Process for the preparation of bismuth germanate of formula Bi4 Ge.sub. O12
BACKGROUND OF THE INVENTION 
     The present invention relates to a process for the preparation of stoichiometric bismuth germanate of formula Bi 4  Ge 3  O 12 . More specifically, it relates to the preparation of a polycrystalline powder of bismuth germanate Bi 4  Ge 3  O 12  having purity and homogeneity characteristics appropriate for the production of bismuth germanate monocrystals usable as scintillators. 
     Hitherto, bismuth germanate powder has been prepared by using dry methods generally based on oxides of bismuth and germanium, preferably working in a fluidized bed or a rotary oven at temperatures above 600°/700° C. 
     On adopting this procedure, there is only a partial synthesis and the yields obtained are generally approximately 65 to 85%. This poor yield is on the one hand due to the limited acid--base interaction between germanium oxide GeO 2  and bismuth oxide Bi 2  O 3  and on the other hand to the appearance of GeO 2  in quadratic form and which is remarkably inert. Thus, at the end of the operation bismuth germanate Bi 4  Ge 3  O 12  is obtained, which is accompanied by other compounds, such as Bi 12  GeO 20 , hexagonal GeO 2  and quadratic GeO 2 , which leads to problems for the subsequent production of high purity bismuth germanate monocrystals. 
     SUMMARY OF THE INVENTION 
     The present invention specifically relates to a process for the preparation of bismuth germanate making it possible to obviate this disadvantage and directly obtain a stoichiometric polycrystalline bismuth germanate powder. 
     The present invention specifically relates to a process for the preparation of bismuth germanate of formula Bi 4  Ge 3  O 12 , wherein it comprises the following successive stages: 
     (a) introducing into an aqueous solution at least one bismuth compound able to release soluble bismuth complexes in solution and at least one germanium compound able to release soluble germanium complexes in solution, 
     (b) bringing the pH and temperature of the aqueous solution in the stability range of the Bi 4  Ge 3  O 12  phase to value such that the bismuth compound and germanium compound simultaneously and respectively release into the solution said soluble bismuth complexes and said soluble germanium complexes in an adequate concentration, 
     (c) maintaining the pH and temperature of the solution at values such that said bismuth and germanium complexes react with one another to form a bismuth germanate Bi 4  Ge 3  O 12  precipitate in the solution, 
     (d) then bringing the pH of the aqueous solution to values such that the concentration in solution of the bismuth complexes becomes negligible, whereas the germanium compound can release soluble complexes, 
     (e) separating the polycrystals of said solution to recover the precipitate of bismuth germanate Bi 4  Ge 3  O 12  obtained. 
     The process according to the invention thus consists of effecting the synthesis in an aqueous medium by proportionally dissolving in the solution the top-up bismuth and germanium compounds, so as to obtain adequate concentrations of the bismuth complexes and the germanium complexes, which react with one another in solution to form a bismuth germanate precipitate. 
     Preferably, at the end of the reaction, the pH of the aqueous solution is progressively brought to value such that the concentration in solution of the bismuth complexes becomes negligible, whereas the germanium compound can release soluble complexes. Thus, by operating with an excess of the germanium compound, it is possible to ensure that all the bismuth has dissolved and reacted with the germanium to form bismuth germinate. 
     By calculating the excess of the germanium compound relative to the stoichiometry in such a way that it appears again in solution in the form of soluble complexes, at the end of the reaction the solid phase is only constituted by a precipitate of bismuth germinate polycrystals, which can easily be separated from the solution, e.g. by decanting. 
     Preferably, the thus separated bismuth germinate polycrystals undergo washing with water, which inter alia eliminates the possible germanium oxide precipitate due to the introduction of an excessive excess of germanium compound. 
     The synthesis process involves several interdependent parameters, including the nature of the compounds used, the temperature and the pH. 
