Rare-earth doped fluorides and process for preparing
View Patent ↗The present invention is directed to rare-earth doped solid state solutions of alkaline earth fluorides having novel luminescence properties, and to a process for preparing them. The invention is useful as identifying markers on articles. Other uses include phosphors for plasma displays, optical frequency multipliers, optical amplifiers and the like.
1. A process comprising combining an aqueous solution of an ammonium fluoride or hydrogen fluoride, or mixtures thereof, with one or more aqueous solutions of the salts of at least two alkaline earth metals, and an aqueous solution of a salt comprising a 3-valent rare earth metal cation, the amount of the rare-earth metal cation being in the range of 0.5 to 20 mol-% of the molar concentration of the total alkaline earth metal cation content, thereby forming a reaction mixture from which is formed a precipitate of a rare-earth doped solid state solution of alkaline earth fluorides represented by the formula
RE x (Ca a Sr b Ba c ) 1−x F 2+x−2y O y
wherein RE represents a three-valent rare-earth element, 0.005≦x≦0.20, and 0≦y≦0.2, a+b+c=1, with the proviso at least two of a, b, and c are not equal to zero; the rare-earth doped multi-valent metal fluoride being characterized by an aqueous solubility of less than 0.1 g/100 g of water.
2. The process of claim 1 wherein the amount of the rare-earth metal cation is in the range of 1 to 10 mol-%.
3. The process of claim 1 wherein the rare earth is Eu +3 , Er +3 , or Tb +3 .
4. The process of claim 1 wherein the alkaline earth metals are calcium and strontium.
5. The process of claim 4 wherein the mole ratio of calcium to strontium ranges from 3:1 to 1:3.
6. The process of claim 1 wherein the rare earth is Eu +3 in an amount in the range of 1 to 10 mol-%, and the alkaline earth metals are calcium and strontium in the mole ratio range of 3:1 to 1:3.
7. The process of claim 1 wherein the one or more aqueous solutions of salts of alkaline earth metals are characterized by a normality, the normality being less than 0.01 N.
8. The process of claim 1 wherein the one or more aqueous solutions of salts of alkaline earth metals are characterized by a normality, the normality being greater than 0.01 N.
9. The process of claim 8 wherein the normality is in the range of 0.1 to 0.8 normal.
10. A process comprising heating in the presence of oxygen a composition comprising a rare-earth-doped solid-state solution of alkaline earth fluorides represented by the chemical formula
RE x (Ca a Sr b Ba c ) 1−x F 2+x−2y O y
wherein RE represents a three-valent rare-earth element, 0.005≦x≦0.20, and 0≦y≦0.2, a+b+c=1, with the proviso at least two of a, b, and c are not equal to zero; to a maximum temperature in the range of 200° C. to 1000° C., followed by cooling, with the proviso that the composition so heated has not previously been exposed to the maximum temperature.
11. The process of claim 10 wherein one of a, b, or c=0.
12. The process of claim 11 wherein a=0.01 to 0.99, b=0.99 to 0.01, and c=0.
13. The process of claim 12 wherein a=0.25 to 0.75, b=0.75 to 0.25, and c=0.
14. The process of claim 10 wherein 0.05≦x≦0.15.
15. The process of claim 10 wherein the rare earth is Eu +3 , Er +3 , or Tb +3 .
16. The process of claim 10 wherein a=0.25 to 0.75, b=0.75 to 0.25, and c=0, 0.05≦x≦0.15, and the rare earth is Eu +3 .
17. The process of claim 10 wherein the rare-earth doped solid-state solution of alkaline earth fluorides is in the form of particles.
18. The process of claim 17 wherein the particles are nano-sized.
19. The process of claim 17 wherein the particles are micro-sized.