Method for preparing single-crystal cubic sesquioxides and uses

Abstract

The present invention relates to a process for the preparation of hulk or thin-film single-crystals of cubic sesquioxides (space group No. 206, Ia-3) of scandium, yttrium or rare earth metals doped or not doped with lanthanide ions having a valency of +III by a high-temperature flux growth technique and to the applications of the nondoped single-crystals obtained according to this process, in particular in the optical field.

Claims

1. A process for the preparation of a bulk or thin-film single-crystal formed of a matrix of a cubic sesquioxide of scandium, yttrium and/or rare earth metal, said matrix being doped or not doped with at least one element of the series of the lanthanides, said single-crystal corresponding to the following formula (I):
(R.sup.1.sub.1xR.sup.2.sub.x).sub.2O.sub.3(I) in which: R.sup.1 is at least one metal with a valency of III selected from the group consisting of scandium, yttrium and the elements of the series of the lanthanides; x is such that 0x<1; R.sup.2 represents at least one element selected from the group consisting of from the series of the lanthanides; said process being carried out in a chemically inert crucible and comprising the following stages: 1) preparing a pulverulent mixture (PM1) comprising at least: a solute composed of at least one sesquioxide of following formula (IIa):
R.sup.1.sub.2O.sub.3 (IIa), in which R.sup.1 represents the same element as R.sup.1, and, when said matrix is doped with at least one element of the series of the lanthanides, of at least one sesquioxide of following formula (IIb):
R.sup.2.sub.2O.sub.3(IIb), in which R.sup.2 represents the same element as R.sup.2, in a molar percentage (x.sub.mp) such that 0<x.sub.mp25 mol %, said sesquioxide of formula (IIa) is present within said solute in a molar percentage 100x.sub.mp, said solute being present within the mixture PM1 in an amount z such that 0<z93 mol %, a primary solvent composed of a mechanical mixture of a compound of following formula (III):
[Li.sub.6(R.sup.1.sub.1xR.sup.2.sub.x)(BO.sub.3).sub.3](III) in which R.sup.1 and R.sup.2 respectively represent the same element as R.sup.1 and R.sup.2 and x is such that 0x<1; 2) preparing a pulverulent mixture PM2 comprising said solute and a synthesis solvent of following formula (IV):
[Li.sub.6(R.sup.1.sub.1xR.sup.2.sub.x(BO.sub.3).sub.3]+(Li.sub.2O and/or B.sub.2O.sub.3 and/or LiX)(IV) in which R.sup.1 and R.sup.2 respectively represent the same element as R.sup.1 and R.sup.2, x is such that 0x<1, and X=F, Cl, Br or I, by addition to the mixture PM1 of at least one pulverulent additive selected from the group consisting of Li.sub.2O, B.sub.2O.sub.3 and LiX with X=F, Cl, Br or I, said mixture PM1 being present within the mixture PM2 in an amount z such that 15z<100 mol %, said additive being present within the mixture MP2 in a total molar amount s such that 0<s85 mol %, and s=100z in the mixture PM2; the molar amount z of the solute within the mixture PM2 being such that z=z.z; 3) bringing the pulverulent mixture PM2 obtained above in stage 2) to a temperature T.sub.PM2 at least equal to the melting point (T.sub.M.p) of said mixture PM2 and 1250 C., in order to bring about the dissolution of the solute in the synthesis solvent of formula (IV) and to obtain a liquid solution of said solute in the synthesis solvent of formula (IV); 4) maintaining the temperature of the liquid solution at the temperature T.sub.PM2 for a period of time of at least 6 hours, with stirring by means of a solid support subjected to rotation around a vertical axis; 5) cooling, in controlled fashion, the liquid solution from the temperature T.sub.PM2 down to a temperature T.sub.Exp between the saturation temperature (T.sub.Sat) of the liquid solution and the critical supersaturation temperature (CT.sub.Super) of the liquid solution or the temperature of solidification of the liquid solution, in order to bring about the controlled crystallization of the expected sesquioxide of formula (I) on said solid support immersed in said liquid solution and subjected to rotation around a vertical axis, said cooling being carried out at a maximum rate of 1 C..Math.h.sup.1; and 6) withdrawing the solid support from the liquid solution and then cooling, in controlled fashion, the sesquioxide of formula (I) crystallized on the solid support from the temperature T.sub.Exp down to ambient temperature, at a maximum rate of 50 C..Math.h.sup.1.

2. The process according to claim 1, wherein the stages 1) and 2) are carried out jointly, the molar amount (z) of the solute within the mixture PM2 then being such that z =z.Math.z mol %, the molar amount of the primary solvent of formula (III) (t) being such that t=z(100z) mol % and the molar amount of additive being s mol %, with s=(100z) %.

3. The process according to claim 1, wherein R.sup.1 is selected from the group consisting of the elements Y, Gd, Tb, Eu, Sc, Lu; the combinations of elements Y/Gd, Y/Sc, Gd/Sc, Lu/Sc, Gd/La, Gd/Tb, Gd/Lu, Y/Lu, Eu/Gd, Eu/La and Eu/Lu.

4. The process according to claim 1, wherein, when x>0, R.sup.2 is selected from the group consisting of the elements Yb, Tm, Er, Pr, Tb, Nd, Ce, Ho, Eu, Sm, Dy, the combinations of elements Yb/Tm, Yb/Pr, Tm/Ho, Er/Yb, Yb/Tb, Yb/Ho, Eu/Sm and Tm/Tb.

