Free and localized excitons in the luminescence spectrum of C60 crystals at high pressure

The luminescence spectra of C₆₀ single crystals are studied at T≅10 K and pressure up to 4.0 GPa. It is observed that as the pressure increases, one fine-structure band in the spectrum intensifies sharply and dominates at pressures P≥1.7 GPa. The pressure shift of this band is much larger than the shift of other bands in the spectrum, and its magnitude correlates with the pressure dependence of the band gap. It is shown that this band could be due to radiative recombination of free Frenkel excitons. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 3, 234–238 (10 August 1998)     

Crystal-field induced mixing of electron states in C60 crystals at high pressure

Optical absorption spectra of thin fullerene (C₆₀) crystals in the range 1.7 to 3.8 eV have been measured at T=300 K and at pressures up to 2.5 GPa. The spectrum shifts toward the red with pressure, and the electron absorption intensity is redistributed among its bands. The intensity of the band associated with the lowest direct electron interband transition monotonically increases with pressure, whereas the intensity of the upper interband feature decreases. Bands related to weak edge absorption in the range between 1.7 and 2.2 eV gradually merge with the band associated with the lowest interband transition, whose intensity rises with pressure. A similar redistribution of intensity among electron transition bands has been observed when comparing the spectrum of an isolated C₆₀ molecule and that of a C₆₀ crystal. The results indicate that the crystal-field induced mixing of electron states is present in solid C₆₀, and they can be discussed in terms of the Craig-McClure model, which was suggested to describe crystal-field induced mixing of electron states in anthracene and naphthalene molecular crystals. Zh. éksp. Teor. Fiz. 113, 313–322 (January 1998)     

Local charge-carrier trapping centers in C60 single crystals

The energy spectrum of local charge-carrier trapping centers in C₆₀ single crystals is studied in terms of the theory of space-charge-limited currents in the Gaussian approximation. The energy distribution of these centers in the energy gap is found, and the type and parameters of this distribution are determined.     

Nature of the luminescence centers in ionic crystals

The main attention is paid to the following questions: 1. The possible models of the luminescence centers in activated alkali halide crystals, 2. the experimental evidence of the presence of various types of the luminescence centers, 3. the nature of the excitations of the simplest luminescence centers in alkali halides activated by mercury-like ions and by some other ions. Mainly two possibilities of views: the quasi-free ions case on the one hand and the quasi-molecular models on the other are discussed in detail.     

Orientation of luminescence centers in self-activated ZnS crystals

Results concerning polarization of the self-activated (SA) luminescence in ZnS crystals are presented. It is assumed that the orientations of absorption centers — circular oscillators (rotators) — and emission centers — rotators and dipoles — are along the crystal 3-fold symmetry axes. The calculated polarization diagrams fit well to experimental results.     

Exited states of the luminescence centers in tungstate crystals

The electronic structures of luminescence centers in ZnWO₄, ZnWO₄:Mo, and ZnWO₄:Cd crystals are calculated by the configuration interaction method using embedded cluster approach. Dependencies of energies of the ground and excited electronic states of the centers on vibration coordinates are computed. The energies and oscillator strengths of radiative transitions in the luminescence centers of regular and doped zinc tungstate crystals are obtained. Formation of emission spectra of ZnWO₄, ZnMoO₄, and CdWO₄ are analyzed using results of the calculations.     

Energy spectrum and phase transitions in C70 fullerite crystals at high pressure

Measurements have been made of the Raman, optical absorption, and luminescence spectra of single crystals and pellets of the fullerite C₇₀ at T=300 K and at pressures up to 12 GPa. The baric shift dω/dP and the Grüneisen parameters of the Raman-active intramolecular phonon modes have been determined. It has been established that the d ω/dP value for certain phonon modes abruptly changes at pressures of P ₁≈2 GPa and P ₂≈5.5 GPa, as do the half-widths of the Raman lines. These features in the Raman spectrum are associated with phase transitions at high pressure. The baric shifts of the absorption and luminescence edges of C₇₀ crystals have been determined and are −0.12 eV/GPa and −0.11 eV/GPa, respectively, for absorption and luminescence. The baric shift of the absorption edge decreases significantly with increasing pressure and is −0.03 eV/GPa at 10 GPa. These data have been used to determine the deformation potential of the fullerite C₇₀, which is about 2.1±0.1 eV. Zh. éksp. Teor. Fiz. 111, 262–273 (January 1997)     

Photoinduced color centers creation in superionic crystals RbAg 4 I 5

Abstract

A new phenomenon of a reversible photoinduced coloration caused by light irradiation is discovered and investigated in superionic RbAg ₄ I ₅ crystals. The reversible photoinduced absorption is found to be a result of irradiation by light with wavelengths in the region from 420 nm to 450 nm. The proposed mechanism of the discovered effect is associated with ambipolar diffusion of screened by mobile ions optically excited electronic carriers. The processes of color centers creation in superionic crystals RbAg ₄ I ₅ due to additive coloring in iodine vapours, ionic implantation and γ-ray irradiation are considered.     

