Dependences of the refractive indices on the proton concentration in H:LiNbO3 waveguides

Proton-exchanged optical waveguides in lithium niobate crystals exhibit a rich variety of structures and phases. It is established that seven H_xLi_1−x NbO₃ crystalline phases with a lithium niobate structure may form under various conditions of proton exchange and post-exchange annealing. A method is proposed to determine the proton concentration in the various phases identified. Relationships are established between the structural parameters, the proton concentration, and the ordinary and extraordinary refractive indices of various H_xLi_1−x NbO₃ crystalline phases. The results can explain various optical phenomena observed in proton-exchanged waveguides and permit prediction of the characteristics of light-guide structures. Zh. Tekh. Fiz. 69, 47–57 (March 1999)     

Optical properties and radiation effects in layered crystals of (CnH2n+nNH3)2MCl4: n = 1, 2, 3; M = Cd,Cu, Mn

Abstract

A new absorption band has been found at 5.10 eV in (C _n H_{2n + 1}NH₃)₂CdCl₄: n = 1, 2, 3 in addition to the absorption bands of CdCl₂ whose electronic structure resembles the former crystals. The energy of the additional peak shifts with temperature by as much as 0.38 eV from 5.10eV at room temperature (RT) to 5.48 eV at liquid nitrogen temperature. This large peak shift is attributed to a structural phase transition between these two temperatures. A new type of electron center has been found in these crystals (M = Cd, Mn; n = 1, 2, 3) irradiated with X-rays at 15 K in addition to the Cl₂ ^?. This shows optical absorption bands (IR bands) in the infrared region of 10 ～ 20 kcm ^?1. The IR bands are assigned to an electron center where an electron is trapped at an ammonium site in the neighborhood of a Cl^? vacancy.     

Disorder in ICE VI at 1 GPa: A neutron diffraction study on in situ grown single crystals

Abstract

Crystallographic structure determinations of H₂O and D₂O ice VI single crystals were performed between 207 K and ambient temperautre at pressures of 0.9 GPa. A neutron pressure cell of the Marburg-type was used. Single crystals were grown in situ under optical control in the pressure cell. Details of the cell construction, especially the design of the sapphire anvils and zero-scattering gaskets, are described.     

First-principles calculations of the electronic structure of fluorite-type crystals (CaF2, BaF2, SrF2, and PbF2) containing Frenkel defects. Analysis of optical and transport properties

The electronic structure of the alkali-earth fluorides CaF₂, BaF₂, SrF₂, and PbF₂ with Frenkel defects is investigated in the tight-binding approximation by the LMTO method. The defect formation and migration energies are calculated. The electronic structure and optical excitations of a H center in a defective fluorite structure are examined. It is shown on the basis of calculations of the binding energies that CaF₂, BaF₂, and SrF₂ are ionic compounds, while the chemical bond in PbF₂ is partially covalent. Possible methods of displacement of interstitial fluorine atoms that lead to the observed optical spectra of an H center are investigated. Fiz. Tverd. Tela (St. Petersburg) 40, 2019–2025 (November 1998)     

Crystal structure and electronic spectrum of SnS2

It is shown that layered metal dichalcogenides are quasi-one-dimensional molecular crystals and form a new class of crystal structures — molecular close packed. Since the minimum structural unit in these crystals is a monomolecular layer, using the symbols employed in atomic close packing to describe them gives a mistaken representation of their structure and symmetry. A new system of notation is proposed which provides complete and exact information about the ordering of the atomic layers in different polytype modifications and about their symmetries. It is found that in molecular close packing and, especially, in tin disulfide, there is not one (as in atomic close packing), but two, simplest structures, 1T and 1H, containing one molecule each in a unit cell and, therefore, two series of superlattices constructed on their basis. An energy model is constructed for the natural superlattice in tin disulfide crystals and the electronic spectra of the 2H, 4H, and 9R polytype modifications are calculated in the Kronig-Penney approximation with rectangular potential barriers as superstructure perturbations of the simplest structures. These results make it possible to explain the observed complicated dependence of the band gaps of these crystals on polytype structure. Fiz. Tverd. Tela (St. Petersburg) 40, 1712–1718 (September 1998)     

