Electronic Structure and d-d Spectrum of Metal-Organic Frameworks with Transition-Metal Ions

The electronic structure of metal-organic frameworks (MOFs) containing transition metal (TM) ions represents a significant and largely unresolved computational challenge due to limited solutions to the quantitative description of low-energy excitations in open d-shells. These excitations underpin the magnetic and sensing properties of TM MOFs, including the observed remarkable spin-crossover phenomenon. We introduce the effective Hamiltonian of crystal field approach to study the d-d spectrum of MOFs containing TM ions; this is a hybrid QM/QM method based on the separation of crystal structure into d- and s,p-subsystems treated at different levels of theory. We test the method on model frameworks, carbodiimides, and hydrocyanamides and a series of M-MOF-74 (M = Fe, Co, Ni) and compare the computational predictions to experimental data on magnetic properties and Mössbauer spectra.

(Mg,Mn,Fe,Co,Ni)O: A rocksalt high-entropy oxide containing divalent Mn and Fe

High-entropy oxides (HEOs) have aroused growing interest due to fundamental questions relating to their structure formation, phase stability, and the interplay between configurational disorder and physical and chemical properties. Introducing Fe(II) and Mn(II) into a rocksalt HEO is considered challenging, as theoretical analysis suggests that they are unstable in this structure under ambient conditions. Here, we develop a bottom-up method for synthesizing Mn- and Fe-containing rocksalt HEO (FeO-HEO). We present a comprehensive investigation of its crystal structure and the random cation-site occupancy. We show the improved structural robustness of this FeO-HEO and verify the viability of an oxygen sublattice as a buffer layer. Compositional analysis reveals the valence and spin state of the iron species. We further report the antiferromagnetic order of this FeO-HEO below the transition temperature ~218 K and predict the conditions of phase stability of Mn- and Fe-containing HEOs. Our results provide fresh insights into the design and property tailoring of emerging classes of HEOs.

Metastable FeCN2@nitrogen-doped carbon with high pseudocapacitance as an anode material for sodium ion batteries

Pseudocapacitive materials are good candidates for fast charging anodes of sodium ion batteries (SIB). However, pseudocapacitive materials with a high surface area face the severe problem of low initial coulombic efficiency. In this work, micro-sized nitrogen-doped carbon (NC) coated and supported polyhedron FeCN₂ networks are designed and synthesized by a facile in situ gel-swelling technique. Impressively, FeCN₂@NC as an SIB anode exhibits excellent rate performance with highly reversible rate capacities of 466 and 303 mA h g^-1 at 0.2 and 10.0 A g^-1, respectively. Furthermore, the FeCN₂@NC anode shows a high initial coulombic efficiency (ICE) of 86% due to a low surface area. Electrochemical tests and density functional theory (DFT) calculation indicate that the metastable character enables the low intercalation/conversion reaction energy for FeCN₂ and further greatly promotes the fast pseudocapacitive storage mechanism for FeCN₂@NC. This work provides evidence that FeCN₂ is a new type of metastability induced pseudocapacitive material with high initial coulombic efficiency.

Nuclear resonant scattering from 193Ir as a probe of the electronic and magnetic properties of iridates

The high brilliance of modern synchrotron radiation sources facilitates experiments with high-energy x-rays across a range of disciplines, including the study of the electronic and magnetic correlations using elastic and inelastic scattering techniques. Here we report on Nuclear Resonance Scattering at the 73 keV nuclear level in ¹⁹³Ir. The transitions between the hyperfine split levels show an untypically high E2/M1 multi-polarity mixing ratio combined with an increased sensitivity to certain changes in the hyperfine field direction compared to non-mixing transitions. The method opens a new way for probing local magnetic and electronic properties of correlated materials containing iridium and provides novel insights into anisotropic magnetism in iridates. In particular, unexpected out-of-plane components of magnetic hyperfine fields and non-zero electric field gradients in Sr₂IrO₄ have been detected and attributed to the strong spin-orbit interaction in this iridate. Due to the high, 62% natural abundance of the ¹⁹³Ir isotope, no isotopic enrichment of the samples is required, qualifying the method for a broad range of applications.