Method for preparing rare-earth halide blocks
The invention relates to a method of preparing a polycrystalline block of a halide of formula A e Ln f X (3f+e) in which Ln represents one or more rare earths, X represents one or more halogen atoms selected from the group consisting of Cl, Br and I, and A represents one or more alkali metals selected from the group consisting of K, Li, Na, Rb and Cs, e, which may be zero, being less than or equal to 3 f , and f being greater than or equal to 1, having a low water and oxyhalide content, in which the method comprises heating a mixture of, on the one hand, at least one compound having at least one Ln—X bond and, on the other hand, a sufficient amount of NH 4 X in order to obtain the oxyhalide content, resulting in a molten mass comprising the rare-earth halide, the heating being followed by cooling, and the heating, after having reached 300° C., never going below 200° C. before the molten mass has been obtained. The blocks thus produced allow very pure single crystals having remarkable scintillation properties to be grown.
1. A method of preparing a polycrystalline block of at least 10 g of a halide of formula A c Ln f X (3f+e) , wherein:
Ln represents one or more rare earth elements;
X represents one or more halogen atoms selected from the group consisting of C 1 , Br and I;
A represents one or more alkali metals selected from the group consisting of K, Li, Na, Rb and Cs;
e, which may be zero, is less than or equal to 3f;
f is greater than or equal to 1; and
the polycrystalline block comprises less than 0.1% by weight of water and less than 0.2% by weight of rare-earth oxyhalides;
the method comprising:
heating a mixture of at least one compound having at least one LN—X bond and NH 4 X and/or NH 4 X provided in a complex to obtain a molten mass comprising the rare-earth halide of the formula A e Ln f X (3f+e) ; and
cooling the molten mass to obtain the polycrystalline block;
wherein:
the NH 4 X and/or the NH 4 X provided in a complex is present in the mixture in an amount sufficient to provide the rare-earth oxyhalides content of less than 0.2% by weight;
heating the mixture comprises heating to a temperature that exceeds a melting point of the rare-earth halide of the formula A e Ln f X (3f+e) ; and
when the mixture is heated to a temperature of 300° C., a temperature of at least 200° C. is maintained until the molten mass is obtained.
2. The method as claimed in claim 1 , wherein:
the compound having at least one Ln—X bond is of a compound having the formula A r Ln s O u X r+3s−2u ;
r is from 0 to 2s;
s is greater than or equal to 1;
u is from 0 to s; and
the compound is optionally complexed with water or NH 4 X.
3. The method as claimed in claim 1 , wherein Ln atoms of the compound having at least one Ln—X bond are linked to atoms of X, oxygen or A.
4. The method as claimed in claim 2 , wherein heating the mixture comprises heating without decreasing temperature until the molten mass is obtained.
5. The method as claimed in claim 1 , wherein heating the mixture comprises carrying out a temperature hold to eliminate gaseous NH 4 X.
6. The method as claimed in claim 5 , wherein heating the mixture comprises heating at a rate of greater than 50° C./hour after completing the temperature hold.
7. The method as claimed in claim 6 , wherein heating the mixture comprises heating at a rate of greater than 100° C./hour after completing the temperature hold.
8. The method as claimed in claim 7 , wherein heating the mixture comprises heating at a rate of greater than 150° C./hour after completing the temperature hold.
9. The method as claimed in claim 1 , wherein heating the mixture comprises heating for less than 10 hours.
10. The method as claimed in claim 9 , wherein heating the mixture comprises heating for less than 6 hours.
11. The method as claimed in claim 10 , wherein heating the mixture comprises heating for less than 4 hours.
12. The method as claimed in claim 1 , wherein:
heating the mixture comprises heating under an inert gas atmosphere;
a sum of the water and contents of the inert atmosphere is less than 200 ppm by weight.
13. The method as claimed in claim 12 , wherein:
the water content of the inert atmosphere is from 10 to 180 ppm by weight; and
the oxygen content of the inert atmosphere is from 0.5 to 2 ppm by weight.
14. The method as claimed in claim 13 , wherein a number of moles of NH 4 X in the mixture is at least the sum of:
A) a number of moles of Ln not linked to oxygen in the at least one compound having at least one Ln—X bond; and
B) three times a number of moles of oxygen atoms linked to Ln in the at least one compound having at least one Ln—X bond.
15. The method as claimed in claim 14 , wherein the number of moles of NH 4 X in the mixture is at least the sum of:
A) three times the number of moles of Ln not linked to oxygen in the at least one compound having at least one Ln—X bond; and
B) five times the number of moles of oxygen atoms linked to Ln in the at least one compound having at least one Ln—X bond.
16. The method as claimed in claim 1 , wherein heating the mixture comprises heating in a crucible made of a material containing at least 20% carbon by weight.
17. The method as claimed in claim 16 , wherein the crucible comprises at least one of carbon, glassy carbon and graphite.
18. The method as claimed in claim 16 , wherein the crucible is coated with a layer of pyrolytic carbon.