S. Erdei

Synthesis and characterization of sol-gel derived hexa-aluminate phosphors

Two refractory phosphors, BaMg 2 Al 16 O 27 : Eu 2+ (BAM) and MgAl 11 O 17.5 : Ce 3+ , Tb 3+ (MAO), have been synthesized both by the conventional solid-state processing route (using oxides as the starting materials) and by reacting precursors made by the sol-gel process using organic precursors. The phases formed were reacted at 1000 °C in (a) steam and (b) steam + AlF 3 . The phosphors were well crystallized with particle sizes in the range of 1–10 μ m. The emission spectra showed the characteristic broad band blue emission of Eu 2+ for BAM and a narrow band green luminescence of Tb 3+ for MAO. The melting points of BAM and MAO were measured to be 1920 ± 20 °C and 1950 ± 20 °C, respectively, using an Ir-strip furnace and optical pyrometer. BAM and MAO phosphor materials are congruently and incongruently melting, respectively. Excellent crystallization via the sol-gel route was found even at 1220 °C. Enhancement of the luminescent output by the steam treatment by some 25% was determined.

Preparation of Eu3+: YVO4 red and Ce3+, Tb3+: LaPO4 green phosphors by hydrolyzed colloid reaction (HCR) technique

Eu3+: YVO4 red and Ce3+, Tb3+: LaPO4 green phosphors were prepared by newly discovered hydrolized colloid reaction (HCR) technique at low temperature (< 100 °C) and atmospheric pressure utilizing subsequent calcining and reductive treatments, respectively. The incorporation of activators (Eu3+ and Ce3+, Tb3+) in these very porous powders was checked by X-ray diffraction (XRD), scanning electron microscopy (SEM) and luminescence investigations.

Possible trends for the growth of low scattering Nd:YVO4 laser crystals; phase relations — growth techniques☆

Abstract The Nd:YVO 4 single crystal is one of the most promising laser hosts for micro and diode-pumped solid state lasers due to its outstanding laser characteristics. However, the growth difficulties encountered can significantly restrain its widespread high-tech applications. The inclusions which are mainly submicron size precipitates, and the color center problems commonly originate from the defects of valency states. The presence of non-pentavalent vanadium ions in the V 2 O 5 starting material, the related valency defects in the YVO 4 compound created by the reaction process, and the in congruent vaporization of vanadium oxides together with YVO 4 melt will primarily determine the congruency as a typical off-stoichiometric effect. This work concludes that the simple Czochralski (CZ) growth technique cannot produce scatterin-free and stoichiometric YVO 4 crystals, nevertheless the top-seeded solution growth (TSSG) and liquid phase epitaxial (LPE) techniques provide possible alternatives for better quality crystal production. Several specific aspects of the phase relations of YVO 4 crystals prepared by CZ, TSSG and LPE techniques are surveyed in this paper.

Micro-probe Raman spectroscopy for detection of inhomogeneities in YVO4 single crystals

Several near quasicongruent undoped YVO4 single crystals grown by the Czochralski (CZ) technique as well as near stoichiometric YVO4 single crystals grown by the top‐seeded solution growth technique, were investigated by micro‐probe Raman spectroscopy using unpolarized argon–ion laser light. The line broadening effects of the Raman active mode at 379 cm−1 indicate that all CZ grown crystals possess wider peak widths than near stoichiometric YVO4. In the latter case a value of 9.23 cm−1 was measured for full width at half‐maximum. Although the annealing process in O2 can improve the oxygen stoichiometry in the originally slightly oxygen deficient CZ grown YVO4 crystals, certain remaining peak broadening effects demonstrate a Y–V related stoichiometry problem by both lower (external) and higher frequency (internal) modes. Radial inhomogeneous distributions were observed in CZ specimens by the shift of external Raman modes at 157 and 162 cm−1 frequencies. In addition, nonassigned low intensity extra Raman peaks appeared over 1000 cm−1 frequencies in all CZ grown samples. The above detected changes in Y–V–O stoichiometry demonstrate a complex phase system around the homogeneity region of quasicongruent YVO4 specimens and enable a more realistic and unified interpretation of the segregated defects and stoichiometry changes in CZ grown YVO4 single crystals than previously available.Several near quasicongruent undoped YVO4 single crystals grown by the Czochralski (CZ) technique as well as near stoichiometric YVO4 single crystals grown by the top‐seeded solution growth technique, were investigated by micro‐probe Raman spectroscopy using unpolarized argon–ion laser light. The line broadening effects of the Raman active mode at 379 cm−1 indicate that all CZ grown crystals possess wider peak widths than near stoichiometric YVO4. In the latter case a value of 9.23 cm−1 was measured for full width at half‐maximum. Although the annealing process in O2 can improve the oxygen stoichiometry in the originally slightly oxygen deficient CZ grown YVO4 crystals, certain remaining peak broadening effects demonstrate a Y–V related stoichiometry problem by both lower (external) and higher frequency (internal) modes. Radial inhomogeneous distributions were observed in CZ specimens by the shift of external Raman modes at 157 and 162 cm−1 frequencies. In addition, nonassigned low intensity extra Raman pe...