5. The process according to claim 1, wherein said process is employed for the preparation of sesquioxides of formula (I) selected from the group consisting of: Tb.sub.2O.sub.3; Gd.sub.2O.sub.3; Eu.sub.2O.sub.3; Y.sub.2O.sub.3; Lu.sub.2O.sub.3; Sc.sub.2O.sub.3; (Tb,Gd).sub.2O.sub.3; (Eu,Gd).sub.2O.sub.3; Lu.sub.2O.sub.3:Yb; Gd.sub.2O.sub.3:Yb; Lu.sub.2O.sub.3:Eu; Gd.sub.2O.sub.3:Eu; Y.sub.2O.sub.3:Eu; Y.sub.2O.sub.3:Er; Gd.sub.2O.sub.3:Tm; Gd.sub.2O.sub.3:Eu.sup.3+; (Y,Gd).sub.2O.sub.3:Pr; (Y,Gd).sub.2O.sub.3:Eu; (Y,Gd).sub.2O.sub.3:Nd; (Y,La).sub.2O.sub.3:Pr; (Gd,La).sub.2O.sub.3:Pr; (Gd,La).sub.2O.sub.3:Yb; (Gd,La).sub.2O.sub.3:Eu; (Gd,La).sub.2O.sub.3:Nd; (Y,La).sub.2O.sub.3:Yb; Y.sub.2O.sub.3:Er:Yb; Y.sub.2O.sub.3:Pr:Yb; Gd.sub.2O.sub.3:Er:Yb; Gd.sub.2O.sub.3:Pr:Yb; Gd.sub.2O.sub.3:Tm:Yb; Lu.sub.2O.sub.3:Tm:Yb; Y.sub.2O.sub.3:Tm:Ho; Y.sub.2O.sub.3m:Yb; Y.sub.2O.sub.3:Tm: Tb; Sc.sub.2O.sub.3:Eu; (Y,Lu).sub.2O.sub.3:Eu and (Gd,Lu).sub.2O.sub.3:Eu.

6. The process according to claim 1, wherein the amount z of solute present within the pulverulent mixture PM2 is such that 5z30 mol %.

7. The process according to claim 1, wherein the amount s of Li.sub.2O and/or of B.sub.2O.sub.3 and/or of LiX present within the pulverulent mixture PM2 is such that 5s30 mol %.

8. The process according to claim 1, wherein x.sub.mp=0 or 0<x.sub.mp10 mol %.

9. The process according to claim 1, wherein the pulverulent mixture PM2 produced during stages 1) and 2) and comprising the solute composed of a sesquioxide of formula (IIa) as a mixture or not with a sesquioxide of formula (IIb) and the synthesis solvent of formula (IV) is prepared according to the process comprising the substages: i) preparing, by mechanical grinding, a pulverulent mixture comprising 6 mol of Li.sub.2CO.sub.3, 6 mol of H.sub.3BO.sub.3, 1+z mol of a sesquioxide of formula (IIa) or of a mixture of a sesquioxide of formula (IIa) and of a sesquioxide of formula (IIb), to which an excess of 20 mol % of at least one additive selected from the group consisting of Li.sub.2O (in the Li.sub.2CO.sub.3 form), B.sub.2O.sub.3 (in the H.sub.3BO.sub.3 form) and LiX, with X=F, Cl, Br or I, is added; ii) subjecting the mixture obtained above in stage i) to a heat treatment comprising: a rise in temperature up to a temperature T1 of from 400 to 500 C., according to a temperature rise gradient of from 120 to 180 C..Math.h.sup.1, a stationary phase during which the temperature T1 is maintained for from 6 to 24 hours, a rise in temperature up to a temperature T2 of from 700 to 800 C., according to a temperature rise gradient of from 120 to 180 C..Math.h.sup.1, a stationary phase during which the temperature T2 is maintained for from 6 to 24 hours, a return to ambient temperature with a cooling gradient of from 120 to 180 C..Math.h.sup.1, in order to obtain a solid material in the form of particles, said material being composed of the synthesis solvent of formula (IV) as a mixture with z mol % of solute; and iii) mechanically grinding the solid material obtained above in stage ii) in order to obtain the pulverulent mixture PM2.

10. The process according to claim 1, wherein, during stage 3), the temperature TPM2 is from 1200 C. to 1250 C. and the rate at which the mixture PM2 is brought to the temperature T.sub.PM2 is 120 C..Math.h.sup.1.

11. The process according to claim 1, wherein, during stage 5), the temperature CT.sub.Super is 1100 C. and the controlled cooling of the liquid solution from the temperature T.sub.PM2 down to a temperature T.sub.Exp is carried out at a rate of from 0.1 to 1 C..Math.h.sup.1.

12. The process according to claim 1, wherein, during stages 4) and 5), the rotational speed of the solid support varies from 5 to 50 revolutions/min.

13. The process according to claim 1, wherein the cooling of stage 5) is carried out at a rate of 0.2 C..Math.h.sup.1 from the temperature T.sub.PM2 down to a temperature CT.sub.Super of 1100 C., after extracting the support from the liquid and positioning the support above the liquid solution, and then the temperature of 1100 C. is maintained for a time of less than 1 hour before carrying out the cooling mentioned in stage 6).

14. The process according to claim 1, wherein the cooling of stage 5) is a heat treatment comprising an alternation of cooling stages and of temperature-rise stages in which the amplitude of each temperature-rise stage is less than or equal to the amplitude of the cooling stage which precedes it.

