JP6246084B2 - Infrared transmitting material - Google Patents

Description

  The present invention is a material for constituting lenses, mirrors, filters, prisms, etc. used in various devices or apparatuses using infrared rays, such as temperature measuring devices, fire detection devices, human body detection devices, thermography, infrared cameras, infrared laser systems, etc. The present invention relates to an infrared transmitting material that can be used.

  Infrared rays are widely used in devices and apparatuses such as temperature measurement devices, fire detection devices, human body detection devices, thermography, infrared cameras, and infrared laser systems. For these devices or apparatuses, a material that transmits infrared rays is essential.

Examples of infrared transmissive materials that can be used for this type of application include Si, Ge, ZnS, ZnSe, KRS-5 (solid solution of TlI and TlBr), single crystals such as CaF ₂ and Al ₂ O ₃ , and chalcogenides. Glass, fluoride glass and GeO ₂ glass are known.

Regarding Ge, for example, Patent Document 1 discloses an infrared zoom lens made of only germanium.
Regarding chalcogenide glass, for example, in Patent Document 2, as chalcogenide glass excellent in infrared transmittance, Ge is 2 to 22%, Sb or Bi is 6 to 34%, Sn or Zn is 1 to 20%, S, Se or A chalcogenide glass containing 58-70% Te is disclosed.

  Patent Document 3 discloses an infrared hybrid lens obtained by integrating chalcogenide glass by mold press molding with an infrared transmitting lens base material made of zinc sulfide (ZnS) or silicon (Si). No. 4 discloses a lens made of chalcogenide glass mainly containing sulfur, selenium, tellurium and the like together with a lens mainly containing germanium.

JP 2011-186071 A JP 2009-161374 A JP 2006-220705 A JP 2014-2182 A

Sandy L. Nguyen.``Photoconductivity in TI6SI4: A Novel Semiconductor for Hard Radiation Detection ''. CHEMISTRY OF MATERIALS, 2013.25, P2868-2877

  When an optical lens is industrially manufactured using an infrared transmitting material, the infrared transmitting material includes 1) excellent infrared transmission characteristics in a wavelength range: λ = 0.78 μm to 14 μm, and 2) a wavelength range: λ = Infrared refractive index having a refractive index of about 2.0 to 4.0 at 0.78 μm to 14 μm, 3) Moldability required to have a melting point of 600 ° C. or lower, and 4) Infrared by moisture absorption Characteristics such as moisture resistance that do not deteriorate the permeability are required.

However, since the materials conventionally proposed as infrared transmitting materials did not satisfy all the characteristics 1) to 4), there were problems in industrial use.
For example, Ge has a problem that the raw material cost is high and the melting point is 938 ° C., making it difficult to mold.
ZnS not only has a narrow transmission wavelength range of 8 μm to 13 μm, but also has a high melting point of 1700 ° C. and sublimates at 1180 ° C., making it difficult to mold.
Since the chalcogenide glass contains Ge, it has a problem that not only the raw material cost increases, but also the transmission wavelength region is as narrow as 8 μm to 12 μm.
In addition, KRS-5 (a solid solution of TlI and TlBr) known as a low melting point infrared transmission material has a problem that it is not only very brittle but also deteriorates infrared transmission due to moisture absorption.

  Therefore, the present invention intends to provide a new infrared transmitting material which is excellent in any of infrared transmission characteristics, infrared refractive characteristics, moldability and moisture resistance.

The present invention relates to an infrared transmitting material comprising a single crystal or a polycrystal containing Tl, S and I, for example, a formula: Tl _{4 + 2x} S _x I ₄ (where x = 0.95 to 1.10). An infrared transmitting material composed of a single crystal or a polycrystal that can be expressed is proposed.

  The infrared transmitting material proposed by the present invention is excellent in all of infrared transmission characteristics, infrared refractive characteristics, moldability and moisture resistance, and is particularly excellent as a material for infrared transmission mold lenses, for example. .

