JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

Temperature Dependence of the Emission of an Improved Manganese-ActivatedMagnesium Germanate Phosphor

LUKE THORINGTONWestinghouse Lamp Division, Research Department, Bloomfield, New Jersey

(Received May 25, 1950)

A number of recent researches have been concerned with magnesium germanate phosphors. An improvedsuch phosphor is reported here having over twice the brightness of the previous compositions as prepared inthis laboratory, and which shows peak luminescence efficiency at temperatures near 350'C. Temperaturedependence of luminescence studies on the improved composition and the others showed little differencebetween them in this respect though the data are in striking contrast with those reported by others. Apartial explanation of the pronounced temperature effect (for temperatures <350 0C) is presented and isbased upon the broadening of the excitation spectrum at elevated temperatures in such a way as to increaseabsorption of the exciting radiation (3650A). Data are presented showing differences in the temperaturedependence relation effected by wave-length of excitation and method of measurement; in addition, curvesare included showing the variation of phosphorescence (t= 1/120 sec.) with temperature as well as curvesshowing excitation and emission characteristics of the phosphor. Finally a practical application of this hightemperature luminescence is demonstrated in the color corrected high pressure mercury lamps the emissioncharacteristics of which are briefly discussed.

INTRODUCTION

1 ED luminescing magnesium germanates activatedwith manganese are interesting phosphors both

from theoretical and practical standpoints. They are ofespecial theoretical interest because of the unusual finestructure found in their luminescence emission spectral(see Fig. 1); they are of practical interest because oftheir ability to function at unusually high temperaturesunder mercury ultraviolet excitation, thus providingfor the first time really suitable phosphors for the colorcorrection of high pressure mercury lamps.

Williams2 was the first to report the fine structure inthe emission of these phosphors though as far back as1936 Leverenz3 had prepared the meta- and ortho-magnesium germanates both of which, however, lumi-nesce only very weakly under photo-excitation. Thediscovery by Williams, 2' 4 that a large excess of MgOabove the ortho- proportions increased the photo-luminescence efficiency of the phosphor by a factor ofabout 5, served to place the material in the category ofuseful phosphors. He reported an optimum Mg/Geratio of 4 for 3650A excitation though a sample con-taining 10 MgO per GeO2 still showed photo-lumines-cence efficiency as high as the original orthogermanate.In a later paper Patten and Williams' have reported onthe temperature dependence of the luminescence of the"double-ortho" formulation and indicate that thermalquenching of luminescence begins in the neighborhoodof 22-100C with peak efficiency near 220C. Otherproperties of the new phosphor as gathered from theliterature' are that it is white in physical appearanceand exhibits an exponential decay of phosphorescence.

It has been found possible in this laboratory to further

1 S. H. Patten & F. E. Williams, J. Opt. Soc. Am. 39, 702 (1949).F. E. Williams, J. Opt. Soc. Am. 37, 302 (1947).H. W. Leverenz, U. S. Pat. 2,066,044 (1936).F. E. Williams, U. S. Pat. 2,447,448 (1948).

5 H. W. Leverenz, Luminescence of Solids (John Wiley and Sons,Inc., New York, 1950).

improve the efficiency of the "double-ortho" magne-sium germanate phosphor by replacing a portion of theMgO with MgF2 . Over-all luminous efficiency of thephosphor is approximately doubled by replacing 0.5MgO with MgF2 thus bringing the total improvement inbrightness to 10 times that of the original ortho-germanate. The present paper describes the preparationof the new fluorogermanate samples, some of theirproperties with emphasis on the temperature de-pendence of luminescence, and finally a practical ap-plication of the high temperature red luminescence.

PREPARATION OF THE SAMPLES

Both plain germanate and fluorogermanate sampleswere satisfactorily prepared using a dry mixing tech-nique in which the required amounts of MgO, MgF2 ,MnCO3 (all C.P. Bakers "Analyzed") and GeO2 (EaglePicher, C.P.) were sieved through 100-mesh silk boltingcloth and ballmilled or ground thoroughly in a mortarbefore firing. However, the wet mixing method of

(_ DULE 10TH PROPORTI NS)

9 HIGH E _ RE ERJR LA P_

(PRINIPAL XCITI X 3I 6 3 0, 4 566A

C 300 C (AR \R10l \

SC -- V - - -t - A-

C, - _ _k _ f - - -4

5C~~~~~~~~~~4'

_ B f LOW PRES RE LUORI CENT LAM (25 7 A IOTAIO

- 6200 6300 6400 65 00 6600 6700WAVELENGTH IN ANGSTROMS

FIG. 1. Spectral distribution of the luminescence ofmagnesium fluorogermanate.