15. The process according to claim 14, wherein the rate of cooling varies from 0.1 to 0.2 C..Math.h.sup.1 and the cooling of stage 5) is carried out according to the following heat treatment: Starting temperature: 1200 C. i) gradient of 0.2 C..Math.h.sup.1 down to 1175 C., ii) gradient of 180 C..Math.h.sup.1 up to 1190 C., iii) gradient of 0.2 C..Math.h.sup.1 down to 1150 C., iv) gradient of 180 C..Math.h.sup.1 up to 1165 C., v) gradient of 0.2 C..Math.h.sup.1down to 1125 C., vi) gradient of 180 C..Math.h.sup.1 up to 1140 C., vii) gradient of 0.2 C..Math.h.sup.1 down to 1100 C., viii) gradient of 180 C..Math.h.sup.1 up to 1115 C., ix) gradient of 0.2 C..Math.h.sup.1 down to 1100 C.

16. The process according to claim 14, wherein the cooling of stage 6) is carried out at a rate of 5 C..Math.h.sup.1 down to 800 C. and then of 30 C..Math.h.sup.1 down to ambient temperature.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a solvent/solute phase diagram in accordance with one embodiment;

(2) FIG. 2 is a diagram of a vertical tubular furnace from example 1, in accordance with one embodiment;

(3) FIG. 3 is a photograph of Gd.sub.2O.sub.3:Yb.sup.3+ crystals obtained from example 1, in accordance with one embodiment;

(4) FIG. 4 is a photograph of a polished crystal obtained from example 1, in accordance with one embodiment;

(5) FIG. 5 is a Laue diffraction diagram from example 1, in accordance with one embodiment;

(6) FIG. 6 is a photograph of Gd.sub.2O.sub.3:Yb.sup.3+ crystals obtained from comparative example 1, in accordance with one embodiment;

(7) FIG. 7 is a photograph of a crystal obtained from comparative example 1, in accordance with one embodiment;

(8) FIG. 8 is a Laue diffraction diagram from comparative example 1, in accordance with one embodiment;

(9) FIG. 9 shows a broad disc composed of several Tb.sub.2O.sub.3 single-crystals obtained from example 2 in accordance with one embodiment;

(10) FIG. 10 is a photograph of a portion extracted from the disc, from example 2 in accordance with one embodiment;

(11) FIG. 11 is a Laue diffraction diagram from example 2, in accordance with one embodiment;

(12) FIGS. 12a and 12b are graphs showing the measurements carried out for 2 different laser sources (HeNe at 632.8 rim (FIG. 12a) and Nd:YAG at 1064 nm (FIG. 12b)) from example 2, in accordance with one embodiment;

(13) FIG. 13 is a photograph of an agglomerate of Tb.sub.2O.sub.3 single-crystals from comparative example 2, in accordance with one embodiment;

(14) FIG. 14 is a Laue diffraction diagram from example 2, in accordance with one embodiment;

DETAILED DESCRIPTION

EXAMPLES

(15) The starting materials used in these examples are powders with a purity of 99.99%. Gd.sub.2O.sub.3, Yb.sub.2O.sub.3, Li.sub.2CO.sub.3 and H.sub.3BO.sub.3 powders sold by Fox Chemicals, Tb.sub.4O.sub.7 powder sold by Oxymet Ltd.

(16) Except for the commercial Tb.sub.4O.sub.7 powder, the other powders were used as received from the manufacturer.

(17) The reactions for the synthesis of the powders of solute and of synthesis solvent of formula (IV) can be carried out simultaneously in the same platinum crucible after prior intimate mixing and grinding of these with one another.

EXAMPLE 1

Crystallogenesis of Gd2O3 Single-Crystals Doped with Ytterbium According to the Process in Accordance with the Invention

(18) In this example, single-crystal cubic gadolinium sesquioxides doped to x.sub.mp=6.67 mol % (x=x=0.0667) with ytterbium: Gd.sub.2O.sub.3:Yb.sup.3+, were prepared by using [Li.sub.6(Gd.sub.0.9333Yb.sub.0.0667)(BO.sub.3).sub.3] primary solvent to which z=20 mol % of solute Gd.sub.2O.sub.3:Yb.sup.3+ were added.

(19) An excess of s=19.35 mol % of Li.sub.2O was subsequently added to this base mixture.

(20) 1) First Stage: Preparation of a Mixture PM2 Composed of [Li.sub.6(Gd.sub.0.9333Yb.sub.0.0667)(BO.sub.3).sub.3], Li.sub.2O and (Gd.sub.0.9333Yb.sub.0.0667).sub.2O.sub.3

(21) 1.1) Preparation of Solute Composed of a Mechanical Mixture of Gadolinium Sesquioxide and Ytterbium Sesquioxide

(22) The solute was prepared by mechanically mixing 93.33 mol % of Gd.sub.2O.sub.3 and 6.67 mol % of Yb.sub.2O.sub.3.

(23) The commercial Gd.sub.2O.sub.3 and Yb.sub.2O.sub.3 powders were mixed according to the stochiometric proportions indicated above and then intimately ground in a mortar in order to obtain the finest particle size possible and the most homogenous mechanical mixture possible. 32.4 g of a solute of Gd.sub.2O.sub.3 doped with ytterbium of formulation (Gd.sub.0.9333Yb.sub.0.0667).sub.2O.sub.3 were thus obtained.

