This work investigates the ${\text{Mn}}^{2+}$ photoluminescence (PL) and electronic structure of ${\left[{\left({\text{CH}}_3\right)}_4\text{N}\right]}_2{\text{Mn}X}_4$ $\left(X:\text{Cl},\text{Br}\right)$ and their variation with pressure through time-resolved spectroscopy in the 0–15 GPa range. The crystal-field excitation and emission spectra are explained on the basis of the ${\text{Mn}X}_4^{2-}\left({\text{T}}_{\text{d}}\right)$ tetrahedra. Their peaks experience large pressure redshifts, which are associated with the big crystal compressibility and the interaction between organic/inorganic $\left({\left[{\text{CH}}_3\right]}_4{\text{N}}^{-}/{\text{Mn}X}_4^{2-}\right)$ tetrahedra. The variation in the Racah parameters and crystal-field splitting with pressure indicates that the excitation and emission redshifts of ${\text{Mn}}^{2+}$ are mainly governed by the increase in the $\text{Mn-}X$ bond covalency (70%) rather than the increase in the crystal-field splitting (30%). Above 6 GPa, pressure induces structural modifications, which are related to aggregation of the ${\text{Mn}X}_4^{2-}$ tetrahedra with change in ${\text{Mn}}^{2+}$ coordination from fourfold ${\text{Mn}X}_4^{2-}\left({\text{T}}_{\text{d}}\right)$ to sixfold ${\text{Mn}X}_6^{4-}\left({\text{O}}_{\text{h}}\right)$. This process involves a drastic change in the PL behavior yielding a simultaneous two-color PL emission: green emission at 520 nm and a red emission around 650 nm. Both emissions experience noticeable redshifts with pressure producing a marked piezo-PL effect. The large pressure range of phase coexistence makes these materials attractive for multiband PL, the wavelengths of which can be tuned through pressure and eventually stabilized at ambient conditions.

We have measured by means of photoluminescence the energy of crystal‐field peaks for RbCdF₃: Mn²⁺ and KZnF₃: Mn²⁺ where the value of the Mn²⁺–F⁻ distance, R, derived by EXAFS is R=2.13±0.01 Å and R=2.08±0.01 Å, respectively. From these data and those for RbMnF₃ and KMnF₃ we have studied the dependence on R of the B, C, and 10 Dq parameters for the MnF⁴⁻₆ complex. This analysis reveals that within the experimental errors, B and C are constant in the range 2.07<R<2.14 Å, in agreement with recent self‐consistent calculations for MnF⁴⁻₆, which also predict that 10 Dq=KR⁻ⁿ, where K and n are constant. The present study confirms this dependence, n being 4.4 which is also in accord to the theoretical predictions. The best values of R derived from optical spectra are found to be R=2.141±0.004 Å (for RbCdF₃: Mn²⁺) and R=2.075±0.004 Å (for KZnF₃: Mn²⁺). The present analysis also points out that by measuring the changes induced on the optical spectrum of MnF⁴⁻₆ in a given lattice we can detect changes in the Mn²⁺–F⁻ distance down to 10⁻³ Å. In this way we have derived the difference, ΔR, between R at room temperature and at 77 K for KZnF₃: Mn²⁺. The obtained value ΔR=(9±1)10⁻³ Å is in agreement with the one ΔR=(10±3.5)10⁻³ Å derived previously from the variations undergone by the isotropic superhyperfine constant A_s. Finally the present results are compared to those for some complexes of Eu²⁺, Co²⁺, Ni²⁺, and Cr³⁺.

We study the effects of pressure on the structural, vibrational, and magnetic behavior of cuproscheelite. We performed powder x-ray diffraction and Raman spectroscopy experiments up to 27 GPa as well as ab initio total-energy and lattice-dynamics calculations. Experiments provide evidence that a structural phase transition takes place at 10 GPa from the low-pressure triclinic phase $\left(P\overline{1}\right)$ to a monoclinic wolframite-type structure $\left(P2/c\right)$. Calculations confirmed this finding and indicate that the phase transformation involves a change in the magnetic order. In addition, the equation of state for the triclinic phase is determined: $V_0=132.8\left(2\right){\text{Å}}^3$, $B_0=139\left(6\right)\text{GPa}$, and $B_0^{\prime }=4$. Furthermore, experiments under different stress conditions show that nonhydrostatic stresses induce a second phase transition at 17 GPa and reduce the compressibility of ${\text{CuWO}}_4$, $B_0=171\left(6\right)\text{GPa}$. The pressure dependence of all Raman modes of the triclinic and high-pressure phases is also reported and discussed.

In this work we present an octahedral perturbative model to explain the dependence of the crystal-field (CF) and the charge-transfer (CT) energy with respect to structural distortions in Jahn-Teller (JT) ${\mathrm{CuCl}}_6^{4\mathrm{-}}$ systems. The method provides a simple way to express the variation of electronic energy to complex distortions of the totally symmetric mode $Q_{a_{1g}}$ and the JT mode $Q_{\theta },$ as a function of the corresponding electron-vibration coupling constants, $[\partial E/\partial Q_i{]}_{O_h}$ ${(i=Q}_{a_{1g}}$ and $Q_{\theta }).\mathrm{}$ A value of 9100 ${\mathrm{cm}}^{\mathrm{-}1}$/Å for the linear JT coupling constant $A_1$ has been obtained for the octahedral ${}^2{E}_g{(x}^2-y^2,{3z}^2-r^2)$ CF state $(e\otimes E)$ from structural correlations along a series of copper compounds. The corresponding JT coupling for the ${}^2{T}_{1u}\left(\pi \right)$ CT state $(e\otimes T),$ $A_2=3000\mathrm{}{\mathrm{cm}}^{\mathrm{-}1}/\AA ,$ has been derived from hydrostatic pressure measurements performed on the ${(\mathrm{C}}_3{\mathrm{H}}_7{\mathrm{NH}}_3{)}_2{\mathrm{CuCl}}_4$ perovskite layer. A noteworthy conclusion of this model is that a redshift of the intense $e_u\left(\pi \right)\to b_{1g}{(x}^2-y^2)$ CT band is possible in axially elongated ${\mathrm{CuCl}}_6^{4\mathrm{-}}$ systems upon anisotropic volume reduction, if the axial distance decreases more rapidly than the equatorial distance by $|\Delta R_{\mathrm{ax}}|>30\mathrm{}|\Delta R_{\mathrm{eq}}|.$ These results are discussed in light of recent pressure experiments carried out in wide-gap CT semiconductors of the $A_2{\mathrm{CuCl}}_4$ family.