Charge transfer excited states of hydroxyl ion impurities in alkali halide crystals

A theoretical investigation of the u.v. absorption anisotropy factor g and the transition energies of OH^? molecule in alkali halide crystals is presented in terms of a charge transfer model and the free OH^? ground wave-function. Comparison of calculated and observed transition energies for OH^? in KBr and NaCl shows reasonable agreement between theory and experiment. For KCl:OH^?, a reasonable result for the transition energy is also obtained through a confinement of the free K4s function used for the one-electron transfer function. The model fails, however, to explain the value of about 0·3 for the anisotropy factor which is observed in a number of host crystals.     

The Stretching Vibration of Hydroxyl Ions in LaGaO3

The presence of hydroxyl ions in LaGaO₃ single crystals has been detected by means of Fourier Transform Infrared (FTIR) absorption spectroscopy. A single narrow absorption line of the OH^? stretching vibration at about 3521 cm^?1 has been observed at T=9k in most of the as-grown crystals. The vibrational frequency decreased abruptly at the orthorhombic-rhombohedral structural phase transition temperature (T_c=418K). The parameters of the anharmonic potential of the OH^? stretching vibration have been determined from the isotopic replica and the overtone frequency and intensity, and found to be in agreement with those measured for hydroxyl ions in other oxide crystals.     

Effective room-temperature broad-band recording of 3D dynamic holograms in CdF<Subscript>2</Subscript>:Ga crystals

Photochromic CdF₂:Ga crystals with bistable impurity centers were effectively used for the dynamic recording of holograms and readout over the visible and near IR spectral regions at spatial frequencies of up to 5000 mm^?1 at room temperature. The diffraction efficiency of the dynamic holograms was as high as 60% at maximum and exceeded 1% when the beams’ intensities were in the ratio 1:100. As one goes from the low temperatures (≤200 K) to 300 K, the peak diffraction efficiency of the dynamic holograms decreases approximately by a factor of 1.5, while the speed of their response and photosensitivity in the long-wavelength spectral region increases by more than an order of magnitude. For the sake of comparison, the dynamic holograms were recorded under the same conditions as the widely used electrooptical SBN crystals. Comparative analysis ascertained a unique combination of the useful features offered by CdF₂:Ga crystals in holography.     

Optically and thermally driven bistability of FH(OH−) centres in KBr: Structure of the bistable configurations

Abstract

Pairs of F centres and OH^? molecules (F_H(OH^?) centres) exhibit a very interesting optical and thermal bistability in KBr. The analysis of our Electron Nuclear Double Resonance (ENDOR) investigation yields structure models. In both bistable configurations the OH^? molecules reside on a next nearest anion site (4^th shell) relative to the F centre. The difference lies in the orientation of the OH^? dipole with respect to the defect pair axis. The thermal bistability will be explained tentatively as being entropy-driven.     

The hydrogen compensated cation vacancy VOH− in magnesium oxide

H-doped MgO single crystals show a number of infrared absorption bands in the OH stretching region. H-doped, high purity, well-annealed MgO crystals (4N grade = 99.99%), grown from the melt, show a prominent group of four equally spaced bands at 3296, 3310, 3325 and 3341 cm^?1, while lower grade 3N MgO crystals exhibit only the first two bands and the third after X-irradiation. Until now these bands were assigned to different types of OH^? ions. The present spectra indicate that the four bands form a quadruplet which is assigned to the OH stretching fundamental of one proton substituting a divalent cation [OH^?V_Mg^″O^2?]^?. In this defect the proton is expected to behave as an off-center impurity, residing probably in an XY₈ multiple well potential in the [111] directions. The 8-fold degenerate ground level will split due to tunneling into A_2u, T_2g, T_1u, A_1g states, giving rise to the observed quadruplet. The quadruplet is expected, if the proton occupies the nearly spherical well of the tetrahedral sites above each face of the octahedron forming the XY₈ system. If the proton moves towards the trigonal sites in the (111) faces, the well becomes elliptical and the quadruplet will be reduced to a doublet.     