(24) 1.2) Preparation of a Mixture PM1 Composed of (Gd.sub.0.9333Yb.sub.0.0667).sub.2O.sub.3 as Solute and of [Li.sub.6(Gd.sub.0.9333Yb.sub.0.0667)(BO.sub.3).sub.3] as Primary Solvent

(25) The mixture PM1 was synthesized according to the following reaction 1:
6 Li.sub.2CO.sub.3+6 H.sub.3BO.sub.3+0.9333 Gd.sub.2O.sub.3+0.0667 Yb.sub.2O.sub.3+0.20 (Gd.sub.0.9333Yb.sub.0.0667).sub.2O.sub.3.fwdarw.2 [Li.sub.6(Gd.sub.0.9333Yb.sub.0.0667)(BO.sub.3).sub.3]+0.20 (Gd.sub.0.9333Yb.sub.0.0667).sub.2O.sub.3+9 H.sub.2O+6 CO.sub.2(Reaction 1)

(26) The commercial Li.sub.2CO, H.sub.3BO.sub.3, Gd.sub.2O.sub.3 and Yb.sub.2O.sub.3 powders were mixed beforehand according to the stochiometric proportions shown by reaction 1 above and then intimately ground in a mortar in order to obtain the finest particle size possible and the most homogeneous mechanical mixture possible.

(27) 133.3055 g of primary solvent and 32.4 g of solute (Gd.sub.0.9333Yb.sub.0.0667).sub.2O.sub.3 were prepared, i.e. z=20 mol % of solute with in the primary solvent, 165.7055 g of mixture PM1 were thus obtained.

(28) 1.3) Preparation of a Mixture PM2

(29) A further 7.8542 g of Li.sub.2CO.sub.3 (i.e., the equivalent of 3.1763 g of Li.sub.2O) were added to the mixture PM1 obtained above in stage 1.2). This corresponds to an addition of s=19.35 mol % of excess of Li.sub.2O with respect to the mixture PM1.

(30) The commercial Li.sub.2CO.sub.3 powder and the powder of the mixture of PM1were mixed and then intimately ground in a mortar in order to obtain the finest particle size possible and the most homogeneous mechanical mixture possible.

(31) The resulting mixture was subsequently heated in a platinum crucible under an air atmosphere according to the following heat treatment:

(32) 1) gradient of 120 C..Math.h.sup.1 up to 500 C. and then a stationary phase lasting 12 h;

(33) 2) gradient of 120 C..Math.h.sup.1 up to 800 C. and then a stationary phase lasting 12 h;

(34) 3) gradient of 180 C..Math.h.sup.1 up to 1250 C. and then a stationary phase lasting 2 h, this being done in order to bring about the dissolution of the solute in the synthesis solvent;

(35) 4) cooling down to ambient temperature with a cooling rate of 180 C..Math.h.sup.1.

(36) The mixture PM2 thus obtained, composed of the synthesis solvent [Li.sub.6(Gd.sub.0.9333Yb.sub.0.0667)(BO.sub.3).sub.3]+Li.sub.2O to which the solute had been added, was subsequently ground using a mortar and a pestle made of agate. A mixture PM2 composed of 136.4818 g of synthesis solvent and 32.4 g of solute, i.e. z=16.14 mol % of solute within the synthesis solvent, was thus obtained.

(37) 2) Second Stage: Crystallogenesis of Gd.sub.2O.sub.3:Yb.sup.3+ Single-Crystals

(38) The mixture PM2 obtained above in stage 2) was subsequently melted under air, in a vertical tubular furnace, by first of all applying a temperature rise gradient of 180 C..Math.h.sup.1 up to 1250 C.

(39) The vertical tubular furnace used in this example is represented in the appended FIG. 2. It comprises a water-cooled stainless steel chamber (1) in which are positioned heating elements (2) and an internal chamber (3) made of refractory material composed of an alumina tube (3a), of Fibrothal and silica wool (3b) and of an alumina washer (3c). The internal chamber (3) includes a heat reflector (4) positioned around a crucible (5) containing the molten reaction mixture (solution) (6), said heat reflector (4) being composed of a backing alumina tube (4a) for the reflector (4) surmounted by a platinum washer (4b) itself surmounted by an alumina washer (4c). On its upper part, the chamber (1) is provided with a leaktight passage (7) guaranteeing the growth atmosphere in the chamber of the furnace, through which passes an alumina rod (8) integral with a mechanical rotation/translation system and with a weighing device (balance having an accuracy of 10.sup.3 g) (9) and comprising, at its end, a solid support for homogenizing and countering sedimentation of the solute (10) by stirring the solute in the solvent for the growth of the single-crystal, said support (10) being immersed in the reaction mixture (6) present in the crucible (5).

(40) The support (10) can be composed of a platinum paddle, of a platinum wire or of an assembly formed by a platinum disc attached horizontally at its centre to a vertical platinum wire itself suspended from an alumina rod or alternatively of an oriented crystal or polycrystalline seed attached to a platinum wire itself suspended from an alumina rod.

(41) In this example, the solid support (10) which was used was a platinum disc attached horizontally at its centre to a vertical platinum wire itself suspended from an alumina rod.

(42) The temperature gradients in the furnace (radial and longitudinal gradients) are of the order of 1 C..Math.cm.sup.1, so that the minimum temperature of the reaction mixture is located at the centre and at the surface of the reaction mixture.

(43) The heat reflector (4), provided with an alumina washer (thickness >1 mm) (4b) as well as a graphite washer (thickness >2 mm) (4c), makes it possible to reduce the temperature gradients. The temperature in the reaction mixture is thus rendered homogeneous.

