Single crystal heat treatment method

1. A single crystal heat treatment method, comprising a step of heating a single crystal of a cerium-doped silicate compound represented by general formula (1) or (2) below in an oxygen-containing atmosphere,

wherein the oxygen-containing atmosphere is an atmosphere having an oxygen concentration of 100 vol %, or an atmosphere containing nitrogen or an inert gas and having an oxygen concentration of 1 vol % or higher but less than 100 vol %,

wherein the single crystal is heated at a temperature ranging from 300° C. to 1150° C. in the heating step

Y 2−(x+y) Ln x Ce y SiO 5   (1)

(wherein Ln represents at least one elemental species selected from a group consisting of elements belonging to the rare earth elements, x represents a numerical value from 0 to 2, and y represents a numerical value greater than 0 but less than or equal to 0.2)

Gd 2−(z+w) Ln z Ce w SiO 5   (2)

(wherein Ln represents at least one elemental species selected from a group consisting of elements belonging to the rare earth elements, z represents a numerical value greater than 0 but less than or equal to 2, and w represents a numerical value greater than 0 but less than or equal to 0.2).

2. A single crystal heat treatment method, comprising a step of heating a single crystal of a cerium-doped silicate compound represented by general formula (3) below in an oxygen-containing atmosphere,

wherein the oxygen-containing atmosphere is an atmosphere having an oxygen concentration of 100 vol %, or an atmosphere containing nitrogen or an inert gas and having an oxygen concentration of 1 vol % or higher but less than 100 vol %,

wherein the single crystal is heated at a temperature ranging from 300° C. to 1150° C. in the heating step

Gd 2−(p+q) Ln p Ce q SiO 5   (3)

(wherein Ln represents at least one elemental species selected from a group consisting of Dy, Ho, Er, Tm, Yb, Lu, Y and Sc, which are rare earth elements having an ionic radius smaller than Tb, p represents a numerical value greater than 0 but less than or equal to 2, and q represents a numerical value greater than 0 but less than or equal to 0.2).

3. A single crystal heat treatment method, comprising a step of heating a single crystal of a cerium-doped silicate compound represented by general formula (4) below in an oxygen-containing atmosphere,

wherein the oxygen-containing atmosphere is an atmosphere having an oxygen concentration of 100 vol %, or an atmosphere containing nitrogen or an inert gas and having an oxygen concentration of 1 vol % or higher but less than 100 vol %,

wherein the single crystal is heated at a temperature ranging from 300° C. to 1150° C. in the heating step

Gd 2−(r+s) Lu r Ce s SiO 5   (4)

(wherein r represents a numerical value greater than 0 but less than or equal to 2, and s represents a numerical value greater than 0 but less than or equal to 0.2).

4. The single crystal heat treatment method of claim 1 , wherein the heating temperature ranges from 700° C. to 1150° C.

5. A single crystal heat treatment method, comprising a step of heating a single crystal of a cerium-doped silicate compound represented by any of general formulae (1), (2), (3) and (4) below in an oxygen-containing atmosphere,

wherein the oxygen-containing atmosphere is an atmosphere having an oxygen concentration of 100 vol %, or an atmosphere containing nitrogen or an inert gas and having an oxygen concentration of 1 vol % or higher but less than 100 vol %,

wherein the single crystal of the cerium-doped silicate compound contains 0.00005 to 0.1 wt %, with respect to the total weight of the single crystal, of at least one elemental species selected from a group consisting of Mg, Ca, Sr, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W, and

wherein the single crystal is heated at a temperature of 700° C. to 1300° C. in the heating Step

Y 2−(x+y) Ln x Ce y SiO 5   (1)

(wherein Ln represents at least one elemental species selected from a group consisting of elements belonging to the rare earth elements, x represents a numerical value from 0 to 2, and y represents a numerical value greater than 0 but less than or equal to 0.2)

Gd 2−(z+w) Ln z Ce w SiO 5   (2)

(wherein Ln represents at least one elemental species selected from a group consisting of elements belonging to the rare earth elements, z represents a numerical value greater than 0 but less than or equal to 2, and w represents a numerical value greater than 0 but less than or equal to 0.2)

Gd 2−(p+q) Ln p Ce q SiO 5   (3)

(wherein Ln represents at least one elemental species selected from a group consisting of Dy, Ho, Er, Tm, Yb, Lu, Y and Sc, which are rare earth elements having an ionic radius smaller than Tb, p represents a numerical value greater than 0 but less than or equal to 2, and q represents a numerical value greater than 0 but less than or equal to 0.2)

Gd 2−(r+s) Lu r Ce s SiO 5   (4)

(wherein r represents a numerical value greater than 0 but less than or equal to 2, and s represents a numerical value greater than 0 but less than or equal to 0.2).

6. The single crystal heat treatment method of any of claims 1 to 3 , wherein the heating step comprises:

a first step of heating the single crystal at a temperature of 300° C. to 1150° C. in the oxygen-containing atmosphere;

a second step of, subsequent to the first step, converting the atmosphere to an oxygen-poor atmosphere while the heating temperature is maintained; and

a third step of, subsequent to the second step, heating the single crystal at a temperature T a (units: ° C.), which is higher than the heating temperature and satisfies the conditions represented by formula (5) below, in the oxygen-poor atmosphere

500 ≦T a <( T m −550)  (5)

(wherein T m (units: ° C.) represents the melting point of the single crystal).

7. The single crystal heat treatment method of any of claims 1 to 3 , wherein the heating step comprises:

a fourth step of heating the single crystal at a heating temperature Tb (units: ° C.) which satisfies the conditions represented by formula (6) below in an oxygen-poor atmosphere;

a fifth step of, subsequent to the fourth step, converting the atmosphere to the oxygen-containing atmosphere; and

a sixth step of, subsequent to the fifth step, heating the single crystal at a temperature of 300° C. to 1150° C. in the oxygen-containing atmosphere

800 ≦T b <( T m −550)  (6)

(wherein Tm (units: ° C.) represents the melting point of the single crystal).

8. The single crystal heat treatment method of claim 6 , wherein the oxygen-poor atmosphere is an atmosphere containing a total concentration of argon and nitrogen gas of 80 vol % or higher, and an oxygen concentration of less than 0.2 vol %.

9. The single crystal heat treatment method of claim 7 , wherein the oxygen-poor atmosphere is an atmosphere containing a total concentration of argon and nitrogen gas of 80 vol % or higher, and an oxygen concentration of less than 0.2 vol %.

10. The single crystal heat treatment method of claim 8 , wherein the oxygen-poor atmosphere is an atmosphere containing hydrogen gas at a concentration of 0.5 vol % or more as a reducing gas.

11. The single crystal heat treatment method of claim 9 , wherein the oxygen-poor atmosphere is an atmosphere containing hydrogen gas at a concentration of 0.5 vol % or more as a reducing gas.

12. The single crystal heat treatment method of any of claims 1 to 3 , wherein prior to the heating step, the single crystal is grown or cooled in an oxygen-free, nitrogen or inert gas atmosphere, or a nitrogen or inert gas atmosphere containing a reducing gas; or is heated in an oxygen-free, nitrogen or inert gas atmosphere, or a nitrogen or inert gas atmosphere containing a reducing gas.

13. The single crystal heat treatment method of claim 2 , wherein the heating temperature ranges from 700° C. to 1150° C.

14. The single crystal heat treatment method of claim 3 , wherein the heating temperature ranges from 700° C. to 1150° C.