372595892

PREPARATIONS

JPRS-UMS-92-003 16 March 1992

V.G. Andreychenko, Odessa State University imeni I. Mechnikov and General and Inorganic Chemistry Insti-tute at the Ukrainian Academy of Sciences]

U DC 546.661.847.23

[Abstract] The self-propagating high-temperature syn-thesis (SVS) method is uscd to produce solid blocks of zinc selenide. The burden is prepared from a standard zinc powder PTs-1 and chemically pure selenium ground in a mortar to a particie size of < 10 pm. Since the ZnSe quality depends primarily on the synthesis conditions as well as the combustion temperaturę and extemal pres-sure, these parameters were manipulated during the process. The results of Chemical and X-ray phase anal-yses of ZnSe alloys, the response intensity of the super-structural ZnSe phase at various wavelengths, infrared transmission spectra measurements, diffuse reflection spectra of polycrystalline ZnSe, and emission spectra of polycrystalline ZnSe are cited. Radiographic studies are performed by the powder method in a DRON-2 under CuK_a radiation while infrared spectroscopy analyses are carried out in a Specord M-80 instrument. Forbidden gap width measurement data attest to the presence of perfect cj7stals in synthesized ZnSe product. Photocon-duction is measured on freshly cleaved surfaces while photoluminescence is analyzed in a special unit under a He-Cd laser. Analytical data indicate that stoichiometric ZnSe with a yield of over 97% can be produced by self-propagating high-temperature synthesis and that the resulting polycrystalline blocks have effective emissivity in the visible and near infrared spectra, making them promising for use in phosphor preparation. Figures 3; tables 2; references 7: 5 Russian, 2 Western.

Photoluminescence of ZnSe/GaAs (100) Heterostructures

927D0068F Moscow IZYESTIYA AKADEMII NAUK SSSR: SERIYA NEORGANICHESKIYE MATERIAŁY in Russian Vol 27 No 12, Dec 91 pp 2520-2522

[Article by N.V. Bondar, A.V. Kovalenko, V.V. Tish-chenko, A.Yu. Mekekechko, Dnepropetrovsk State Uni-versity named after the 300th anniversary of the unifi-cation of Ukrainę and Russia]

UDC 541.148

[Abstract] Chemical precipitation from the gaseous phase, epitaxy from organometallic compounds, and molecular beam epitaxy are the most promising methods of producing heterostructures; consequently, the effect of growth parameters (TRP) on the structure and photoluminescence spectra (FL) of epitaxial zinc selenide layers deposited on GaAs (100) substrates by the Chemical gaseous phase precipitation method is studied. Multiply purified (by zonę melting) ZnSe powder was vaporized in a quartz horizontal reactor in a purified hydrogen current. A 1 pm-thick cleaved ZnSe/GaAs (100) film was examined by microphotography and reflection and photoluminescence spectra of two epitaxial ZnSe films on GaAs (100) substrates were plotted. As a result of the studies, bar charts characterizing the effect of the growth process parameters on the photoluminescence spectra of epitaxial ZnSe/GaAs (100) layers at a 300K temperaturę under ultraviolet excitation are plotted; the charts make it possible to identify the optimum growth parameters as being a 235-310°C temperaturę rangę and a carrier gas ratę of 0.9-1.4 1/min. The exciton photoluminescence process is intensified under exposure to a He-Cd laser whose scattered radiation is aimed at the GaAs sub-strate. At a 4.2K temperaturę, only excitons are present in the ZnSe/GaAs photoluminescence spectra. Figures 3; references 8: 1 Russian; 7 Western.

Eu₂Ti₂0₇ Phase Produced at High Pressures and Its Ferroelectric Properties

927D0068J Moscow IZYESTIYA AKADEMII NAUK SSSR: SERIYA NEORGANICHESKIYE MATERIAŁY in Russian Vol 27 No 12, Dec 91 pp 2597-2599

[Article by A.M. Sych, S.Yu. Stefanovich, Yu.A. Titov, T.N. Bondarenko, V.M. Melnik, Kiev State University imeni T.G. Shevchenko, Scientific Research Institutc of Physical Chemistry imeni L.Ya. Karpov, and Institute of Materials Science Problems at the USSR Academy of Sciences]

UDC 548.312.3

[Abstract] The ability of titanates of rare earth elements (r.z.e.) to crystallize as two modifications, pyrochlore (PKh) and flaky perovskite-like structures (SLPS) with a polar symmetry, is discussed and an attempt is madę to produce an individual high-pressure phase (f.v.d.) of Eu₂Ti₂0₇ with a flaky perovskite-like structure and investigate its ferroelectric properties. Eu₂Ti₂0₇ with a pyrochlore structure was converted to a polar modifica-tion by a two-stage treatment at a 3-4 GPa pressure and a 298K temperaturę and 8 x 10⁹ Pa pressure at a 2,020K temperaturę. A radiographic analysis demonstrates that the high-pressure phase of Eu₂Ti₂0₇ with a flaky perovs-kite-like structure is crystallized in a monoclinic syng-ony. The temperaturę dependence of the relative second harmonie generation signal strength of high-pressure phases is examined; it is shown that the phase transition in the ferroelectric high-pressure phase of Eu₂Ti₂0₇ with a flaky perovskite-like structure is similar to the second-kind phase transition. The linear character of the second harmonie signal strength as a function of temperaturę is characteristic of intrinsic ferroelectrics near the phase transition. Eu₂Ti₂0₇ has a Curie temperaturę of close to 1,520K and a spontaneous polarization of 2.7 pCl/cm². Figures 1; references 7: 5 Russian; 2 Western.