Novel cerium/lanthanum/terbium (LaCeTb) mixed phosphates, especially useful in luminescence for the production of green phosphors, have the formula (I): EQU La.sub.x Ce.sub.y Tb.sub.1-x-y PO.sub.4 in which x ranges from 0.4 to 0.6 and x+y is greater than 0.8, display a lightness (L*) of greater than 98% after calcination at a temperature above 700.degree. C. in air, and the cerium (IV) and/or terbium (IV) contents of which are very low, even after calcination in air at a temperature above 500.degree. C.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to rare earth (RE) mixed phosphates 
especially useful for the production of phosphor materials, and, more 
particularly to cerium lanthanum terbium mixed phosphates especially 
useful as green phosphor precursors, as well as to a process for the 
production of such precursors. 
2. Description of the Prior Art 
Since 1970 it has been known to this art that rare earth mixed phosphates 
and especially those of cerium, lanthanum and terbium exhibit an 
advantageous luminescence property. Thus, numerous cerium lanthanum 
terbium phosphates, also designated "LaCeTb phosphates" have been 
developed with different concentrations of lanthanum, cerium and terbium, 
as have various processes for the production thereof. 
Such processes for the production thereof can be divided into two basic 
categories, namely, processes which entail a "dry route", and processes 
which entail a "wet route". 
The dry-route processes, described especially in JP 62/007,785, WO 
82/04,438, JP 62/089,790, JP 59/179,578 and JP 62/000,579 include forming 
a mixture of rare earth oxides or phosphating a rare earth mixed oxide by 
calcination in the presence of diammonium phosphate. 
The "wet-route" processes, described especially in JP 57/023,674, JP 
60/090,287 and JP 62/218,477, entail a direct synthesis of a rare earth 
mixed phosphate or of a mixture of rare earth phosphates by digestion of a 
solid compound (carbonate, oxide) with H.sub.3 PO.sub.4 to precipitate the 
phosphates. 
U.S. Pat. No. 3,507,804 describes a process for the production of lanthanum 
terbium double phosphate by precipitating the phosphate from a solution of 
rare earth nitrates and adding phosphoric acid thereto. However, the 
phosphate or phosphates obtained are very difficult to filter. 
These different processes produce mixed phosphates requiring, for their 
application in luminescence, a heat treatment at a high temperature, 
approximately 1,200.degree. C., under a reducing atmosphere. Indeed, in 
order that the LaCeTb phosphate provide a green phosphor, the cerium and 
the terbium must be in the 3+ oxidation state. 
In addition, the efficiency of the phosphatation must be close to 100% in 
order to obtain a phosphor which is as pure as possible and to obtain a 
maximum emission efficiency, and this requires numerous precautions and a 
relatively long treatment in the case of the "dry-route" processes. 
SUMMARY OF THE INVENTION 
Accordingly, a major object of the present invention is the provision of 
improved rare earth (cerium, lanthanum, terbium) mixed phosphates which 
can be converted into green phosphor by calcination at high temperature 
and under a nonreducing atmosphere, together with an improved process for 
the production of such mixed phosphates by a wet route resulting in a rare 
earth mixed phosphate of high purity and which improved process avoids or 
conspicuously ameliorates the above disadvantages and drawbacks to date 
characterizing the state of this art. 
Briefly, the present invention features cerium lanthanum terbium mixed 
phosphates having the general formula: 
EQU La.sub.x Ce.sub.y Tb.sub.1-x-y PO.sub.4 (I) 
in which y+x is greater than 0.8, and x ranges from 0.4 to 0.6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
More particularly according to the present invention, the subject 
phosphates are characterized in that they exhibit a cerium (IV) and 
terbium (IV) concentration which is very low after calcination at a 
temperature above 500.degree. C. in air. 
Such low proportion is demonstrated by a colorimetric test that includes 
determining the characteristic coordinates of the color of the substance 
in the CIE 1976 system (L*, a* , b* ) as defined by the International 
Commission on Illumination, and listed in the French Standards (AFNOR) 
compendium colorimetric color No. X08-012 (1983). These coordinates are 
determined using a colorimeter marketed by the Pacific Scientific Company. 
