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`VIZIO 1013
`VIZIO 1013
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`

`

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`
`VIZIO 1013
`VIZIO 1013
`
`

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`G. Blasse, B. C. Grabmaier
`
`Luminescent
`
`Materials
`
`With I71 Figures and 3] Tables
`
`Springer—Verlag
`Berlin Heidelberg New York
`London Paris Tokyo
`Hong Kong Barcelona Budapest
`
`VIZIO 1013
`VIZIO 1013
`
`

`

`Prof. Dr. G. Blasse
`
`a
`\‘ ‘r L‘! I7 53
`
`Debye Institute
`University Utrecht
`Postbox 80.000
`
`3508 TA Utrecht
`
`.-
`
`_ U-
`
`The Netherlands
`
`Prof.. Dr. B. C. Grabmaier
`
`Siemens Research Laboratories
`
`ZFE BT MR 22
`
`D-81730 Miinchen
`
`Germany
`
`also with Debye Institute
`
`University Utrecht
`
`ISBN 3-540-58019-0 Springer—Verlag Berlin Heidelberg New York
`ISBN 0-387-58019-0 Springer—Verlag New York Berlin Heidelberg
`
`Library of Congress Cataloging-in-Publication Data
`Blasse, G. Luminescent materials I G. Blasse, B.C. Grabmaier. p. cm.
`Includes bibliographical references and index.
`ISBN 3-540-58019-0. -- ISBN 0-387-58019-0 (U.S.)
`I. Phosphors. 2. Luminescence.
`I. Grabmaier, B. C., 1935- ll. Title.
`QC476-7-B53 1994 620.1’ l295——dc20 94-20336 CIP
`
`This work is subject to copyright. All rights are reserved, whether the whole or part of the
`material is concerned, specifically the rights of translation, reprinting,
`re—use of illustrations,
`recitation, broadcasting, reproduction on microfilms or in other ways, and storage in data banks.
`Duplication of this publication or parts thereof is only permitted under the provisions of the
`German Copyright Law of September 9, 1965, in its current version, and a copyright fee must
`always be paid.
`
`© Springer.Verlag Berlin Heidelberg 1994
`Printed in Germany
`
`in this publication does not imply, even in the
`The use of registered names, trademarks, etc.
`absence of a specific statement, that such names are exempt from the relevant protective laws
`and regulations and therefore free for general use.
`
`Typesetting with TEX: Data conversion by Lewis & Leins, Berlin
`SPIN: 10187460
`023020 - 5 4 3 2 1 0 - Printed on acid-free paper
`
`VIZIO 1013
`VIZIO 1013
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`

`

`Preface
`
`Luminescence is just as fascinating and luminescent materials (are) just as important
`
`as the number of books on these topics are rare. We have met many beginners in
`
`these fields who have asked for a book introducing them to luminescence and its
`
`applications, without knowing the appropriate answer. Some very useful books are
`
`completely out of date, like the first ones from the late l940s by Kroger, Leverenz and
`
`Pringshcim. Also those edited by Goldberg (1966) and Rich] (I971) can no longer
`
`be recommended as up—to—date introductions.
`
`In the last decade a few books of excellent quality have appeared, but none of
`
`these can be considered as being a general introduction. Actually, we realize that it
`
`is very difficult to produce such a text in view of the multidisciplinary character of
`the field. Solid state physics, molecular spectroscopy, ligand field theory, inorganic
`chemistry, solid state and materi'als chemistry all have to be blended in the correct
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`proportion.
`
`Some authors have tried to obtain this mixture by producing multi—authored books
`
`consisting of chapters written by the specialists. We have undertaken the difficult task
`of producing a book based on our knowledge and experience, but written by one
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`hand. All the disadvantages of such an approach have become clear to us. The way in
`
`which these were solved will probably not satisfy everybody. However, if this book
`
`inspires some of the investigators just entering this field, and if it teaches him or her
`how to find his way in research, our main aim will have been achieved.
`
`The book consists of three parts, although this may not be clear from the table of
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`contents. The first part (chapter I) is a very general introduction to luminescence and
`
`luminescent materials for those who have no knowledge of this field at all. The second
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`part (chapters 2-5) gives an overview of the theory. After bringing the luminescent
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`center in the excited state (chapter 2: absorption), we follow the several possibilities
`of returning to the ground state (chapter 3: radiative return; chapter 4: nonradiative
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`return; chapter 5: energy transfer and migration). The approach is kept as simple as
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`possible. For extensive and mathematical treatments the reader should consult other
`books.
`
`Part three consists of live chapters in which many of the applications are discussed,
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`viz. lighting (chapter 6), cathode—ray tubes (chapter 7), X—ray phosphors and scintil-
`lators (chapters 8 and 9), and several other applications (chapter 10)- These chapters
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`discuss the luminescent materials which have been, are or may be used in the appli-
`
`cations concerned. Their performance is discussed in terms of the theoretical models
`presented in earlier chapters. In addition, the principles of the application and the
`preparation of the materials are dealt with briefly. Appendices on some, often not-well-
`understood, issues follow (nomenclature, spectral units, literature, emission spectra)-
`
`VIZIO 1013
`VIZIO 1013
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`

