Abstract:
A method of producing boracites is disclosed in which an aqueous mixture of a bivalent metal halide and a group 1a element borate is heated under an inert atmosphere.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a new and improved method for the synthesis of boracites. 
     Boracites are a class of compounds of the general formula M 3  B 7  O 13  X where M is a divalent metal and X is a monovalent anion. 
     Boracites exhibit pyroelectric and ferroelectric properties and are useful for example as infrared detectors. 
     In the past the boracites have been synthesized generally by vapor transport methods, for example, by the method as described by Schmid, H. J. Phys. Chem. Solids 26, 973, 1965. However this method has the disadvantages of producing a low yield of the boracite, causing the boracites to be contaminated by complex metal borates and requiring a complex and sophisticated experimental arrangement. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a new and more improved method for the synthesis of boracites. It is further an object of this invention to provide a new method for the synthesis of boracites which produces improved yields. It is still a further object of this invention to provide a method for the production of boracites which produce boracites essentially free of complex metal borates contamination. Finally it is object of this invention to provide a method of producing boracites wherein a less complex arrangement may be employed and in which there is no need for drying the reactants. 
     According to the invention the boracites are prepared by heating together in an aqueous medium a mixture of a metaborate of an element of Col. 1A of the Periodic Table and a metal halide. 
     The reaction involved in the method of the instant invention is as follows: 7GBO 2  +3MX 2  +H 2  O→M 3  B 7  O 13  X+5GX+2GOH. However when G is hydrogen, water need not be employed in the reaction. 
     The reaction is carried out under an inert atmosphere at a temperature of 200° to 300° C. and a pressure of about 5 to 33 atmospheres. 
     G may be any element of Col. 1A of the Periodic Table, however, it preferably is a metal such as lithium, sodium or potassium. X is a halogen of the group consisting of bromine, iodine and chlorine. 
     M is bivalent metal such as nickel, zinc, copper, iron manganese, cobalt or magnesium. 
     As the inert atmosphere there may be employed for instance atmospheres of argon, helium, or nitrogen. 
     The method of the instant invention has the advantages over the prior art method of requiring less sophisticated equipment, the use of lower temperatures for instance 200°-300° C. as compared to temperatures of about 1000° C. and the preparation of boracites of a very high yield that are essentially free of metal borate contamination. Further the method of the instant invention requires much shorter reaction times then the method of the prior art. Finally the method of the instant invention is much more versatile than that of the prior art in that it is applicable to the metals which also exhibit monovalent oxidation states, such as copper. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will now be described in greater detail with reference to the following example. 
     EXAMPLE 
     34.8 g of LiBO 2 , 81.8 g of NiBr 2 .3H 2  O and 36.4 g of H 2  O were placed in a 250 cc paar pressure reaction vessel. The reaction mixture was heated to a temperature of 270° C. and a pressure of 300 psi for 60 hours and then rapidly cooled to room temperature. The product was washed with boiling water to dissolve unreacted material as well as water soluble by-products. The boracite yield was estimated at 87%. Some nickel borate (about 5%) of material which was estimated to be NiB 2  O 4 . (H 2  O)X was also found. When the reaction was repeated by the employment of a deoxygenated water solvent and employing an inert atmosphere such as argon the yield of the boracite was increased to 95%. 
     The boracite produced by this example was found to have the formula Ni 3  B 7  O 13  Br. 
     The following additional boracites were prepared according to the method of the example, the details and the results of which syntheses are shown in the following table. 
     
                       TABLE______________________________________RESULTS OF BORACITE SYNTHESES            Reactants (g)Bor-                 t                         Yieldacite T (K)  P(atm.) (h) Atm.  MX.sub.2                               LiBO.sub.2                                     H.sub.2 O                                          (%)______________________________________Ni-Br 540    11      60  air   65.6 34.8  1.8  38Ni-Br 540    11      60  argon 65.5 34.8  1.8  55Ni-Br 540    11      60  oxygen                          65.5 34.8  1.8  12Ni-Br 540    26      60  air   65.5 34.8  21.8 87Ni-Br 540    26      60  argon 65.5 34.8  21.8 95Ni-Br 490    20      40  argon 65.5 34.8  21.8 95Zn-Br 570    33      24  argon 135.2                               69.6  16.0 98Zn-Cl 525    18      24  argon 81.8 69.6  16.0 95Cu-Cl 525    18      26  argon 80.6 69.6  16.0 91Mn-Cl 500     5      24  argon 75.6 69.6  14.0 95Co-Br 475     6      18  air   66.2 34.8  9.0  --Mg-Cl 475     5      18  air   57.7 17.4  9.0  nm*Mn-I  475     5      18  air   45.0 17.4  9.0  nm*______________________________________ T = reaction temperature in °K., P = pressure employed in atmosphere and t = reaction time in hours. * = not measured 
    
     As will be noted from the results shown there is a very significant improvement in the yield of the boracites when an inert atmosphere is employed as compared with the results when air or oxygen is employed.