The structural transformation of monoclinic [(R)-C5H14N2][Cu(SO4)2(H2O)4]·2H2O into orthorhombic [(R)-C5H14N2]2[Cu(H2O)6](SO4)3: crystal structures and thermal behavior

Single crystals of [(R)-C₅H₁₄N₂][Cu(SO₄)₂(H₂O)₄]·2H₂O (1) were grown through the slow evaporation of a solution containing H₂SO₄, (R)-C₅H₁₂N₂ and CuSO₄·5H₂O. These crystals spontaneously transform to [(R)-C₅H₁₄N₂]₂[Cu(H₂O)₆](SO₄)₃ (2) over the course of four days at room temperature. The same single crystal on the same mounting was used for the determination of the structure of (1) and the unit cell determination of (2). A second single crystal of the transformed batch has served for the structural determination of (2). Compound 1 crystallizes in the noncentrosymmetric space group P2₁ (No. 4) and consists of trimeric [Cu(SO₄)₂(H₂O)₄]^2? anions, [(R)-C₅H₁₄N₂]²⁺ cations and occluded water molecules. Compound 2 crystallizes in P2₁2₁2 (No. 18) and contains [Cu(H₂O)₆]²⁺ cations, [SO₄]^2? anions and occluded water molecules. The thermal decompositions of compounds 1 and 2 were studied by thermogravimetric analyses and temperature-dependent X-ray diffraction.     

Luminescence properties of calcium-iodide crystals

The effect of the conditions of preparation, temperature, and the action of x rays on the luminescence properties of calcium-iodide scintillation crystals is investigated. On the basis of the results of a study of the spectral characteristics of CaI₂ and CaI₂:H₂ crystals for optical and x-ray excitation in the temperature range 90–400 K, also taking into account the results of a study of the luminescence properties of CaI₂ crystals activated by Cl⁻, Br⁻, OH⁻, and Ca²⁺ impurities, it is suggested that the 236-nm band observed in the excitation spectra of crystals of calcium iodide may be caused by an uncontrollable hydrogen impurity. The luminescence of these crystals with maximum at 395 nm is ascribed to radiative recombination of excitons trapped at H⁻ ions. Zh. Tekh. Fiz. 69, 135–136 (January 1999)     

Tunneling investigations of single crystals of the single-layer cuprate Bi2Sr2CuOz in high magnetic fields

The coexistence of a superconducting gap and a normal-state gap (pseudogap) is observed in tunneling experiments with high-quality single crystals of the single-layer cuprate Bi₂Sr₂CuO_z. At temperatures T< T _c the normal-state gap, whose width is close to that of the super-conducting gap, does not vanish in magnetic fields H>H _c2. The large smearing of the gap structure in the tunneling spectra and the high conductance of tunnel junctions at zero bias voltage are due to the strong angular dependence of the superconducting-and normal-state gaps. The results of the investigations are in good agreement with recently published angle-resolved photoemission data. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 3, 217–222 (10 August 1998)     

Spectral-kinetic characteristics of transient absorption in calcium fluoride crystals

The method of absorption spectroscopy with nanosecond resolution is used to study transient absorption in nominally pure crystals of CaF₂ irradiated by a pulse of fast electrons. Optical absorption caused by self-trapped excitons appears in the form of a superposition of bands with half-widths of no more than 0.2 eV in both the electron and hole components. These groups of bands are assigned to absorption of F, H pairs in various configurations. We observe a regular change in the spectral-kinetic characteristics depending on the distance between the components of the pair. Fiz. Tverd. Tela (St. Petersburg) 39, 1197–1201 (July 1997)     

Erbium in silicon carbide crystals: EPR and high-temperature luminescence

Erbium ions have been incorporated for the first time in bulk 6H-SiC crystals during growth, and they were unambiguously identified from the ¹⁶⁷Er EPR hyperfine structure. High-temperature luminescence of erbium ions at a wavelength of 1.54 μm has been detected. The observed luminescence exhibits an increase in intensity with increasing temperature. The observation of Er luminescence in 6H-SiC offers a promising potential for development of semiconductor light-emitting devices at a wavelength within the fiber-optics transparency window. Fiz. Tverd. Tela (St. Petersburg) 41, 38–40 (January 1999)     