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VOLUME 40, NUMBER 9 SEPTEMBER, 1950

LUKE THORINGTON

_LINESEPR T EE AfT E -L ESC CE FE VENIS

90 _ i OF HINES FRO a H I LA P

70 N

F i - M} GNESIUM I LUOR GERM NATE

S MADE9Ut GER SPLAT a 25 - - - - ~-- -~----

MAG A N

A 0

e I.I .5 30 4 6CS .55 i . 4 S .50

I WAVELENGTH IN MICRONS

FIG. 2. Excitation spectrum of magnesium germanates(from reflectivity data).

Williams 2 in which a solution of soluble salts of themagnesium and manganese was used to form a slurrywith GeO2 which was then evaporated to dryness andground preparatory to firing, was found somewhatsuperior for the plain germanates though not for thefluorogermanates. The samples reported here whichcontain no fluorine were prepared by the wet methodand those containing fluorine by the- dry method.Molar proportions used were 4 MgO; 1 GeO 2 : 0.01 Mnfor the plain germanates and 3.5 MgO; 0.5 MgF2 ; 1GeO2: 0.01 Mn for the fluorogermanates. The inti-mately mixed and finely ground raw batches were firedin platinum containers in an electric wirewoundalundum muffle furnace (air atmosphere) at tempera-tures between 1000-1200C for about 30 min., afterwhich they were again ballmilled (large batches) orground in a mortar. For best results samples were thenrefired at about 1100C for 30 min. to several days andthen re-ground to pass 100 mesh bolting cloth. Samplesof concern here were fired at 1080TC for 16 hours forthis second firing. Firings in excess of about 2-3 hoursproduce a gradual improvement in brightness amount-ing to about 10-15 percent for times up to about threedays. The exact nature of this long term change is notunderstood at present but is under investigation. X-raydiffraction powder patterns of the finished samplesshowed the well-crystallized magnesium orthogermanatestructure with a very slight amount of free MgO.

SOME PROPERTIES OF MAGNESIUMGERMANATE PHOSPHORS

The plain germanate samples as prepared aboveshowed a slight yellowish tinge while those containingfluorine were of a decided yellow color (magnesiumorthogermanate: 0.01 Mn is a pure white). All lumi-nesced a brilliant red under excitation by both longand short wave mercury ultraviolet radiation. Absorp-tion spectra of the phosphors were obtained from diffusereflectance measurements made with the Beckmanspectrophotometer and reflectance attachment using

the tungsten and hydrogen sources and are plotted inFig. 2 as representing excitation spectra. Absorptionand excitation are apparently the same function ofwave-length for these phosphors as will be seen later.The reason for the yellowish color of the samples isfound in the pronounced absorption maximum (orreflection minimum) at about 4200A; another broaderand more intense absorption band is maximum atabout 2800A. These bands are definitely attributable tomanganese as they are absent in samples identical inevery other respect but containing no manganese.

Luminescence emission spectra were measured withan automatic recording spectroradiometer consisting

_ CITAll N SINUM AL 60 c p. ;CH- LAM CORNIN, 98_3

FILTEV

DTECTI N P 22+ COF RING 2418FILTIER

BR N ELCTROKo POTNTIOM ETER

Mg G O' .5 M

4~~~~~~~~~~

IC

-zoo -100 0 100 200 300 400I TEMPERATURE C

FIG. 3. Temperature dependence of the luminescence ofmagnesium germanate phosphors.

essentially of a Beckman monochromator, cam cor-rected 1P22 photo-multiplier and cathode followercircuit feeding a Brown "Electronik" strip chart re-corder having its chart drive synchronized with theprism drive of the monochromator. The spectral bandwidth transmitted was about 50A at 7000A and about30A at 6000A. Emission spectra of the phosphorsappear to be the same for excitation by the mercury2537, 3650, or 4358A lines and consist of a narrow bandbetween 6000-6900A with five sharp peaks occurringat about 6260, 6340, 6425, 6535, and 6595A. Thesevalues are approximately 20-30A less than those ob-tained spectrographically by Williams' for the plaingermanate; fluorine apparently has no effect on thewave-length position of the peaks. The effect of tem-perature on the emission spectrum is seen from Fig. 1to be that of effectively smoothing out the weakerbands, broadening the two remaining, shifting themtoward longer wave-lengths, and enhancing the shorterwave-length band relative to the longer. The highestpeaks of the two spectra were equated in this figureso as to show more clearly the changes in distributionalthough the actual heights may not be far from equal,judging from the temperature dependence of the in-tegrated spectral emission (Fig. 3). The luminosity ofthe luminescence spectrum is thus seen to increase with

