Phosphor and process for preparing same

A phosphor of the fluoride type mixed with potassium and yttrium doped with divalent europium, characterized in that it responds to the formula EQU K.sub.(1-x) Eu.sub.x Y.sub.3 F.sub.10+x wherein x is less than 0.06. The invention also concerns a process for preparing this product which may be used in a laser.

The invention concerns a new phosphor and a process for preparing, and 
applications of, the phosphor. 
Luminescence is a property of numerous substances which emit light under 
the effect of an excitation. The excitation may be of diverse origin, such 
as mechanical, electrical, thermal or others. Advantageously, the 
excitation is of luminous origin. Depending upon whether it is a 
transitory or a residual manifestation, one speaks of "fluorescence" or 
"phosphorescence". In the English literature, these bodies are generally 
designated "phosphors". 
The article of A.L.N. STEVELS published in the Journal of Luminescence 
12/13 (1976) pp. 97-107 describes in detail compositions useful as 
phosphors. 
It has already been proposed to use as phosphors fluorides mixed with 
potassium and yttrium doped with trivalent europium. As is known "doping" 
is an operation which consists in substituting in a part of a composition 
an ion given by a weak quantity of another ion called a "dopant" or 
"activator". In this case, the doping is effected on yttrium by mixing 
yttrium trifluoride, europium trifluoride and potassium fluoride in such 
proportions that after a convenient thermal treatment, one obtains a phase 
of formula: 
EQU KY.sub.3-x Eu.sub.x.sup.(+3) F.sub.10 
That phase is luminescent and exhibits the classic emission spectrum of the 
trivalent europium ion, said line spectrum being in the visible region. 
The invention is another type of phosphor also of the fluoride type mixed 
with potassium and yttrium, but doped with europium. It is characterized 
in that it corresponds to the formula: 
EQU K.sub.(1-x) Eu.sub.x Y.sub.3 F.sub.10+x 
The phase K.sub.(1-x) Eu.sub.x.sup.+2 Y.sub.3 F.sub.10+x has a crystal 
structure identical to that of the matrix KY.sub.3 F.sub.10 (similar to 
fluorite, CaF.sub.2) up to a value of x equal to 0.06, determined by 
radiocrystallographic analysis with the aid of a diffractometer C.G.R. 
.theta.60. The optimal luminescence is obtained where x is between 0.01 
and 0.03 and, in contrast to materials of formula K Y.sub.3-x 
Eu.sub.x.sup.+3 F.sub.10, one obtains an emission of intense monochromatic 
luminescence in the near ultraviolet about 3600 Angstroms. 
A process for preparing the phosphor of the type in which the matrix is 
constituted of potassium fluoride doped with divalent europium and yttrium 
trifluoride is characterized as follows: 
The divalent europium is introduced in the form of a composition selected 
from the group consisting of europium difluoride EuF.sub.2, europium 
trifluoride EuF.sub.3 reduced in situ, and a mixture of europium 
trifluoride and metallic europium in stoichiometric proportions. One heats 
the mixture to 900.degree. C. under reduction conditions. Finally the 
mixture is permitted to slowly cool and the composition is obtained at 
ambient temperature. 
If one slightly heats the mixture in a graphite crucible in a hydrogen 
atmosphere or in a platinum or gold ampoule, one obtains better results in 
comparison with the sealed ampoules of nickel. This last compound 
(nickel), owing to the fact of its reducing properties, causes the 
transformation of the divalent europium ion to the trivalent ion. 
The heating operation is advantageously carried out for 15 hours, after 
which slow cooling is permitted to obtain the composition.