Formation and stabilization of F centers following direct generation of self-trapped excitons in KCl crystals

The spectrum of luminescent F centers generated in high-purity KCl crystals by 7–10.2-eV photons has been measured at 230 K. The pulsed annealing of these centers (250–550 K), as well as the dependence of the efficiency of stable F-center generation on irradiation temperature (80–500 K) has been studied. The efficiencies of F ⁻ and Cl ₃ ⁻ -center generation are maximum under direct optical creation of self-trapped excitons in the region of the Urbach intrinsicabsorption tail. Besides the exciton decay with formation of F centers and mobile H centers, a high-temperature exciton decay channel which involves creation of cation defects stabilizing the H centers has been revealed. Fiz. Tverd. Tela (St. Petersburg) 41, 433–441 (March 1999)     

Introducing Ga2O3 thin films as novel electron blocking layer to ZnO/p-GaN heterojunction LED

N-ZnO/Ga₂O₃/p-GaN heterojunction light-emitting diode (LED) was fabricated by metal-organic chemical vapor deposition. Compared with the n-ZnO/p-GaN structure, the deep level visible emission at 525?nm was completely suppressed while UV emission at ~392?nm was significantly improved in ZnO/Ga₂O₃/p-GaN structure. The role of Ga₂O₃ in n-ZnO/Ga₂O₃/p-GaN heterojunction LED was discussed in detail.     

Deep impurity states and intrinsic defects in photorefractive Cd1−x FexTe crystals

AbstractStudies were made of the low-temperature optical and photoelectric properties of Cd_1−x Fe_xTe crystals (x=0.0038) which provided information on the optical quality of these crystals and the nature of their inhomogeneity, and also revealed deep Fe²⁺ impurity centers and singly charged acceptor complexes. It was established that these complexes, which include doubly charged cadmium vacancies and ionized donors, are anisotropic. It was shown that their anisotropy is determined by the nature of the donor atom and its position in the crystal lattice (at a cationic or anionic site). Since these crystals contain real deep impurity centers and acceptor complexes, a mechanism is proposed for the photorefractive effect in these crystals. It was observed for the first time that the photorefractive properties of CdTe crystals containing impurity 3d elements may exhibit anisotropy unrelated to that of the electrooptic effect. Fiz. Tverd. Tela (St. Petersburg) 40, 1216–1220 (July 1998)     

2D degenerate electron gas at Ba/<Emphasis Type="Italic">n</Emphasis>-AlGaN and Ba/<Emphasis Type="Italic">n</Emphasis>-GaN interfaces

Ba/n-GaN(0001) and Ba/n-AlGaN(0001) interfaces were investigated for the first time by means of ultraviolet photoelectron spectroscopy. The spectra of the photoemission from a valence band along with the spectra of the core levels of Ga 3d, Al 2p, and Ba 4d were studied. The formation of a 2D degenerate electron gas (an accumulated layer on the n-GaN and n-AlGaN surfaces during adsorption of Ba atoms) was revealed.     

After effects of ec decay of57Co in Mg(1-x)FexSO4.7H2O mixed crystals

After effects due to the EC decay of⁵⁷Co in the mixed crystals (Mg,Fe)SO₄·7H₂O were studied. When the Fe concentrationx=Fe/(Fe+Mg) is 0.4≈0.5, the surroundings of⁵⁷Fe have the characteristics of orthorhombic MgSO₄·7H₂O although the crystal structure is monoclinic. This fact suggests a possibility of transient release of the Jahn-Teller transformation.     

