Photoluminescence and radiationless processes in Mn2+-doped Ca1−xSrxF2 fluorites as a function of pressure and temperature. A structural correlation study

Temperature and pressure dependence of the luminescence of Eu(2+)-doped fluoride crystals Ba(x)Sr(1-x)F(2) (x = 0, 0.3, 0.5 and 1): experiment and model

This paper summarizes experimental evidence of anomalous luminescence in Eu(2+)-doped fluoride crystals Ba(x)Sr(1-x)F(2) (x = 0, 0.3, 0.5 and 1). Luminescence, luminescence excitation spectra and luminescence kinetics obtained at ambient and high hydrostatic pressure at various temperatures are discussed. Hydrostatic pressure was shown to cause a redshift of normal [Formula: see text] emission and anomalous luminescence. The experimental data shows the existence of temperature- and pressure-induced spectral transformations where the anomalous luminescence is replaced by normal emission of Eu(2+) centers. We present a model that predicts a strong electron-lattice coupling of the trapped excitons as well as the pressure effect of the spectral transformation from anomalous to normal emission.

Molecular structure and dynamics of off-center Cu2+ ions and strongly coupled Cu2+–Cu2+ pairs in BaF2 crystals: Electron paramagnetic resonance and electron spin relaxation studies

X-band and Q-band electron paramagnetic resonance (EPR) spectra of Cu(2+) in BaF(2) crystal were recorded in the temperature range of 4.2-200 K. Spin-Hamiltonian parameters of single Cu(2+) complexes and of Cu(2+)-Cu(2+) pairs were derived and discussed. A special attention was paid to the dimeric species. Their molecular ground state configuration was found as having antiferromagnetic intradimer coupling with the singlet-triplet splitting J=-35 cm(-1). The zero-field splitting being D=0.0365 cm(-1) at 4.2 K increases with temperature as an effect of thermal population of excited dimer configurations. Electron spin echo (ESE) method was used for measurements of electron spin lattice and phase relaxation. The spin-lattice relaxation data show that except for coupling to the host lattice phonons the Cu(2+) ions are involved in local mode motions with energy of 82 cm(-1). Phase relaxation (ESE dephasing) of single Cu(2+) ions is due to spin diffusion at low temperatures. This relaxation is hampered for temperatures higher than 30 K due to the triplet state population of neighboring Cu(2+)-Cu(2+) dimers, which disturb dipolar coupling between Cu(2+) ions. For higher temperatures the relaxation is dominated by Raman T(1) processes. Fourier transform ESE spectrum displays dipolar Cu-F splitting which allowed determination of the off-center shift of Cu(2+) as delta(s)=0.132 nm. The dynamical effects observed in EPR spectra and in electron spin relaxation both for single Cu(2+) ions and Cu(2+)-Cu(2+) pairs are discussed as due to jumps between six off-center positions in the crystal unit cell and jumps between various dimer configurations.

Pressure-induced two-color photoluminescence in MnF(2) at room temperature

A novel two-color photoluminescence (PL) is found in MnF(2) at room temperature under high pressure. Contrary to low-temperature PL, PL at room temperature is unusual in transition-metal concentrated materials like MnF(2), since the deexcitation process at room temperature is fully governed by energy transfer to nonradiative centers. We show that room-temperature PL in MnF(2) originates from two distinct Mn(2+) emissions in the high-pressure cotunnite phase. The electronic structure and the excited-state dynamics are investigated by time-resolved emission and excitation spectroscopy at high pressure.