Sharp lines due to Cr³⁺ and Mn²⁺ impurities in insulators: going beyond the usual Tanabe-Sugano approach

Understanding Pressure Effects on Structural, Optical, and Magnetic Properties of CsMnF4 and Other 3dn Compounds

The pressure dependence of structural, optical, and magnetic properties of the layered compound CsMnF4 are explored through first-principles calculations. The structure at ambient pressure does not arise from a Jahn-Teller effect but from an orthorhombic instability on MnF63- units in the tetragonal parent phase, while there is a P4/n → P4 structural phase transition at P = 40 GPa discarding a spin crossover transition from S = 2 to S = 1. The present results reasonably explain the evolution of spin-allowed d-d transitions under pressure, showing that the first transition undergoes a red-shift under pressure following the orthorhombic distortion in the layer plane. The energy of such a transition at zero pressure is nearly twice that observed in Na3MnF6 due to the internal electric field and the orthorhombic distortion also involved in K2CuF4. The reasons for the lack of orthorhombic distortion in K2MF4 (M = Ni, Mn) or CsFeF4 are also discussed in detail. The present calculations confirm the ferromagnetic ordering of layers in CsMnF4 at zero pressure and predict a shift to an antiferromagnetic phase for pressures above 15 GPa consistent with the reduction of the orthorhombicity of the MnF63- units. This study underlines the usefulness of first-principles calculations for a right interpretation of experimental findings.

Probing site-selective doping and charge compensating defects in KMgF3: insights from a hybrid DFT study

Design of optoelectronic materials with tunable properties using activators and defect clusters has become one of the prime interests of current research. In this study, detailed Density Functional Theory based calculations have been presented to investigate the geometries and electronic structures of various possible defect clusters using Eu-KMgF3 as a probe which has numerous technological and industrial applications. Using a more reliable hybrid density functional, we have calculated defect formation energies and thermodynamic transition levels to get knowledge about the site selectivity of Eu. It has been observed that the electronic structure of Eu-KMgF3 is not only dependent on the site of doping but also on the oxidation state of Eu (2+/3+). The present study also investigates the relative stability of different kinds of defects and defect clusters under various synthetic growth conditions. The ultimate aim is to find out the microscopic origin of the fundamental optical properties of Eu-KMgF3 and provide an unambiguous explanation of available experimental results. Thus, it has been revealed that doping with Eu results in the spontaneous formation of intrinsic defects, which contribute to the observed optical behaviour. We have also extended our study to investigate the role of codoping with Li in determining the geometry and electronic structure of Eu-KMgF3 aiming to explain its impact on the optical properties. Thus, a complete presentation of the influence of the activator in the absence and presence of lattice defects on the optical properties of KMgF3 has been accomplished in the current study. We strongly believe that the present study will be helpful in designing tunable phosphor materials by a defect-controlled synthesis strategy.

Direct Observation of Cr3+ 3d States in Ruby: Toward Experimental Mechanistic Evidence of Metal Chemistry

The role of transition metals in chemical reactions is often derived from probing the metal 3d states. However, the relation between metal site geometry and 3d electronic states, arising from multielectronic effects, makes the spectral data interpretation and modeling of these optical excited states a challenge. Here we show, using the well-known case of red ruby, that unique insights into the density of transition metal 3d excited states can be gained with 2p3d resonant inelastic X-ray scattering (RIXS). We compare the experimental determination of the 3d excited states of Cr³⁺ impurities in Al₂O₃ with 190 meV resolution 2p3d RIXS to optical absorption spectroscopy and to simulations. Using the crystal field multiplet theory, we calculate jointly for the first time the Cr³⁺ multielectronic states, RIXS, and optical spectra based on a unique set of parameters. We demonstrate that (i) anisotropic 3d multielectronic interactions causes different scaling of Slater integrals, and (ii) a previously not observed doublet excited state exists around 3.35 eV. These results allow to discuss the influence of interferences in the RIXS intermediate state, of core–hole lifetime broadenings, and of selection rules on the RIXS intensities. Finally, our results demonstrate that using an intermediate excitation energy between L₃ and L₂ edges allows measurement of the density of 3d excited states as a fingerprint of the metal local structure. This opens up a new direction to pump-before-destroy investigations of transition metal complex structures and reaction mechanisms.

Large Differences in the Optical Spectrum Associated with the Same Complex: The Effect of the Anisotropy of the Embedding Lattice

Transition-metal complexes with a well-defined geometry are usually considered to display almost the same properties independently of the system where they are embedded. Here we show that the above statement is not true depending on the anisotropy of the host lattice, which is revealed in the form of the electric field created by the rest of lattice ions over the complex. To illustrate this concept we analyze the origin of the surprisingly large differences in the d-d optical transitions of two systems containing square-planar CuF₄^2- complexes, CaCuF₄, and center II in Cu²⁺-doped Ba₂ZnF₆, even though the Cu²⁺-F^-distance difference is just found to be 1%. Using a minimalist first-principles model we show that the different morphology of the host lattices creates an anisotropic field that red-shifts the in vacuo complex transitions to the 1.25-1.70 eV range in CaCuF₄, while it blue-shifts them to the 1.70-3.0 eV region in Ba₂ZnF₆:Cu²⁺. This particular example shows how the lattice anisotropy strongly alters the optical properties of a given transition-metal complex. This knowledge opens a new path to tune the spectra of this large family of systems.

Fe doped LaGaO3: good white light emitters

Photoluminescence emission spectra from Fe doped LaGaO₃. The luminescence due to ultra violet He–Cd laser is shown in the inset.