0000000000505675
AUTHOR
Zorana Dohčević-mitrović
Raman scattering study of the anharmonic effects in CeO2−ynanocrystals
We have studied the temperature dependence of the F2g Raman mode phonon frequency and broadening in CeO2−y nanocrystals. The phonon softening and phonon linewidth are calculated using a model which takes into account the three-and four-phonon anharmonic processes. A detailed comparison of the experimental data with theoretical calculations revealed the predominance of four-phonon anharmonic processes in the temperature dependence of the phonon energy and broadening of the nanocrystals. On the other hand, three-phonon processes dominate the temperature behavior of phonons in polycrystalline samples. The anti-Stokes/Stokes peak intensity ratio was also investigated and found to be smaller for…
Phonon properties of the spinel oxideMgTi2O4with theS=1/2pyrochlore lattice
We study the phonon dynamics of ${\mathrm{MgTi}}_{2}{\mathrm{O}}_{4}$ spinel by measuring the Raman and infrared reflectivity spectra in a wide frequency $(100--3000{\mathrm{cm}}^{\ensuremath{-}1})$ and temperature (10 K--300 K) range. The reflectivity spectra are analyzed by a fitting procedure based on a model which includes both Drude and phonon oscillator contributions to the dielectric constant. The phonon assignment is done from comparison between experimental data and shell model lattice dynamical calculations. We find two infrared-active ${F}_{1u}$ symmetry modes superimposed on the free carrier continuum, and four Raman-active modes of ${1A}_{1g},$ ${1E}_{g},$ and ${2F}_{2g}$ symme…
Short range charge/orbital ordering in La1−xSrxMn1−zBzO3 (B = Cu,Zn) manganites
We have measured the reflectivity spectra of La1−x SrxMn1−zBzO3 (B = Cu, Zn; 0.17 x 0.30; 0 z 0.10) manganites over wide frequency (100–4000 cm −1 )a nd temperature (80–300 K) ranges. Besides the previously observed infrared active modes or mode pairs at about 160 cm −1 (external mode), 350 cm −1 (bond bending mode) and 590 cm −1 (bond stretching mode), we have clearly observed two additional phonon modes at about 645 and 720 cm −1 below the temperature T1 (T1 < TC), which coincides with the phase transition temperature when the system transforms from ferromagnetic metallic into a ferromagnetic insulator state. This transition is related to the formation of short range charge/orbitally orde…
F-centre luminescence in nanocrystalline CeO2
Nanocrystalline CeO2 powders were synthesized by two cost-effective methods: the self-propagating room temperature (SPRT) method and the precipitation method. Differently prepared samples exhibited different temperature-dependent photoluminescence (PL) in the ultraviolet and visible regions. The PL signals originated from different kinds of oxygen-deficient defect centres with or without trapped electrons (F0, F+ or F++ centres). The temperature-dependent PL spectra were measured using different excitation lines, below (457, 488 and 514?nm) or comparable (325?nm) to the ceria optical band gap energy, in order to investigate the positions of intragap localized defect states. Evidence for the…
Optical phonons in theNaTiSi2O6oxide withS=12spin chains
. The infrared-active phonon frequencies are obtained by Kramers-Kronig analy-sis of the reflectivity data. The assignment of the observed modes is done using lattice dynamical calculationsbased on the valence shell model. A phase transition at about 210 K is manifested through a dramatic changeof the mode frequency and broadening, and the appearance of new phonon modes. Analyzing the phononfrequency and damping vs temperature we have found that the phase transition leaves a fingerprint in thephonon dynamics of NaTiSi
Novel phase transitions in B-site doped manganites
We have examined the infrared reflectivity and the electrical resistivity of La 1-x [Sr(Ba)] x Mn 1-z [Cu(Zn)] z O 3 samples in ferromagnetic metallic and insulator regime. Several phase transitions are observed, the most obvious being the transition from a ferromagnetic metallic to a ferromagnetic insulator phase that is related to the formation of short-range orbitally ordered domains. The temperature T 1 of the phase transition is dependent on doping concentration and for optimally doped samples (∼32% of Mn 4+ ions) we have found T 1 0.93 T C .
Effect ofA-site andB-site substitution on the infrared reflectivity spectra ofLa1−yAyMn1−xBxO3(A=Ba,Sr;B=Cu,Zn,Sc;0<y<~0.3;0<~x<~0.1)manganites
We have measured the infrared reflectivity spectra of ${\mathrm{La}}_{1\ensuremath{-}y}[\mathrm{Sr}(\mathrm{Ba}){]}_{y}{\mathrm{Mn}}_{1\ensuremath{-}x}[\mathrm{Cu}(\mathrm{Zn},\mathrm{Sc}){]}_{x}{\mathrm{O}}_{3}$ $(0lyl~0.3,0l~xl~0.10)$ manganites, in a wide frequency $(100\char21{}4000{\mathrm{cm}}^{\ensuremath{-}1})$ and temperature (80 K\char21{}300 K) range. The reflectivity spectra were analyzed by a fitting procedure based on a model that includes Drude, midinfrared electronic, and phonon oscillator contributions to the dielectric constant. Six infrared active ${(3A}_{u}$ and ${3E}_{u})$ modes of rhombohedral symmetry are clearly observed. We assigned all observed modes according to e…