OH)2(NO3)4]·(NO3)·4CH3CN·3H2O (2), [Dy4(L)4(µ4-OH)(µ3-OH)2(NO3)4]·(NO3)·6CH3CN·H2O (3), and [Ho4(L)4(µ4-OH)(µ-OH)2(NO3)4]·(NO3)·8CH3CN·3CH3OH·2H2O (4). All four complexes contain a monocationic tetranuclear core with a unique seesaw topology. The tetranuclear assembly is formed through the coordination of four [L](-), one µ4-OH, two µ3-OH, and four chelating nitrate ligands, with a charge-balancing nitrate counteranion. Magnetic studies reveal a weak antiferromagnetic coupling throughout the series. Compound 1 can be modeled well with an isotropic exchange between all centers parametrized by J = -0.09 cm(-1). Compound 3 exhibits slow magnetic relaxation at low temperatures.