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`vi
`
`Preface
`
`We are very grateful to Mrs. Jessica Heilbrunn (Utrecht) who patiently typed the
`manuscript and did not complain too much when correction after correction appeared
`over many months. Miss Rita Bergt (Miinchen) was of help in drawing some of the
`figures. Some of our colleagues put original photographs at our disposal.
`This book would not have been written without discussions with and inspiration
`
`from many colleagues over a long period of time. These Contacts, some oral, some
`via written texts, cover a much wider range than the book itself. In the preparation of
`this book our communication with Drs. P-W. Atkins, F. Auzel, A. Bril, C.W.E. van
`
`Eijk, G-F- Imbusch, C.K. Jergensen, and B. Smets has been very useful.
`For many years we have enjoyed our work in the field of luminescence. We hope
`that this book will help the reader to understand luminescence phenomena, to design
`new and improved luminescent materials, and to find satisfaction in doing so.
`
`Spring I994
`
`G. Blasse, Utrecht
`B.C. Grabmaier, Mijnchen
`
`VIZIO 1013
`VIZIO 1013
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`

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`Table of Contents
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`Chapter 1 A General Introduction to Luminescent Materials
`Chapter 2 How Does a Luminescent Material Absorb Its Excitation Energy?
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`2.]
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`General Considerations .
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`2.2 The Influence of the Host Lattice .
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`The Energy Level Diagrams of Individual Ions .
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`The Rare Earth Ions (4f-5d and Charge—Transfer Transitions) .
`2.3.5
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`2.3.7
`Other Charge—Transfer Transitions .
`2.3.8
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`2.4 Host Lattice Absorption .
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`Chapter 3 Radiative Return to the Ground State: Emission
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`33
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`3.]
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`Introduction .
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`3.2 General Discussion of Emission from a Luminescent Center .
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`Some Special Classes of Luminescent Centers .
`3.3.]
`Exciton Emission from Alkali Halides .
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`3.3.4
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`3.3.5
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`3.3.7
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`d” Complex Ions .
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`3.3.8
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`3.3.9
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`VIZIO 1013
`VIZIO 1013
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`viii
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`Table of Contents
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`Chapter 4 Nonradiative Transitions
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`4.1
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`4.2
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`Introduction .
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`4.2.2
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`Efficiency .
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`Chapter 6 Lamp Phosphors
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`6.1
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`Introduction .
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`The Preparation of Lamp Phosphors .
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`Cathode—Ray Tubes: Principles and Display .
`7.1
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`7.4 Outlook .
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`VIZIO 1013
`VIZIO 1013
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`

`

`Table of Contents
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`ix
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`Chapter 3 X-Ray Phosphors and Scintillators (Integrating Techniques)
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`8.1
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`Introduction .
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`Computed Tomography .
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`Preparation of X—ray Phosphors .
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`Table of Contents
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`l0.4 Electroluminescence .
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`. 227
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`VIZIO 1013
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`

`

`CHAPTER 6
`
`Lamp Phosphors
`
`6.1 Introduction
`
`The previous chapters presented an outline of the phenomenon of luminescence in
`solids. They form the background for the following chapters which discuss lumines-
`cent materials for several applications, viz. lighting (Chapter 6), television (Chap-
`ter 7), X-ray phosphors and scintillators (Chapters 8 and 9), and other less—general
`applications (Chapter 10). These chapters will be subdivided as follows:
`
`— the principles of the application
`— the preparation of the materials
`— the luminescent materials which were or are in use or have a strong potential to
`become used; a discussion of their luminescence properties in terms of Chapters
`2-5
`
`— problems in the field.
`
`The emphasis will be on the materials in view of the topic of this book.
`
`6.2 Luminescent Lighting [1-3]
`
`Luminescent lighting started even before the Second World War*. The ultraviolet
`radiation from a low-pressure mercury discharge is converted into white light by
`a phosphor layer on the inner side of the lamp tube. These lamps are much more
`efficient than the incandescent lamp: a 60 W incandescent lamp yields 15 lmfW, a
`standard 40 W luminescent lamp 80 lmI'W.
`A luminescent lamp is filled with a noble gas at a pressure of 400 Pa, containing
`0.8 Pa mercury. In the discharge the mercury atoms are excited. When they return to
`the ground state, they emit (mainly) ultraviolet radiation- About 85% of the emitted
`radiation is at 254 nm and 12% at 185 nm. The remaining 3% is found in the longer
`wavelength ultraviolet and visible region (365, 405, 436 and 546 nm).
`
`* We use the term luminescent lighting instead of the generally used fluorescent lighting, since
`most of the luminescent materials that are used do not show fluorescence (which is defined as
`an emission transition without spin reversal, i.e. AS : 0; see also Appendix III).
`
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`