(44) After thermalization, the solid support was immersed by translation along the axis of the furnace (1) and of the crucible (5) into the reaction mixture down to a height of 1 mm from the bottom of the crucible, so that only the platinum constituting the disc and the wire of the support (10) is in contact with the molten reaction mixture and so that the end of the alumina rod (8) (point of attachment between the platinum wire and the rod) is at least at more than 1 cm from the surface of the reaction mixture at a minimum.

(45) The platinum disc was immersed by translation in the reaction mixture at the centre of the crucible. Stirring by rotation around the axis of the rod, of the order of 30 revolutions/min, was carried out for 24 hours at 1250 C. with the aim of thoroughly homogenizing the dissolved entities (solute) in the synthesis solvent and of preventing them from sedimenting, if appropriate.

(46) In view of the high viscosity of the molten bath, sufficient stirring, of the order of 20 rev/min or more, proved to be necessary in order to keep the entities dissolved throughout the liquid phase and to alleviate the effects of sedimentation of the solute.

(47) Crystal growth was carried out according to the following heat treatment programme:

(48) Starting temperature: 20 C. gradient of 120 C..Math.h.sup.1 down to 1250 C. and then a stationary phase lasting 24 h, gradient of 180 C..Math.h.sup.1 down to 1200 C. and then a stationary phase lasting 4 h, gradient of 0.2 C..Math.h.sup.1 down to 1175 C. and then a stationary phase lasting 0.1 h, gradient of 180 C..Math.h.sup.1 up to 1190 C. and then a stationary phase lasting 0.1 h, gradient of 0.2 C..Math.h.sup.1 down to 1150 C. and then a stationary phase lasting 0.1 h, gradient of 180 C..Math.h.sup.1 up to 1165 C. and then a stationary phase lasting 0.1 h, gradient of 0.2 C..Math.h.sup.1 down to 1125 C. and then a stationary phase lasting 0.1 h, gradient of 180 C..Math.h.sup.1 up to 1140 C. and then a stationary phase lasting 0.1 h, gradient of 0.2 C..Math.h.sup.1 down to 1100 C. and then a stationary phase lasting 0.1 h, gradient of 180 C..Math.h.sup.1 up to 1115 C. and then a stationary phase lasting 0.1 h, gradient of 0.2 C..Math.h.sup.1 down to 1100 C. and then a stationary phase lasting 0.1 h, no pulling from the solution, extraction of the platinum disc above the molten bath at 1100 C., gradient of 0.5 C..Math.h.sup.1 down to 1000 C., gradient of 2 C..Math.h.sup.1 down to 800 C., gradient of 60 C..Math.h.sup.1 down to ambient temperature.

(49) A photograph of the Gd.sub.2O.sub.3:Yb.sup.3+ crystals thus obtained is given in the appended FIG. 3. In this figure, the Gd.sub.2O.sub.3:Yb.sup.3+ crystals were placed on a sheet of graph paper. It is observed that the crystals exhibit a size of the order of a centimetre in their large length. A photograph of a polished crystal of more than 25 mm.sup.2 in surface area and 1.5 mm thickness not exhibiting any inclusion is presented in the appended FIG. 4. The working volume of such a crystal is consequently very high.

(50) The Laue diffraction diagram is represented in the appended FIG. 5. In this figure, the measurements were made on an oriented (2 1 1) natural face of a Gd.sub.2O.sub.3:Yb.sup.3+ single-crystal on a goniometer sold under the reference GM WS series X-ray by Delta Technologies International, using a molybdenum anticathode and a CCD camera of the Photonics Science brand. This diagram is in accordance with the expected theoretical structure and confirms the cubic structure of the Gd.sub.2O.sub.3:Yb.sup.3+ single-crystal obtained in this example.

COMPARATIVE EXAMPLE 1

Crystallogenesis of Gd2O Single-Crystals Doped with Ytterbium According to a Process NOT in Accordance with the Invention

(51) As comparative example to example 1 above, Gd.sub.2O.sub.3 single-crystals doped to 6.67% (molar) with ytterbium were prepared according to a process not in accordance with the invention, that is to say by using [Li.sub.6(Gd.sub.0.9333Yb.sub.0.0667)(BO.sub.3).sub.3] solvent (mixture not comprising Li.sub.2O), to which 20 mol % of Gd.sub.2O.sub.3:Yb.sup.3+ solute of formulation (Gd.sub.0.9333Yb.sub.0.667).sub.2O.sub.3 were added according to the process described in international application WO 2012/055075.

(52) 1) First stage: Preparation of a Solute Composed of a Mechanical Mixture of Gadolinium Sesquioxide and Ytterbium Sesquioxide

(53) The solute was prepared by mechanically mixing 93.33 mol % of Gd.sub.2O.sub.3 and 6.67 mol % of Yb.sub.2O.sub.3 according to the same process as that of stage 1) of example 1 above.

