Jakub Hrubý

The synthesis of cubic boron nitride (cBN) solids by consolidation of cBN powders at high pressure and high temperature (HPHT) is challenging, as it leads to the hexagonal BN (hBN) phase. Here we report an HPHT synthesis of polycrystalline fluorinated cBN (F-cBN) solids that show twinned nanodomains, the presence of F at the grain boundaries, and excellent mechanical hardness and second-order nonlinear susceptibility. In contrast to pure diamagnetic cBN, we found that F-cBN is paramagnetic at room temperature, as confirmed by electron paramagnetic resonance spectroscopy, suggesting the role of defects in tuning the magnetic properties of cBN. Density functional theory calculations reveal that F stabilizes the cBN phase by passivating the dangling bonds and, in addition, forms substitutional defects (FN) leading to FN-VB defect complexes that generate local magnetic moments. These properties of F-cBN might lead to new applications of cBN and also open a pathway to synthesize new metastable phase materials with emergent properties.

Magnetic exchange coupling is difficult to foster in polynuclear lanthanide (Ln) complexes and poorly understood. While coupling Ln ions through closed-shell ligands is inherently weak due to the contracted 4f orbitals, placing open-shell ligands instead has proven to promote orders of magnitude stronger coupling, giving rise to single-molecule magnets (SMMs) innate to real magnetic memory effect in the case of the anisotropic Ln ions. Notably, the impact of radical bridges with differing oxidation states on magnetic blocking remains unexplored due to lack of Ln SMMs with radicals in two distinct charge states. Herein, the first dilanthanide complexes (Ln = Gd, Dy) containing fluoflavine (flv) bridges, [(Cp*₂Ln)₂(μ-flv^z)]X, (where X = [Al(OC{CF₃}₃)₄]^- (z = 1-•), 1-Ln; X = 0 (z = 2-), 2-Ln; X =[K(crypt-222)]⁺ (z = 3-•), 3-Ln) are reported. 1-Ln and 3-Ln, comprising the flv^1-• and flv^3-• radical bridges, were investigated via single-crystal X-ray diffraction (SCXRD), ultraviolet-visible (UV-vis) spectroscopy, Superconducting Quantum Interference Device (SQUID) magnetometry, high-field electron paramagnetic resonance (HF-EPR) spectroscopy and broken-symmetry density functional theory (BS-DFT) calculations. 1-Dy and 3-Dy constitute the first SMMs innate to radicals in two differing oxidation states. 1-Dy exhibits a spin-reversal barrier U_eff of 28.36 cm^-1 and open magnetic hysteresis loops below 3 K. By contrast, 3-Dy displays a much higher U_eff of 143(2) cm^-1 and open hysteresis loops until 9.5 K, representing a record for dilanthanide SMMs containing an organic radical bridge. The boost in SMM properties in 3-Dy is attributed to spin-phonon coupling and improved frontier orbital structure. This study paves the way for advanced design strategies of polynuclear Ln SMMs.

The synthesis of molecules with strong coupling between electronic and nuclear spins represents an important challenge in molecular quantum information science. Here, we report the synthesis and characterization of the divalent lutetium metallocene complexes Lu(Cp^Me₅)(Cp^iPr₅) (Cp^Me₅ = pentamethylcyclopentadienyl; Cp^iPr₅ = pentaisopropylcyclopentadienyl), Lu(Cp^iPr₄Et)₂ (Cp^iPr₄Et = ethyltetraisopropylcyclopentadienyl), and Lu(Cp^iPr₄)₂ (Cp^iPr₄ = tetraisopropylcyclopentadienyl). The molecular structures of these complexes, as determined through single-crystal X-ray diffraction, feature a common bent sandwich geometry, with average Cp-Lu-Cp angles ranging from 159.9° to 152.6°. Analysis of continuous-wave electron paramagnetic resonance (EPR) spectra for the complexes reveals nearly isotropic g tensors with only a slight deviation from that of a free electron. Moreover, an extremely large splitting of the eight-line spectra indicates the presence of strong hyperfine coupling, and simulations provide isotropic hyperfine coupling constants of A_iso = 4.38, 4.30, and 4.17 GHz across the series, where the value of A_iso is found to decrease as the Cp-Lu-Cp angle becomes more acute. Notably, these values are the largest yet observed for any lanthanide complex. Moreover, EPR and computational analysis show that the large values of A_iso stem from large s-orbital character─up to 41.2%─in the corresponding singly occupied molecular orbitals. To our knowledge, this degree of s-character in a molecular orbital is the largest yet reported for an open-shell isolable complex. These results outline a general strategy toward the isolation of paramagnetic molecules with strong hyperfine coupling and highly isotropic doublet electronic ground states.

