Structural investigations and luminescence properties of nanocrystalline europium-doped yttrium silicates prepared by a sol-gel technique

Luminescent yttrium silicate powders doped with europium ions (Eu(x)y(2-x)SiO(5)) were obtained by a sol-gel method after thermal treatment at 1300 degrees C. The crystalline phases retrieved from X-ray diffraction (XRD) patterns were quantitatively evaluated through Rietveld analysis, which showed that the formation and stability of high-temperature crystalline phases is affected by the europium ions. In fact, the X-2 monoclinic oxyorthosilicate Y2SiO5 is the dominant phase in all the samples and decreases on increasing europium content, while the relative amount of the additional apatite and disilicate phases increase. Small amounts of cubic yttria are also present. The Rietveld analysis evidences also an increase of the cell volume of the phases, except for the disilicate one, indicating that trivalent europium ions enter preferentially into the X-2 oxyorthosilicate, apatite and yttria structures. TEM observations indicate that the samples exhibit a rather large distribution of particle sizes from values lower than 20 nm up to higher than 100 nm. The emission spectra in the D-5(0) -> F-7(0) region allowed to assign the occupation of the sites for the europium ions in the various phases in good agreement with XRD results. These materials appear to be interesting as phosphors for application in luminescent devices.