Paramagnetic defects induced by mechanical stress in calcium sulfide phosphor

Paramagnetic defects induced by mechanical stress in calcium sulfide phosphor D. Caurant, D. Gourier, N. Demoncy, I. Ronot, and M. PhamThi Citation: Journal of Applied Physics 78, 876 (1995); doi: 10.1063/1.360278 View online: http://dx.doi.org/10.1063/1.360278 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/78/2?ver=pdfcov Published by the AIP Publishing

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Paramagnetic defects sulfide phosphor

induced

by mechanical

stress in calcium

0. Caurant,@ 0. Gourier,a) N. Demoncy, and I. Ronot Laboratoire de C’himie Appliquke de 1‘Etat Solide, URA 1466 CNRS, Ecole Nationale Sup&ieure de Chimie de Paris, I1 rue Pierre et Marie Curie, 75231 Paris Cedex 05, France

M. Pham-Thi Thomson-CSE Lnboratoire Central de Recherches, Domaine de Corbeville, 91404 Orsay Cedex, France

(Received 23 January 1995; accepted for publication 28 March 1995) The effect of uniaxial pressing and grinding on pure and europium-doped CaS powders prepared by the alkaline polysulfide flux method is studied by electron spin resonance (ESR). F+ centers are generated in the bulk of CaS particles and their ESR spectra can be easily separated from those of other paramagnetic species by using a 90” out-of-phase detection. This is possible because of the very long transverse relaxation time Ts of F+ centers. It is thought that the primary defects induced by mechanical stress are bulk F centers, which are next partially converted into Ff centers by reduction of metal ion impurities (Eu3’ ,Crs’,Cu2’) and native hole centers. Heat treatments in air of mechanically stressed powders show that F+ centers firstly diffuse to the particle surface forming low-symmetry F,f centers for temperatures ranging from 300 to 700 K. Atmospheric oxygen then transforms F: centers into ESR silent O- ions except if they are stabilized by Naf impurities originating from the flux, For temperatures higher than 700 K, these centers are then converted into paramagnetic SO, centers. The reason for the strong decrease of Eu2+ emission intensity (X=645 nm) following mechanical stress is discussed. The study of the fine structure of Mn2’ natural impurity shows that local order is strongly disturbed in CaS particles by mechanical stress. 0 1995 American

Institute

of Physics.

I. INTRODUCTION Alkaline-earth sulfides doped with various activators such as lanthanide and transition metal cations are still widely studied for their optical properties.1’2 CaS phosphors doped with EL?’ or Ce3’ particularly appear to be excellent candidates for their cathodoluminescent applications in television tube?-” Luminescence efficiency of CaS red phosphor is of the same order of magnitude as the Eu-doped oxysulfide presently used for television screen,* whereas activator concentrations are lower for CaS. For instance, europium contents are about 5 and 0.1 mol. %, respectively, in Y202S and CaS, which has implications with regard to the cost of phosphors.” CaS(Eu2”) gives a red light near 645 nm attributed to the interconfigurational emission bandsrg 4f65d1+4f75du of Err”‘. However, alkaline-earth sulfide powders are known for their sensitivity to water and humidity, producing emission of HsS. This is a practical problem for powder conservation and handling. Among alkaline-earth sulfides, calcium sulfide seems to be less sensitive against atmospheric water than MgS and BaS, for instance. CaS phosphor powders are usually prepared by heating a mixture of calcium salts (sulfates, carbonates) or oxides and activators in an atmosphere of hydrogen sulfide or carbon disulfide.6 A method using calcium sulfate reduction in a sodium sulfate flux with carbon has also been proposed.4 However, all these methods generally give powders which are an agglomeration of very small particles, sensitive to water because of their large specific surface. Kato and Okamato6T7have developed a sulfurizing flux ‘tAuthors to whom correspondence should be addressed.

method already known for the Y,O,S synthesis.r2 In this case calcium carbonate or sulfate powders containing an appropriate amount of activator are mixed with a flux made of sodium or potassium carbonate and sulfur. CaS particles of good crystallinity and large size are formed in the NazS, or K& melt by a crystal nucleation and growth process, with a relatively good stability against water.“‘* Calcium sulfide crystallizes in the cubic NaCl-type structure and is an indirect gap semiconducto?‘3 with a fundamental absorption band around 265 nm.’ Point defect energy calculations performed by Pandey et aZ.14 showed that Schottky pairs are the most proeminent defect type. However, the calculated formation energies are still very high (>7 eV) for Schottky pairs, thus intrinsic point defect concentrations are very low and Caz’ and S2- vacancies are only controlled by aliovalent impurities. In order to explain the thermoluminescent properties of CaS and CaS(Ce3’) powders, Ghosh et aZ.t’,t6 proposed that sulfide and calcium vacancies act, respectively, as donor and acceptor levels. These authors have observed electron spin resonance (ESR) signalsatg=1.9998 andg=2.0018 afterUVirradiationat low temperatures which were attributed to electrons temporarily trapped in S2- vacancies. They observed stable f;’ centers at g = 2.003 2 at room temperature (one electron permanently trapped in an anionic vacancy) only after prolonged x-ray irradiation.2.‘7,‘8 Crystalline quality of particles and application of mechanical stress are known to strongly influence the intrinsic point defect concentration, such as vacancies. This modifies the luminescence properties of the phosphor and a decrease of the emission intensity is generally observed. An important

[This article 676 is copyrighted as indicated the15article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: J. Appt. Phys. 78 in(2), July 1995 0 1995 American institute of Physics 134.157.146.58 On: Mon, 17 Mar 2014 12:06:46

EuZ+

(4f6

5dl

tion allowed us to observe multiple quantum (up to live) transitions for Mn”+ impurities in CaS powders.“’ We also show in the present work that the long transverse relaxation time T2 of Fe-type centers in CaS enables selective detection if the phase sensitive detector is set 90” out-of-phase with respect to the field modulation. The other defects with short T, are only observed if the detection is set in phase with respect to the modulation. This feature provides an efficient method for the separation of overlapping ESR spectra containing F’ -type centers. The enhanced resolution and the separation of the ESR spectra allowed us to monitor the evolution of the F’ centers and their reaction with atmosphere during thermal treatment.

