LABORATORY
CRYSTAL GROWTH

HEAD: Prof. Marin Gospodinov, Dr.Sci.
tel: 7144-263; e-mail: crystlab@issp.bas.bg

TOTAL STAFF: 8
RESEARCH SCIENTISTS: 7

Assoc.Prof. S.Dobreva, Ph.D.
Assoc.Prof. T.Milenov, Ph.D.
Research Scientist P.Rafailov, Ph.D.
Research Scientist M.Veleva
Research Scientist L.Yankova
eng. N.Petkov

RESEARCH ACTIVITIES:

    The research work of the Laboratory of Crystal Growth was focussed on three main fields:

-growth of Bi₁₂MO₂₀ (M= Ge, Si and Ti) crystals with application in the field of non- linear optics and optical information storage
-investigation of two and three dimensional defects in fcc and bcc structures
-growth and investigation of complex oxides with perovskite and spinel structure.

    Changes of the optical activity in BSO crystal induced by thermal annealing treatments are studied. The annealing in vacuum and hydrogen atmosphere causes decreasing of the optical rotation. It is supposed that the annealing induces reduction of Bi ions to metallic bismuth on the surface as well as oxygen and bismuth diffusion and evaporation. Annealing in oxygen leads to partial recovery of the oxygen content in the crystal as well as the optical activity tends to approach the level of un- annealed crystal [1].

    Absorption, light- induced absorption, photoconductivity and dark conductivity measurements are performed on Bi₁₂TiO₂₀ (BTO) crystals doped with different concentrations of ruthenium. It is found that a higher ruthenium addition shifts the optical absorption to the near- IR spectral region in comparison with non- doped BTO crystals and the absorption changes increase with increasing Ru content. The dark conductivity follows the Arrhenius law with an activation energy of 0.89 eV. From the measured light- induced absorption and nonlinear photoconductivity the participation of a shallow level in the charge transport mechanism in Ru- doped BTO crystals is suggested. Highly Ru- doped BTO samples posses infrared sensitivity and holographic gratings are successfully recorded at 790 and 825 nm, respectively [2].

    Bi₄Ge₃O₁₂ single crystals doped with different concentration of Ru were grown by the Czochralski technique. Optical absorption bands induced by illumination with ultraviolet light at room temperature. The absorption coefficient values increased with increasing Ru concenrtation. The induced photochromic effect was completely reversible: all samples, recovered their transparency after thermal treatment. Prelliminary holographic recording experiments showed a fast response time and a relatively slow erase time during the reading process [3].

    (SnS₂)_x- (SnSe₂)_1-x layered crystals were grown using the vertical Bridgman technique for x= 0.0- 1.0. IR spectra of these compounds were obtained using a Bruker- spectrometer. The structural properties were studied using transmission electron microscopy (TEM). The variation of the structural and optical properties was studied as a function of the composition x [4].

    The core region in a Bi₁₂SiO₂₀ crystal doped with Mn was examined by X-ray double-crystal diffraction topography. Specific features were observed in the topographies as lines and contrast differences that point to defects occupying the central part of the crystal. We discuss the nature of these defects and propose an explanation in terms of stacking faults arranged in different structures [5].

    Crystals Pr_0,7Sr_0,28Pb_0,02MnO₃ and Pr_0,7Sr_0,3MnO₃ are successfully grown. The structure, component and phase composition of the grown crystals are investigated.

    The magnet susceptibility dependence of Pr_0,7Sr_0,28Pb_0,02MnO₃ on the temperature is measured and the type of conductivity is determined in different temperature intervals. The ferromagnetic Curie temperature T_c is determined (Т_с = 380 К). The difference between this result and the result obtained in LaMnO₃ crystals (Т_с=240-260 К) is explained with differences in the oxygene octahedral surrounding of Mn- ions [6].

    Hall effect measurements of perovskite manganite Pr_0,7Sr_0,3MnO₃ have been obtained at a magnetic field of 0.6 T in the temperature range 77- 400 K. The results show peculiarities in the Hall- coefficient behaviour. This is likely to be due to the co- existance of charge ordering and ferromagnetism. Obviously, the transformation between different magnetic phases with increasing temperature is significant for the physical properties, and influences the current transport in the compound [7].

    The Raman spectra of CaFe₂O₄ were measured with several exact scattering configurations between 20 and 520 K and the symmetry of all observed Raman lines was determined. The А_g and В_2g lines were assigned to definite photon modes by comparison to the results of lattice dynamical calculations. No anomaly of phonon parameters was observed near the magnetic ordering temperature ТN = 160 К [8].

