JP6037871B2 - Crystal growth method for rare earth sesquisulfides - Google Patents

Description

In the field of chemical technology, the present invention relates to sulfides, in particular, high-temperature transformation γ-Ln ₂ S ₃ having a Th ₃ P ₄ structure (Ln is a general term for Y, Sc, and lanthanoid rare earths, hereinafter the same). ), And a method for crystallizing and growing rare earth sesquisulfide crystals doped by Sn doping.

  Rare earth sesquisulfides have a very high affinity with oxygen atoms at high temperatures, so oxygen and moisture can interact with sulfides and convert to oxysulfides to crystallize and grow crystals. There is a need to carry out the process in a closed exhaust reactor.

  The melting point of rare earth sesquisulfides is very high, and in the melting point region at about 2000 ° C., there is no suitable structural material that is inert to the melt and vapor and at the same time provides high hermeticity to the reactor. Therefore, there are technical difficulties in growing sulfide from the melt. Furthermore, in order to prevent the loss of S (sulfur) due to thermal dissociation of the sulfide melt, it is necessary to create a required sulfide vapor pressure in various atmospheres.

Another feature in the crystallization and growth of rare earth sesquisulfides is the presence of multiple multiple transformations. In rare earth sesquisulfides, from the low temperature side, polymorphs such as α phase, β phase, and high temperature transformation γ-Ln ₂ S ₃ having a Th ₃ P ₄ structure exist at 1300 ° C. or higher. Among these, high-temperature transformation γ-Ln ₂ S ₃ having a Th ₃ P ₄ structure exhibits high thermoelectric properties and is most advantageous for technical applications.

However, for its growth, temperatures above 1300 ° C. are required (lower boundary of thermodynamic stability), in which case quartz is unsuitable as a reactor. To grow a high-temperature transformation γ-phase crystal having a Th ₃ P ₄ structure of rare earth sesquisulfide using a quartz reactor, the temperature of the lower boundary is lowered to ensure the stability of the high-temperature transformation morphology. Special methods or low temperature methods must be developed.

  Below, the well-known example of the manufacturing method of a rare earth sesquisulfide crystal is shown.

Crystal growth of γ-Ln ₂ S ₃ from the melt at a temperature of about 1800 to 2400 ° C. under 1 atm sulfur vapor pressure produces a large crystal lump with a Th ₃ P ₄ structure and a diameter of 15 mm and a length of 20 mm. It is known (see Non-Patent Document 1).

  In this case, powdered sesquisulfide is placed in a graphite or amorphous graphite crucible having a conical bottom, and this crucible is placed in a quartz reactor of a specific form. A chamber containing sulfur is present in the upper part of the reaction vessel. By heating the sulfur to the boiling temperature, the required sulfur vapor partial pressure can be created in the reaction vessel.

  The inside of the crucible is selectively heated to about 2000 ° C. using high-frequency heating to dissolve the rare earth sesquisulfide, but the temperature of the inner wall of the quartz reaction vessel is set to 1200 by a heat-resistant material that shields from the crucible. It can be kept below ℃. The quartz reaction vessel is withdrawn through the HF-reducer and crystal growth proceeds for several hours.

  Crystal growth from the melt has the following drawbacks. That is, i) Since a high temperature is used, the crystallization time must be shortened, and as a result, the crystal size becomes small. Otherwise, the quartz reaction vessel and the amorphous graphite crucible will rapidly deteriorate. ii) The crystal structure is usually sulfur deficient and deviates greatly from the stoichiometric ratio. iii) Since internal stress is generated through uneven heating of the crucible, additional annealing is required to heat to 1300 ° C. under the required sulfur vapor partial pressure.

Next, it is known to grow rare earth sulfides from the gas phase by chemical reaction at 1150 to 1200 ° C. using I ₂ as a transport material (see Non-Patent Document 2). In this case, a powder of any rare earth sesquisulfide or a mixture of multiple rare earth sesquisulfides (metal mixed sulfide) is sealed with solid I ₂ and placed in the high temperature zone (1200 ° C.) of the quartz reactor.

In this high temperature zone, volatile components with high vapor pressure (LnI ₃ and sulfur) are produced. In the low temperature zone (1150 ° C.), crystals grow due to the gas transport of the volatile components (LnI ₃ and sulfur) and the chemical reaction between them. By this method, needle-like crystals (1.0 × 2.2 × 14 mm size) of a composite sulfide of Ln ₂ S ₃ can be obtained.