`6-2. Luminescent Lighting [I-3]
`
`I09
`
`
`
`"ig. 6.1. Cross section of a low-pressure luminescent lamp. 1 glass tube; 2 luminescent powder;
`3 cathode; 4 lamp cap
`
`The lamp phosphor converts the 254 and 185 nm radiation into visible light
`
`(Fig. 6.1). It is in direct contact with the mercury discharge which rules out many
`
`potential candidates. For example, sulfides cannot be used in lamps since they react
`
`with mercury. A lamp phosphor should absorb the 254 and 185 nm radiation strongly
`and convert the absorbed radiation efficiently, i.e. their quantum efficiency should be
`
`high.
`
`A luminescent lighting lamp has to emit white light, so that the sun, our natural
`
`lighting source,
`
`is imitated. The sun is a black body radiator, so that its emission
`
`spectrum obeys Planck's equation:
`
`A/\"5
`
`5"" :
`
`<6-"
`
`Here A and B are constants, A the emission wavelength and Tc the temperature of
`the black body. With increasing Tc the color of the radiator moves from (infra)red
`
`into the visible. In luminescent lamp terminology, “white” is used for 3500 K light,
`“cool—white” for 4500 K, and “warm-white” for 3000 K.
`
`According to the principles of colorimetry, each color can be matched by mixing
`
`three primary colors- It is possible to represent colors in a color triangle [2]. Most
`currently used is the chromaticity diagram standardized by the Commission Interna-
`
`tionale d’Eclairage. It is depicted in Fig. 6.2- For a definition of the color coordinates
`
`x and 3/, see Refs. [2] and [3]. The real colors cover an area enclosed by the line
`
`representing the spectral colors and the line connecting the extreme violet and the
`
`extreme red. The points within this area represent unsaturated colors.
`
`The color points corresponding to Eq. (6.1) are given by the black body locus
`
`(BBL)- Colors lying on the BBL are considered to be white. White light can be
`
`generated in different ways. The simplest one is to mix blue and orange. However,
`it is also possible to mix blue, green and red- Blending a number of emission bands
`into a continuous spectrum also yields, of course, white light. All these examples of
`
`color mixing are used in lamps, as we will see below.
`
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`

`1 10
`
`6. Lamp Phosphors
`
`0.8
`
`Fig. 6.2. CIE ehromaticity diagram with black body locus (BBL). See also text. Reproduced
`with permission from Ref. [3]
`
`Apart from the color point, there is another important lamp characteristic, viz.
`the color rendition. This property depends on the spectral energy distribution of the
`emitted light. It is characterized by comparing the color points of a set of test colors
`under illumination with the lamp to be tested and with a black body radiator. The color
`rendering index (CR1) equals 100 if the color points are the same under illumination
`with both sources. Under illumination with a lamp with low CR1, an object does not
`appear natural to the human eye.
`In addition to the Iow—pressure mercury lamp discussed above, there is the high-
`pressure mercury lamp (Fig. 6.3). The gas discharge is contained in a small envelope
`sunounded by a larger bulb. The phosphor coating is applied to the inside of the outer
`bulb, so that there is no Contact with the discharge.
`In the high-pressure lamp the discharge also shows strong lines at 365 nm. The
`ideal phosphor for this lamp should, therefore, not only absorb short-wavelength ultra-
`violet radiation, but also long-wavelength. Further this discharge shows a considerable
`amount of blue and green emission. However, it is deficient in red. The phosphor has
`to compensate for this deficiency, so that it should have a red emission.
`The phosphor temperature in the high-pressure lamp increases to 300°C, so that
`the emission should have a very high quenching temperature.
`Since high—pressure lamps are used for outdoor lighting, the requirements for color
`rendering are less severe than for low—pressure lamps. However, if the phosphor is
`left out, red objects appear to be dull brown: this not only makes human skin look
`terrible, but also finding a red car in a parking lot problematic.
`
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`

