Luminescence mechanism of tungstate phosphors

Self Cite

Electron paramagnetic resonance has been observed at single crystals of calcium tungstate doped with lead. After exposure to X-ray irradiation a t the temperature of liquid air paramagnetic centres appear. One group of lines originates from the Vrcentre in the tungstates. The resonance of Pb3+ ions a t Ca2+ sites is reported. This spectrum is characterized by a pronounced hyperfine interaction of the unpaired 6s electron of the Pb3+ ion with the nuclear magnetic moment of the PbZo7 isotope. Superhyperfine interaction with four surrounding tungsten nuclei is observed indicating covalent bonding effects with the tungstate groups as ligands and a treatment of the complex in terms of molecular orbitals is proposed. The thermal annealing of the centres detected by E P R is correlated with the thermoluminescence glow curve.Messungen der paramagnetischen Elektronenresonanz sind an mit Blei dotierten Kalziumwolframat-Einkristallen vorgenomnien worden. Paramagnetische Zentren werden nach Rontgenbestrahlung der Proben bei der Temperatur der fliissigen Luft nachgewiesen. Eine Liniengruppe stellt das Vk-Zentrum in den Wolframaten dar; die Resonanz der Pb3+-Ionen auf Ca2+-Platz wird berichtet. Dieses Spektrum zeichnet sich durch eine ausgepragte Hyperfeinwechselwirkung des ungepaarten 6s-Elektrons des Pb3+-Ions mit dem magnetischen Kernmoment des Pb207-Isotopes aus. Reobachtet wird auBerdem eine Superhyperfeinwechselwirkung mit vier umgebenden Wolframkernen, was auf kovalente Bindungsanteile rnit den Wolframatgruppen als Liganden und eine Behandlung im Bild der Blolekulbahntheorie hinweist. Das mit der E P R festgestellte thermische Verschwinden der paramagnetischen Zentren ist mit der Thermolumineszenz-Glowkurve korreliert.

Section: Resultsmentioning

confidence: 99%

“…The measured deviation in the azimuthal angle q~ is easily understood. The state of the sinall polaron was postulated for the V,-centre (self-trapped hole) [4]. Stabilization of the hole takes place via polarization of the neighbourhood of the defect electron and not by an defect or impurity as trapping centre.…”

Section: Resultsmentioning

confidence: 99%

EPR Investigations on CaWO₄:Pb Single Crystals

Born

Hofstaetter

Scharmann

1970

Self Cite

“…The emission bands of the Sm 3+ , at the wavelength close to that of this Sm-doped ZnWO 4 , are found in many crystals, such as, apatite structure [23], alumina films [24], Y 2 O 3 and Gd 2 O 3 phosphors [25], and tungstate single crystals [16,17].…”

Section: Resultsmentioning

confidence: 99%

“…In the past few years, there have been many papers concerned with the preparation and photoluminescence properties of rare earth-doped tungstates crystals [13][14][15][16][17]. In these, the trivalent samarium can generate red emissions in some tungstate crystals [16,17], that may make the tungstates emit blue-green light and become a potential white-light phosphor. However, Sm-doped tungstates in the forms of powder and film, especially Sm-doped ZnWO 4 material, have not been reported for the luminescence application.…”

mentioning

confidence: 99%

Preparation and luminescence property of Sm‐doped ZnWO₄ powders and films with wet chemical methods

2008

Sm‐doped ZnWO4 powders and films have been prepared with wet chemical methods. XRD analysis indicated that single‐phase Sm‐doped ZnWO4 were formed above a calcining temperature of 400 °C, and the particle size and intensity of XRD peak of the powders were varied with calcining schedules. SEM micrographs indicated that powders and films had uniform morphology and a grain size of about 10–150 nm, which increased with increasing calcining temperature. Compared with the ZnWO4 powders synthesized with the same method, Sm‐doped ZnWO4 powders showed obviously less crystallinity and smaller grain size and more dispersive morphology. The Sm‐doped ZnWO4 and undoped materials showed broad emissions in the blue‐green bands. The Sm‐doped ZnWO4 materials showed red emissions near 567 nm, 603 nm, and 641 nm. The intensities of the blue‐green emission and the red emission were increased with the crystallinity and the grain size of the powders. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

“…The most recent suggestion rcgarding the fundamental mechanism of energy transfer is that of Born et al [6] who have postulated that small polarons are involved and that paramagnetic centres similar to Vk-centres in the alkali halides play an important role in the intrinsic luminescence in calcium tungstate.…”

Section: Introductionmentioning

confidence: 99%

Ultra‐Violet Excited Paramagnetic Centres in Calcium Tungstate

Sayer

Lynch

1970

Two sets of paramagnetic species having a spectroscopic splitting factor slightly higher than the free electron value are produced in CaWO, doped with copper after irradiating the crystal with ultra-violet light. One set of lines is identified as being due to a hole trapped a t a single tungsten site and is found to bleach out on warming the crystal through the thermoluminescence glow peak a t 160 OK. The other set of lines increases in intensity first with the emission of light in the 160 "K peak and then again with the 260 O K peak and bleaches out on heating the crystal through the thermoluminescence glow peak a t 400 OK. The copper does not take part in the excitation process directly. It is suggested that the low temperature paramagnetic species arises from the loss of an electron from the tungstate complex.Zwei Gruppen paramagnetischer Zentren mit einem spektroskopischen Aufspaltungsfaktor, der etwas grol3er ist als der Wert fur ein freies Elektron, werden in kupferdotierteni CaWO, nach Bestrahlung der Kristalle mit ultraviolettem Licht erzeugt. Die eine Gruppe von Linien wird einem Loch zugeschrieben, das an ein einzelnes Wolframatom gebunden ist. Diese Gruppe verschwindet beim Erwarmen des Kristalls iiber das Thermolumineszenz-Emissionsmaximum bei 160 "K. Die Intensitat der anderen Gruppe steigt mit der Lichtemission des 160 "K-Maximums und dann nochmals mit der des 260 OK-Maximums an. Die Linienintensitat verschwindet bei weiterem Erwarmen des Kristalls iiber das Thermolumineszenz-Emissionsmaximum bei 400 O K . Die Kupferatome nehmen nicht direkt am AnregungsprozeLi teil. E s wird vermutet, dal3 das paramagnetische Zentrum bei tiefer Temperatur durch Verlust eines Elektrons des Wolframatkomplexes entsteht.