Composition nanodefects in doped lithium fluoride crystals

This paper presents the results of investigations into the optical properties of LiF crystals doped with oxides of different metals (Li, W, Ti, Fe). It has been proved that, during the growth of crystals, nanodefects containing polyvalent dopant ions and oxygen in different states (O^2?, OH^?, O ₂ ^? ) are formed in the crystalline matrix. It has been shown that these nanodefects are sinks of electronic excitations and determine the direction and efficiency of radiation-induced processes.     

Uncommon temperature dependence of the rotational motion of OH− centers in NaCl

Abstract

The correlation time τ_D of the rotational motion of OH^? centers in NaCl was studied for two different OH^?-concentrations (44 ppm and 220 ppm) between 0.5 K and 16 K by ²³Na nuclear spin relaxation measurements. The correlation time in the sample containing 44 ppm OH^? is given by phonon-assisted tunneling of isolated defects, whereas the OH^?-motion in the sample containing 220 ppm can be described by a coupled ensemble of quantum mechanical hindered rotators below a critical temperature which is determined by the strength of the interaction between the OH^? dipoles.     

New hydrogen associated paramagnetic centre in LiNbO3

A new hydrogen associated paramagnetic centre (g=2.0028±0.0005, A=3±0.5 mT at 77K) was observed in LiNbO₃. This centre is identified as an OH^2? ion, produced as a result of an electron capture by a diamagnetic OH^? ion, substituting the O^2? ion in LiNbO₃. An experimental procedure for detecting the EPR spectrum of the new centre is established. It is based on the observation of the more rapid destruction of O^? and Nb⁴⁺ ions under UV-irradiation at 77 K as compared to the OH^2? ions obtained after γ-irradiation of LiNbO₃ crystals at 77 K.     

Intrinsic photorefractive centers in LiNbO3:Fe

In LiNbO₃:Fe,Me crystals (Me = K, Mg, Zn) elementary holograms are recorded and erased with frequency-doubled, ordinarily polarized pulses of a Q-switched Nd:YAG laser. At high light intensities (I > 10⁹ Wm^–2) holographic sensitivities and saturation values of refractive index changes grow with increasing intensity and a contribution to photo-conductivity appears increasing quadratically withI. The intensity-dependent parts of the holographic quantities are diminished by increasing the lithium content of the samples or by doping the crystals with magnesium or zinc. Experimental results can be described by a two-center charge transport model, which is the Fe^2+/3+ model supplemented by shallow traps, namely niobium ions occupying lithium sites.     

Lattice site of Er in LiNbO3:Mg,Er crystals

Abstract

The lattice site of Er has been determined in LiNbO₃:Mg, Er crystals by the Rutherford backscattering channeling technique. It has been shown unambiguously that Er occupies Li site in the crystal lattice. OH absorption band has not been found in the 3500–3525 cm^?1 region. The RBS result confirms the assumption that OH^? ions may form defect complexes only with those trivalent impurities which replace niobium.     

Room‐temperature scavengers for macromolecular crystallography: increased lifetimes and modified dose dependence of the intensity decay

The advent of highly intense wiggler and undulator beamlines has reintroduced the problem of X‐ray radiation damage in protein crystals even at cryogenic temperatures (100 K). Although cryocrystallography can be utilized for the majority of protein crystals, certain macromolecular crystals (e.g. of viruses) suffer large increases in mosaicity upon flash cooling and data are still collected at room temperature (293 K). An alternative mechanism to cryocooling for prolonging crystal lifetime is the use of radioprotectants. These compounds are able to scavenge the free radical species formed upon X‐ray irradiation which are thought to be responsible for part of the observed damage. Three putative radioprotectants, ascorbate, 1,4‐benzoquinone and 2,2,6,6‐tetramethyl‐4‐piperidone (TEMP), were tested for their ability to prolong lysozyme crystal lifetimes at 293 K. Plots of relative summed intensity against dose were used as a metric to assess radioprotectant ability: ascorbate and 1,4‐benzoquinone appear to be effective, whereas studies on TEMP were inconclusive. Ascorbate, which scavenges OH radicals (k_OH = 8 × 10⁹ M^?1 s^?1) and electrons with a lower rate constant (k_e‐(aq) = 3.0 × 10⁸ M^?1 s^?1), doubled the crystal dose tolerance, whereas 1,4‐benzoquinone, which also scavenges both OH radicals (k_OH = 1.2 × 10⁹ M^?1 s^?1) and electrons (k_e‐(aq) = 1.2 × 10¹⁰ M^?1 s^?1), offered a ninefold increase in dose tolerance at the dose rates used. Pivotally, these preliminary results on a limited number of samples show that the two scavengers also induced a striking change in the dose dependence of the intensity decay from a first‐order to a zeroth‐order process.     