`6-3. The Preparation of Lamp Phosphors
`
`1 ll
`
`
`
`
`it I:
`_,p.-5
`
`
`
`I
`
`-'.I'____J I
`“ll
`
`Fig. 6.3. Cross section of a high-pressure luminescent lamp. I glass bulb; 2 luminescent powder;
`3 quartz envelope for gas discharge; 4 lamp cap; 5 electrodes
`
`6.3 The Preparation of Lamp Phosphors
`
`The lamp bulbs are coated with phosphor by using a suspension of phosphor powder
`particles. A lamp phosphor is therefore prepared as a powder. In principle this is
`done by standard solid state techniques in which intimate mixtures of starting mate-
`rials are fired under a controlled atmosphere [4]. As a simple example we consider
`MgWO4: it is prepared by mixing basic magnesium carbonate and tungsten trioxide
`in open silica crucibles at about l000°C. Much more complicated is the case of the
`calcium halophosphate phosphor Ca5(PO4)3(F,Cl): Sb,Mn which is made by firing a
`mixture of CaCO3, CaHPO4, CaF2, NH4Cl, Sb3O3 and MHCO3. Actually the history
`of the preparation of this material is a beautiful illustration that increasing control and
`knowledge yields results: the light output of this phosphor has increased considerably
`during a long period of time. For more details the reader is referred to Chapter 3 in
`Ref. [2]-
`
`The luminescent activator concentration is of the order of 1%. Therefore high-
`quality starting materials and a Clean production process are prerequisite for obtaining
`luminescent materials with a high efficiency. The controlled atmosphere is necessary
`to master the valence of the activator (for example Eu“ or Eu“) and the stoichiom—
`etry of the host lattice. Also the pa1'ticle—Size distribution of the phosphor needs to be
`controlled; this depends on the specific material under consideration.
`
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`1 I2
`
`6. Lamp Phosphors
`
`In order to obtain homogeneous phosphors it is often necessary to leave the simple
`solid state technique. -Coprecipitation may be of importance, especially if the activator
`and the host lattice ions are chemically similar. This is, of course, the case with rare-
`earth activated phosphors. For example, YZO3 :Eu3+ can be prepared profitably by
`eoprecipitating the mixed oxalates from solution and firing the precipitate [5]. Actually
`the mixed oxides have become available commercially.
`
`Usually phosphors decline slowly during lamp life [2]. This can be due to several
`processes;
`
`— photochemical decomposition by 185 nm radiation from the mercury discharge
`(an illustrative approach to this problem is given in Ref. [6])
`— reaction with excited mercury atoms from the discharge
`
`—— diffusion of sodium ions from the glass.
`
`Quite often, coarse phosphors appear to be more stable than fine-grained phos-
`phors. Obviously a high specific surface makes the phosphors more sensitive to in-
`teraction with radiation, mercury, and so on. This does not Come as a surprise.
`
`6.4 Photoluminescent Materials
`
`6.4.]. Lamp Phosphors for Lighting
`6.4. I. 1'. Early Phosphor:
`
`In the early period of luminescent lighting (l938—1948), a mixture of two phos-
`phors was used, viz. MgWO4 and (Zn,Be)2SiO4 :Mn2+. The tungstatc has a broad
`bluish—white emission band with a maximum near 480 nm (Fig. 6.4) and can be effi-
`ciently excited with short wavelength ultraviolet radiation- The emission spectrum of
`(Zn,Be)2SiO4 : Mn“ is given in Fig. 6.5. It covers the green to red part of the visible
`spectrum.
`The phosphor MgWO4 is an example of a luminescent material with 100% ac-
`tivator concentration, since each octahedral tungstatc group in the lattice is able to
`luminesce. However, there is no concentration quenching. This is due to the large
`
`CO‘ “/04
`
`
`
`550
`
`.__.__l.__
`600 M.
`
`400
`
`45b
`
`J
`500
`km}
`
`Fig. 6.4. Emission spectra of MgWO4 and CaWO4
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`6.4. Photoluminescent Materials
`
`I 13
`
`300
`
`.400
`
`500
`A-Th-r
`
`500 um
`
`Fig. 6.5. Emission spectra of ZH2SlO4 : Mn2+ (8) and (Zn,Be);SiO4 : Mn2+ (C). Curve A gives
`the diffuse reflection spectrum of ZngSiO4 : Mn2+
`
`Stokes shift of the emission which brings the relaxed emitting state out of resonance
`with the neighbors. This, in turn, can be related to the nature of the optical transition
`which is a charge-transfer transition in the tungstate group. Therefore AR in Fig. 2.3
`is large. This yields not only a strongly Stol<es—shifted emission, but also a very broad
`emission which is of importance for the coior rendering.
`The broadness of the (Zn,Be)gSiO4:Mn2+ emission is due to another reason.
`Actually the emission of beryllium—free Zn2SiO4 : Mn“ is narrow (Fig. 6.5). Let us,
`therefore, start with the latter phosphor which shows a bright—green emission.
`
`Both Zn2SiO4 and BegSiO4 hav