The disadvantages of this method are as follows. That is, i) In the rare earth sesquisulfide growth part, the reactivity of LnI ₃ vapor with respect to quartz is high, so that the crystallization time has to be shortened, and the crystal size becomes small. ii) Due to the low vapor pressure of LnI ₃ , the transport rate is low, thus the crystal size is small and the product yield is low. iii) Since the crystal growth field is less than 1200 ° C., a low temperature phase (α phase or β phase) may occur.

  In addition, in order to grow high-temperature phase crystals of rare earth sesquisulfides below the phase transition temperature, doping of rare earth sesquisulfides with s block elements and d block elements has been attempted.

By sulfiding a composite mixture of La ₂ S ₃ , SnO ₂ , and CaCO ₃ with Ar + sulfur vapor pressure flow, and by doping Sn and Ca into La ₂ S ₃ to form a γ phase at 900 ° C. It has been reported that a uniform γ phase can be obtained even at 900 ° C. which is lower than the transition temperature (see Patent Documents 1 and 2).

The conventional method closest to the method of the present invention is a method of growing rare earth sesquisulfide from an LnI ₃ —S solution using LnI ₃ as a solvent (see Non-Patent Document 3).

A mixture of RE, S, and I _{2 in} an atomic ratio of 1: 1: 1 is placed as a starting charge in a graphite crucible and further placed in a quartz reactor, and the reactor is evacuated and sealed. The starting temperature is 600 ° C. and the reactor is held at this temperature for 1-2 hours to synthesize the ternary compound LnSI. Thereafter, the temperature rises to 1100 to 1275 ° C. (the temperature rise depends on the type of Ln), LnSI is decomposed, and Ln ₂ S ₃ crystals are precipitated in the LnI ₃ melt.

  The latter phase representing the melt is a solvent and crystal growth appears to be a recrystallization of the rare earth sulfide. The grown crystals are separated from the solvent by water or ethanol treatment. In this method, no doping effect was observed.

The disadvantages of this method are as follows. (1) The reactivity of LnI ₃ vapor and quartz reactor is high, and the growth process time must be greatly shortened. As a result, the crystal size is in the range of 1 × 1 × 10 mm at the maximum. . (2) Since the reaction temperature is not higher than the phase transition temperature, it is impossible to grow a high-temperature transformation γ phase of a sulfide having a cubic Th ₃ P ₄ structure having a lower stable boundary exceeding 1275 ° C.

  (3) In this method, when La, Er, Tm, and Y are used as Ln, the decomposition temperature of the ternary phase LnSI is 1250 ° C. or higher, which is not suitable for growing rare earth sesquisulfides. (4) The loss of rare earth during the leaching process of the final product is as large as about 33%.

  Some of the known methods described above are in common with the method of the present invention. It is the introduction of starting components into a heat-resistant crucible, the installation of the crucible in a quartz reactor, the exhaust and sealing of the reactor, and the heating in the furnace.

Japanese Patent No. CN101200604 Japanese Patent No. RU2388773

A. A. Kamarzin, K .; E. Mironov, V.M. V. Sokolov, Yu. N. Malovsky, I.M. G. Vasilyeva // J. Cryst. Growth. 1981, V52, P619-622 M.M. Leiss // J. Phys. C: Solid State Phys. V13, P151-158 A. W. Sleight and C.I. T. T. Prewitt // Inorg. Chem. 1968. V7, No. 11, P2282-2288

The object of the present invention is to i) growth of a high temperature transformation γ phase of Ln ₂ S ₃ having a Th ₃ R ₄ type cubic structure; ii) increase in crystal size; iii) various combinations of starting compounds To provide a method for growing a sulfide, especially a rare earth sesquisulfide crystal from a melt, which enables expansion of the method and iv) minimization of S (sulfur) deficiency in each step of the crystal process. is there.

The inventors have used a novel solvent, SnS, to separate SnS from grown crystals, and using a combination of doping process and crystallization that allows the formation of high temperature transformation gamma phases of sulfides. We tackled the problem of growing rare earth sesquisulfide crystals from a melt of Ln ₂ S ₃ and SnS.

