Influence of mechanochemical processing to luminescence properties in Y2O3 powder

Abstract

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...

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