First-principles study of large-amplitude dynamic Jahn–Teller effects in vanadium tetrafluoride

Does Cr3+ Occupy Tetrahedral Sites and Luminesce in Oxides? A First-Principles Exploration

The Cr³⁺ activators have been adopted to produce desired near-infrared broadband emission via ligand field engineering by choosing hosts with appropriate sites. First-principles calculations help to analyze the site, valence, and luminescent mechanism of the activators. Our calculations on Mg₂Al₄Si₅O₁₈:Cr elucidate that the activators are dominated by Cr³⁺ at tetrahedral Al and octahedral Mg sites, while the experimentally reported near-infrared emission previously assigned to tetrahedral sites is actually produced by Cr³⁺ at the octahedral site. Meanwhile, our results show that the emission energies of Cr³⁺ activators at octahedral sites can be well predicted. Moreover, further calculations show that the quenching of the ⁴T₂ → ⁴T₁ transition of Cr³⁺ at a tetrahedral site is general due to nonradiative relaxation pathways mediated by sublevels split off from the ⁴T₁ multiplet states by intrinsic or Jahn-Teller distortions. Our work shows that the sophisticated first-principles calculations put together here can be effective in exploring Cr³⁺ and potentially more general activators in crystals, which benefit the design and optimization of luminescent materials.

Angular Jahn-Teller Effect and Photoluminescence of the Tetrahedral Coordinated Mn2+ Activators in Solids-A First-Principles Study

First-principles calculations based on density functional theory have been performed to investigate the electronic structure, excited-state Jahn-Teller distortion, and photoluminescence of the multielectron d⁵ system of the strongly covalent tetrahedral coordinated Mn²⁺ activator in solids. The electronic structure of the ⁴T₁ and ⁴A₁/⁴E excited states is analyzed, and Slater's transition-state method and occupation matrix control methodology are applied to deal with the spin contamination in the lower-spin excited states, which is due to the mixing of the ground state of the same spin projection number. In a series of covalent tetrahedral coordinations, the ⁶A₁ → ⁴T₁ and ⁴A₁/⁴E excitations and the ⁴T₁ → ⁶A₁ emission energies are obtained and compared to the reported experimental results. The nephelauxetic effect follows O^2- < S^2- ≈ Se^2- < N^3-, and the larger nephelauxetic effect and crystal field strength lead to the red-shifted emission of nitride phosphors. The Jahn-Teller distortion of the ⁴T₁ states is dominated by the e-type angular distortion of the [MnL₄] moiety (L being the ligand), which accounts for the small Stokes shift of tetrahedral coordinated Mn²⁺. The results show that the ground- and excited-state electronic and geometric structures and the luminescent property of tetrahedral coordinated Mn²⁺ can be reliably predicted. The method can be further explored to interpret and discriminate the luminescent properties of materials containing a variety of different Mn²⁺ sites and complexes and even other transition metals.

Jahn-Teller Effect in Orthorhombic Manganites: Ab Initio Hamiltonian and Roto-vibrational Spectrum

For the first time, using three different electronic structure methodologies, namely, CASSCF, RS2c, and MRCI(SD), we construct ab initio adiabatic potential energy surfaces (APESs) and nonadiabatic coupling term (NACT) of two electronic states (⁵E_g) of MnO₆^9- unit, where eight such units share one La atom in LaMnO₃ crystal. While fitting those APESs with analytic functions of normal modes (Q_x, Q_y), an empirical scaling factor is introduced considering the mass ratio of eight MnO₆^9- units with and without one La atom to explore the environmental (mass) effect on MnO₆^9- unit. When the roto-vibrational levels of MnO₆^9- Hamiltonian are calculated, peak positions computed from ab initio constructed excited APESs are found to be enough close with the experimental satellite transitions [ J. Exp. Theor. Phys. 2016, 122, 890-901] endorsing our earlier model results [ J. Chem. Phys. 2019, 150, 064703]. In order to explore the electron-nuclear coupling in an alternate way, theoretically "exact" and numerically "accurate" beyond Born-Oppenheimer (BBO) theory based diabatic potential energy surfaces (PESs) of MnO₆^9- are constructed to generate the photoelectron (PE) spectra. The PE spectral band also exhibits good peak by peak correspondence with the higher satellite transitions in the dielectric function spectra of the LaMnO₃ complex.

Strong static and dynamic Jahn-Teller and pseudo-Jahn-Teller effects in niobium tetrafluoride

We present a first-principles study of the static and dynamic aspects of the strong Jahn-Teller (JT) and pseudo-JT (PJT) effects in niobium tetrafluoride, NbF₄, in the manifold of its electronic ground state, ²E, and its first excited state, ²T₂. The complex topography of the full-dimensional multi-sheeted adiabatic JT/PJT surfaces is analyzed computationally at the complete-active-space self-consistent-field (CASSCF) and multireference second-order perturbation levels of electronic structure theory, providing a detailed characterization of minima, saddle points, and minimum-energy conical intersection points. The calculations reveal that the tetrahedral (T_d) configuration of NbF₄ undergoes strong JT distortions along the bending mode of e symmetry, yielding tetragonal molecular structures of D_2d symmetry with T_d → D_2d stabilization energies of about 2000 cm^-1 in the X̃²E state and about 6400 cm^-1 in the òT₂ state. In addition, there exists strong X̃²E-òT₂ PJT coupling via the bending mode of t₂ symmetry, which becomes important near the crossing seam of the X̃²E and òT₂ potential energy surfaces. A five-state five-mode JT/PJT vibronic-coupling Hamiltonian is constructed in terms of symmetry-invariant polynomial expansions of the X̃²E and òT₂ diabatic potential energy surfaces in the e and t₂ bending coordinates. The parameters of the Hamiltonian are determined by a least-squares fit of its eigenvalues to the CASSCF ab initio data. The vibronic spectra and the time evolution of adiabatic electronic population probabilities are computed with the multi-configuration time-dependent Hartree method. The complexity of the spectra reflects the effects of the exceptionally strong E × e and T₂ × e JT couplings and (E + T₂) × (e + t₂) PJT coupling. The time evolution of the populations of the adiabatic electronic states after the initial preparation of the òT₂ state reveals the femtosecond nonadiabatic dynamics through a multidimensional seam of conical intersection. These results represent the first study of the static and dynamical JT/PJT effects in the X̃²E and òT₂ electronic states of NbF₄.