  The present invention has been made based on the results, and the gist thereof is as follows.

(1) A step of putting starting components into a thermally stable crucible, a step of placing the crucible in a quartz reaction vessel, a step of evacuating and sealing the reaction vessel, and heating the vessel apparatus in a furnace In the crystal growth method including the step of:
(A) (a1) a rare earth sesquisulfide (a rare earth is one selected from the group consisting of Y, Sc and a lanthanoid rare earth group (hereinafter referred to as Ln)) and tin sulfide, or (a2) A mixture of a rare earth sesquisulfide and tin sulfide containing a plurality of rare earths (two or more selected from the above Ln), or (a3) a stoichiometric amount of rare earth to produce rare earth sesquisulfide and tin monosulfide A mixture of element (one or more selected from the above-mentioned Ln), Sn, and S, and in addition to that, excess SnS as a solvent is used as a starting component
(B) heating the quartz reaction vessel containing the starting components to a temperature higher than the melting point of SnS and holding until the resulting melt is completely homogenized;
(C) forming a temperature gradient in the reaction vessel to cause SnS evaporation and mass transfer from the crucible towards the relatively cooler portion of the reaction vessel;
(D) enabling growth of sulfide crystals containing a high temperature transformation γ-Ln ₂ S ₃ having a Th ₃ P ₄ structure;
A method of growing a rare earth sesquisulfide crystal characterized by crystallizing and growing a rare earth sulfide crystal accompanied by sulfide doping with Sn.

  (2) As a starting component, a bottom surface of a crucible using a mixture of rare earth sesquisulfides (rare earth is one selected from the rare earth group of Y, Sc and lanthanoids (hereinafter collectively referred to as Ln)) and tin sulfide. The method of growing a rare earth sesquisulfide crystal as described in (1) above, wherein the crystal is crystallized and grown from the seed crystal placed on the substrate.

  According to the present invention, compared with the conventional method, the following becomes possible.

  i) Growing crystals of sulfide compounds, especially rare earth sesquisulfides doped with Sn (II).

ii) Growing crystals in the form of cubic high-temperature transformation γ phase having a Th ₃ P ₄ type structure.

  iii) growing rare earth sesquisulfide crystals having a larger size.

  iv) Growing crystals of various combinations of starting products using all rare earth series.

  v) Growing crystals in all steps of the crystallization process with minimal S (sulfur) deficiency.

In the crucible of quartz reaction vessel, it is a diagram showing a state in which the mixture was housed in Ln ₂ S ₃ and SnS of the starting components. The whole quartz reaction vessel shown in FIG. 1 is heated to a temperature higher than the melting point of SnS and held until the generated melt is completely homogenized, and then a temperature gradient is formed in the reaction vessel. relatively cause evaporation and mass transfer of SnS toward the low temperature part of a diagram showing crystallization crystals of Ln ₂ S _3, a state in which grown.

  The requirements for the rare earth sesquisulfide crystal growth method of the present invention (hereinafter sometimes referred to as the “method of the present invention”) different from the conventional method will be described in detail below.

  The conventional method and the method of the present invention are common in that the starting components are put into a thermally stable crucible, the crucible is placed in a quartz reaction vessel, the reaction vessel is evacuated and sealed, and the furnace is heated. The procedure is as follows.

  However, the method of the present invention functions as follows. This will be described with reference to FIGS.

As shown in FIG. 1, a mixture of Ln ₂ S ₃ and SnS as starting components 3 (or a mixture of Sn and S with a stoichiometric amount of a rare earth element for producing rare earth sesquisulfide and tin monosulfide). ) Is introduced into a crucible 2 of graphite or amorphous graphite and placed in a quartz reaction vessel 1.

  The quartz reaction vessel 1 is evacuated, sealed, and placed in a furnace (not shown) that can be controlled in two zones, a low temperature part and a high temperature part, as shown in FIG. In this case, the mixture is heated to about 1100 ° C. at 2000 ° C./hour, and for the element-derived mixture at 50 ° C./hour.

In the quartz reaction vessel 1 in which the entire furnace is at a high temperature of about 1100 ° C., Ln ₂ S ₃ is completely dissolved in the SnS melt, and a Ln ₂ S ₃ —SnS melt 4 is generated. This temperature is maintained until the melt 4 is homogenized. Neither melt 4 nor vapor interacts with quartz at this temperature. This has been observed through experiments.

