Phil. 11ag. 18,-159 (1334). STOVES:P l d . 31ag. 22: 113T (1936). ( 5 ) .JI;voss : R t p o r t on Bard S p c c l ) of ~ ~lliatotriic Jfolecrtlcs. London (1032). (6) SLTHERLAKD:Proc. Indian -4cad. Sci. 8,341 (1935). ( 7 ) SUTHERLAND: J. Cheiii. Phys. 8, 161 (1940).
(3)
(,'LARK:
(4)
('IARK
ASD
Cambridge University I'rese,
T H E ?tlAGNESIUhI TUSGSTATE PHOSPHOR GORTOX R. POSD.1 Rescarch Laboratory, General E l e c t r i c Companij, 1 Riuer Road. 8chc?~ectadij6,-Ti?L' 170rk
Received Aprrl 19, 19.44
One of the component phosphors used in the manufacture of the fluorescent lump is magnesium tungstate ( 3 , 4, 5 , 6). It is prepared by firing a mixture of magnesium oxide u-ith tungstic oxide (W03),without the addition of any foreign actiyator. The writer's attention n-as brought to it several years ago by T. E. Foulke of the General Electric Company, who had found that the greatest brightness resulted by firing a mixture of about 2 moles of magnesium oxide per mole of tungstic oxide. Such a curious departure from the proportions of the recognized compound, IlgTTO4, led to an investigation in this laboratory with the aim of finding if any unique compound were involved. It developed, however, a3 will be discussed in this paper, that, n hen the phosphor was prepared from this most favorable composition or in fact from any other composition by firing a t a suitable temperature to render it fluorescent, it yielded an x-ray diffraction pattern identical with that of MgW04. However, when the mixture comprised 2 or more moles of magnesium oxide per mole of tungstic oxide, and was fired for an extended time a t 1250°C. or above, another compound was formed which had an entirely different diffraction pattern and which was non-fluorescent. EXPERIMESTAL
Suitable firing conditions for preparation of the phosphor consisted in heating the mixture a t 1100°C. for 1 hr. Firing at lower temperatures produced almost equal brightness of fluorescence, but correspondingly longer times were required -4 hr. at lOOO"C.,15 hr. at 9OO"C., and 70 hr. a t 470°C. Somewhat higher brightness values, especially a t the optimum concentration, would have resulted from following in detail the direction3 giren by Foulke, but even n-ith the simple firing schedule used there was a notable increment in brightness that characterized those mixtures approximating 2 moles of magnesium oxide, as is brought out in the first twQ vertical columns of table 1. Khen the phosphors Trere refired at 1250"C., the fluorescence was greatly reduced or destroyed altogether, as is brought out in the table. On an additional
304
GORTON R. FONDA
refiring a t 950"C., fluoresccnce reappeared a t appreciable brightness valucs that approached those resulting from the initial firing a t 1100°C. In the case of mixtures containing 1.44 moles of magnesium oxide or less, firing at 1250°C. produced a highly sintered mass, verging on fusion, which was completely fused by firing at 1400°C. There was also a change t o a yellowish color, but without loss in weight. For mixtures with 1.93 moles of magnesium oxide or more, the product remained as a white pon-der a t both 1250°C. and 1460"C., with no sintering whatever.
I
FLUORESCENCE BRIGRTKESS
per
p e r ceni
CBIE,
1 .uo
89
1.15 1.41 1.93
88
0 0 25 0 0 0
100 100 79 ti6
2.88 5.75
TABLI.: 2 E f f e c t o j compositio?i and $ring t e m p e r a t i u c
I
7 :$
34
c r y s t a l l i n e pattern
7rpo7c
CRPSTALLINE PATTERN
1 h r . a t 1100°C
+
1.00 1.44 1.93 2.88 5.i5
.
_
_
_
+
1 hr. a t 1100°C. 1 hr. a t 1250T. 15 h r . a t 900°C.
1 h r . a t 1250°C.