Microwave absorption in a Y1Ba2Cu3O7−x single crystal near the superconducting transition

Y₁Ba₂Cu₃O_7−x single crystals are investigated near T _c ≈92 K in fields 0<H⩽9 kOe using a modified ESR spectrometer. The temperature modulation method is used for the first time, together with the traditional magnetic modulation method, to detect microwave responses in single crystals. Superconducting-transition peaks shifted relative to one another in temperature and differing in shape are observed in the temperature dependence of the corresponding signals ∂R/∂H and ∂R/∂T (R — microwave absorption). The evolution of these peaks as a function of the field and the angle π between H and the c axis of the single crystal is traced. It is shown that the difference in the temperature dependences of the derivatives ∂R/∂H and ∂R/∂T is due to the broadening of the superconducting transition characteristic of HTSCs. Fiz. Tverd. Tela (St. Petersburg) 41, 14–17 (January 1999)     

Width of the ferromagnetic resonance line in highly dispersed powders of crystalline and amorphous Co-P alloys

The resonance characteristics (inhomogeneous FMR linewidth ΔH) in highly dispersed (d=0.1–3 μm) powders of crystalline and amorphous Co-P alloys are investigated as a function of the composition, particle size, and atomic structure. It is established that ΔH for powders of amorphous Co-P alloys is two to three times larger than ΔH for crystalline Co-P powders. According to the investigations performed, this is caused by thermodynamically stimulated segregation of nonmagnetic Co₂P inclusions, apparently an effective relaxation channel, in the amorphous state of Co-P powders. Fiz. Tverd. Tela (St. Petersburg) 41, 464–467 (March 1999)     

Solid solutions of paradichlorobenzene in paradibromobenzene with vacancies present in their structure

The results of polarization studies of the low-frequency optical Raman spectra of solid solutions of paradibromobenzene with p-dichlorobenzene (10% paradichlorobenzene) are reported. The spectra of lattice vibrations of the mixed crystals are calculated, and it is shown that vacancies can be present in their structure, in agreement with the experimental spectra. The existence of vacancies is manifested by the appearance of secondary lines in the low-frequency spectrum, in the vicinity of 70 cm⁻¹. The intensities of the lines are related to the number of vacancies. The diffusion activation energies in mixed crystals of paradibromobenzene with paradichlorobenzene with vacancies present in their structure are calculated at different temperatures on the basis of the method of atom-atom potentials. It is shown that the activation energy in these mixed crystals is not as dependent on the temperature variation and the crystallographic direction as in crystals of the constituent components. Fiz. Tverd. Tela (St. Petersburg) 39, 1564–1566 (September 1997)     

Electronic level diagram and structure of metastable centers in CdF2:Ga and CdF2:In semiconducting crystals

A study is reported of the optical properties of wide-gap, predominantly ionic cadmium fluoride crystals in photo-and thermally stimulated transformations of metastable indium and gallium centers. An analysis of these properties leads one to a conclusion of gallium having two metastable states (two types of deep centers). The deep-center binding energies and the barriers separating the shallow (hydrogenic) and deep centers have been determined for both impurities. Configuration-coordinate diagrams are developed, and microscopic models for the deep centers are proposed. It is concluded that these centers are identical with the metastable DX centers in typical semiconducting crystals. Thus cadmium fluoride is the most ionic among the crystals where DX centers have thus far been found. The potential of using such crystals for optical information recording is discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 2130–2136 (December 1997)     

On the possibility of investigating the dispersion of the energy bands of a crystal using resonant x-ray emission spectra

It is shown that, when the dispersion of the energy bands of crystals is investigated by resonant x-ray emission (RXE), an ambiguity can arise in the determination of k _tr — the transition point in k space — as a result of the characteristic features of the band structure and the value of the matrix elements. It is proposed that calculations of the distribution of partial contributions to interband optical transitions in the Brillouin zone (BZ) be used as a means of obtaining a simplified estimate of k _tr. The basic properties of this distribution are examined for lithium hydride, which is a convenient model object having a simple band structure and interband transitions which have been studied in detail. It is shown that the k points of the general position, which form a complex figure in the BZ, make the main contribution to the interband transitions. The contribution of high-symmetry points is much smaller mainly because of their small representativeness. This make it easier to understand the reason for the possible nonmonotonic variation of k _tr in RXE measurements of the dispersion of the energy bands of crystals or the sharp changes in the resonance-fluorescence intensity. Fiz. Tverd. Tela (St. Petersburg) 41, 1597–1601 (September 1999)