Using first-principles density functional calculations, we study the electronic and magnetic properties of the ferromagnetic insulating double perovskite compound ${\mathrm{La}}_2{\mathrm{NiMnO}}_6$, which has been reported to exhibit an interesting magnetic field sensitive dielectric anomaly as a function of temperature. Our study reveals the existence of very soft infrared active phonons that couple strongly with spins at the Ni and Mn sites through modification of the superexchange interaction. We suggest that these modes are the origin for the observed dielectric anomaly in ${\mathrm{La}}_2{\mathrm{NiMnO}}_6$.

The conversion of solar energy into chemical energy or high-value chemicals has attracted considerable research interest in the context of the global energy crisis. Hydrogen peroxide (H₂O₂) is a versatile and powerful oxidizing agent widely used in chemical synthesis and medical disinfection. H₂O₂ also serves as a clean energy source in fuel cells, generating electricity with zero-carbon emissions. Recently, the sustainable production of H₂O₂ from water and oxygen using covalent organic frameworks (COFs) as photocatalysts has attracted considerable attention; however, systematic studies highlighting the role of linkages in determining photocatalytic performance are scarce. Under these circumstances, herein, we demonstrate that varying the imine and hydrazone linkages within the framework significantly influences photocatalytic H₂O₂ production. COFs with high-density hydrazone linkages, providing optimal docking sites for water and oxygen, enhance H₂O₂ generation activity (1588 μmol g⁻¹ h⁻¹ from pure water in the air), leading to highly efficient solar-to-chemical energy conversion.

In the present Rapid Communication, we explore superconductivity in $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_2$ and $\mathrm{La}\mathrm{Ni}{\mathrm{O}}_2$ employing a first-principles derived low-energy model Hamiltonian, consisting of two orbitals: Ni $x^2-y^2$, and an axial orbital. The axial orbital is constructed out of Nd/La $d$, Ni $3z^2-r^2$, and Ni $s$ characters. Calculation of the superconducting pairing symmetry and pairing eigenvalue of the spin-fluctuation mediated pairing interaction underlines the crucial role of the interorbital Hubbard interaction in superconductivity, which turns out to be orbital selective. The axial orbital brings in material dependence to the problem, making $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_2$ different from $\mathrm{La}\mathrm{Ni}{\mathrm{O}}_2$, thereby controlling the interorbital Hubbard interaction-assisted superconductivity.

Double perovskites, which can host two different transition metal cations at the B-sites of its perovskite derived structure, provide the possibility to explore the interplay of localized 3d transition metals and the relatively delocalized 4d or 5d transition metals within the same structure. This interplay gives rise to extraordinary magnetic properties. In this overview article, we summarize our recent computational efforts in understanding the curious properties of magnetic double perovskites with 3d and 4d/5d transition metal ions, make predictions on new functionalities in known compounds of this family, engineering functionalities via chemical modifications/doping and prediction on a new set of magnetic 3d-4d/5d double perovkite compounds, which may be synthesized to explore and expand further this interesting family.

The quest for Kitaev quantum spin liquids has led to great interest in honeycomb quantum magnets with strong spin-orbit coupling. It has been recently proposed that even Mott insulators with $3d$ transition-metal ions, having nominally weak spin-orbit coupling, can realize such exotic physics. Motivated by this, we study the rhombohedral honeycomb cobaltates ${\mathrm{CoTiO}}_3,$ ${\mathrm{BaCo}}_2{\left({\mathrm{PO}}_4\right)}_2$, and ${\mathrm{BaCo}}_2{\left({\mathrm{AsO}}_4\right)}_2$, using ab initio density-functional theory, which takes into account realistic crystal-field distortions and chemical information, in conjunction with exact diagonalization numerics. We show that these ${\mathrm{Co}}^{2+}$ magnets host $J_{\mathrm{eff}}=1/2$ local moments with highly anisotropic $g$ factors, and we extract their full spin Hamiltonians including longer-range and anisotropic exchange couplings. For ${\mathrm{CoTiO}}_3$, we find a nearest-neighbor easy-plane ferromagnetic $XXZ$ model with additional bond-dependent anisotropies and interlayer exchange, which supports three-dimensional Dirac nodal line magnons. In contrast, for ${\mathrm{BaCo}}_2{\left({\mathrm{PO}}_4\right)}_2$ and ${\mathrm{BaCo}}_2{\left({\mathrm{AsO}}_4\right)}_2$, we find a strongly suppressed interlayer coupling, and significant frustration from additional third-neighbor antiferromagnetic exchange mediated by P or As. Such bond-anisotropic $J_1\text{−}{J}_3$ spin models can support collinear zigzag or coplanar spiral ground states. We discuss their dynamical spin correlations which reveal a gapped Goldstone mode and argue that the effective parameters of the pseudospin-$1/2$ models in these two materials may be strongly renormalized by coupling to a low-energy spin exciton. Our results call for reexamining proposals for realizing Kitaev spin liquids in the honeycomb cobaltates.

With the aim of manipulating the mechanical properties of the recently discussed two-dimensional material MXene, we investigate the effect of alloying. We consider substitutional doping of B and V at Ti and C sites of ${\mathrm{Ti}}_2\mathrm{C}$. Calculations of quantities such as in-plane stiffness, Young's modulus, and critical strain through rigorous first-principles technique establish that B doping is highly effective in improving the elastic properties. Oxygen passivation of B-doped ${\mathrm{Ti}}_2\mathrm{C}$ in addition to improved elastic properties also exhibits reasonably high critical strains making them ideally suited for applications in flexible devices. Our study further reveals the presence of strong spin-phonon coupling in unpassivated ${\mathrm{Ti}}_2\mathrm{C}$ compounds which influences the mechanical behavior. The damage of ${\mathrm{Ti}}_2\mathrm{C}$ in its magnetic ground state of A-type antiferromagnetic structure is found to occur at much higher strain than that of the nonmagnetic ${\mathrm{Ti}}_2\mathrm{C}$.