~
_
+
_
A h
4
13 13
h
I3 T MgO
A
+ MgO
With the assistance of E. T. Asp of this laboratory, x-ray diffraction patterns with molybdenum K, radiation were obtained for these wrious products. Two different patterns resulted, A and B, as is disclosed in table 2. For mixtures containing the highest content of magnesium oxide, some lines of the magnesium oxide pattern were found as well. In no case were there any lines of tungstic oxide (WOa) observable. When phosphors were fired in platinum boats for over 50 hr. a t 110O"C., no alterations in fluorescence brightness or in x-ray pattern mere observable. The lattice spacings corresponding t o these two types of x-ray pattern are given in table 3. The lattice spacings of pattern h were found to be identical with those rcported by Broch (1) for R4gW04, having monoclinic structure. Broch reports
305
11AGSESIUM TUSGSTATE PHOSPHOR
also Some other lines which were not Sound in our samples, presumably because they were too n-eak. Inasmuch as pattcrn . I was found also for prccipitated TABLI: 3 Lattice spaczngs correspondtiig to the x-rail p a t t e r n s os the 1wo ,forms of ?nagnesauni t u n g s l a t e ~PATTERU
A
PATTERN
3.60 2.90 2.46 2.34
f s
3.20 3.01 2.71 2.32
1.571 1.494 1.424 1.362 1.310 1.259 1.202 1.175 1.112 1.100 1.078
f
111
m
m m IV
1.482 1.450 1.410 1.280
B
111
f s
In
f f \V
f
in f
f f f f f
1;1c. 1. Excitation and emission spectra of iiiagriesium tungstate phosphor. Curve A , efficiency of excitation; curve B, fluorescence spectrum.
MgW04, it seems saSe to assume that the phosphor is essentially monoclinic MgJF-04, with which an excess OS magnesium oxide may advantageously be associated.
30G
GOIWOX E. FONDA
Pattern 13 hears no relation t o any compound of magnesium or tungsten that is on record. It appear" therefore to be a unique compound. The spectral distribution of fluorescence for the phosphor was determinul u ith a spectrophotometer by exciting it with 2537 1.and measuring thc energy emitted by comparison with that froin a calibrated tungsten incandescent lnmp. With the aid of Frank B. Quinlan, the cficiency of excitation was deterniincd under radiation from a monochromator, measuring the radiant energy of the exciting line with a thermopile and the fluorescence lumens in a 4.5-cni. sphere with a photovoltaic cell and eye-correcting filter (2). The rrsulting curve3 are gircn in figure 1. The breadth of the excitation band yielding high efficiency of conversion iq noteworthy. 'In efficiency very clobe to the formation of on:> quantum of fluorescence energy from one quantum of radiant, exciting m w g y exiPts throughout the spectral rangc cxtcnding from 2500 to 2950
x.
DISCUSSIOS
Sormal magnesium tungstate, having the composition lIg\\'Oc and thc monoclinic structure, is itself converted into a phosphor by suitable firing. Allthough no foreign activator i* required for this, it is noteworthy that a higher fiuorescence yield results when an e s c e ~ of s magnesium oxide i. present during hring. It is difficult to conceive that the preaence of bIg0 groups loosely bound to the &lgNOccryital could have any effect upon fluorescence brightness. It seems preferable to aisume either that the exceqs hIg0 is incorporated in the RlgW04 lattice, with the 0 ion replacing a \TO4 ion, despite the absence of any alteration in lattice size, or else that atoms of Rlg and of 0 are introduced interstitially into the Mgn'04 lattice. The lo+ of fluorescence that was ohm-ved by refiring a t 1250°C. those phosphors containing 0.44 mole excess MgO or less can be explained as the result of partial conversion into a colloidal, glassy state, on-ing to the fact that the melting point of MgWOa lies in the neighborhood of this temperature. This view is supported by the appearance of the line3 in the x-ray pattern which had become broken and more diffuse. The reconversion into a phosphor that took place on refiring at the reduced temperature of 900°C'. iq of course due to devitrification that led to recrystallization. For those phosphors containing 1.93 moles of lIgO or more it is evident that a drastic alteration in structure was produced by the firing a t 1250°C. Inasmuch as the compound corresponding to pattern B did not appear a t this firing teniperature until this composition was reached, i t can be assumed that i t has the formula 31gaW0,. Jt differs from the monoclinic l'IgWO4, not only in structure but also in being still infusible a t 1460°C. and in being non-fluorescent. It is stable only above 1200°C. or thereabouts 'and is reconverted into the fluorescent, monoclinic form by refiring at (300°C. An interesting contrast to magnesium tungstate is shon-n in the behavior of calcium tungstate, the most familiar member of the tungstate group of phosphors. It can be prepared by firing a mixture of the oxides. In crystalline form i t belongs to the tetragonal system and has the scheelite structure. Its characteristic x-my pattern persisted after firing for BO hr. a t 1100°C and after 3 hr. a t 1200°C.
