Molecular and Electronic Structures of M2O7 (M = Mn, Tc, Re)

Performance of the r2SCAN Functional in Transition Metal Oxides

We assess the accuracy and computational efficiency of the recently developed meta-generalized gradient approximation (metaGGA) functional, restored regularized strongly constrained and appropriately normed (r²SCAN), in transition metal oxide (TMO) systems and compare its performance against SCAN. Specifically, we benchmark the r²SCAN-calculated oxidation enthalpies, lattice parameters, on-site magnetic moments, and band gaps of binary 3d TMOs against the SCAN-calculated and experimental values. Additionally, we evaluate the optimal Hubbard U correction required for each transition metal (TM) to improve the accuracy of the r²SCAN functional, based on experimental oxidation enthalpies, and verify the transferability of the U values by comparing against experimental properties on other TM-containing oxides. Notably, including the U-correction with r²SCAN increases the lattice parameters, on-site magnetic moments, and band gaps of TMOs, apart from an improved description of the ground state electronic state in narrow band gap TMOs. The r²SCAN and r²SCAN+U calculated oxidation enthalpies follow the qualitative trends of SCAN and SCAN+U, with r²SCAN and r²SCAN+U predicting marginally larger lattice parameters, smaller magnetic moments, and lower band gaps compared to SCAN and SCAN+U, respectively. We observe the overall computational time (i.e., for all ionic+electronic steps) required for r²SCAN(+U) to be lower than SCAN(+U). Thus, the r²SCAN(+U) framework can offer a reasonably accurate description of the ground state properties of TMOs with better computational efficiency than SCAN(+U).

A 70-Year-Old Mystery in Technetium Chemistry Explained by the New Technetium Polyoxometalate [H7 O3 ]4 [Tc20 O68 ] ⋅ 4H2 O

[H₇ O₃ ]₄ [Tc₂₀ O₆₈ ] ⋅ 4H₂ O [1] was prepared from an aqueous Tc₂ O₇ solution concentrated over anhydrous H₂ SO₄ . [Tc₂₀ O₆₈ ]^4- is the first polyanionic species to be reported for Tc. The unit cell contains one centrosymmetric [Tc₂₀ O₆₈ ]^4- polyanion as well as hydronium ions and water molecules. The core of the structure consists of four Tc(V)O₆ octahedra that form a square Tc₄ O₄ ring. The four Tc(V)O₆ octahedra are decorated by sixteen Tc(VII)O₄ tetrahedra. Calculations show the bonding within the Tc₄ O₄ ring to consist of a 3-center bond formed between each neighboring pair of Tc atoms and their bridging oxygen. Calculations also indicate that a strong d→d electronic transition at 513 nm is the origin of the red color of [1]. The characterization of red HTcO₄ solutions by X-ray absorption spectroscopy has complemented the description of this compound in aqueous solution. The formation mechanisms in solution, including the possible role of technetium's radioactivity in the formation of [1], are discussed.

Colossal Density-Driven Resistance Response in the Negative Charge Transfer Insulator MnS_{2}

A reversible density driven insulator to metal to insulator transition in high-spin MnS_{2} is experimentally observed, leading with a colossal electrical resistance drop of 10^{8}  Ω by 12 GPa. Density functional theory simulations reveal the metallization to be unexpectedly driven by previously unoccupied S_{2}^{2-} σ_{3p}^{*} antibonding states crossing the Fermi level. This is a unique variant of the charge transfer insulator to metal transition for negative charge transfer insulators having anions with an unsaturated valence. By 36 GPa the emergence of the low-spin insulating arsenopyrite (P2_{1}/c) is confirmed, and the bulk metallicity is broken with the system returning to an insulative electronic state.

Decoupling Lattice and Magnetic Instabilities in Frustrated CuMnO2

The AMnO₂ delafossites (A = Na, Cu) are model frustrated antiferromagnets, with triangular layers of Mn³⁺ spins. At low temperatures (T_N = 65 K), a C2/m → P1̅ transition is found in CuMnO₂, which breaks frustration and establishes magnetic order. In contrast to this clean transition, A = Na only shows short-range distortions at T_N. Here, we report a systematic crystallographic, spectroscopic, and theoretical investigation of CuMnO₂. We show that, even in stoichiometric samples, nonzero anisotropic Cu displacements coexist with magnetic order. Using X-ray/neutron diffraction and Raman scattering, we show that high pressures act to decouple these degrees of freedom. This manifests as an isostuctural phase transition at ∼10 GPa, with a reversible collapse of the c-axis. This is shown to be the high-pressure analogue of the c-axis negative thermal expansion seen at ambient pressure. Density functional theory (DFT) simulations confirm that dynamical instabilities of the Cu⁺ cations and edge-shared MnO₆ layers are intertwined at ambient pressure. However, high pressure selectively activates the former, before an eventual predicted reemergence of magnetism at the highest pressures. Our results show that the lattice dynamics and local structure of CuMnO₂ are quantitatively different from nonmagnetic Cu delafossites and raise questions about the role of intrinsic inhomogeneity in frustrated antiferromagnets.