Thus, after calcination at 700.degree. C. in air, the LaCeTb phosphates of 
the invention exhibit a lightness, denoted by the L* coordinate, higher 
than 98% and advantageously ranging from 99% to 99.9%. 
This L* coordinate makes it possible to measure the white color of the 
product, which is directly related to the presence of colored species in 
the product, such as cerium and/or terbium in the 4+ oxidation state. 
The phosphates of the invention also exhibit a* and b* color coordinates 
ranging from approximately -0.5 to 0.5, preferably from -0.25 to 0.50. 
These excellent values of lightness L* , a* and b* are also obtained 
employing calcination temperatures ranging from 700.degree. to 900.degree. 
C. This reflects an exceptional stability of the compounds of the 
invention in a nonreducing atmosphere. 
The presence or absence of cerium and/or of terbium in the 4+ oxidation 
state can also be assessed by a surface analysis of the phosphates by the 
XPS technique described especially in Praline et al, Journal of Electron 
Spectroscopy and Related Phenomena, 21, pp. 17 to 30 and 31 to 46 (1980). 
Thus, in the energy range corresponding to the 3d electrons of cerium, the 
phosphates of the invention exhibit two doublets characteristics of the 3+ 
oxidation state and the absence of a satellite situated at 32.7 V from the 
first peak, characteristic of a 4+ oxidation state. 
The mixed phosphates of the invention have cerium and terbium atoms which 
are stabilized in the 3+ oxidation state, permitting any atmosphere 
whatever to be employed during the calcination. As the cerium and the 
terbium are in the 3+ state, the mixed phosphates of the invention make it 
possible to produce green phosphors having a high luminescence property. 
Furthermore, the present invention also features cerium lanthanum terbium 
mixed phosphates of general formula (I) described above, containing, 
before calcination at a temperature above 500.degree. C., not more than 5% 
by weight of ammonium ions, advantageously less than 2%. 
These ammonium ions are removed by thermal decomposition or evaporation 
during the calcination of the product. 
The mixed phosphates of the invention exhibit a specific surface area 
greater than 50 m.sup.2 /g after heat treatment at a temperature below 
300.degree. C. 
This specific surface area, measured by the so-called B.E.T. method, which 
is determined by nitrogen adsorption in accordance with ASTM standard 
D3663-78 established from the Brunauer-Emmett-Teller method described in 
The Journal of the American Chemical Society, 60, 309 (1938), 
advantageously ranges from 50 m.sup.2 /g to 100 m.sup.2 /g. 
The mixed phosphates of the invention may also contain additives, such as 
other rare earths or metallic elements. 
This invention also features a process for the preparation of the cerium 
lanthanum terbium mixed phosphates described above. 
This process comprises mixing a solution of soluble lanthanum, cerium and 
terbium salts with phosphate ions and controlling the pH of the 
precipitation medium at a pH value higher than or equal to 2, then 
filtering off and washing the precipitate and optionally drying it to 
provide a mixed phosphate of general formula (I) which may contain 
absorbed ammonium groups. This mixed phosphate may then be calcined at a 
temperature above 500.degree. C. and under any atmosphere whatever to 
provide the compound of general formula (I). 
According to the invention, the precipitate is maintained in the 
precipitation medium, after completion of the mixing, for a period of time 
ranging from approximately 15 min to approximately 10 hours, when the pH 
of the precipitation medium ranges from approximately 2 to 6. This aging 
period, typically designated "maturation", permits a rearrangement of the 
precipitated species. The product obtained is thus filterable. 
Although the precipitate obtained when the pH of the precipitation medium 
is controlled at a value above 6 is filterable, this filterability can be 
improved by a maturation of the precipitate equivalent to that used when 
the pH of the precipitation medium ranges from 2 to 6. 
This maturation stage can be carried out at any temperature whatever, for 
example at a temperature ranging from 15.degree. C. to 100.degree. C., 
advantageously at the precipitation temperature, preferably with stirring. 
The pH of the medium may be controlled or may be allowed to change freely. 
By "controlled pH" is intended a period of maintaining the pH at a certain 
value by addition of basic or acidic compounds or of a buffer solution. 
