Date

December 1980

Document Type

Dissertation

Degree Name

Ph.D.

Department

Dept. of Materials Science

Institution

Oregon Graduate Center

Abstract

The compound La [subscript x] Sr [subscript 1-x] TiO [subscript 3] with X up to 40 at. % was observed to be homogeneous by microscopic examination. The observed X-ray powder diffraction pattern corresponds to the cubic perovskite structure. The plot of lattice constant a [subscript o] vs X indicated a Vegard's law dependence within experimental error. Gravimetric measurements on pure and lanthanum doped SrTiO [subscript 3] have shown that the reversible change of oxygen content, between specified states of oxidation and reduction, is proportional to the dopant concentration. These measurements indicate that the donor dopants are ionically compensated by additional oxygen uptake in the oxidized state. The gravimetrical measurements are explained by a model involving a shear structure. The electrical conductivity of polycrystalline SrTiO [subscript 3] with different Sr to Ti ratios and with donor- and acceptor-dopants were determined for the oxygen partial pressure range of 10[superscript 0] to 10[superscript -22] atm., and temperature range of 800-1050°C. For SrTiO[subscript 3] with (Sr/Ti) = 1, the data were found to be proportional to the minus 1/6 power of the oxygen partial pressure for the oxygen pressure range 10[superscript minus 15] - 10[superscript minus 22] atm, proportional to P[subscript O2, superscript minus ¼] for the oxygen pressure range 10[superscript minus 8] - 10[superscript minus 15] atm., and proportional to P[subscript O2, superscript minus ¼] for the oxygen pressure range > 10[superscript minus 3] atm. These data are consistent with the presence of a very small amount of acceptor impurity in SrTiO [subscript 3]. The deviation from ideal (Sr/Ti) ratio is found to be accommodated by neutral vacancy pairs. For the donor doped SrTiO[subscript 3], two kinds of charge compensation were observed according to the oxygen partial pressure, via an electronic or a lattice defect in a La[subscript x]Sr[subscript 1 minus x]TiO[subscript 3] or a La[subscript x]Sr[subscript 1 minus x]TiO[subscript 3+x/2] solid solution, respectively. For the acceptor (Fe, Al)-doped samples, the p to n transition is shifted to lower PO[subscript 2] as compared to the undoped SrTiO[subscript 3]. The conductivities in the acceptor-doped samples are lower in the n-type region, and are higher in p-type region, as compared to the values in the undoped SrTiO[subscript 3]. The Raman spectra of SrTiO[subscript 3] were recorded as a function of temperature using an argon-ion laser as exciter. At room-temperature the Raman spectrum was found to be second-order in agreement with the selection rules for the cubic perovskite structure. The bands in the second-order spectrum originate largely from pairs of phonons with wave vectors near the Brillouin zone boundary. On cooling the SrTiO [subscript 3] below 120K, five additional bands appear in the spectra, which are due to first-order modes of the tetragonal structure. The energies of phonon branches are deduced, and compared with the results of neutron inelastic scattering experiments. It is shown that Raman scattering from polycrystalline solid contains all of the essential features of the Raman single crystal spectrum. Factor group analysis were made for the Ruddlesden-Popper phases (nSrTiO [subscript 3] SrO) up to n = 25. These analyses did not yield a simple band multiplicity, even though the space group D [superscript 17, subscript 4h] [I4/mmrn] was preserved by stacking the cubic perovskite (SrTiO [subscript 3]) blocks. Not all the Raman active modes predicted by factor group analysis were actually observed for these phases and this may be due to poor coupling of the vibrations between individual segments of the enlarged unit cell.

Identifier

doi:10.6083/M47D2S29

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