Attaining sub-Kelvin temperatures remains technologically challenging and often relies on the scarce resource ³He, unless employing adiabatic demagnetization refrigeration. Herein, the active coolant typically consists of weakly coupled paramagnetic ions, whose magnetic interaction strengths are comparable in energy to the relevant temperature regime of cooling. Such interactions depend strongly on inter-ion distances, fundamentally hindering the realization of dense coolants for sub-Kelvin refrigeration. We present a magnetically concentrated triangular coordination network, Eu_0.9Ba_0.1I₂(pyrazine)₃, featuring the large s = 7/2 moment of Eu(II). Electron paramagnetic resonance, magnetization, and heat capacity measurements reflect antiferromagnetic correlations between the Eu(II) ions and a dominant easy-plane magnetic anisotropy. The ensuing geometric frustration prevents entropy-annihilating magnetic order down to at least 0.17 K. This remarkably low operational temperature for a low-dimensional, molecule-based magnetic refrigerant opens possibilities for on-chip cryogenic refrigeration, especially in scenarios where established inorganic refrigerants cannot be utilized.

Attaining very-low temperatures remains technologically challenging and often relies on the scarce resource 3He, unless employing adiabatic demagnetisation refrigeration. Herein, the active coolant typically consists of weakly coupled paramagnetic ions, whose magnetic interaction strengths are comparable in energy to the relevant temperature regime of cooling. Such interactions depend strongly on inter-ion distances, fundamentally hindering the realisation of dense coolants for sub-Kelvin refrigeration. We present a magnetically concentrated triangular coordination network, Eu0.9Ba0.1I2(pyrazine)3, featuring the large s = 7/2 moment of Eu(II). Electron paramagnetic resonance, magnetization, and heat capacity measurements reflect antiferromagnetic correlations between the Eu(II) ions and a dominant easy-plane magnetic anisotropy. The ensuing geometric frustration prevents entropy-annihilating magnetic order down to at least 0.17 K, that is, a remarkably low working temperature for a relatively dense magnetic refrigerant.

We present a proof-of-concept magnetic resonance detection of organic radical 2,2-diphenyl-1-picrylhydrazyl via a nanostructured on-chip graphene quantum dot bolometer. In a common electron paramagnetic resonance setup, the microwave propagates from a source into a sample and back to a detector. Yet, by using on-chip detection, it is possible to skip the whole detection arm and, thus, mitigate lab expenses on instrument and cooling costs. The bolometric detection was demonstrated at a frequency of 151 GHz and a temperature of 15 K.

We present a theoretical and experimental study of two tetracoordinate Co(II)-based complexes with semi-coordination interactions, i.e., non-covalent interactions involving the central atom. We argue that such interactions enhance the thermal and structural stability of the compounds, making them appropriate for deposition on substrates, as demonstrated by their successful deposition on graphene. DC magnetometry and high-frequency electron spin resonance (HF-ESR) experiments revealed an axial magnetic anisotropy and weak intermolecular antiferromagnetic coupling in both compounds, supported by theoretical predictions from complete active space self-consistent field calculations complemented by N-electron valence state second-order perturbation theory (CASSCF-NEVPT2), and broken-symmetry density functional theory (BS-DFT). AC magnetometry demonstrated that the compounds are field-induced single-ion magnets (SIMs) at applied static magnetic fields, with slow relaxation of magnetization governed by a combination of quantum tunneling, Orbach, and direct relaxation mechanisms. The structural stability under ambient conditions and after deposition was confirmed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Theoretical modeling by DFT of different configurations of these systems on graphene revealed n-type doping of graphene originating from electron transfer from the deposited molecules, confirmed by electrical transport measurements and Raman spectroscopy.