Kinetics of luminescence in ZnS crystals as studied by centers discrimination

It was shown, by alternating the unpolarized and polarized UV-excitation of ZnS crystals, that the excited S-A centers accumulated during the rise of luminescence do not contribute to the steady emission.     

Edge cathode luminescence of zinc-selenide single crystals at high levels of excitation

Investigating cathode-luminescence spectra over a broad range of temperature at high levels of excitation (G 10²⁷ cm^–3 · sec^–1) permits the establishment of the experimental conditions for the observation of a multiplasmon structure in studying electron-hole plasma of zinc-selenide single crystals.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 69–73, July, 1989.     

Martensitic fcc-to-hcp transformation observed in xenon at high pressure

Angle-resolved x-ray diffraction patterns of Xe to 127 GPa indicate that the fcc-to-hcp transition occurs martensitically between 3 and 70 GPa in diamond-anvil cells without an intermediate phase. These data also reveal that the transition occurs by the introduction of stacking disorder in the fcc lattice at low pressure, which grows into hcp domains with increasing pressure. The small energy difference between the hcp and the fcc structures may allow the two phases to coexist over a wide pressure range. Evidence of similar stacking disorder and incipient growth of an hcp phase are also observed in solid Kr.     

Phase transformations in amorphous fullerite C60 under high pressure and high temperature

First phase transformations of amorphous fullerite C₆₀ at high temperatures (up to 1800 K) and high pressures (up to 8 GPa) have been investigated and compared with the previous studies on the crystalline fullerite. The study was conducted using neutron diffraction and Raman spectroscopy. The amorphous fullerite was obtained by ball-milling. We have shown that under thermobaric treatment no crystallization of amorphous fullerite into С₆₀ molecular modification is observed, and it transforms into amorphous-like or crystalline graphite. A kinetic diagram of phase transformation of amorphous fullerite in temperature–pressure coordinates was constructed for the first time. Unlike in crystalline fullerite, no crystalline polymerized phases were formed under thermobaric treatment on amorphous fullerite. We found that amorphous fullerite turned out to be less resistant to thermobaric treatment, and amorphous-like or crystalline graphite were formed at lower temperatures than in crystalline fullerite.     

Properties of C60 polymerized under high pressure and temperature

60 polymers. Pure and mixed phase polymeric samples were synthesized by simultaneously subjecting microcrystalline C₆₀ powder or pellets to various pressures () and temperatures (). The optical spectra of the orthorhombic, tetragonal, and rhombohedral C₆₀ polymer phases are observed to be quite distinct and rich. These spectra exhibit numerous lines and an overall downshift in frequency relative to C₆₀ is observed, consistent with a loss of double bonds from the fullerene cage. The LDMS spectra of a sample synthesized at under hydrostatic conditions and , exhibited a succession of clear peaks at mass numbers corresponding to , similar to the LDMS data on the C₆₀ photopolymer. This is taken as further evidence for interfullerene bonds in these high-pressure polymers. The XRD pattern of this sample indicates the presence of a strong texture in the sample. Received: 14 November 1996/Accepted: 8 January 1997     

Peculiarities of absorption and luminescence spectra of nitrogen-vacancy color centers in diamond crystals

We have studied photoluminescence spectra of nitrogen-vacancy (NV) centers upon their pumping by short-wavelength visible and near-UV radiation. We have shown that NV⁰ centers, as distinct from NV^? centers have an absorption line in the UV range. Inversion of the zero-phonon line of NV^? centers has been shown and interpreted.     

Excitation spectra of self-activated luminescence in ZnSe crystals under pressure

A narrow excitation band observed for S-A luminescence in ZnSe crystal is attributed to free exciton absorption. Some overlap with a higher characteristic band of low intensity is considered for low temperature spectra.This band shifts under pressure toward higher energies with a coefficient dE/dP = (7.5±0.3) meV Kb^-1 at 300 K and (7.4±0.5) meV Kb^-1 at 85 K.The pressure shift of the excitation due to edge absorption is (7.0± 0.5) meV Kb^-1.