edited by JOHN H. WOTlZ Southern Illinois University Carbondale. Illinois 62901
Discoveries by ~ccident:Yttrium Oxysulfide Phosphors Ronald A. Brown' The Polytechnic of North London Hollowoy, London N78DB England Frequently science progresses by chance or accidental occurrences (I).One area which seems particularly prone to such happenings is the chemistry of phosphor systems. Even the earliest recorded luminescent material, the so-called Stone of Bologna, came about through a combination of chance events. In 1603Vincenzo Cascariolo (2), a cobbler who was also a natural scientist, found a very heavv silvery rock on Monte Patemi, near Hologna, while searching for the philosopher's stone. Folluwinr the usual akhemical procedures (:ascariolo pulverized thisrock, mixed with charcoal, and heated. The spongt:-like product had the unusual property of absorbing the sun's rays by day and givingoff a feehle blue radiation at night. According to present day concepts Cascariolo had converted the mineral barite, a naturally occurring form of barium sulfate, into the corresponding.metal sulfide by means of the reaction BaS04 2C BaS + 2C02. Notice that untreated BaS is not itself luminescent. In fact, the effect resulied from traces of heavy meld impurities, including Hi, Cu, or hln (31, which happened to he present with the harite. In rnrl? termina,lo:y n material with the attributes of the Rolornian itone mmc to t ~ eknown as a pho.iphorus, from the (;reek wurdi meaning "bringer o i light." Full~u,ingBrand's extraction t n m urine of matter that emitted iis own light. .Iohann Klshaltz ( 4 ) in I t i X referred to this new chemical element as phosphorus. A process involving emission of light as a result of surface oxidation or hydrolytic attack, such as that occurring with elemental P, nowadays is designated chemiluminescence. Eventually, substances with the same ability as the Stone of Bologna to glow in the dark hecame known as phosphors. Today the name "phosphor" is applied to any solid material that can convert various forms of energy into visible, or near visible, radiation. Luminescence is the
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732 1 Journal of Chemical Education
reneric term for the processes of conversion. The purpose of the present note is to provide some background information on the discovery of one particular class of phosphors, namely those hased o n t h e oxys"lfides of yttrium or a rare earth element. Perhaps an appropriate place to start is with the introduction of YV04:Eu in color television receivers in 1964 ( 5 ) . ,\Ithough the preparation and crystal .;trucrure of yttrium ~orthuvanadatewere reported at least ;is lung o w a, 1933 161. it is only cump~rativelyrriently that thiscomp~undh3s heen utilived as a cnrhodduminescmt material. In IIW van llitert c.1 al. 171 .. -~ . , investieated the swthi\iis ot Y\'O,:F:u in single crystal t'orm, together with its tluorescent pmpertiei under ~~ltruviolet excitation. Some Id months 131er I.evinennd I'alilla (5),and their coworkers a t General Telephone and Electronics Lahoratories (8). . .. announced the use of YV04:Eu as a redemitting phosphor in color kinescopes. Like most novel and worthwhile concepts this application seemed "obvious" in retrospect. At the-time the announcement had quite an impact. The use of rare earths in color television stimulated renewed interest in what had become an almost neglected branch of chemistry. A frantic search for new luminescent materials hased on compounds of yttrium or the lanthanons was initiated in many parts of the world. At RCA Electronic Components and Devices in Lancaster, Pennsylvania, the Materials Group was one of the many centers caught up in this search for new rare earth phosphors. Chance played a role even a t the very start of the program. One part ofthe group was asked to work on yttrium systems, whereas another section was allocated lanthanum salts. Martin Robert Royce was given the assignment of developing new phosphors hased on compounds of yttrium. One of the early choices, possibly influenced by the widespread uses for
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Between 1965and 1971Ronald Brown carried out research on the synlhesis and characterization of luminescent materials as a Member of the Technical Staff at RCA Electronic Components in Lancaster, Pennsylvania. During this period he was a co-worker of Martin R. Royce.
sulfide-type luminophors (9),was to prepare yttrium sulfide activated with europium. A conventional method of preparation of metal sulfides is by reaction of the corresponding metal oxide with a sulfidizing agent such as hydrogen sulfide, a t elevated temperatures (10).Frequently oxalates or carbonates are used as precursors, since the formation of an oxide in situ yields a reactive intermediate. On 24 Septemher 1964, Royce decided to attempt the synthesis of Y&Eu by heating a coprecipitated (yttrium, 3 atomic % europium) oxalate in a stream of hydrogen sulfide for 1 hr a t llOO°C. It was evident a t once that the resultant new product was brighter than the then current phosphors. Those who have heen involved in a search for novel materials in any field will know that it is an extremely rare occurrence to obtain something that is an improvement over existing products. Consequently there was some interest in identifying this new system, and in optimizing its performance in color kinescopes. Chemical analysis and X-ray powder diffraction studies showed that the oxide had been only partially converted to sulfide. As a result the host lattice was not yttrium sulfide hut yttrium oxysulfide, so the new phosphor was actually Y202S:Eu. Initial investigations indicated that this material was a line emitter with the principal emission a t 62fi0 A; the color (red to orange) and efficiency were dependent on the europium content; the cathodoluminescent efficiency was some 30% higher than the existing (zinc, cadmium) su1fide:silver standard. As recognition, Martin Royce was presented with the 1967 David Sarnoff award in Engineering. Although a new phosphor had been discovered, this was not the end of the project. There is a large gap between making the first few grams of sample in the laboratory and manufacturing ton quantities by a process which is commercially viable. From its inception in September 1964, until its commercial release in February 1967, Royce and his colleagues in various parts of RCA were involved in a development program, with the objective of ohtaining a convenient means of manufacturing Y202S:Eu having a specified luminous efficiency, color, and particle size. One important task was the need to eliminate HzS in the firing stage. Hydrogen sulfide is relatively expensive; its use requires specialized furnace equipment a t high cost. The process finally chosen for the large-scale production of YzOrS:0.048 Eu involved a solid state reaction in which oxides of yttrium and europium were fired in a flux mixture composed of sodium carbonate, sulfur, and various additives (11). At the time of its introduction in color receivers yttrium oxysulfide was some 40% brighter than the yttrium vanadate which it replaced (12). Over the years Yz02S:Eu has proved very stable to processing and reclamation conditions employed in color kinescopes (13). Today yttrium oxysul-
fide:europium is used almost exclusively as the red component in color television. Shrader and Yocom (14) have developed solid solutions of (Y,La)?O?S:Eu as phosphors; rare earth oxysulfides activated with various'lanthanons are now a useful family of cathodoluminescent materials (15). Notice that many of these compounds have proved of henefit in other applications, such as display devices, X-ray scintillation counters, fluorescent lamps, fundamental studies on electronic structures of rare earth ions. As an aside, it might he worthwhile pointing out that as long ago as 1947 Pitha and coworkers (16) prepared lanthanum oxysulfide containing either Pb-In or Pb-Eu as coactivators. Unfortunately these workers emoloved ultraviolet. rather than cathode rav. excitation. i t ;meeting held during April 1954, ~ e n d e r s o n(17) emphasized that luminescence was larrelv an exnerimental science, if not a craft. More recently s&; (18) has expressed a similar sentiment. "Research, development, and technology of phosphors is today still a combination of art, intuition, and fundamental knowledge." Althourh phosphors were the very first of the electroni~allyactive-mater