  The required temperature gradient (low temperature part: about 1050 ° C. and high temperature part: about 1100 ° C.) along the quartz reaction vessel 1 is formed, and the SnS vapor from the melt 4 is solidified in the low temperature part, whereby the crucible 2 Mass transfer of SnS occurs from the inside to the low temperature part (see FIG. 2).

Due to the mass transfer of SnS, the concentration of Ln ₂ S ₃ in the melt 4 increases until reaching the liquidus of the eutectic diagram of the Ln ₂ S ₃ + SnS system without any intermediate layer, Isothermal crystals of rare earth sulfides crystallize (M. Guittard, M. Julien-Pouzol, S. Jaulemes // Mate. Res. Bull. 1976, V11, P 1073-1080).

The evaporation rate of SnS can be controlled by the temperature of the two zones of the high temperature part and the low temperature part, the temperature gradient, the geometry of the reaction vessel, and other factors, and thus the crystal size can also be controlled. As shown in FIG. 2, since the SnS solidified body 6 can be collected in the low temperature portion of the quartz reaction vessel 1, the SnS melt as a solvent is completely vaporized from the crucible 2, and only the grown Ln ₂ S ₃ crystal 5 is obtained. Can be recovered.

In this method, since there is no damage or deterioration of the crucible and the reaction vessel due to the chemical side reaction, there is no limitation on the time of the growth process. In this method, not only crystallization but also doping of rare earth sulfide with Sn (II) is performed. As a result, a crystal having a Th ₃ P ₄ structure is formed at about 1100 ° C.

In the method of the present invention, since no composite compound was found in the Ln ₂ S ₃ + SnS system, the crystal growth of sulfide compounds is possible for all rare earth series. Therefore, it is possible to grow multi-element rare earth sulfides using a mixture of suitable sulfide powders as starting products.

  Another advantage of this method compared to the conventional method is that SnS can be completely evaporated from the crucible, thus eliminating the need for a separate process to separate the sulfide crystals from the solvent. This process allows complete conversion of rare earth elements to sulfide crystals and complete collection of pure SnS solidified on the side of the crucible in the reaction vessel. The recovered SnS enables repeated use.

  There is a possibility that the orientation crystallization is performed together with the seed crystal to increase the size of the growing crystal.

Example 1
To obtain a sulfide crystals 0.3 g, in a molar _{ratio, SnS: La 2 S 3 =} 0.95: 0.05 mixture of SnS and La ₂ S _3, and placed in amorphous graphite crucible. The crucible is placed in a quartz reaction vessel and evacuated to 10 ⁻² to 10 ⁻³ Torr and sealed.

The reaction vessel is placed in a furnace having two zones, and the entire reaction vessel is heated to 1100 ° C. at a rate of about 2000 ° C./hour. The reactor is held at this temperature for 12 hours until La ₂ S ₃ is completely dissolved in the SnS melt.

  Thereafter, the temperature of the low temperature part of the reaction vessel is lowered to 1050 ° C. This temperature gradient causes SnS mass transfer from the crucible to the relatively cooler portion of the reaction vessel.

The relatively cold part functions as a collector for SnS, holding the reaction vessel in that state for 72 hours and then quenching the reaction vessel in ice water. Dark red crystals remain in the amorphous graphite crucible, which are phase homogeneous and have a size of 5 × 5 × 0.1 mm ³ . Other characteristics are shown in Table 1.

(Example 2)
To obtain a sulfide crystals 0.3 g, in a molar _{ratio, SnS: Dy 2 S 3 =} 0.95: 0.05 mixture of SnS and Dy ₂ S _3, and placed in amorphous graphite crucible. The crucible is placed in a quartz reaction vessel and evacuated to 10 ⁻² to 10 ⁻³ Torr and sealed.

The reaction vessel is placed in a furnace having two zones, and the entire reaction vessel is heated to 1100 ° C. at a rate of about 2000 ° C./hour. The reaction vessel is held at this temperature for 12 hours until Dy ₂ S ₃ is completely dissolved in the SnS melt.

