Abstract:
Single crystals, useful as laser emission compounds, having the formula ARF 4  wherein A is potassium, sodium, or lithium, and R is holmium, yttrium, erbium, or dysprosium are prepared by a direct congruent melt crystal growth process that yields crystals exhibiting substantially superior physical and chemical characteristics compared to those grown by conventional methods.

Description:
RELATED APPLICATIONS 
     This is the latest of a series of patent applications disclosing various applications of the concept of reactive atmospheric processing that have been filed by the inventors and others which have a common assignee. 
     In U.S. application Ser. No. 275,130, filed July 26, 1972, now U.S. Pat. No. 3,826,817, granted July 30, 1974, by A. C. Pastor and R. C. Pastor, the use of dissociated carbon halide gas to convert a metal salt to the corresponding metal halide is disclosed. 
     In U.S. application Ser. No. 334,179, filed February 20, 1973, now abandoned, by R. C. Pastor, a process whereby molten metal halides are passed through a bed of carbon particles to remove oxide and hydroxide impurities is disclosed. 
     In U.S. application Ser. No. 349,516, filed Apr. 9, 1973 by R. C. Pastor, the treatment of metal halide laser window surfaces by exposure to gaseous nascent halogens for the purpose of improving structural properties and optical transmission is disclosed. 
     In U.S. application Ser. No. 351,214, filed Apr. 16, 1973 By R. C. Pastor and A. Timper, we disclosed that alkali metal halides may be purified, prior to their use as starting materials for the growth of alkali metal halide crystals, by scrubbing the material in its molten form with gaseous nascent halogens which correspond to the halide anion. 
     In U.S. application Ser. No. 416,899, filed Nov. 19, 1973, now abandoned, by R. C. Pastor, an improved alkali metal halide purification method is disclosed whereby the crystal growth starting materials are purified by scrubbing with halide-source species in their vapor phase and cast in the form of an ingot to be used directly in subsequent halide crystal growth processes. 
     In U.S. application Ser. No. 446,581, filed Feb. 27, 1974, by R. C. Pastor, Kaneto Arita and Morton Robinson, an invention directed to the preparation of laser windows from single metallic component halide crystals purified by regrowth from the congruent melt is disclosed. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention is concerned with the preparation of single crystals from the congruent melt of dual component metallic fluorides for use as laser emission materials. 
     2. Description of the Prior Art 
     Crystals of the formula ARF 4  have been used as laser emission materials prior to this invention. LiYF 4 , in particular, has been grown by a process based on the accepted proposition that the material melts incongruently. &#34;Phase Equilibria in the System LiF-YF 3 , &#34; by R. E. Thoma et al., J. Phys. Chem. 65, 1096 (1961). This proportion of incongruency is extended to many rare earth compounds, LiF--RF 3 , where R ranges from Eu to Ho. &#34;The Rare Earth Halides,&#34; by R. E. Thoma, Progress in Science and Technology of the Rare Earths, Vol. 2, edited by LeRoy Eyring (Pergamon, 1966). Accepting the incongruency behavior, LiYF 4  has been grown. &#34;Single Crystal Growth and Some Properties of LiYF 4 ,&#34; by W. A. Shand, J. Cryst. Growth, 5, 143 (1969). However, the incongruent growth process has two outstanding drawbacks. The first is a forced slow rate of growth, necessitated by the composition mismatch, and the second is the change of growth temperature as a function of the amount of crystal formed from a given starting amount of melt. In the case of LiFY 4 , incongruent growth is conducted on the Li rich LiYF These  drawbacks are avoided in our congruent growth process which yields crystals of vastly improved quality at a substantially faster growth rate. 
     According to E. P. Chicklis et al. in Research and Development Technical Report TRECOM-00130F, Sanders Associates, Inc., January, 1973, entitled &#34;Development of Multiply Sensitized Ho:YLF as a Laser Material,&#34; crystals of αβ--YLF (LiY 1-x-y-z  Er x  Tm y  Ho z  F 4 ) are grown by the Top-Seeded Solution technique, a modification of the Czochralski technique, in an atmosphere of highly purified helium. In the system LiF--YF 3 , incongruently melting LiYF 4  crystallizes as the primary phase over the composition range 51-75 mol % LiF. Although in the corresponding system LiF--ErF 3 , LiErF 4  is congruently melting, in the mixed crystal system LiYF 4 .LiErF 4  the incongruent melting behavior of LiYF 4  prevails. Thulium and holmium are not present in sufficient amounts to effect the observable phase equilibrium. E. P. Chicklis et al &#34;Development of Multiply Sensitized Ho:YLF as a Laser Material&#34;, Research and Development Technical Report TRECOM-0013-F Sanders Associates, Inc, January 1973. 
     In U.S. Pat. No. 3,649,552 it was disclosed, by one of the applicants herein, that an HF purge during crystal growth would lead to the production of ultra high purity rare earth fluoride crystals and solid state solutions of rare earth fluorides and alkaline earth fluorides. The process disclosed has become known as the &#34;Reactive Atmosphere Processing&#34; method. We have found that this process can be utilized to remove the peritectic character of ARF 4  compounds, thereby allowing one to promote crystal growth from the congruent melt and form dual component single crystals of rare earth fluorides and alkali fluorides. This discovery is contra to published literature and commonly accepted beliefs of those who are skilled in the art of rare earth halide crystal growth. 
     The &#34;Reactive Atmosphere Process&#34; was applied to the growth of calcium fluoride-rare earth fluoride fluorescent compounds in U.S. Pat. No. 3,769,230. The present invention discloses that the techniques utilized to grow Ca 2  Er 5  F.sub. 19 can be adopted to grow congruent crystals of equimolar alkali metal-rare earth fluorides such as LiYF 4  contra to published literature which states that these materials melt incongruently. 
     SUMMARY OF THE INVENTION 
     We have discovered that a reactive atmosphere processing method can be used to promote the growth of single crystals from the congruent melt of equimolar alkali metal fluorides and rare earth fluorides thereby forming compounds having the formula ARF 4 , wherein A is lithium and R is holmium, yttrium, erbium or dysprosium. Single crystals of these compounds have, heretofore, been grown from the peritectic melt with all of the problems associated with this method. Crystals grown from the congruent melt by our process are substantially superior to those grown by conventional crystal growth techniques. This congruent melt technique, while contrary to the teachings of prior art in ARF 4  crystal growth, has been shown to facilitate greater control over the growth process whereby crystals of increased purity are obtained. As such, these crystals are particularly useful as laser emitters and also as laser window materials for 2-6 μm range laser applications. 
    
