CC BY
16УДК 539.23;539.216.1
K. Dragounova
Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Czech Republic, Prague
PHOTOLUMINESCENCE OF Cr-DOPED NANOCRYSTALLINE SrTiO3
Materials with the perovskite-type structure (ABO3) find wide range of technical applications owing to their ferroelectric, piezoelectric and optical properties. Passing from bulk to the nanocrystalline material, the properties may change noticeably due to the increasing role of surface layer. For that reason the nanocrystalline materials are now extensively studied. The brief overview of available knowledge on photoluminescence of Cr-doped nanocrystalline SrTiO3 and on the use of Cr3+ ions as a spectroscopic probe in study of phase transitions in this material is presented.
SrTiO3 belongs to the incipient ferroelectrics keeping paraelectric phase until the lowest temperatures. Their permittivity is high and increases with decreasing temperature according to Curie-Weiss law
e = 1/(T - Tc)
as for ferroelectrics at temperatures above ferroelectric phase transition. Nonetheless, high sensitivity of properties on content of defects and action of external force fields (pressure, electric field) was observed for these materials at low temperatures. Moreover, suitable defects and external force fields can even induce the ferroelectric phase transition. The origin of spontaneous polarization is connected with mutual shift of negative and positive ions at the transition from paraelectric cubic phase to the low-temperature ferroelectric phase. This transition is caused by the crystal structure instability towards so-called soft mode of the heat oscillations. Soft mode frequency is considerably temperature-dependent. Phase transitions in perovskite-type materials induced by soft mode are characterized only by small shift of ions to their positions in cubic phase. In the case of SrTiO3, the transition is predominantly associated with the tilt and rotation of [TiO6]8- octahedron. The oscillations of Ti4+ ions against O2- ions contribute mainly to the soft phonon mode. For that reason the Cr^ ions substituting Ti4+ site in SrTiO3 strongly interact with the soft phonon mode, which is of the essential importance to study of ferroelectric phase transitions in ABO3 materials by means of Cr3+ ions as a spectroscopic probe.
The Cr3+ ions belong to the transition-metal ions with 3d3 electronic configuration. Their luminescence in Cr-doped materials originates from transitions of electrons between energy levels of incompletely filled 3d shell. The emission spectra of Cr3+ ions in SrTiO3 crystals consist of the sharp R-line in the near infrared spectral region (794 nm for 20.4 K [1]), which corresponds to the 2Eg ^ 4A2g zero-phonon transition, and the vibronic sidebands. The interaction of Cr3+ ions with soft phonon mode results in the anomalous temperature shift of the R-line position to higher energy side with increasing temperature (+45 cm-1 between 4. 2 K and 200 K [1]) that makes their use as the probes for study of ferroelectric phase transitions possible. It was found out that the R-line shift Av is proportional to the reciprocal permittivity 1/e of SrTiO3 at the temperatures below 107 K and hence to
the square ra2 of soft phonon mode frequency, i. e. Av ~ ra2 ~ 1/e, as follows from Lydan-Sachs-Teller law. The anomalous temperature behavior of the R-line position is ascribed to the local configurational instability of Cr3+ ions in the excited 2Eg state towards oscillations connected with the soft TO1 mode. This instability leads to displacement of the Cr3+ ion from the substitution Ti4+ site in the SrTiO3 lattice. The mechanism responsible for the local configuration instability is probably dynamic Jahn-Teller effect on soft phonon mode occurring at the degenerate excited state 2Eg with long lifetime.
At the low temperatures, R-line splitting to the doublet is observed in consequence of splitting of the excited 2Eg state by the tetragonal crystal field below the structural phase transition from cubic to the tetragonal phase (Oh1 ^ D4h18) at ~105 K. However, the R-line splitting is already hidden in the emission spectra at 77 K due to decrease of the doublet splitting and increase of width of the doublet lines with increasing temperature and only single line with half-width of 5 cm-1 is observed.
Similar behavior of the Cr3+ R-line is observed for SrTiO3 ceramics as well. There are two significant differences in comparison with SrTiO3 crystals nevertheless. First, an additional inhomogeneous broadening of the R-lines is observed due to the inhomogenity inherent to the ceramics. Second, the R-line temperature shift is proportional to the reciprocal permittivity of the crystal rather than to that of the ceramic. Moreover, R-line splitting is somewhat smaller than that of the bulk SrTiO3 crystals indicating smaller distortion of [TiO6]8- octahedron below (Oh1 ^ D4h18) phase transition.