The pH of the medium will thus vary by approximately one pH unit around 
the fixed target value. 
In the present invention, this pH control is advantageously effected by the 
addition of a basic compound, as more fully described below. 
The filtered product may be especially dried by processes employing 
spraying and drying of the droplets, such as an atomization technique. It 
is thus possible to produce a product which has particles of a mean 
diameter which advantageously ranges from approximately 1 .mu.m to 10 
.mu.m with a narrow particle size distribution. 
Thus, the process for the preparation of mixed phosphates according to the 
invention permits the calcination of this product to be carried out under 
any atmosphere, whether reducing or nonreducing, or even an oxidizing 
atmosphere. The calcined product obtained will contain cerium and terbium 
in the 3+ oxidation state, cerium and terbium in the 4+ oxidation state 
being present only in trace amounts, or being completely absent. 
The solution of lanthanum, terbium and cerium salts may contain other metal 
salts such as, for example, salts of other rare earths, to provide LaCeTb 
phosphates doped with other elements. 
The control of the pH of the precipitation medium may be carried out by the 
addition of a compound during the mixing of the solution of the rare earth 
ions and of the phosphate compound. This compound is generally a basic 
compound. 
Thus, if the phosphate is added to the rare earth solution, the basic 
compound is added simultaneously with the phosphate to control the pH at a 
value above 2. 
Similarly, when the solution of rare earth compounds is added to a 
phosphate solution, the basic compound is added simultaneously to control 
the pH at a value which is higher than 2 and advantageously constant. 
This control of the pH at a value above 2 and advantageously ranging from 2 
to 10 makes it possible to obtain a nongelatinous and filterable 
precipitate of LaCeTb phosphate with or without a maturation stage, 
depending on the pH range. 
The precipitation is preferably carried out in an aqueous medium at a 
temperature which is not critical and which advantageously ranges from 
room temperature (15.degree. C.-25.degree. C.) to 100.degree. C. 
The concentrations of the rare earth salts are not critical. Thus, the 
total rare earth concentration, expressed as rare earth oxide, 
advantageously ranges from 0.01 mol/l to 2 mol/l. 
The rare earth salts which are suitable per the present invention are 
especially the salts which are soluble in an aqueous medium, such as, for 
example, nitrates, chlorides, acetates, carboxylates or a mixture thereof. 
The phosphate ions mixed with the rare earth solution are contributed by 
pure or dissolved compounds such as, for example, phosphoric acid, alkali 
metal phosphates or those of other metallic elements which yield a soluble 
compound with the anions associated with the rare earths. 
In a preferred embodiment of the invention, the phosphate ions are added in 
the form of ammonium phosphate because the ammonium cation will decompose 
during the calcination, thus enabling a mixed phosphate of high purity to 
be obtained. 
Among the ammonium phosphates, diammonium or monoammonium phosphates are 
the preferred compounds of the invention. 
The phosphate ions are added such as to provide a PO.sub.4 .tbd./RE molar 
ratio higher than 1 and advantageously ranging from 1.1 to 3. 
Exemplary basic compounds which are suitable according to the invention 
include the metal hydroxides or ammonium hydroxide, or any other basic 
compound, the species of which will not form any precipitate upon being 
added to the reaction mixture by combining with one of the species present 
in this mixture, and permitting a control of the pH of the precipitation 
medium. 
In another preferred embodiment of this invention, this basic compound is 
advantageously a compound which is easily removable either with the liquid 
phase of the reaction mixture and washing of the precipitate, or by 
thermal decomposition when the mixed phosphate is calcined. 
Thus, the preferred basic compound of the invention is ammonia, 
advantageously in the form of an aqueous solution thereof. 
The cerium lanthanum terbium mixed phosphates of the invention display 
luminescence after having been subjected to a heat treatment generally 
above 500.degree. C. and advantageously from 700.degree. C. to 
1000.degree. C. 
However, such luminescence can be improved even further by a heat treatment 
employing "fluxes", these treatments being conventionally employed in the 
art for the production of phosphors. 
They permit, inter alia, an adaptation of the phosphor to the intended use. 
These phosphors based on LaCeTb phosphates are especially useful in lamps. 