SETT BOOKS
30i
and its fluorescence remained unaltered. At 1400°C. it sintered don-n in 15 min. to a semi-fused mass! hut it still maintained the scheelite pattern and considerable fluorescence. SL7\11\I.4RT
The magnesium tungstate phosphor always exhibits the monoclinic structure of I\lgTT704,even in the presence of an excess of magnesium oxide xvhich, a,t a concentration approaching 1mole, serves to gire a not,ableincrease in fluorescence brightness. Khen fired for a long period at 1250°C. it loses its fluorescence in one of t8xoTmys, depending upon the concentration of magnesium oxide: (a) by conversion into a semi-glassy state, if it contains 0.5 mole or less of excess magnesium oxide; (b) by alteration into another crystalline modification, having presumably the formula I\Ig,TT05, if it cont'ains 1 mole or more of excess magnesium oxide. lMg,W06 differs from MgW04 not only in being non-fluorescent but also in being stable only above 1200°C. or thereabouts and in having a melting point xhich lies ab0.i-e 1460°C. The quantum efficiency of excitation of the phosphor is near unit,y for t,he range 2500-295Oa. REFERESCES (1) BROCH, EIX.\R: Z . physik. Chem. lB,415 11928). ( 2 ) FOSDA, G . R . : J. Phys. Chem. 43,5 i 4 (1939). (3) FOTLKE! T . E . : U. S.patent 2:203,6S2 (June 11, 1940); 2,232,780 (February 25, 1941). ( 4 ~ 1F R E R I C HR Y., : Saturn-issensciinften 26, 681 (1938). ( 5 ) THAYER, R.1.: ASD BARKES, B. T . : J. Optical SOC.Ani. 29,131 (1939). (6) LTTERHOETES, W , : Elekfrisclie Gaaentladu,igslampeii. Julius Springer, Berlin (1938).
S E W BOOKS A Text-Book of Inorganic Chemistry. By F. EPHRAIM.Fourth English edition by P. C. L. Thorne and E. R. Roberts. 24 s 16 cni.; sii 921 pages. London and Edinburgh: Gurney and Jackson, 1943. Price: 28 shillings net. This work, now published in the fourth English edition, is probably well known to most readers. It is a n advanced test-book in which the treatment follows a n order of topics rather than a grouping of individual elements, with chapters on theory. I n considering compounds. the order is usually t h a t of Valence rather than the periodic law order. The book is deservedly popular and can be recommended t o advanced students. The reviewer has noticed a number of places in the text which seem to call for consideration n-hen a new edition is required. It is impossible t o notice all of these in a short review, but some indication may be given of the kind of thing which is meant. The theoretical side of the book is the least satisfactory, although i t has been improved in the English editions as compared v i t h the original. The chapters on atomic structure and valence contain a number of errors and obscure statements; for example, the force between the electron and nucleus is said t o be quantized, the uncertainty principle is obscurely stated (page 3), i t is said t h a t the outer electron shell of 8 of the inert gases is confined t o those of low atomic number, a special parachor value is given for semi-polar double bonds, etc. The short account of the quantum theory of valence on pages 53-57 is good as far as i t goes, but the statement of the quantum-mechanical basis on page 53 is defective, and the symbols s, p , d for the electrons are not defined. The assertion on page 56 t h a t
+