Study of the paraelectric behavior of OH− ions in alkali halides with optical and caloric methods-I. Statics of dipole alignment

The electric field induced alignment of substitutional OH^? ions in a variety of alkali halide crystals was studied, using electro-optical and electro-caloric techniques. As basis for the electro-optical studies, the u.v. absorption of OH^? in various crystals was thoroughly investigated at different temperatures, and in several cases oscillator strength values were determined. The energy position of the u.v. absorption of OH^? in 13 alkali halides follows closely an Ivery-relation, which is discussed in terms of a charge-transfer model. The observed electric field induced zero-moment changes of the u.v. absorption, which depend strongly on light polarization, field direction and temperature, can be quantitatively accounted for by a paraelectric alignment model of permanent dipoles with moment p. The anisotropy of the OH^? electro-dichroism reveals <100> dipole orientation in NaCl, KCl, KBr, RBCl, RbBr and RbI, <110> orientation in KI, and <111> orientation in CsBr. The determined dipole moment values p show little variation with the host material, which excludes sizeable contributions to p from off-center shifts of the OH^? ion. The observed saturation of the electro-dichroism indicates a peculiar ‘mixed’ polarization behavior of the optical transition, which will be discussed. Reversible electro-caloric measurements, performed on several OH^? systems, reveal a field dependence and anisotropy in the high field range, which yield dipole orientation and dipole moment values in agreement with the electrooptical results.     

The orange luminescence band in MgO crystals

Nominally pure MgO crystals exhibit a bright orange luminescence when excited with u v or visible laser light This luminescence has been characterized using a variety of measurements with pulsed and cw laser sources Its amplitude increases with temperature to about 400 K. while the lifetime of emission decreases from 270 μs at 290 K to 9 μs at 433 K This emission appears to originate in hole recombination with Fe²⁺ ions Hole trapping at V^?_OH and V^?_Al centers may occur as an intermediate step Bleaching with 514 5-nm Ar⁺ laser light is reversible upon exposure to u v radiation Present data are compared with those obtained previously on the thermoluminescence of irradiated MgO crystals.     

Infrared spectra of hydrogen impurities in BaTiO3 crystals

The IR spectra in the 2.8 μm region of the OH^? impurities in thick single domain crystals have been measured for different polarization geometries and various temperatures. Simple calculations of the Coulomb interactions between hydrogen and lattice ions show that these interactions are much more important than the hydrogen bonding ones for determining the positions of the hydrogen ions. The appearance of only one strong non-dichroic OH^? stretching line is due to an almost equal population of the proton axial sites. The side bands are interpreted as combinations of the stretching mode with a low-frequency translational mode. The large broadening of the main line just above T_c supports the model which attributes a considerable order—disorder character to the ferroelectric phase transition.     

Hydrogen-deuterium exchange induced by an electric field in α-Al2O3 single crystals

Abstract

Hydrogen and deuterium are observed in α-Al₂O₃ crystals in the form of OH^? and OD^? radicals, respectively, which absorb in the infrared region. Infrared-absorption measurements were used to monitor diffusion of deuterons and protons in α-Al₂O₃ single crystals under the application of a moderate electric field parallel to the crystallographic c-axis, in the temperature range of 973—1333K. A linear dependence of the percent of exchange with both annealing time and applied voltage is observed, indicating that ionic conduction was taking place. The activation energy for the H⁺  D⁺ exchange was determined to be 2.4 eV, less than half the value obtained by pure thermal means, suggesting that under the application of an electric field the deuteron (proton) diffusion mechanism is different.     