In order to further illustrate the present invention and the advantages 
thereof, the following specific examples are given, it being understood 
that same are intended only as illustrative and in nowise limitative. 
EXAMPLE 1 
To a solution of monoammonium phosphate heated to 80.degree. C., a solution 
of rare earth nitrates was added, containing an overall 0.2 mol/l 
concentration of rare earths, and comprised of: 0.112 mol/l of 
La(NO.sub.3).sub.3, 0.062 mol/l of Ce(NO.sub.3).sub.3 and 0,026 mol/l of 
Tb(NO.sub.3).sub.3. 
The PO.sub.4 .tbd./RE molar ratio was 1.5. The pH during the precipitation 
was controlled at 2 by adding aqueous ammonia. 
The reaction mixture was subjected to a maturation of 4 hours at 80.degree. 
C. 
The precipitate was then recovered by filtration and washing with water. 
The recovered product was a white powder whose characteristics were as 
follows (after drying at 110.degree. C.): 
(a) X-ray spectrum (see Figure of Drawing), 
(b) specific surface area: 65 m.sup.2 /g, measured at 200.degree. C., 
(c) NH.sub.4.sup.+ ion content: 1% by weight 
The powder had the formula Za.sub.0.56 Ce.sub.0.31 Tb.sub.0.13 PO.sub.4. 
This powder was subjected to a heat treatment at 900.degree. C. in air. 
X-ray analysis evidenced that the product was an LaCeTb orthophosphate of 
monoclinic crystalline structure. This product was compact aggregates of 
approximately 250 nm formed by the aggregation of elementary crystallites 
ranging from 20 to 150 nm in size. 
EXAMPLE 2 
A solution of diammonium phosphate was added to an aqueous solution 
containing 0.50 mol/l of rare earth nitrates to provide a PO.sub.4 
.tbd./RE molar ratio of 1.5. This reaction was carried out at 25.degree. 
C. The pH of the precipitation medium was controlled at 8.4 by adding 
aqueous ammonia. 
After recovery of the precipitate, washing and drying, the product obtained 
exhibited physicochemical characteristics similar to those described in 
Example 1. The product calcined at 900.degree. C. in air was evaluated for 
luminescence. 
EXAMPLE 3 
The procedure of Example 1 was repeated, but the solution of rare earth 
nitrates had a concentration of 2 mol/l. 
The morphology of the product obtained was similar to that from Example 1 
with crystallite sizes ranging from 80 to 200 nm. 
Luminescence Test 
The LaCeTb phosphates produced by the process of the invention were 
analyzed to determine their luminescence property. 
The luminescence was determined by means of a "Bentham".RTM. spectrometer 
making it possible to obtain an emission spectrum of the specimen excited 
with a low-pressure mercury vapor lamp at a wavelength of 254 nm. The 
calculation of the integral of the intensities of emission between two 
wavelengths is designated "brightness." 
These two wavelengths were 540 and 560 nm. 
The results obtained after heat treatment of the LaCeTb phosphates of 
Examples 1 to 3 at a temperature of 900.degree. C. for 8 hours are 
reported in the Table below: 
TABLE 
______________________________________ 
EXAMPLE 1 2 3 
______________________________________ 
Brightness 111 122 147 
(ua) 
______________________________________ 
Colorimetry Test 
After calcination at 700.degree. C. for 8 hours, the LaCeTb phosphate of 
Example 1 was tested in the colorimeter described above according to the 
procedure indicated in French AFNOR standard No. X08-012 of 1983. 
The L*, a* and b* coordinates of the CIE system (L*, a* , b*) were 
determined and had the following values: 
L*=99.0% 
a*=-0.1% 
b*=-0.1% 
With the same product calcined at 900.degree. C. for 8 hours, the results 
were as follows: 
L*=99.4% 
a*=-0.1% 
b*=+0.4% 
While the invention has been described in terms of various preferred 
embodiments, the skilled artisan will appreciate that various 
modifications, substitutions, omissions, and changes may be made without 
departing from the spirit thereof. Accordingly, it is intended that the 
scope of the present invention be limited solely by the scope of the 
following claims, including equivalents thereof.