Luminescence of KCaSO4Cl:X, Y (X=Eu or Ce; Y=Dy or Mn) halosulfate material
Highlights
► A new KCaSO4Cl host is invented as a luminescent material. ► Present phosphors can be useful in scintillation, TL dosimetry as well as in lamp industry. ► Polycrystalline KCaSO4Cl:Eu, Dy; Ce, Dy and Ce, Mn have been studied for its PL characteristics. ► Effects of Dy and Mn co-doping on the PL characteristics of KCaSO4Cl:Eu or Ce have been studied. ► An efficient energy transfer from Eu2+ to Dy3+, Ce3+ to Dy3+ and Ce3+ to Mn2+ has been observed.
Introduction
The luminescence properties of co-activator compounds have received considerable interest. The main interest has been received concerned with the utilization of efficient energy migration on sensitizer to activator. Energy transfer phenomena have led to the development of new and efficient photoluminescence materials. The Eu doped solid state materials usually show strong broadband luminescence with a short decay time of the order of some tens of nanoseconds. Luminescence is very strongly dependent on the host lattice and can occur from the ultraviolet to the red region of the electromagnetic spectrum. Eu emission is intense enough to find important industrial applications in, for example, the tricolor low-pressure mercury fluorescence lamps.
Luminescent materials are used for various purposes such as making lamps [1], [2], [3], CR tubes and TV screens [4], electroluminescent lamp and display panels, LEDs, detectors for X-ray imaging [5], [6], [7], [8], scintillation detectors [9], laser crystals [10], paints [11] and solar concentrators [12]. The luminescent properties of Eu3+ compounds obtained in the presence of Eu2+ are of considerable interest in recent years.
Eu2+ emission results from two types of transitions. The most common is that due to 4f65d→4f6 (8S7/2). As the position of the band corresponding to 4f65d configuration is strongly influenced by the host, the emission can be anywhere from 365 nm (e.g. in BaSO4) to 650 nm (e.g. in CaS). Blasse [13] has listed the Eu2+ doped compounds, which shows that the emission color of Eu2+ can vary in a broad range, from ultraviolet to red. Since the 4f→5d transition is an allowed electrostatic dipole transition, the absorption and emission of Eu2+ are very efficient in many hosts, which makes the Eu2+ doped phosphors of practical importance.
CaF2:Mn does not give any fluorescence under the UV excitation while CaF2:Ce gives a characteristic Ce3+ fluorescence emission with UV light excitation .The combination of Ce and Mn in the CaF2 lattice however gives brilliant Mn2+ fluorescence emission in addition to that of Ce3+ on UV excitation due to energy transfer from Ce3+ to Mn2+ ions. Similar enhancement in the fluorescence efficiency of Dy and Mn ions was reported in CaSO4:Ce, Dy and CaSO4:Ce, Mn [10]. However, the application of a similar energy transfer mechanism leading to enhanced sensitivity in the case of thermoluminescence (TL) and photoluminescence (PL) is made continuously. During the course of the present investigation on TL in sulfate based phosphors, we have reported several sulfate phosphor possessing properties useful for TL dosimetry of ionizing radiations [14], [15], [16], [17], [18], [19].
Energy transfer from donor to acceptor plays an important role in luminescence and solid state lasers. It is important to get different regions of the visible spectrum in a matrix in which different activation ions can be structurally for cation sites. In recent years there has been a growing interest in stoichiometric sulfate compounds as host lattices for efficiently transporting excitation energy from a sensitizer to an activator. This has resulted in the production of new and efficient photoluminescent materials.
In recent years Moharil and co-workers [20], [21], [22], [23] have reported several phosphors on rare earth (RE) ions doped mixed sulfate. Other than sulfates and mixed sulfates, some investigations are going in progress on halosulfate-based materials. Klement [24] synthesized the halosulfate Na6Ca4(SO4)6F2 and characterized this compound by X-ray powder diffraction (XRD), and also the compounds Na6Pb4(SO4)6Cl2 [25], [26], Na6Cd4(SO4)6Cl2 [27] and Na6.45Ca3.55(SO4)6 (FxCl1–x)1.55 [28]. Recently we have reported new halosulfate phosphors [29], [30]. The luminescence properties and energy transfer of KZnSO4Cl:Ce, Dy and KMgSO4Cl:Ce, Dy has been studied.
Section snippets
Experimental
The samples KCaSO4Cl (pure and doped) were prepared by a solid state diffusion method. While preparing KCaSO4Cl:Eu, Dy, KCaSO4Cl:Ce, Dy and KCaSO4Cl:Ce, Mn the constituents KCl, CaSO4 and sulfate salt of europium and dysprosium were taken in a stoichiometric ratio and crushed in a crucible for 1 h. Then this material was heated at 800 °C for 8 h, resulting in the compound of KCaSO4Cl:Eu, Dy in powder form. The samples were then slowly cooled at room temperature. The resultant polycrystalline mass
XRD and particle size
Fig. 1 shows the X-ray diffraction (XRD) pattern of KCaSO4Cl material. The XRD pattern did not indicate the presence of the constituents CaSO4 or KCl and other likely phases, which is an indirect evidence for the formation of the desired compound. These results indicate that the final product was formed in homogeneous form.
When materials are less than approximately 100 nm in size, appreciable broadening in the X-ray diffraction lines occurs. The observed line broadening can be used to estimate
Conclusion
It can be concluded from the results presented and discussed here that in the present material the dopant ions Eu2+ instead of giving a single emission peak at around 450 nm, as in the case of the conventional material, give two bands at around 400 and 450 nm. Since the 450 nm emission in the PL may be an indicator of good TL material. In KCaSO4Cl matrix singly Dy doped emission is not observed. Eu2+ plays an important role in PL emission in the host lattice, which could be attributed to the
Acknowledgment
We are thankful to University Grant Commission (UGC), New Delhi, for financial support.
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