    A re- entrant novel phase has been observed in the hexagonal HoMnO₃ in the presence of magnetic field, in the temperature ranges defined by the plateau of dielectric constant anomaly. The dielectric plateau evolves with field from a narrow sharp dielectric peak at the Mn- spin rotation transition at 32.8 K in zero magnetic field. Such a field induced dielectric plateau anomaly appears both in the temperature sweep at a constant field and in the field sweep at a constant temperature without detectable hysteresis. This is attributed to the indirect coupling between the ferroelectric and antiferromagnetic orders, arising from an antiferromagnetic domain wall effect, where the magnetic order parameter of the Mn- subsystem has to change sign across the ferroelectric domain wall in the compound, that influences the ferroelectric domains via local magnetostictive effect [9].

1. V. Marinova, M. Veleva and D. Petrova, The influence of annealing at different temperatures and atmospheres on the optical rotation of Bi₁₂SiO₂₀ crystals, Optical Materials 24 (2003) 595.
2. V. Marinova, S.H. Lin, V. Sainov, M. Gospodinov and K.Y. Hsu, Light- induced properties of Ru doped Bi₁₂TiO₂₀ crystals, Journal of Optics A, 5 (2003) S500
3. V. Marinova, S.H. Lin, K.Y. Hsu , Mei- Li Hsieh, M. Gospodinov and V. Sainov, Optical and holographic properties of Bi₄Ge₃O₂₀ crystals doped with ruthenium, J. of Mater. Sci. 14 (2003) 857
4. M.M. Gospodinov, V. Marinova, E. Polychroniadis, D. Papadopoulos, K. Kampas, A.N. Anagnostopoulos, K. Kyritsi, Growth and characterization of (SnS₂)_x - (SnSe₂)_1-x mixed crystals, Mater. Res. Bull. 38 (2003) 177
5. T.I.Milenov, P.A.Botev, P.M.Rafailov and M.M.Gospodinov, X-ray diffraction topography observations of the core in Bi₁₂SiO₂₀ crystals doped with Mn, J. of Mater. Sci. Eng B (2003)- (in press)
6. K. Kalaydjiev, M. Gospodinov, S. Dobreva, M. Kirov, M. Baychev, Chr. Popov, M. Bushev, Growth of Pr_0.7Sr_0.28Pb_0.02MnO₃ single crystals and determination of the phase transition temperature, J. of Mat. Sci.: Materials in Electronics 14, (2003) 763.
7. D. Dimitrov, V. Lovchinov, M. Gospodinov, S. Dobreva, Hall-effect studies of Pr_0.7Sr_0.3MnO₃ , J. of Mat. Sci.: Materials in Electronics 14 (2003) 765.
8. N. Kolev, M. N. Iliev, V. N. Popov and M. Gospodinov, Temperature-dependent polarized Raman spectra of CaFe₂O₄, Solid State Comm. 128, 4 (2003) pp.153-155.
9. B. Lorenz, A.P. Litvinchuk, M. Gospodinov, C.W. Chu, A field-Induced Re-Entrant Novel Phase and A Ferroelectric-Magnetic Order Coupling in HoMnO₃, Phys. Rev. Lett. (2003) - in press.
10. B. Guttler, B. Mihailova, R. Stosch, U. Bismayer and M. Gospodinov, Local phenomena in relaxor-ferroelectric PbSc_0.5B^"_0.5O₃ (B^"= Nb, Ta) studied by Raman spectroscopy, Journal of Molecular Structure, 661-662C (2003) pp. 469-479.

1. Growth of Bi₁₂MeO₂₀ (Me = Si, Ge, Ti) and investigation of their physical properties.
2. Investigation of two- and three- dimensional defects in bcc and fcc crystals
3. Growth and study of complex oxides with perovskite and spinel structures.

1. F 1207: Growth of crystals of complex oxides with mixed valence. Investigations of their structure, defects, electrical, optical and magnetic properties.
2. F1308: Growth and investigation of wide band gap semiconductor and oxide crystals with application in photorefractive optics and detectors of X- and gamma- rays.

1. "Static moments of short- lived exotic nuclei studied via hyperfine interactions of the nuclear spins with crystalline electric field gradients."

1. "Growth and investigation of photorefractive oxide crystals and wide band gap semiconductor materials as SiC and GaN etc. Institute of Common and Inorganic Chemistry, Russian Academy of Sciences (RAS), Moscow, Russia.
2. "Synthesis and investigation of complex oxide crystals: LaPbMnO₃, Bi₁₂SiO₂₀, Bi₁₂TiO₂₀, Bi₁₂GeO₂₀, Gd₃RuO₇", Phys.- Tech. Inst. "Yoffe", (RAS), St.Petersburg, Russia.
3. "Growth and characterisation of oxide crystals for optical applications - Research Institute for Solid State Phisics and Optics, Budapest, Hungary.
4. "Research of the real electronic structure and photochromism of complex oxides" - Institute of physics, Polish Academy of Sciences, Warszawa, Poland.

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