The emission spectra of Cr3+ luminescence in nanocrystalline SrTiO3:Cr powders are also similar to that of the bulk crystals. Nonetheless, the R-line and vibronic sidebands are shifted to a higher energy side at the given temperature. This shift is explained by the coupling of the excited 2Eg state with minimum of the conduction band, whose energy increases with decreasing nanocrystal size. Furthermore, the inhomogeneous broadening, increasing with a decrease in nanocrystal size, makes any observations of the R-line splitting in the tetragonal phase impossible. Such behavior of R-line width originates from the increasing inhomogenity and related larger random electric fields and strains in the nanocrystals that is probably related to the increasing effect of the surface
Решетневские чтения
layer on a nanocrystal structure. An example of the emission spectrum of Cr3+ luminescence for nanocrystalline SrTiO3:Cr powder with the average particle size 5 nm is on the see figure. In this case, the R-line position was determined to be 789.4 nm at 11 K.
è 1.5-
I
8 i,o-
I
I 0,50,0770 780 790 800 810
wavelength [nm]
Emission spectrum of nanocrystalline SrTiO3:Cr (5 nm). Spectrum was recorded at 11 K
It seems that ferroelectric phase transition can appear in SrTiO3 particles with suitable shape and size. The study of the luminescence of Cr3+ impurity probe in nanocrystalline SrTiO3:Cr powders evidenced so far increasing instability towards a ferroelectric phase transition in small SrTiO3 nanoparticles with their size reduction and probably even the emergence of low-temperature ferroelectric phase transition in sufficiently small SrTiO3:Cr nanoparticles. The question requires further investigations nevertheless [2; 3].
References
1. Stokowski S. E., Schawlow A. L. // Phys. Rev. 178; 1969.
2. Trepakov V. A., Potucek Z., Makarova M. V. et al. // Nazev. Phys. Condens. Matter 21. 2009. P. 375303.
3. Potucek Z., Trepakov V. A., Makarova M. V. et al. // Nazev, Mat. Science and Technology 25, 2009. P. 11.
К. Драгунова
Чешский технический университет в Праге, Чешская Республика, Прага
ФОТОЛЮМИНЕСЦЕНЦИЯ НАНОКРИСТАЛЛИЧЕСКИХ СОЕДИНЕНИЯ SrTiO3
С ВКРАПЛЕНИЯМИ ХРОМА
Материалы с перовскитовой структурой (ABO3) находят широкое применение, благодаря их ферроэлек-трическим, пьезоэлектрическим и оптическим свойствам. Переходя от основных к нанокристаллическим материалам, свойства могут заметно изменяться из-за усиления свойств покрытия. По этой причине в настоящее время широко изучается применение нанокристаллических материалов. Представлен краткий обзор существующих исследований по фотолюминесценции нанокристаллических соединений SrTiO3 с вкраплениями хрома и по использованию ионов сг3+ в качестве спектроскопической пробы при исследовании фазовых переходов.
© Dragounova К., 2011
УДК 53
T. Koubsky
Czech Technical University, Faculty of Nuclear Research and Physical Engineering, Department of Solid State Engineering, Czech Republic, Prague
SIMULATION OF CHEMICAL STABILITY OF UAM-069 AND ONE OF THE DEGRADATION
PRODUCTS*
For nuclear hydrometallurgical separation process development, it is necessary to demonstrate the stability of the extracting systems. The m-xylylene-bis-diglycolamide (compound 1) is used in this process but its instability is undesirable. It is known that one of the degradation products (2) is significantly more stable while having comparable extraction properties. The simulation of chemical stability showed significantly less localized density of the HOMO for 2, which corresponds to the lower preference of the ether oxygen atom to be the weak point.
One of the strategies used for spent nuclear fuel management is the hydrometallurgical treatment of the high level liquid waste (HLLW). Among others, it comprises actinide and lanthanide group separation. In this process the compound 1 UAM-069 (m-xylylene-bis-diglycolamide - figure 1) is used for good selective properties of its diglycolamide group. The degradation of
1 in the radioactive and acidic environment leads clearly to undesirable effects caused by decrease of concentration. Unlike the other fragments, the major degradation product (2 - see figure 2) also acts as an efficient extractant but its stability against the hydrolysis is higher. The main reason of the degradation is an electrophilic (acidic) attack of hydrogen cations.
^Computational results obtained using software programs from Accelrys Software Inc. The ab initio calculations performed with the DMol3 program with the BLYP, PBE and RPBE xc-functionals.