(54) 2) Second Stage: Preparation of the Mixture for the Synthesis

(55) The mixture for the synthesis was prepared according to the following reaction 1:
6 Li.sub.2CO.sub.3+6 H.sub.3BO.sub.3+0.9333 Gd.sub.2O.sub.3+0.0667 Yb.sub.2O.sub.3.fwdarw.2[Li.sub.6(Gd.sub.0.9333Yb.sub.0.0667)(BO.sub.3).sub.3]+0.20 (Gd.sub.0.9333Yb.sub.0.0667).sub.2O.sub.3+9 H.sub.2O+6 CO.sub.2(Reaction 1)

(56) The commercial Li.sub.2CO.sub.3, H.sub.3BO.sub.3, Gd.sub.2O.sub.3 and Yb.sub.2O.sub.3 powders were mixed according to the stochiometric proportions shown by reaction 1 above and then intimately ground in a mortar in order to obtain the finest particle size possible and the most homogeneous mechanical mixture possible. The mixture was subsequently heated in a platinum crucible under an air atmosphere according to the following heat treatment:

(57) 1) gradient of 120 C..Math.h.sup.1 up to 500 C. and then a stationary phase lasting 12 h;

(58) 2) gradient of 120 C..Math.h.sup.1 up to 800 C. and then a stationary phase lasting 12 h;

(59) 3) gradient of 180 C..Math.h.sup.1 up to 1250 C. and then a stationary phase lasting 2 h;

(60) 4) cooling down to ambient temperature with a cooling rate of 180 C..Math.h.sup.1.

(61) The mixture of [Li.sub.6(Gd.sub.0.9333Yb.sub.0.0667)(BO.sub.3).sub.3] and (Gd.sub.0.9333Yb.sub.0.0667).sub.2O.sub.3 thus obtained was subsequently ground using a mortar and a pestle made of agate.

(62) 3) Third Stage: Crystallogenesis of the Gd.sub.2O.sub.3:Yb.sup.3+ Single-Crystals

(63) 133.3055 g (80 mol %) of the solvent [Li.sub.6(Gd.sub.0.9333Yb.sub.0.0667)(BO.sub.3).sub.3] obtained above in stage 2) and 32.400 g (20 mol %) of the solute prepared above in stage 1) were mixed and intimately ground together in a mortar in order to obtain the finest particle size possible and the most homogeneous mechanical mixture possible. This mixture was subsequently placed in a platinum crucible.

(64) The solution composed of [Li.sub.6(Gd.sub.0.9333Yb.sub.0.0667)(BO.sub.3).sub.3] (primary solvent) and of the solute of formulation (Gd.sub.0.9333Yb.sub.0.0667).sub.2O.sub.3 was subsequently melted under air, in the same vertical tubular furnace as that used above in example 1, by applying first of all a temperature rise gradient of 180 C..Math.h.sup.1 up to 1200 C.

(65) Gd.sub.2O.sub.3:Yb.sup.3+ was crystallized in the furnace described above in example 1, according to the protocol described in example 1, by using the same support as above in example 1, under an air atmosphere and according to the same heat treatment programme as that employed above in example 1.

(66) Gd.sub.2O.sub.3:Yb.sup.3+ single-crystals were obtained on the platinum paddle with a horizontal disc, a photograph of which is given in the appended FIG. 6. In this figure, the Gd.sub.2O.sub.3:Yb.sup.3+ crystals were placed on a sheet of graph paper. It is observed that the crystals obtained are more numerous and exhibit a lower mean size than that of the crystals obtained in example 1 above (in their large length). A photograph of a crystal in the centimetre range is represented in the appended FIG. 7. In this figure, it is observed that the crystal obtained according to the process not in accordance with the invention, that is to say by using a synthesis solvent not containing Li.sub.2O, comprises numerous inclusions which have the consequence of limiting the working volume of the crystal, that is to say the volume where there is no inclusion.

(67) The Laue diffraction diagram is represented in the appended FIG. 8. The measurements were made on an oriented (2 1 1) natural face of a Gd.sub.2O.sub.3 single-crystal on a goniometer sold under the reference GM WS Series X-ray by Delta Technologies International, using a molybdenum anticathode and a CCD camera of the Photonics Science brand. This diagram is in accordance with the expected theoretical structure and confirms the cubic structure of the pure Gd.sub.2O.sub.3 single-crystal obtained.

EXAMPLE 2

Crystallogenesis of Tb2O3 Single-Crystals According to the Process in Accordance with the Invention

(68) In this example, pure single-crystal cubic terbium sesquioxides (x.sub.mp=0 and x=0): Tb.sub.2O.sub.3, were prepared by using [Li.sub.6Tb(BO.sub.3).sub.3] as primary solvent to which z=17 mol % of solute Tb.sub.2O.sub.3 were added.

(69) An excess of s=20 mol % of Li.sub.2O was subsequently added to this base mixture.

(70) 1) First Stage: Preparation of the Solute Tb.sub.2O.sub.3

(71) Before it was used, the commercial Tb.sub.2O.sub.3 powder was heated beforehand under argon/H.sub.2 (5%) separately in order to obtain a dehydrated, perfectly stochiometric Tb.sub.2O.sub.3 powder exhibiting only terbium atoms having a valency of +III, according to the following reaction (2):
Tb.sub.4O.sub.7.fwdarw.2Tb.sub.2O.sub.3+0.5 O.sub.2(Reaction 2)

(72) The heat treatment was as follows:

(73) 1) gradient of 180 C..Math.h.sup.1 up to 500 C. and then a stationary phase lasting 3 h;

(74) 2) gradient of 180 C..Math.h.sup.1 up to 850 C. and then a stationary phase lasting 3 h;

(75) 3) gradient of 180 C..Math.h.sup.1 up to 1250 C. and then a stationary phase lasting 48 h;

(76) 4) cooling down to ambient temperature with a cooling rate of 120 C..Math.h.sup.1.

(77) This heat treatment was carried out in a platinum crucible.