The manner in which the invention is realized and the advantages which flow 
therefrom will be brought out better by the following example, which is 
intended to be indicative and not limiting, concerning the particular 
composition K.sub.(1-x) Eu.sub.x.sup.+2 Y.sub.3 F.sub.10+x wherein x=0.03. 
First one prepares all of the europium difluoride (EuF.sub.2) by reducing 
the trifluoride (EuF.sub.3) with hydrogen dried over phosphoric anhydrate. 
This reduction is effected by successive steps: (1) 2 hours under vacuum 
at 15.degree. C.; (2) 3 hours under hydrogen at 800.degree. C.; (3) 15 
hours under hydrogen at 1100.degree. C.; (4) 8 hours under hydrogen at 
1300.degree. C. 
For entirely preventing the process of hydration, the later manipulations 
are carried out in a box with gloves. 
One mixes with the obtained europium difluoride, in the desired proportions 
and in solid phase, potassium fluoride (KF) and yttrium fluoride 
preliminarily dehydrated at 500.degree. C. under a nitrogen stream, then 
again under a secondary vacuum. 
The mixture is then finely ground, for example in an agate mortar, and then 
is placed in an ampoule of nickel sealed with an oxyacetylene torch. The 
nickel, which provides the reduction properties, prevents the 
transformation of divalent europium to trivalent europium. The ampoule is 
then heated to 900.degree. C. for 15 hours in a tubular oven, and then 
slowly cooled just to ambient temperature. 
As for the composition KY.sub.3 F.sub.10 doped with trivalent europium, the 
product obtained is a non hygroscopic white powder with uniform melting 
(at about 1000.degree. C.) and without polymorphic transformation. This 
permits consideration of the growth of single crystals of important height 
by drawing, with very good chances of success. 
One makes optical studies of the composition under the following 
conditions: Excitation Spectrum: 
hydrogen lamp so that the continuous spectrum is in the ultraviolet; 
a Hilger & Watt "Monospek 1000" monochrometer (grating of 1200 traces per 
millimeter blazed at 5000 Angstroms and of linear dispersion of 8.2 
Angstroms per millimeter); 
the measurements are made from 4.degree. Kelvin to 550.degree. Kelvin with 
the aid of two cryostats, the first for working from 550.degree. to 
77.degree. K., the second therebelow. Emission Spectra: 
The same apparatus as above, but the excitation source is replaced by a low 
pressure mercury vapor lamp which emits radiation principally of 2537 
Angstroms. 
It has been determined that the composition according to the invention 
K.sub.1-x Eu.sub.x Y.sub.3 F.sub.10+x at temperatures lower than 
550.degree. K. (277.degree. C.), under ultraviolet excitation, presents an 
emission of luminescence whose spectrum of very high resolution is 
constituted of three ultraviolet lines which are very fine and very close 
together. The intensity of the principal component at 3585 Angstroms 
appears quite grandly up to a temperature in the neighborhood of 
400.degree. K. (127.degree. C.). 
In the case of the luminescent composition according to the invention 
K.sub.1-x Eu.sub.x Y.sub.3 F.sub.10+X, all of the energy emitted in the 
form of radiation at the time of the deexcitation is concentrated in a 
very narrow spectral domain, principally at the line 3585 Angstroms of 
which the larger by half-higher is lower by 10 Angstroms. In return the 
greater part of the other active compositions of divalent europium, the 
energy emitted in the form of radiation is principally dispersed in a 
spectral interval the larger one of several hundred Angstroms; one very 
feeble part is found as a narrow ultraviolet emission. Finally, in the 
composition where trivalent europium is incorporated in the same matrix 
KY.sub.3 F.sub.10, the energy emitted in the form of radiation is 
dispersed together with the visible spectrum in the grouping of lines. 
As elsewhere, the excitation of the luminescence is made by optical pumping 
with the strength and wide ultraviolet absorption band constituted by the 
energy levels of the excited configuration 4f.sup.6 -5d, then by 
non-radiative disexcitation across the level .sup.6 P.sub.7/2 of the 
configuration 4f.sup.7 from which is effectuated the emission of the 
luminescence. 
The emission of luminescence is favored in the composition according to the 
invention K.sub.1-x Eu.sub.x Y.sub.3 F.sub.10+x by: 
(1) the strong probability of absorption transitions occurring during the 
optical pumping; 
(2) the low energy variation between the lowest level of the configuration 
4f.sup.6 -5d and the emitter level .sup.6 P.sub.7/2 of the configuration 
4f.sup.7 ; 
(3) the absence of emission of luminescence upon leaving the levels of the 
configuration 4f.sup.6 -5d; 
(4) the elevated value of the probability of the non-radiative transitions 
between the levels 4f.sup.6 -5d and the emitter level .sup.6 P.sub.7/2 ; 
(5) the elevated value of the duration of life of the emitter level which 
is greater than 2.8 milliseconds at ambient temperature. 
The optical and crystallo-chemical characteristics of the composition 
according to the invention K.sub.1-x Eu.sub.x Y.sub.3 F.sub.10+x permits 
consideration of its utilization in an ultraviolet laser. The powdered 
form of this composition may be put to good use with all of the techniques 
calling for an intense monochromatic ultraviolet radiation (3585 A), such 
as fluorescence or photocopying. 
Other applications may be cited, such as those enumerated in the article in 
Journal of Luminescence, 13/13 (1976) pp. 97-107 cited above, notably as 
an initiator of polymerization, for medical lamps, sensitizers for other 
luminescent cations, etc. 
Finally, the cost of the compositions according to the invention is 
relatively moderate and their manufacture does not necessitate complex and 
onerous apparatus.