Preparation of YVO4 powder from the Y2O3 + V2O5 + H2O system by a hydrolysed colloid reaction (HCR) technique

Prior to the formation of YVO4 in the Y2O3 + V2O5 + H2O system, two intermediate, partially hydrophobic, complex colloidal mixtures with metastable characteristics can be produced at room temperature and atmospheric pressure. The ball-milled system, having both hydrophobic and hydrophilic species, transforms into the stable yttrium orthovanadate phase due to intensive hydrolysis. At room temperature an orange mixture (possessing dispersed Y2O3 and 4Y2O3−P(OH)pp+·2VO3−, Y2O3−p(OH)pp+·6VO3−·xH2O-like heteroaggregations) formed by 20 h mixing at pH ca. 4.0 transforms slowly, another red-brown heavily flocculated colloidal mixture (with dispersed Y2O3 and Y2O3−p(OH)pp+·V10O286−·yH2O-like aggregation) formed by 70 h mixing at pH ca. 4.5 transforms rapidly into YVO4 in water. During additional mixing of highly diluted red-brown mixtures this transformation can be completed at room temperature. At elevated temperatures (50–95 °C) the orange mixture precipitates into a red-brown decavanadate-type precipitatium which subsequently can also rapidly hydrolyse into an orthovanadate phase in the diluted aqueous systems. Both vanadium excess meta-and decavanadate-type aggregations exhibit amorphous character by X-ray diffraction.The semi-hydrophobic colloidal structure can modify the dissociation mechanism, which prevents the system from returning to the starting oxides, and gives a new HCR technique for YVO4 preparation with a simple hydrolysis process at low temperatures and atmospheric pressure.

Segregation in YVO4 single crystals grown by Czochralski technique

Abstract SEM-EDS investigations on cleavaged (100) planes of YVO 4 single crystals grown from the melt indirectly indicate a very limited homogeneous solid solution region in the Y 2 O 3 -V 2 O 5 − x phase system around the 50.7 ± 0.2 mol% Y 2 O 3 quasi congruent composition corresponding to the maximum melting (freezing) point at slightly different rates of oxygen deficiency. Two different samples grown by Czochralski technique from stoichiometric starting compositions exhibited either many precipitates with excess V or Y depending upon the sintering and growth conditions employed.

Raman study of oxygen deficient YVO4 single crystals

Oxygen deficiency which is an inherent problem in melt grown YVO{sub 4} single crystals was investigated by Micro-Probe Raman Spectroscopy (MPRS). External vibrational modes at 157 cm{sup {minus}1}, 162 cm{sup {minus}1} and selected internal vibrational modes at 379 cm{sup {minus}1}, 840 cm{sup {minus}1} of full Raman spectra were compared with different YVO{sub 4} crystals grown by the Czochralski (CZ) technique both with and without annealing in an O{sub 2} atmosphere as well as by top seeded solution growth technique (TSSG) in directly produced oxygen deficiency-free form. Special differences of above mentioned frequencies and over 1,000 cm{sup {minus}1} were observed in the Raman spectra, which could prove that the growth technologies generated different rates of oxygen deficiency in addition to critical intrinsic segregation effects in YVO{sub 4} which significantly determine the crystal quality.

Trends in the growth of stoichiometric single crystals

Stoichiometry problems in binary or more complex compounds can impair the electric, ferroelectric, magnetic, optical and other characteristics in these single crystals. For many compounds of interest, congruent melting compositions have been detected which are non-stoichiometric thereby giving rise to unwanted effects like thermodynamic instabilities, precipitates, intrinsic impurities and deviations from unity of the segregation coefficient of cations. To eliminate or significantly reduce these defects, specific growth techniques were considered and introduced. Traditional growth methods combining the advantage of flux-growth techniques assisted by specific chemical control of stoichiometry were explored. Specific examples of near-stoichiometric YVO 4 and LiNbO 3 growth are discussed, and future trends for the growth of stoichiometric single crystals considered.

UV absorption edge position for characterization of YVO4 crystals growth by Czochralski and TSSG techniques

Abstract The UV absorption edge of YVO 4 crystal with a slight yttrium excess grown by Czochralski (CZ) technique, after annealing in O 2 atmosphere, was compared with oxygen deficiency-free YVO 4 crystals grown by the top seeded solution growth (TSSG) technique. The measurements show that the TSSG crystal has higher transparency in near-UV than the CZ sample, demonstrating a better near-stoichiometric composition in the TSSG-YVO 4 single crystal.

Hydrolyzed colloid reaction (HCR) technique for preparation of YVO4, YPO4 and YVxP1−xO4

Abstract YVO 4 , YPO 4 and YV x P 1− x O 4 phosphor host materials were synthesized by a special hydrolyzed colloid reaction (HCR) technique from Y 2 O 3 +V 2 O 5 +H 2 O, Y 2 O 3 +P 2 O 5 +H 2 O and Y 2 O 3 +V 2 O 5 +P 2 O 5 +H 2 O systems at low temperature ( T C ) and atmospheric pressure. Complex colloidal mixtures with metastable characteristics produced by intensive ball milling were transformed into YVO 4 , YPO 4 and YV x P 1− x O 4 by simple hydrolysis.