    
     DESCRIPTION OF THE INVENTION 
     We have found that the peritectic like breakdown during cool down in the preparation of equimolar alkali metal rare earth halide crystals can be attributed to the presence of OH -  impurities which promote an incongruent melt behavior. Suppression of the OH -  content of halide melts tends to raise the melting points and render possible congruent melt behavior. 
     The OH -  may be suppressed or removed entirely by causing the ratio of P H .sbsb.2 /P HX  to be as low as possible in the following exchange: 
     
         X.sup.-(c) + H.sub.2 O(g) ⃡ OH.sup.-(c) + HX(g) (1) 
    
     where (c) stands for the melt or crystal at the operating temperature of congruent growth and (g) for the vapor phase. We can write the equilibrium relation, ##EQU1## which shows that the process parameter, P H .sbsb.2O /P HX , of crystal growth is the key to the control of C(c), or the OH -  content. The equilibrium constant, K, is fixed once the nature of the metal halide, HX, is specified because at a total pressure of one atmosphere the melting point (operating temperature for congruent growth) is also fixed. The limit to the choice in the value of P HX  is set by the solubility of HX in (c), to avoid the problem of trapped bubbles in the crystal. The value of P H .sbsb.2O depends on the dewpoint of the source HX and the outgassing of surfaces in the growth apparatus. 
     In practice, we have found that compounds, of the formula ARF 4 , can be prepared directly from the congruent melt according to the following equation: 
     
         AF + RF.sub.3 = ARF.sub.4                                  (3) 
    
     wherein A is lithium and R is holmium, yttrium, erbium, or dysprosium. Stoichiometric amounts of each starting material are placed into a suitable crucible, such as vitreous carbon or platinum, and inserted into a furnace. The furnace is then thoroughly outgassed via the application of a vacuum. Dry helium is introduced at a rate of approximately 2 STP-liters/minute whereby the furnace is then returned to atmosphere conditions. A reactive atmosphere is introduced by causing HF to flow into the furnace at approximately 3 STP-liters/hr. The temperature of the furnace is then increased to approximately 100°C above the anticipated melting point of ARF 4  and allowed to remain at that temperature for approximately 24 hours whereby the HF reacts with residual oxides found in the AF or RF 3  srarting materials. The constant flow of HF also causes the furnace to be purged of any H 2  O vapor or oxygen which could react with AF or RF 3  to form additional oxides. Approximately 6 hours is allowed for the molten equimolar mixture of AF and RF 3  to react in the presence of HF and the temperature is then lowered to the proximity of the anticipated melting point of ARF 4 . Crystal nucleation is initiated by lowering or causing a platinum rod to immerse into the melt as taught by Czochralski. The nucleated crystal is then slowly withdrawn to facilitate a single crystal growth. We obtain excellent results with LiYF 4 , for example when a pull rate of from 3-12 mm/hr is used with a nucleation rod rotation rate of from 15-30 rpm. 
     After a crystal of the desired length has been obtained, the melt temperature and pull rate is then increased to pull the crystal from the melt. The temperature of the crystal and furnace is allowed to cool to room temperature prior to the termination of the HF flow and subsequent purge of the system with dry He. The crystal and rod may then be removed from the furnace and separated. 
     U.S. Pat. No. 3,769,230 issued to M. Robinson et al. on Oct. 30, 1973 contains detailed drawings of the Vertical Bridgman apparatus which is also applicable to the growth of ARF 4  crystals. Contra to well established beliefs, the equimolar mixture of fluorides of alkali metals and rare earth metals exhibit congruent melting characteristics when treated in a manner disclosed by this patent. This alteration of or change in melting behavior is believed to have been caused by the ability of this process to eliminate anion inpurities and significantly lower the OH -  concentration in the materials. 
     This process is also effective for growing single crystals of the formula AR&#39;F 4  wherein R&#39; is a combination of rare earth ions. 
     In view of the above disclosure and the utility of preparing large ultra pure single crystals of the formula ARF 4 , the scope of our invention will now be understood.