     The bismuth supply compounds used are those able to release soluble complexes under the conditions referred to in stages (b) and (c) of the inventive process. Among the latter, reference can be made to hydrated bismuth nitrate Bi(NO 3 ) 3 ,5H 2  O or a bismuth oxide. 
     The germanium supply compounds used are those able to release soluble complexes under the conditions referred to in stages (b) and (c) of the inventive process. Among these reference can be made to amorphous germanium oxide, hexagonal germanium oxide and ammonium germanates. 
     The temperatures used must be sufficiently low to simplify to the greatest possible extent the industrial preparation process of the high purity polycrystalline powder. However, they must not be to low so as to be able to contribute in the thermal stability range in aqueous solution of bismuth germanate Bi 4  Ge 3  O 12  to a synthesis reaction speed Kinetics adequate for meeting industrial requirements. Generally temperatures between 60° and 120° C. are used. 
     The pH conditions must ensure the relative stability of the bismuth germanate Bi 4  Ge 3  O 12  and simultaneously supply in solution concentrations of bismuth complexes and germanium complexes adequate to permit an appreciable reaction speed. The minimum concentrations of the complexes in solution are generally 0.5 g/liter. 
     During the synthesis reaction corresponding to (b) and (c) of the process according to the invention, the pH is preferably between 0.5 and 5. At the end of the synthesis, corresponding to (d) of the inventive process, the pH is preferably between 4 and 8. 
     The process according to the invention has numerous advantages compared with prior art processes. Low temperature synthesis inter alia makes it possible to better control the stoichiometry and microscopic homogeneity of the mixed compound formed, whereas in the hot reaction of the germanium oxide and the bismuth oxide there are problems of solid diffusion into the grains, of selective evaporation, of inhomogeneities due to the possible presence of numerous phases including Bi 12  GeO 20 , Bi 4  Ge 3  O 12 , hexagonal GeO 2 , and quadratic GeO 2  which is remarkably inert. 
     Moreover, provided that high purity products are used for forming the reaction medium, it is well known that low temperature synthesis in solution constitutes an excellent purification means. This process also makes it possible to use reduced quality starting products. 
    
    
     SPECIFIC DESCRIPTION OF THE INVENTION 
     Other features and advantages of the invention will become more apparent on reading the following non-limitative example. The example illustrates the preparation of bismuth germanate Bi 4  Ge 3  O 12  from bismuth nitrate of formula Bi(NO 3 ) 3 ,5H 2  O and hexagonal germanium dioxide GeO 2 . 
     300 g of bidistilled water are placed in a 500 cc beaker, placed on a heating magnetic stirrer. This is followed by the introduction of 150 g of Bi(NO 3 ) 3 ,5H 2  O and 24.9 g of hexagonal GeO 2 , which corresponds to a GeO 2  excess of approximately 2 g/liter with respect to the stoichiometric weight of the bismuth germanate. Stirring takes place, followed by heating to a temperature of 72° C. and bringing the pH to a value of 2.5 by the addition of ammonia, whilst checking the pH of the solution with a pH meter. Under these conditions, in solution concentrations of the bismuth complexes and the germanium complexes of several grams per liter are obtained. These soluble complexes react with one another to form a bismuth germanate, whilst releasing nitric acid, which is proportionally nutralized by adding ammonia, so as to maintain the pH at a value of 2.5, whilst ensuring a continuous release of bismuth and germanium complexes in solution and permitting the reaction to proceed. At the end of the synthesis, i.e., after two to three hours, all the bismuth has dissolved and the addition of ammonia leads to an increase in the pH. This addition is slowly continued to bring the pH to 5, which makes it possible to stabilize the bismuth germanate precipitated in the solution. 
     The solution is then cooled again and the mechanical stirring is interrupted to separate the bismuth germanate polycrystals by decanting. The solution is then eliminate and the polycrystals undergo two successive rinsing operations with hot water to eliminate the ammonium nitrate and germanium dioxide in excess. This leads to approximately 100 g of monophase bismuth germanate Bi 4  Ge 3  O 12  having satisfactory qualities for producing scintillators.