Stoichiometry dependence of the OH- absorption band in LiNbO3 crystals

Hydrogen in the form of OH^- ions can be incorporated into LiNbO₃ single crystals during the growth process. The infrared absorption band due to the OH^- defects has been measured at room temperature and a considerable change of the band shape has been observed depending on the crystal composition. The structure of the band found in nearly stoichiometric crystals indicates three slightly different proton sites which can be explained on the basis of the LiNbO₃ crystal structure.     

Relaxation phenomena in TlGa<Subscript>0.99</Subscript>Fe<Subscript>0.01</Subscript>Se<Subscript>2</Subscript> single crystals

The relaxation electronic phenomena occurring in TlGa_0.99Fe_0.01Se₂ single crystals in an external dc electric field are investigated. It is established that these phenomena are caused by electric charges accumulated in the single crystals. The charge relaxation at different electric field strengths and temperatures, the hysteresis of the current-voltage characteristic, and the electric charge accumulated in the TlGa_0.99Fe_0.01Se₂ single crystals are consistent with the relay-race mechanism of transfer of a charge generated at deep-lying energy levels in the band gap due to the injection of charge carriers from the electric contact into the crystal. The parameters characterizing the electronic phenomena observed in the TlGa_0.99Fe_0.01Se₂ single crystals are determined to be as follows: the effective mobility of charge carriers transferred by deep-lying centers μ_f=5.6×10^?2 cm²/(V s) at 300 K and the activation energy of charge transfer ΔE=0.54 eV, the contact capacitance of the sample C _c =5×10^?8 F, the localization length of charge carriers in the crystal d _c =1.17×10^?6 cm, the electric charge time constant of the contact τ=15 s, the time a charge carrier takes to travel through the sample t _t =1.8×10^?3 s, and the activation energy of traps responsible for charge relaxation ΔE _σ = ΔE _Q = 0.58 eV.     

Der Einfluß von OH−-Ionen auf Absorptionsspektrum und Ionenleitfähigkeit von KCl-Einkristallen

Air-grown KC1 crystals (undoped or doped with KOH) contain two types of OH^? impurities:

1. a)
   a free OH^? ion, probably in a substitutional position,     
   
   

Electrolytic coloration and spectral properties of OH- and Cu-codoped NaCl crystals

OH⁻- and Cu⁺-codoped NaCl crystals are colored electrolytically by using a pointed anode and a flat cathode at various temperatures and under various voltages. Absorption and fluorescence spectral properties of colored crystals are investigated. Absorption, excitation and fluorescence spectra reveal existence of V₂, V₂^m, V₃, OH⁻, U, U_A, O²⁻–V_a⁺, O₂⁺, Cu⁺, Cu⁰, Cu⁻ and OH⁻-perturbed Cu⁺ color centers in colored crystals. Formations of color centers are explained.     

OH impurities in co-doped LiNbO3:Cr :ZnO congruent crystals

Infrared optical absorption has been used to study OH⁻impurities into congruent co-doped LiNbO₃:Cr³⁺:ZnO crystals doped with different Zn²⁺ concentration. The OH⁻ IR absorption spectra present three bands that can be associated with different OH⁻ complex centres available in the lattice. For crystals with lower Zn²⁺ concentrations (<4.7%) only one IR absorption band centred at 2867 nm (3490 cm⁻¹) is reported which is associated with the OH⁻ unperturbed vibration. For crystals with higher Zn²⁺ concentrations (>4.7%), two new bands associated with OH⁻vibration in distortion environment are reported. These bands are centred at 2827 nm (3537 cm⁻¹) and 2847 nm (3512 cm⁻¹) and can be associated with OH⁻-Zn²⁺ and Cr³⁺(Li⁺)-OH⁻-Zn²⁺(Int.) complex centres, respectively. Electron paramagnetic resonance (EPR) has been used to identify the Cr³⁺ centres in the lattice of the doped LiNbO₃:ZnO crystals.