--+4f7)

II. EXPERIMENT 600

an

700

ht nm) FIG. 1. Luminescence spectra of europium-doped (0.075 mol. 95) CaS(K,Eu) phosphor under 250 nm excitation at room temperature. (a) Native powder; (b) ground powder; (c), (d), (e) powder after,,grinding and thermal treatment in air at 700 K during 2 min, 5 min, and 2 ti,.respectively, followed by rapid cooling at 300 K; (f~ powder after grinding and thermal treatment in air at 700 K during 2 h and slow cooling. :.

decrease of luminescence intensity- has been observed for doped zinc sulfide”,zO and rare-earth oxysulfides” that have been submitted to mechanical stress. Ghosh et aZ.16observed from thermally stimulated luminescence exp$riments on CaS powders that the concentration of trap centers (sulfide and calcium vacancies) strongly decreases when these materials are prepared with the sulfurizing flux method using alkaline polysulfide, rather than with the classical methods which generally produce particles of poor quality. However, in both cases the luminescence efficiency is of the same order of magnitude.“-s This indicates that p,oint defect concentrations of native powders are too low to influence the optical properties. III the present work, we have observed a very strong effect of attrition on the Eu2+ emission band intensity for europium-doped calcium sulfide [Figs. l(a) and l(b)]. At the same time, the color of CaS powder changes from white to pink in the case of undoped powders and a new ESR signal attributed to F’ centers is observed. Such color changes for CaS22,23and rare-earth oxysulfides’l have already been reported in literature. They may be attributed to color centers such as Fi and F centers. The formation of Fi centers in CaS powders by different mechanical stress techniques such as grinding,24-27 mechanical compression,23 or explosive shock” have already been reported in the literature. Thermal treatment only partially restores luminescence properties of mechanically stressed phosphors [Figs. 1(c)-l (fj]. In this work, we have investigated in detail the paramagnetic point defects induced by grinding and uniaxial pressing of CaS and CaS(Euj powders, and their evolution under thermal treatment. CaS powders synthetized by polysulfide flux method are particularly suited for this investigation because the very good crystallinity of the grains ensures an improved resolution of the ESR spectra, resulting from the lack of strain broadening effect. For example, this very high resolu-

Almost all the polycrystalline CaS samples were prepared by the alkaline polysulfide tlux method.‘.” Calcium carbonate was mixed with sodium or potassium carbonate or an equimolar mixture the two with an excess of sulfur. For europium-doped CaS powders, J&O, is initially mixed with CaCO, (Eu 0.1 mol. %). The Na,S, and K2Sx fluxes are formed and calcium sulfide particles are generated during heating under a nitrogen tlux during about 2 h at 1273 K. After slow cooling, the CaS microcrystals are separated from the alkaline polysulfide phase, which is very soluble in water, by washing the product with a potassium hydroxide solution (pH= 11). CaS(Na), CaS(K), and CaS(Na,K) notations have been adopted to indicate the nature of the alkaline flux used for synthesis. In each case various samples have been pre pared (see Table I). A CaS sample has been also prepared using a classical technique. In this case a CaCOs powder is reduced under dihydrogen sulfide atmosphere during 6 h at 1473 K. This sample shows the ESR signal of 0.1”~ which is a natural impurity, and the notation CaS(Cu) will be hereafter adopted. Powder grain size and morphology were studied by scanning electron microscopy &EM) and a Coulter multisizer granulometer. The morphology of CaS grains strongly depends on the nature of the alkaline polysulfide used for the synthesis.“,30 Powders prepared with K,S, or K$,+Na$, fluxes exhibit agglomerates of irregular cube-octahedral grains, whereas isolated pseudospherical grains with a lower size distribution are formed with Na$, flux. X-ray diffraction patterns of phosphor powders do not show a significant difference between the three fluxes except slightly broader diffraction peaks for CaS(K). Thus CaS of very good crystallinity is obtained with grain sizes ranging between 10 and 20 pm. CaS(Cu> grains are-agglomerated and exhibit irregular shapes. Moreover, their mean diameter is lower (5 pm) than for the powders elaborated using the polysulfide flux method. However, x-ray diffraction patterns do not show any difference between the two synthetic methods. The impurity content of the powders was measured using a plasma torch technique. Chemical analysis of various CaS powders is reported in Table I., A clear correlation is observed between the Na and the K content of the powders and the composition of the polysulfide fluxes. This may be due to flux traces remaining on CaS particle surfaces after synthesis and washing. However, as indicated by Pham-Thi

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 134.157.146.58 On: Mon, 17 Mar 2014 12:06:46 Caorafit et al. 877 J. Appl. Phys., Vol. 78, No. 2, 15 July 1995

I. Elementaryconcentrations of impurities(ipm mol.) in undoped andin europium-doped CaSpowderselaborated in potassium,sodium,or mixed pofysulfideflux. The corresponding samplesare labelledCaS(Na),CaS(K),and CaS(Na,K).The samplereferred to as CaS(Cujhasbeenelaboratedby sulfurizationof CaC03underH,S.

TABLE

Elementarymolar concentration h.m mol.) Na K htn Fe cr CU Eu

CaSWJ 923