A novel type of sub-lattice of the Jahn-teller (Jt) centers was arranged in ti-doped barium hexaferrite Bafe 12 o 19. In the un-doped crystal all iron ions, sitting in five different crystallographic positions, are fe 3+ 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 fe 2+ predominantly in tetrahedral coordination, resulting in doubly-degenerate states subject to the ⊗ E e problem of the JT effect. The arranged JT complexes, Fe 2+ o 4 , 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 methodology is worked out to retrieve the numerical values of all the main parameters of the six-dimensional adiabatic potential energy surface (APES) of a polyatomic system with a quadratic T-term Jahn-Teller effect (JTE) from ultrasound experiments. The method is based on a verified assumption that ultrasound attenuation and speed encounter anomalies when the direction of propagation and polarization of its wave of strain coincides with the characteristic directions of symmetry breaking in the JTE. For the SrF2:Cr crystal, employed as a basic example, we observed anomaly peaks in the temperature dependence of attenuation of ultrasound at frequencies of 50-160 MHz in the temperature interval of 40-60 K for the wave propagating along the [110] direction, for both the longitudinal and shear modes, the latter with two polarizations along the [001] and [110] axes, respectively. We show that these anomalies are due to the ultrasound relaxation by the system of non-interacting Cr 2+ JT centers with orthorhombic local distortions. The interpretation of the experimental findings is based on the T2g ⊗ (eg + t2g) JTE problem including the linear and quadratic terms of vibronic interactions in the Hamiltonian and the same-symmetry modes reduced to one interaction mode. Combining the experimental results with a theoretical analysis we show that on the complicated six-dimensional APES of this system with three tetragonal, four trigonal, and six orthorhombic extrema points, the latter are global minima, while the former are saddle points, and we estimate numerically all the main parameters of this surface, including the linear and quadratic vibronic coupling constants, the primary force constants, the coordinates of all the extrema points and their energies, the energy barrier between the orthorhombic minima, and the tunneling splitting of the ground vibrational states. To our knowledge, such a based-on-experimental-data numerical reconstruction of the APES of a JTE problem in the five-dimensional space of all active tetragonal and trigonal displacements is realized here for the first time.
The experimental evidence of the influence of the structural phase transition on the elastic and optoelectronic properties of CH3NH3PbI3 single crystals has been reported. A peak in the attenuation for longitudinal and shear ultrasonic waves and a step-like anomaly in their velocity have been found near the structural the orthorhombic-to-tetragonal phase transition (160 K). The narrow hysteresis observed in the temperature dependences of the elastic properties confirms that this is the first-order phase transition. A redshift of the absorption threshold (of about 110 meV) has been revealed both in the photocurrent (PC) and in the photoluminescence (PL) spectra with increasing temperature from 140 to 160 K. In the orthorhombic phase of CH3NH3PbI3 single crystals, the fine exciton structure in the PC spectrum has been found with an exciton binding energy of 19–25 meV. The peculiarities of the PC and PL spectra near the phase transition temperature (160 K) have been observed within a wider temperature range in contrast to the elastic anomalies that can be explained by the complex structure of the near-surface region. It may be essential for the further development of optoelectronic devices based on hybrid halide perovskites.
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