(78) The Tb.sub.2O.sub.3 powder was then subsequently used in the crystal synthesis in the hour which followed the end of the heat treatment mentioned above in order to prevent any reuptake of moisture.

(79) 2) Second Stage: Preparation of a Mixture PM1 Composed of Tb.sub.2O.sub.3 and of [Li.sub.6Tb(BO.sub.3).sub.3] as Primary Solvent

(80) The mixture PM1 was prepared according to the following reaction 3:
6 Li.sub.2CO.sub.3+6 H.sub.3BO.sub.3+1.17 Tb.sub.2O.sub.3.fwdarw.2[Li.sub.6Tb(BO.sub.3).sub.3]+0.17 Tb.sub.2O.sub.3+9 H.sub.2O+6 CO.sub.2(Reaction 3)

(81) The commercial Li.sub.2CO.sub.3 and H.sub.3BO.sub.3 powders and the Tb.sub.2O.sub.3 powder as prepared above in stage 1) were mixed according to the stochiometric proportions shown by reaction 3 above and then intimately ground in a mortar in order to obtain the finest particle size possible and the most homogeneous mechanical mixture possible.

(82) 98.2793 g of primary solvent and 19.5344 g of solute Tb.sub.2O.sub.3 were prepared, 117.8137 g of mixture PM1 are thus obtained.

(83) 3) Third stage: Preparation of a Mixture PM2 by Addition of Li.sub.2O in Molar Excess to the Mixture PM1 Described Above in Stage 2)

(84) A further 5.8018 g of Li.sub.2CO.sub.3 (i.e., the equivalent of 2.3463 g of Li.sub.2O) were added to the 117.8137 g of mixture PM1 obtained above in the preceding stage. This corresponds to an addition of 20 mol % of excess of Li.sub.2O with respect to the mixture PM1.

(85) The commercial Li.sub.2CO.sub.3 powder and the powder of the mixture PM1 were mixed and then intimately ground in a mortar in order to obtain the finest particle size possible and the most homogeneous mechanical mixture possible.

(86) The resulting mixture was subsequently heated in a platinum crucible under an atmosphere of argon/H.sub.2 (5 vol %) according to the following heat treatment:

(87) 1) gradient of 120 C..Math.h.sup.1 up to 500 C. and then a stationary phase lasting 12 h;

(88) 2) gradient of 120 C..Math.h.sup.1 up to 800 C. and then a stationary phase lasting 12 h;

(89) 3) gradient of 180 C..Math.h.sup.1 up to 1250 C. and then a stationary phase lasting 2 h;

(90) 4) cooling down to ambient temperature with a cooling rate of 180 C..Math.h.sup.1.

(91) The mixture PM2 was subsequently ground using a mortar and a pestle made of agate. 120.16 g of mixture PM2 composed of 100.6256 g of synthesis solvent and 19.5344 g of Tb.sub.2O.sub.3, i.e. z=13.6 mol % of solute in the synthesis solvent, were thus obtained.

(92) 4) Fourth Stage: Crystallogenesis of the Tb.sub.2O.sub.3 Single-Crystals

(93) The mixture PM2 obtained above in the preceding stage was subsequently placed in a platinum crucible.

(94) The mixture PM2 was subsequently melted under argon, in a vertical tubular furnace identical to that used above in example 1, by applying first of all a temperature rise gradient of 180 C..Math.h.sup.1 up to 1235 C.

(95) A paddle in the form of a shovel was immersed by translation in the reaction mixture at the centre of the crucible. Stirring by rotation around the axis of the rod, of the order of 30 revolutions/mm, was carried out for 24 hours at 1235 C. with the aim of thoroughly homogenizing the dissolved entities (solute) in the synthesis solvent and of preventing them from sedimenting, if appropriate.

(96) In view of the high viscosity of the molten bath, sufficient stirring, of the order of 20 rev/min, proved to be necessary in order to keep the entities dissolved throughout the liquid phase and to alleviate the effects of sedimentation of the solute. Crystal growth was carried out according to the following heat treatment programme:

(97) Starting temperature: 20 C., gradient of 180 C..Math.h.sup.1 up to 1235 C. and then a stationary phase lasting 24 h, cooling at a rate of 0.2 C..Math.h.sup.1 down to 1175 C., rise at a rate of 120 C..Math.h.sup.1 up to 1210 C., cooling at a rate of 0.2 C..Math.h.sup.1 down to 1160 C. no pulling from the solution over the temperature range from 1235 C. to 1160 C., extraction of the paddle above the molten bath at 1160 C., gradient of 7 C..Math.h.sup.1 down to 800 C., gradient of 30 C..Math.h.sup.1 down to ambient temperature.

(98) A broad disc composed of several Tb.sub.2O.sub.3 single-crystals was thus obtained at the end of approximately 20 days. The photograph of this disc is given in the appended FIG. 9. In this figure the Tb.sub.2O.sub.3 disc has been placed on graph paper (1 square=1 mm). It is observed that the crystals exhibit a size of the order of a centimetre. The appended FIG. 10 presents a photograph of a portion extracted from the disc, this portion exhibiting dimensions of more than 551.2 mm.sup.3 without any inclusion.

(99) Furthermore, the Laue diffraction diagram obtained by the Laue method is represented in the appended FIG. 11. The growth axis of the disc was determined as being (6 2 1). The measurements were made on a goniometer sold under the reference GM WS series X-ray by Delta Technologies International, using a molybdenum anticathode and a CCD camera of the Photonics Science brand. This diagram is in accordance with the expected theoretical structure and confirms the cubic structure of one of the Tb.sub.2O.sub.3 single-crystals obtained.