580

MAGNESIUM GERMANATE PHOSPHOR

increasing temperature and is due both to the spectralbroadening and to the preferential growth of the bandat 6340A.

As mentioned above, the red luminescence of mag-nesium germanate and fluorogermanate is excited byradiation absorbed within the bands shown in Fig. 2.For both 2537A and 3650A excitation the fluoro-germanate has over twice the brightness of the un-substituted samples as measured with a WestonPhotronic cell with Viscor filter. Part of this increasemay be attributed to the intensification of the absorp-tion bands by the fluorine as illustrated in the figure.That the absorption and excitation are one and thesame for photo-excitation of these phosphors in thewave-length range shown was confirmed experimentallyby measuring the relative luminescence evoked by theindividual lines of a high pressure mercury H-1 quartzlamp of fairly well-known energy distribution. Thelines shown in Fig. 2 represent the relative amount ofluminescence excited by the corresponding mercurylines from this lamp. When the values are divided bythe phosphor absorption at each wave-length, the rela-tive energy of each line is obtained thus demonstratinga luminescence efficiency which varies as the spectralabsorption of exciting radiation. The actual quantumefficiency of the fluorogermanate was measured at

2537 A / _ - -

7-- 365 A

(CH 4 LAI+9 63FITER) /

60-

EXCTION SINUS 0IALj60c. . -\

DETE ION: P22 2 8 Fl ERI0 ~~~~~~BRam N EL (IR I). TEN I.MET

-20-0 -0 0 00 20 300 40_ T__ EMPERUE I DEGR CENTIGRADE

FIG. 4. Effect of exciting wave-length on the temperature depend-ence of the luminescence of magnesium fluorogermanate.

400C and for 2537A excitation using the method ofThayer6 and was found to be near 0.90; efficiency for3650A excitation at 400 would probably be only half asgreat because of the decreased absorption.

TEMPERATURE DEPENDENCE OF LUMINESCENCEMEASUREMENTS

Two general methods of obtaining the temperaturedependence data were used. In one method the outputsof the photo-multiplier measuring the luminescence(1P21) and the one measuring ultraviolet intensity(1P28) were alternately presented on a cathode-ray

6 N. Thayer, Trans. Electrochem. Soc. 87, 424 (1945).

screen and the two traces photographed on the sameframe for the different temperatures of interest. Theultraviolet source was operated on a stabilized half-wave rectified 60-cycle supply so as to obtain more of thephosphorescence "tail" for future study (Fig. 8).Values plotted were the peak values of fluorescence vs.temperature. The other method differed only in thatmeters replaced the oscilloscope so that average valuesof luminescence and excitation intensity were measuredinstead of instantaneous peak values, and that theexciting source was operated on a full wave 60-cyclea.c. supply. In every case the phosphor to be measuredwas thinly coated on a rhodium or platinum surfacethe temperature of which could be varied from about- 180C to 450TC; samples were measured in vacuum(< ,4 Hg). Temperatures were measured with an ironconstantan thermocouple peened into a hole in thephosphor mounting block and either a Leeds andNorthrup potentiometer or a Tagliabue Celect-Rayrecorder-controller. Provision was made for isolatingvarious spectral regions of the luminescence emissioneither by the use of appropriate filters or a large Hilgerconstant deviation spectrometer.

It is important in reporting temperature dependenceof luminescence data that all pertinent factors bespecified. Some of the factors found thus far thatinfluence the type of relation obtained are (1) intensityof excitation (2) wave-length of excitation, (3) wave-length at which emission is measured, (4) impurities inthe phosphor, and (5) technique of measurement(together with mode of excitation- pulsed or steady)e.g., whether instantaneous values of fluorescence oraverage values of luminescence are measured for pulsedexcitation. The first factor appears important only inphosphors showing bimolecular phosphorescence decay7

and is, therefore, of little concern here since magnesiumgermanate exhibits an exponential decay of phosphores-

FIG. 5. Temperature dependence of the long and shortwave-length emission of magnesium fluorogermanate.

