Synthesis and physical properties of some new phosphors



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Abstract

SYNTHESIS AND PHYSICAL PROPERTIES OF SOME NEW PHOSPHORS


Part I
Halophosphate Phosphors The most widely used phosphor in fluorescent lamps, to day, is the halophosphate. Halophosphates have the general formula M10(PO4)6X2 ,where M is alkaline earth metal and X is halogen and they crystallize in the apatite structure. The apatite structure tolerates extensive substitution for metal ion, phosphorous and halogen, permitting the preparation of a considerable number of chemical combinations. In the present work, thorium and tungsten have been substituted for stontium, in strontium halophosphate and the effect of this substitution on the crystal structure and optical properties of the phosphors has been studied. Further, the effect of sensitizer and activator on the fluorescent emission of these phosphoros has also been studied. The basic halophosphate matrix has been prepared by heating appropriate mixture of the constituents at 11000 C according to the following equation.
9(Sr,M)CO 3 + 6(NH 4 )2HPO 4 + SrF 2 -> 3(Sr,M) 3 (PO 4 ) 2 .SrF 2 + 12NH 3 + 9CO 2 + 9H 2 O Where M= Th or W. In the second stage, antimony and manganese have been introduced in the basic matrix as sensitizer and activator respectively. In strontium-thorium fluorophosphates (Sr1-X ThX/2 X/2)10(PO4)6F2 system nine compositions in the range 0 < x < 0.18 and in strontium-tungsten fluorophosphates (Sr1-xW X/3 2X/3)10(PO4)6F2 system, seven compositions in the range 0 < x < 0.11. have been studied where represents the metal Vacancy. The concentration of antimony has been kept constant at 3 per cent whereas the concentration of manganese has been varied from 0.5 to 4 per cent. The x-ray powder diffraction technique has been employed to see the completion of the reaction and to determine the extent of substitution in the mixed halophosphate matrix. The 'd' values and lattice parameters have been reported. The visually estimated intensities of different diffraction lines have been compared with the calculated intensities for different planes, using different models. With the help of this comparison, the distribution of the substituted cations on the two sites in the apatite structure has been determined, The measurement of spectral energy distribution and intensity of emission has been done with the following assembly: A Uv lamp providing 2537A0 wavelength as excitation source, phosphoroscope, constant deviation spectrometer,photomultiplier tube, spot galvanometer. Graphs have been plotted for relative intensity as a function of emission wavelength. From the X-ray diffraction results, it has been concluded that thorium and tungsten ions are substituted for strontium at site(I) leaving a metal vacancy at site(II). From the results of spectral energy distribution studies, It has been found that substitution of thorium and tungsten, fop strontium, in the Sr halophosphate activated with Sb and Mn, does not shift the emission peaks of the phosphors but increases the intensity to a considerable extent, at the optimum concentration of the substituted ions. A plausible mechanism for this has been proposed. Part II - Sulphide phosphors Lately, some mixed sulphides have received close attention, because of their interesting properties. Work done so far in this field concerns mostly with substitution of alkaline earths having the same crystal symmetry as the host lattice. In the present work, a mixed phosphor (Ca +Cd)S has been studied in which the crystals of CaS and CdS belong to different classes of symmetry. The effect of substitution of one cation for the other in the host matrix, on the structural and optical properties of the phosphor has been studied. The effect of concentration of activators on the luminescence of this mixed phosphor has also been studied. Synthesis of mixed sulphide phosphor has been done in two stages. In the first stage, appropriate mixtures of CaS and CdS have been fired in H2S atmosphere at about8500 C. CaS has been prepared by passing H2S over heated CaCO3 while Cds has been prepared by usual precipitation Method. ln the second stage, copper and manganese, have been Introduced independently as well as in presence of each other, as activators. About nine compositions of the host matrix, having different proportions of calcium and cadmium have been prepared. Copper percentage has been varied from 0.1 to 0.5 whereas manganese percentage has been varied from 0.5 to 2.5 The completion of the reaction and extent of substitution has been checked by x-ray diffraction method. The 'd' values and lattice parameters of the phosphor have been reported. It has been shown that cubic CaS tolerates the substitution of Cd upto 20 per cent but hexagonal Cds does not tolerate the substitution of Ca for Cd. It has been found that the activators, copper and manganese, show bright yellow and orange-red fluorescence respectively, when excited by UV source having 3650A0 radiation. When copper and manganese both are present together, an arranged fluorescence is observed. From the spectral energy distribution studies, graphs showing the relative intensity against emission wavelength have been plotted. The graphs have been resolved into various gaussian curves. With the help of previously reported data, the different peaks, due to the activators have been identified; the mode of substitution of the activator has been proposed. In ease of copper alone the peak at 6940A0 has been attributed to the Cu2+ at substitutional site while the emission peaks at lower wavelengths are attributed either to Cu+ substitutional or Cu+ interstitial .A model for the mechanism of emission has been proposed. In case of manganese alone, definite energy transitions have been assigned to the peaks of emission. In case of phosphors, containing copper and manganese together, the concept of resonance transfer, from senstizer to activator, has been proposed to explain the observed sharp and single peak of emission. Further more, it has been proposed that cadmium does not act as an activator impurity but merely modifies the host lattice. Part III Fluoride phosphors Fluoride compounds have been known to show fluorescence since a very long time. However, the systematic study of the luminescent fluoride has been started quite recently. Some simple fluorides have already been used as fluorescent host materials. In the present work, attempt has been made to prepare a mixed fluoride NaYF4 phosphor activsted with rare earth ions particularly europium and terbium. The host NaYF4 has been prepared by the precipitation method. A solution of hot NaF has been added to a soiution of YCl3 to precipitate NaYF4. Later, the setivstore Eu3+ and Tb3+ have been added independently as well as in presence of each other the Mixtures have been fired at 10000 C for about four to six hours in N2 atmosphere. The effect of the concentration of the activators has also been studied. The structure of NaYF4 has been determined by x-ray diffraction method. There are two structures proposed for NaYF4 is literature. The two forms are temperature dependent. The one formed at higher temperature is cubic while the other one formed at low temperature has unknown structure. It has been found that the NaYF4 prepared by precipitation method also shows the cubic structure. The 'd' values, intensities of the diffracted lines and lattice parameters have been reported. In the spectral energy distribution studies, the microphotometer tracings have been recorded for the fluorescence spectra of the samples. It has been found that only europium serves as a good activator. Terbium does not serve as an activator nor does it act as a sensitizer to europium on the contrary it is found to quench the fluorescence of eurupium. The spectra have been analyzed and the energy transitions have been assigned to the various peak wavelengths observed la the fluorescence spectra. Crystal field effect on the energy levels of Eu 3+ ion in NaYF 4 has been considered and it has been proposed that the substitution of europium ions in the NaYF 4 crystal distorts the octahedral symmetry and gives rise to a lower symmetry C 2 . The observed lines in the fluorescence spectra have been accounted for as due to C 2 symmetry.


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