Studying the properties of complex molecules on surfaces is still mostly an unexplored research area because the deposition of the metal complexes has many pitfalls. Herein, we probed the possibility to produce surface hybrids by depositing a Co(II)-based complex with chalcone ligands on chemical vapor deposition (CVD)-grown graphene by a wet-chemistry approach and by thermal sublimation under high vacuum_. Samples were characterized by high-frequency electron spin resonance (HF-ESR), XPS, Raman spectroscopy, atomic force microscopy (AFM), and optical microscopy, supported with density functional theory (DFT) and complete active space self-consistent field (CASSCF)/N-electron valence second-order perturbation theory (NEVPT2) calculations. This compound’s rationale is its structure, with several aromatic rings for weak binding and possible favorable π–π stacking onto graphene. In contrast to expectations, we observed the formation of nanodroplets on graphene for a drop-cast sample and microcrystallites localized at grain boundaries and defects after thermal sublimation.

Herein, we report on investigations of magnetic and spectroscopic properties of three heterobimetallic Fe(II)–Co(II) coordination compounds based on the tetracoordinate {CoP₂X₂} core encapsulated by dppf metalloligand, where X = Cl (1), Br (2), I (3), dppf = 1,1′-ferrocenediyl -bis(diphenylphosphine). The analysis of static magnetic data has revealed the presence of axial magnetic anisotropy in compounds (1) and (2) and this was further confirmed by high-frequency electron spin resonance (HF-ESR) spectroscopy. Dynamic magnetic data confirmed that (1) and (2) behave as field-induced Single-Ion Magnets (SIMs). Together with bulk studies, we have also tested the possibility of depositing (2) as thick films on Au(111), glass, and polymeric acetate by drop-casting as well as thermal sublimation, a key aspect for the development of future devices embedding these magnetic objects.

The development of stable and reproducible inorganic pigments is noteworthy for industrial applications mainly considering more intense shades and low toxicity. Among the various candidates to substitute non‐hazardous red and yellow pigments, CeO ₂ and CeO ₂ :Pr have been attracting attention because of their opacity and high‐temperature stability besides being environmental‐friendly and health‐friendly. In this study, nanostructured CeO ₂ and CeO ₂ :Pr samples were synthesized using the polymeric precursor method and structural and optical characterizations were performed. Scanning electron microscopy reveals the morphology of CeO ₂ nanoparticles in which the particle size ranges from 22 to 28 nm as a function of the annealing temperature. Pr‐doping does not show influence on the particle size. XRD results show that CeO ₂ and CeO ₂ :Pr samples crystallize in the cubic fluorite lattice with Fm 3 m space group. Raman spectra show the fluorite F _2g mode, confirming the XRD results. With Pr‐doping and the annealing of the samples, two bands are observed between 550 and 600 cm ⁻¹ , which are related to the defects in the fluorite structure associated with oxygen vacancies. XPS spectra reveal an increase in the ratio of Ce ³⁺ ions depending on the annealing temperature and Pr‐doping. This increase is associated with the carbon removal from the lattice by annealing. This behavior causes a change in the hue of the powders as the annealing temperature increases. According to diffuse reflectance and colorimetric measurements, CeO ₂ shows a light‐yellow color due to the O 2p­Ce 4f transitions whose b* parameter mainly decreases with annealing, becoming almost white. The CeO ₂ :Pr sample exhibits a red‐orange color because of the electronic transitions between 4f ² → 5d ¹ states of Pr ³⁺ . Upon annealing, L* and b* parameters decrease, resulting in a red‐brown shade. The charge compensation or charge transfer is responsible for the modification of the hue of these pigments.