7 G. R. Fonda and F. Seitz, editors, Preparation and Charac-teristics of Solid Luminescent Materials (John Wiley and Sons,Inc., New York, 1948) p. 112.

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LUKE THORINGTON

90 --- \- -

M% ZnG. F D0i0 Mn F .01M

4C _ _;C l _ ~~~Me t0

- EXC TION SINUDDIDAL 60cp. s.CH-A LAMF + 9 3 FILTER

,20 ~~DETE lION IP22 + FOWN ELEC RONIKI

10 ~ ~~~~~P TENTIOMETR

-10 0 10 0 00 400 -

TEMPERATURE IN DEGREES CENTIGRADE

FIG. 6. Effect of zinc on the temperature dependence of theluminescence of magnesium fluorogermanate.

cence.5 However, the intensity level was kept constantthroughout these measurements for both long and shortwave excitation.

Effects (2), (3), and (4) above are illustrated inFigs. 4-6 respectively, the curves of which were ob-tained by the second measuring technique described.Effect (4) is also shown in Fig. 3 for the case of fluorinesubstitution though it is not as pronounced as for thezinc substitution shown in Fig. 6. The different curvesobtained for the same phosphor using the two measuringtechniques are shown in Fig. 7 for 2537A excitation.The dotted curve in the lower-half of the figure repre-sents the temperature dependence of the phosphores-cence intensity 1/120 sec. after excitation has ceased(see Fig. 8); values are relative to the peak in the tem-perature vs. fluorescence curve. Just what is measuredin the oscilloscope technique is clear from Fig. 8 whichshows the cathode-ray traces obtained for the excitationand emission at two different temperatures of thephosphor. At the higher temperature the long termphosphorescence is almost completely quenched whilethe peak fluorescence is actually higher than at the lowtemperature. It is the variation in this fluorescence peakwith temperature that is plotted in Fig. 7 along with thecurve for the same phosphor obtained using the methodof measuring average values of luminescence.

DISCUSSION

From Figs. 1 and 2 magnesium germanate andfluorogermanate activated with manganese appear tobelong to that class of phosphors in which both theexcitation and emission processes take place in spectralbands characteristic of the activating impurity and inthis respect are similar to Kroger's8 magnesium ortho-titanate activated with manganese. The titanatephosphor also shows fine structure in its luminescenceemission and an excitation spectrum which correspondto those found in the germanates though in the titanate

8 F. A. Kroger, Some Aspects of the Luminescence of Solids(Elsevier Publishing Company Inc., New York, 1948), p. 64.

both excitation and emission are shifted slightly towardlonger wave-lengths due probably to the effect of thehost crystal. Kroger offers an abundance of evidencein favor of the existence of the manganese as Mn4+ ionsin the titanate and it would thus appear likely that thesame is true for germanate phosphors rather than thatit is in the divalent state as stated by Williams.'Oxidation effects for the germanates were somewhatsimilar to those reported for the titanates except thatno green luminescence of the reduced samples was ob-served (see reference 5, p. 110). Since the transitionsresponsible for the absorption and emission processesin the Mn4 + ion are associated with electrons in anincomplete inner shell shielded somewhat from ex-ternal perturbations5 8 one would expect the effects onthese processes of thermal vibrations of the host latticeto be less than for Mn'+ ions where the optical electronsare not shielded. That this is true for germanate phos-phors is evidenced by the fine structure of the emissionand perhaps to some extent by the relatively high effi-ciencies of these phosphors at elevated temperatures(compare, for example, MnI+ activated silicates).

If both the absorption and emission processes of thegermanate phosphors are attributable to the Mn ac-tivator as suggested already, then one would expect thatchanges in the absorption spectrum with temperaturesimilar to those for the emission spectrum (Fig. 1)might be observed. That the absorption does change isconfirmed visually and by measurement of the diffusereflectance of the 3650A (CH-4+9863 filter) excitationas the temperature of the phosphor is increased. Atelevated temperatures (300C) the phosphor is verynoticeably yellower than at room temperature whichmeans that absorption in the 4200A band (Fig. 2)becomes more intense with increasing temperature.Absorption (within the band peaking at 2800A) forthe exciting radiation used in the temperature de-pendence measurements (3650A) more than doubles inthe temperature interval 25-450TC. It is hoped thatcomplete spectral absorption curves at elevated tem-

FIG. 7. Temperature dependence of luminescence curves obtainedusing different measuring techniques. Excitation: 2537A.

