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In high purity Y 2 O 3 powders presence of small amount of erbium ions in cation sites was identified due to the luminescence bands observed by Raman spectrometer. The decrease of the luminescence intensity during high-energy ball-milling was explained. The mechanochemical processing induced the changes of Y 2 O 3 microstructure introducing distortion of the crystal lattice and oxygen vacancies in the cubic phase, substitution of tungsten at the cation sites in c-Y 2 O 3 , and incomplete phase transition of Y 2 O 3 from the cubic to the monoclinic structure. The influence of the particle-size decrease, caused by ball-milling, to luminescence was also considered.Keywords: Oxide materials; Mechanochemical processing; Microstructure; Luminescence IntroductionYttrium sesquioxide (Y 2 O 3 ) ceramics have been intensively investigated for different technological purposes. For decades yttrium oxide has been an important material in ceramic industry, from constituent of ceramic superconductors [1,2], to wellknown YSZ ceramics [3,4]. Y 2 O 3 is also used in electronic applications as a part of metal-oxide-semiconductor heterostructures in MOS transistors [5][6][7][8]. It also plays an important role in preparation of novel light-emitting materials. Y 2 O3 is used as a refractory matrix in rare earth ion doped laser materials [9]. It is selected as a host because of favorable thermomechanical properties (high melting point, phase stability, low thermal expansion). A very good substitution of dopants is insured by similar crystal-chemical constraints for rare earth ions and yttrium [9,10]. Y 2 O 3 structure corresponds to the C-type cubic bixbyite structure, space group Ia3 [11,12] with 16 formula units in the elementary cell. There are 32 cation (six-fold coordinated) sites available for substitution of lanthanide ions: eight are centrosymmetric with C3i symmetry and 24 noncentrosymmetric with C 2 symmetry (Fig. 1a). The sites with C 3i symmetry have a smaller crystal field, so Stark splitting of 4f orbital is smaller [13]. Moreover, since C 3i site has an inversion center, the selection rules forbid the electron-dipole (ED) transitions, but thermal fluctuations of Y 2 O 3 lattice destroy the perfect C 3i symmetry locally [14], and the ED transitions can occur. There is no center of inversion in the C 2 site and the luminescence of incorporated lanthanide ions is predominantly connected with this site. At the pressure of 10-13 GPa cubic Y 2 O 3 exhibits the first-order reconstructive (irreversible) phase transition [15] to monoclinic structure, space group C2/m [16]. Monoclinic structure has 6 Y 2 O 3 formula units in the elementary cell with 3 nonequivalent cation sites (seven-fold coordinated) and Cs symmetry (Fig. 1b). Bihari et. al [17] observed three sets of luminescence lines in doped monoclinic Y 2 O 3 , which correspond to three nonequivalent cation sites. In this work the luminescence bands was observed in high purity Y 2 O 3 (99.99%). The origin of the luminescence bands was identified and their propert...

The structural stability of some nanostructured titanates was investigated in terms of their subsequent processing and possible applications. With the aim to investigate their mechanochemical stability, we applied high-energy ball milling and studied the resulting induced phase transitions. Hydrogen titanates with two different morphologies, microcrystals and nanotubes, were taken into consideration. The phase-transition sequence was studied by Raman spectroscopy and X-ray powder diffraction, while the morphology and crystal structure, on the nanoscale, were analyzed by high-resolution transmission electron microscopy. During the mechanochemical treatment of both morphologies, the phase transitions from hydrogen titanate to TiO2 anatase and subsequently to TiO2 rutile were observed. In the case of hydrogen trititanate crystals, the phase transition to anatase starts after a longer milling time than in the case of the titanate nanotubes, which is explained by the larger particle size of the crystalline powder. However, the phase transition from anatase to rutile occurred more quickly in the crystalline powder than in the case of the nanotubes.

The metamict state and recrystallization of fergusonite in metamict natural samples were studied by thermal methods (TGA-DTA), X-ray powder diffraction (XRD), Raman spectroscopy (RS), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and electron microprobe (EPMA). Two metamict mineral samples assumed to be fergusonite were investigated in order to identify the original premetamict crystal structure, and to reveal recrystallization mechanisms. The TEM data and partly RS provided evidence of the partial preservation of the original structure in the investigated minerals, which seem to be X-ray amorphous. The collected data indicated the fergusonite recrystallization from the metamict mineral originally having fergusonite structure or from parent pyrochlore, which was substantially altered during metamictization.Two recrystallization mechanisms were recognized: (a) epitaxial growth occurring at the boundary between preserved premetamict fragments and completely metamictized areas, and (b) nucleation-crystal growth mechanism occurring in completely amorphous areas of the minerals, and resulting in recrystallization of the original mineral as well as in the crystallization of a new mineral with a modified chemical composition as compared to the initial matrix.

A series of thin silicon films with different degrees of crystallinity were prepared by decomposition of silane gas highly diluted with hydrogen, in radiofrequency glow discharge. The crystallite size, shape, and the portion of crystalline phase were investigated by highresolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), Raman spectroscopy (RS), and X-ray powder diffraction (XRD). The absorption coefficient (a) was calculated from the measurement of UV-vis-transmittance. By using RS, the volume fractions of the crystalline phase were estimated from the ratio of the integrated intensities of transversal optical (TO)-related crystalline and amorphous bands. These results were in excellent agreement with the mean crystallite sizes measured in HRTEM images and crystallite sizes refined from XRD measurements. The red shift of absorption, appearing as a result of the increase of the crystal fraction, depends on the size and distribution of nanocrystals.

Protonated titanate nanotubes (TiNT-H) were surface-modified with (3-aminopropyl)trimethoxy silane (APTMS) by novel method suitable for syntheses of large amounts of materials with low costs. Usage of prepared nanotubes for polymer reinforcement was studied. Since the thermal stability of nanofiller was important to preserve their functional properties, stability was studied by in situ high temperature measurements. The most thermally stable nanotubes, silanized for 20 min, were used for preparation of epoxy-based nanocomposites. The nanofiller formed smaller (few hundreds of nm) and larger (few μm) aggregates in polymer matrix, and the amount of aggregates increased with increasing nanofiller content. APTMS modified titanate nanotubes bonded well with the epoxy matrix since amine groups on the TiNTs surface can react with epoxy group to form covalent bonds between the matrix and the nanofiller. Very small addition (0.19 -1.52 wt%) of nanotubes significantly increased the glass transition temperature and the modulus in rubbery state of the epoxy based polymer. Smaller nanofiller content lead to larger increase in these parameters and therefore better dynamic mechanical properties due to the smaller amount of large aggregates. APTMS-modified titanate nanotubes were proven as promising nanofiller in epoxy based nanocomposites.