(100) This crystal was also tested in order to measure its Verdet constant. It should be remembered that a nonabsorbing isotropic diamagnetic or paramagnetic medium, or more generally one not exhibiting a difference in absorption between two right and left circular polarization forms of light, brings about a simple rotation of the polarization of the electric field of a rectilinearly polarized electromagnetic wave when this medium is subjected to a magnetic induction field B applied along the direction of propagation of this wave. This phenomenon, known as the Faraday effect, results from the magnetic circular birefringence induced in the medium, (solely) the single pass effect of which is similar to that of natural circular birefringence, also known as optical activity. The classical theory of the electron (Lorentz model) in combination with the Maxwell equations makes it possible to show that the Faraday rotation is both proportional to the amplitude of the magnetic induction field B and to the length l of the material traversed, so that =V().Math.l.Math.B, where V() is a constant dependent on the material and a function of the wavelength : this is the Verdet constant.

(101) The crystal was positioned at the centre of the air gap of an electromagnet delivering a continuous magnetic field which can range from 0 mT to 1200 mT. An analyser and a polarizer were positioned in crosswise fashion on either side of the assembly, in order to have extinction of the light at the downstream detector. An upstream laser source, the wave vector of which is collinear with the magnetic field, was then passed. At each value B of the magnetic field, the analyser was rotated by an angle (rad) in order to again obtain the extinction of the light at the detector.

(102) The results obtained are represented in the appended FIG. 12. The two graphs show the measurements carried out for 2 different laser sources (HeNe at 632.8 nm (FIG. 12a) and Nd:YAG at 1064 nm (FIG. 12b)). In these figures, the angle (in rad) is a function of the magnetic field B (in mT). The linear regression of the straight lines obtained gives the Verdet constant V().

(103) Three samples Tb.sub.2O.sub.3-1, Tb.sub.2O.sub.3-2 and Tb.sub.2O.sub.3-3 prepared according to the process described the present in example were thus tested at 632.8 nm and one sample (Tb.sub.2O.sub.3-2) was tested at 1064 nm.

(104) The V() values for Tb.sub.2O.sub.3 were compared with those of TGG, which is currently the most efficient material in terms of values of Verdet constant (V.sub.TGG(632.8 nm)=134 rad.Math.T.sup.1 .Math.m.sup.1; V.sub.TGG(1064 nm)=40 rad.Math.T.sup.1 .Math.m.sup.1).

(105) The results obtained show that the values of Verdet constant for Tb.sub.2O.sub.3 according to these 2 wavelengths are at least three times greater than that of TGG.

COMPARATIVE EXAMPLE 2

Crystallogenesis of Pure Tb2O3 Single-Crystals According to a Process NOT in Accordance with the Invention

(106) In this example, pure single-crystal cubic terbium sesquioxides Tb.sub.2O.sub.3 were prepared according to a process not in accordance with the invention, that is to say by using [Li.sub.6Tb(BO.sub.3).sub.3] as solvent for the synthesis (solvent not comprising Li.sub.2O) with 17 mol % of solute Tb.sub.2O.sub.3.

(107) 1) First Stage: Preparation of the Solute Tb.sub.2O.sub.3

(108) The solute Tb.sub.2O.sub.3 was prepared as indicated above in stage 1) of example 2.

(109) The Tb.sub.2O.sub.3 powder was then subsequently used in the crystal synthesis in the hour which followed the end of the heat treatment mentioned above in order to prevent any reuptake of moisture.

(110) 2) Second Stage: Preparation of the Solvent [Li.sub.6Tb(BO.sub.3).sub.3] for the Synthesis

(111) The solvent [Li.sub.6Tb(BO.sub.3).sub.3] for the synthesis was prepared according to reaction 3 described above in stage 2) of example 2, following the same protocol.

(112) 83.8504 g of solvent [Li.sub.6Tb(BO.sub.3).sub.3] for the synthesis were thus obtained, which solvent was subsequently ground using a mortar and a pestle made of agate.

(113) 3) Third Stage: Crystallogenesis of the Tb.sub.2O.sub.3 Single-Crystals

(114) 83.8504 g (83 mol %) of [Li.sub.6Tb(BO.sub.3)] obtained above in stage 2) and 16.6662 g (17 mol %) of Tb.sub.2O.sub.3 prepared above in stage 1) were mixed and intimately ground together in a mortar in order to obtain the finest particle size possible and the most homogeneous mechanical mixture possible. This mixture was subsequently placed in a platinum crucible.

(115) Crystal growth was subsequently carried out under the same conditions as those described in detail above in stage 4) of example 2.

(116) An agglomerate of Tb.sub.2O.sub.3 single-crystals was thus obtained; the photograph of this is given in the appended FIG. 13. In this Figure, the Tb.sub.2O.sub.3 crystals were placed on graph paper (1 square=1 mm). It was observed that the crystals exhibit a size of the order of a few millimetres. Furthermore, the Laue diffraction diagram obtained by the Laue method is represented in the appended FIG. 14. The measurements were made on an oriented (1 1 0) natural face of a Tb.sub.2O.sub.3 single-crystal on a goniometer sold under the reference GM WS series X-ray by Delta Technologies International, using a molybdenum anticathode and a CCD camera of the Photonics Science brand. This diagram is in accordance with the expected theoretical structure and confirms the cubic structure of one of the Tb.sub.2O.sub.3 single-crystals obtained.