A novel type of sub-lattice of the Jahn-Teller (JT) centers was arranged in Ti-doped barium hexaferrite BaFe₁₂O₁₉. In the un-doped crystal all iron ions, sitting in five different crystallographic positions, are Fe³⁺ in the high-spin configuration (S = 5/2) and have a non-degenerate ground state. We show that the electron-donor Ti substitution converts the ions to Fe²⁺ predominantly in tetrahedral coordination, resulting in doubly-degenerate states subject to the $$E\otimes e$$E ⊗ e problem of the JT effect. The arranged JT complexes, Fe²⁺O₄, their adiabatic potential energy, non-linear and quantum dynamics, have been studied by means of ultrasound and terahertz-infrared spectroscopies. The JT complexes are sensitive to external stress and applied magnetic field. For that reason, the properties of the doped crystal can be controlled by the amount and state of the JT complexes.

A method is constructed that uses ultrasonic experiments to evaluate the parameters of the Jahn-Teller (JT) effect in impurity centers in crystals. The method is based on measurements of temperature dependent attenuation and phase velocity and does not require assumptions about mechanisms of relaxation. The results are illustrated by measurements performed on the impurity system ZnSe:Cr(2+), in which the Cr(2+) ion has a threefold degenerate T term in the ground state, subject to the [Formula: see text] JT problem. Ultrasound propagation anomalies show that the main JT distortions of the tetrahedral environment of the Cr(2+) ion are of tetragonal E type and hence the lowest branch of the adiabatic potential energy surface (APES) is formed in accordance with the [Formula: see text] problem. With dopant concentration 3.8 × 10(18) cm(-3) the modulus of the constant of linear vibronic coupling to tetragonal E type vibrations is determined by two independent experiments: |F(E)| = 5.49 × 10(-5) dyn revealed from attenuation measurements, while a slightly different value |F(E)| = 5.57 × 10(-5) dyn emerges from phase velocity measurements. Contributions of other active vibronic modes to the elastic modulus C(l) = (C(11) + C(12) + 2C(44))/2 are analyzed and it is shown that the influence of the totally symmetric mode is negligible. Using additional information about this system obtained from independent sources, we also estimated the primary force constant in the E direction (K(E)≈(1.4-4.2) × 10(4) dyn cm(-1)) and orthorhombic and trigonal saddle points of the APES in the five-dimensional space of the tetragonal and trigonal coordinates, their stabilization energies being E(JT)(O)≈81-450 cm(-1) and E(JT)(T)≈48-417 cm(-1), respectively (the variations of the K(E), E(JT)(O) and E(JT)(T) values are due to different literature data for E(JT)(E)). With these data the APES of the JT linear [Formula: see text] problem for the Cr(2+) ion in the ZnSe:Cr(2+) system is revealed.

New research results on phenomena due to the existence of electronic resonance energy levels and hybridized states at impurities of transition elements in semiconductors are presented. The data show that the thermal conductivity and ultrasonic parameters of mercury selenide containing iron impurities have resonance anomalies due to the influence of these impurities. A consistent and detailed interpretation is offered for the set of observed effects of hybridized states in mercury selenide with iron impurities. The proposed interpretation of the data obtained on other systems is discussed.

Temperature dependences of phase velocity and absorption of ultrasound were investigated in ZnSe crystal doped with ${\mathrm{Cr}}^{2+}$ ions at the frequencies $32–158\mathrm{MHz}$. Absorption peak and variation of phase velocity found in the temperature interval $10–20\mathrm{K}$ were interpreted as manifestation of the Jahn-Teller effect. Temperature dependences of relaxation time, relaxed, and unrelaxed moduli $C=\frac{1}{2}(C_{11}+C_{12}+2C_{44})$ were restored. The dependences were compared with the results of such investigations in $\mathrm{Zn}\mathrm{Se}:{\mathrm{Ni}}^{2+}$ and analyzed regarding lattice stability of the crystals. Fitting of the temperature dependence of the relaxation time was done with account of thermal activation over the potential barrier, quantum tunnelling with phonon emission, and two phonon process analogous to Raman scattering. Thermal activation proved to be the dominant mechanism of relaxation above $6\mathrm{K}$ and quantum tunnelling—below this temperature.

We present a temperature and laser-power dependent photoluminescence (PL) study of methylammonium lead iodide (CH₃NH₃PbI₃) single crystals in the orthorhombic phase. At temperatures below 140 K, we revealed the multi-component PL emission. In addition to a free exciton with an energy of 1.65 eV, we found emission bands with peaks approximately equal to 1.6 eV, 1.52 eV, and 1.48 eV. Analysis of the thermal evolution of the intensities, peak positions, and linewidths of all the PL bands allowed one to determine their origin. We attributed the PL peak with energy of 1.6 eV to a bound exciton, while the free exciton-bound exciton splitting energy is 50–60 meV. The PL emission with an energy of 1.52 eV can be explained by the donor-acceptor pair (DAP) recombination, where donor and acceptor defects have a depth of about 12 meV and 120 meV, respectively. MA (CH₃NH₃) interstitials (MA i + ) and lead vacancies (V P b 2 − ) are the most suitable for the DAP transition to occur in CH₃NH₃PbI₃ crystals. The 1.48 eV PL emission is consistent with the recombination of self-trapped excitons, and interstitial iodine is likely to be an active trap source. We found the variation of the self-trapped depth from 15 meV (at T < 80 K) to 53 meV (at T > 80 K) with increasing the temperature. Although the multi-component PL emission in CH₃NH₃PbI₃ single crystals appears at low temperatures, defects and excitonic traps that cause this emission can affect the photophysics of hybrid perovskites at higher temperatures.