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MAGNESIUM GERMANATE PHOSPHOR

peratures may be obtained soon. However, the presentdata are sufficient to account (at least partially) for thepronounced dependence of the 3650A excited lumines-cence upon temperature for temperatures below that atwhich the characteristic thermal quenching begins.From Fig. 4 it is seen that the luminescence is less tem-perature dependent for 2537A than for 3650A excitationfor temperatures below the quenching point. Thismight be explained if the effect be attributed to abroadening and shift of the excitation spectrum and ifthe two exciting wave-lengths were located in suchpositions as to be affected differently by such a change.That they are is seen from Fig. 2; thus a broadeningand/or a shift of the 2800A band toward longer wave-lengths would affect absorption at 3650A much morethan at 2537A which is near the peak of the band.Garlick and Gibson9 have observed a similar effect withself-activated zinc sulfide phosphor.

The high temperature portion of the temperaturedependence curves of the germanate phosphor showsthe very rapid decay of luminescence with increasingtemperature which is characteristic of most luminescentsystems. Theoretical aspects of the phenomenon havebeen discussed by others5 7. 8

,10 but no satisfactory

theory seems to have yet met with general acceptanceand none is advanced here. Generally speaking, tem-perature dependence of luminescence data in the litera-ture has been so lacking in specifications of experi-mental technique that it is hard to know just what hasbeen measured and, therefore, harder still to understandhow the theory could fit the data. Thus it would seemunsafe to attempt to fit the curves presented here withany relation derived from fundamental considerationswithout first correcting for the spectral selectivity of thefilter-photo-tube combination used for detecting thechanges which are plotted. Apparently the only tem-perature dependence data that would be of reallyunquestionable theoretical value would be that in whichactual energy distributions of the luminescence aremeasured' for the various temperatures or that inwhich the total integrated luminescence is measured vs.-temperature using an effectively non-selective receiver.

A PRACTICAL APPLICATION OF THE HIGHTEMPERATURE LUMINESCENCE OFMAGNESIUM FLUOROGERMANATE

For a phosphor to be of practical advantage in thecolor correction of the well-known high pressure mer-

9 G. F. J. Garlick and A. F. Gibson, Nature 161, 359 (1948).10 F. E. Williams and Henry Eyring, J. Chem. Phys. 15, 289

(1947).

+175 C

EXCI TATION

-

COXW

a:W

W

I SC1T20 - I SQ12o

TIME

FIG. 8. Variation with time and temperature of theluminescence of magnesium fluorogermanate.

cury lamps" it should possess the following character-istics: (1) It should be as white as possible so as notto subtract materially from the visible mercury emis-sion.. (2) It should respond efficiently to all wave-lengths of the mercury ultraviolet generated in the highpressure arc. (3) It should possess (2) at elevated tem-peratures preferably near 400-500TC. (4) It shouldluminesce in the spectral ranges where mercury is de-ficient, i.e., primarily the red and (for complete colorcorrection) also in the bluegreen, and (5) It should bethermally and photo-chemically stable at the tempera-tures and radiation densities encountered in the highpressure lamps. Magnesium fluorogermanate: Mn ap-pears to be the first phosphor to fulfill all of theserequirements in sufficient degree to warrant using it insuch an application. A thin coating of this phosphorapplied to the inside of the evacuated T-20 outer bulbof a 400 watt EH-1 lamp considerably improves thecolor rendering properties of the source with only afew percent over-all lumen loss. When the phosphoris coated on an outer bulb of optimal design an actualincrease in efficiency of the lamps results in addition tothe red correction which amounts to about 2600 totallumens radiated between 6000-7000A or 12 percentof the total visible radiation-approximately the sameproportion of "red" as present in daylight! Thus there ismade available a whole new series of efficient lamps ofgood color distribution for such applications as streetlighting, technicolor and television studio set lighting,and high bay factory lighting to name but a few. Amore complete specification of these lamps will bepublished elsewhere.

u E. W. Beggs, Trans. Illum. Eng. Soc. 42 (4), 435.

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