  Thereafter, the temperature of the low temperature part of the reaction vessel is lowered to 1050 ° C. This temperature gradient causes SnS mass transfer from the crucible to the relatively cooler portion of the reaction vessel.

The relatively cold part functions as a collector for SnS, holding the reaction vessel in that state for 72 hours and then quenching the reaction vessel in ice water. Dark red crystals remain in the amorphous graphite crucible, which are phase homogeneous and have a size of 1 × 1 × 0.5 mm ³ . Other characteristics are shown in Table 1.

Example 3
In order to obtain 1.2 g of crystals, metal Lu powder, metal Sn crystal grains, and sulfur crystallites are placed in an amorphous graphite crucible at an atomic ratio of 0.10 Lu + 0.95 Sn + 1.10 S. The preparation procedure was performed in a dry chamber in an atmosphere of P _H20 = 0.3 ppm and P ₀₂ = 1.0 ppm.

The crucible was placed in a quartz reaction vessel, and the reaction vessel was evacuated to a maximum of 10 ⁻² to 10 ⁻³ Torr and sealed. The reaction vessel was placed in a furnace having two zones, and the entire reaction vessel was heated to 1100 ° C. at a rate of about 50 ° C./hour. The reaction vessel was held at this temperature for 4 hours, after which the temperature in the cold part of the reaction vessel was lowered to 1050 ° C.

This temperature gradient caused SnS mass transfer from the crucible to the relatively cooler portion of the reaction vessel. The reaction vessel was kept in this state for 72 hours, and then the reaction apparatus was quenched in ice water. Pale brown crystals remained in the amorphous graphite crucible, which were phase homogeneous and the size was 0.5 × 0.5 × 0.5 mm ³ . Other characteristics are shown in Table 1.

  As described above, according to the present invention, the following is possible as compared with the conventional method.

  i) Crystal growth of a sulfide compound, particularly a rare earth sesquisulfide doped with Sn (II).

ii) Growing a cubic HT-type crystal having a Th ₃ P ₄ type structure.

  iii) growing rare earth sesquisulfide crystals having a larger size.

  iv) Growing crystals of various combinations of starting products using all rare earth series.

  v) Growing crystals in all steps of the crystallization process with minimal S (sulfur) deficiency.

  Therefore, the present invention has high industrial applicability.

1 Quartz reaction vessel 2 Crucible 3 Starting component (Ln ₂ S ₃ / SnS mixture)
4 Ln ₂ S ₃ -SnS melt 5 Ln ₂ S ₃ crystal 6 SnS solidified body

Claims (2)

The step of introducing starting components into a thermally stable crucible, the step of placing the crucible in a quartz reaction vessel, the step of evacuating and sealing the reaction vessel, and the step of heating the reaction vessel in a furnace In a crystal growth method including:
(A) (a1) a rare earth sesquisulfide (a rare earth is a mixture of Y, Sc and a rare earth group of lanthanoids (hereinafter collectively referred to as Ln)) and tin sulfide, or
(a2) a mixture of a rare earth sesquisulfide containing a plurality of rare earths (two or more selected from Ln) and tin sulfide, or
(a3) Stoichiometric amount of rare earth elements (one or more selected from the above-mentioned Ln), Sn, and S for generating rare earth sesquisulfide and tin monosulfide, and a surplus as a solvent in addition thereto Using a mixture of SnS as a starting component,
(B) heating the quartz reaction vessel containing the starting components to a temperature higher than the melting point of SnS and holding until the resulting melt is completely homogenized;
(C) forming a temperature gradient in the reaction vessel to cause SnS evaporation and mass transfer from the crucible towards the relatively cooler portion of the reaction vessel;
(D) enabling growth of sulfide crystals containing a high temperature transformation γ-Ln ₂ S ₃ having a Th ₃ P ₄ structure;
A method of growing a rare earth sesquisulfide crystal characterized by crystallizing and growing a rare earth sulfide crystal accompanied by sulfide doping with Sn.   As a starting component, a rare earth sesquisulfide (rare earth is one selected from Y, Sc, and a lanthanoid rare earth group (hereinafter referred to as Ln)) and tin sulfide is used and placed on the bottom of the crucible. 2. The method for growing a rare earth sesquisulfide crystal according to claim 1, wherein the crystal is crystallized and grown from the seed crystal as a starting point.

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