Introduction At ambient conditions titanium monoxide, TiO y , with a wide range of homogeneity of the B1 lattice structure is stable only with vacancies on both the titanium and oxygen sublattices. Even for the stoichiometric composition of TiO the vacancy content is large and equal to about 15-16 at.% on the two sublattices [1]. The vacancies can be eliminated only at the very high pressure of 8 GPa and high temperature of 1920 K [2, 3]. The elimination of the vacancies is accompanied by an increase in the lattice constant from 0.41796 to 0.42062 nm [2]. At low temperatures the titanium, the oxygen atoms and the vacancies are ordered and form a superstructure with space group C2/m [4,5]. In order to understand the stability of the structure with vacancies on both sublattices the electron structure and the charge state of such vacancies has been studied. In this paper electron microdiffraction which yields information about the distribution of lattice vacancies and positron annihilation techniques which are sensitive to the charge state of lattice vacancies were employed.
Red phosphor Sr2ZnSi2O7:Eu3+ nanoparticles with an average diameter of 20 nm were successfully synthesized via a low-temperature hydrothermal route in order to understand the underlying relationship between size and luminescent properties. The nanometer-sized particles result in a distinct improvement in chromaticity and a high quenching concentration. According to emission spectra, the relative intensity of the 5D0 --> 7F2 to 5D0 --> 7F1 transitions in nanometer-sized phosphors is higher than that of the corresponding bulk material. The better chromaticity results from the more distorted lattices and relatively lower crystal symmetry around the Eu3+ ions, which is ascribed to the large surface area due to the nanometer size of the phosphor. Moreover, the nanometer-sized Sr2ZnSi2O7:Eu3+ red phosphor exhibits a shorter fluorescent lifetime and a blue-shift in excitation spectra compared to that of its bulk counterpart. These results indicate that size-induced enhancement of luminescent properties is an efficient way to obtain red phosphors with better chromaticity.
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