Non-Patent Document 1 describes Tl ₆ SI ₄ , but only describes an action or effect that can directly convert X-rays into an electrical signal. Regarding light transmittance in the infrared region, It is not described at all.

a) is an XRD profile obtained by powder X-ray diffraction measurement of the crystal obtained by zone melting in Example 1, and b) is an X-ray obtained by powder X-ray diffraction measurement of the purified crystal finally obtained in Example 1. C) is an X-ray profile obtained by powder X-ray diffraction measurement of the single crystal finally obtained in Example 2. 2 is an X-ray diffraction pattern by single crystal X-ray diffraction measurement of the purified crystal finally obtained in Example 1. FIG. It is the figure which showed the infrared region external transmittance in the mid-infrared region-far-infrared region of the purified crystal finally obtained in Example 1. 3 is an XRD diffraction pattern by single crystal X-ray diffraction measurement of the purified crystal finally obtained in Example 2. FIG. It is the figure which showed the infrared region external transmittance in the mid-infrared region-far-infrared region of the refined crystal finally obtained in Example 2. It is the figure which showed the infrared region external transmittance in the mid-infrared region-far-infrared region before and after the humidity resistance test of KRS-5. It is the figure which showed the infrared region external transmittance in the mid-infrared region-far-infrared region before and after the moisture resistance test of the Tl ₆ SI ₄ polycrystal obtained in Example 1. It is the figure which showed the external transmittance in the visible light region-the mid-infrared region of the Tl ₆ SI ₄ polycrystal obtained in Example 1.

  Next, the present invention will be described based on an embodiment. However, the present invention is not limited to the embodiment described below.

<Infrared transmitting material>
An infrared transmitting material (hereinafter referred to as “the present infrared transmitting material”) according to an example of the embodiment of the present invention is made of a single crystal or a polycrystal including Tl, S, and I.

  Since this infrared transmissive material may be a single crystal or polycrystal containing Tl, S and I, it may contain components other than Tl, S and I. However, it is preferable that Tl, S, and I are the main components, and Tl, S, and I occupy 96% by mass or more, especially 99% by mass or more, and especially 99.99% by mass or more (including 100% by mass). Those are particularly preferred.

  When a single crystal containing Tl, S and I is compared with a polycrystal containing Tl, S and I, the single crystal is superior in terms of internal transmittance, but the infrared transmission characteristics In addition, it has been confirmed that it is inferior in terms of infrared region refraction, hardness characteristics, moldability, moisture resistance, and the like.

The infrared transmitting material is preferably a single crystal or a polycrystal that can be represented by the formula: Tl _{4 + 2x Sx} I ₄ (where x = 0.95 to 1.10).
The single crystal or polycrystal that can be represented by the formula: Tl _{4 + 2x} S _x I ₄ (where x = 0.95-1.10) is excellent in the wavelength range: λ = 0.78 μm to 37 μm. Infrared transmission characteristics, wavelength range: λ = 0.78 μm to 14 μm refractive index of 2.0 to 4.0, excellent infrared refractive index, excellent hardness characteristics, melting point at 440 ° C. In addition, it has not only the property of being moldable, but also has moisture resistance that does not deteriorate infrared transmission due to moisture absorption, and is particularly excellent as an infrared transmission material.

In the single crystal or polycrystal that can be represented by the formula: Tl _{4 + 2x} S _x I ₄ , the stoichiometry x = 1.0 is within a predetermined range, that is, x = 0. It is not easy to control to .95 to 1.10. As will be described later, in zone melt refining (band refining), thallium iodide is likely to become rich in thallium iodide due to evaporation and re-mixing, and therefore, special measures as described later are required. For example, in zone melting refining, the inside of the ampoule is made an inert gas atmosphere, and the heating temperature is made as low as possible, specifically 440 to 450 ° C., thereby suppressing the evaporation of thallium iodide and reducing the number of refining times. After performing at least 50 times, it is necessary to devise such that only the high-purity tip is taken out and placed in another ampoule and further purified 50 times or more.

The infrared transmitting material may be a single phase having a crystal structure represented by the formula: Tl _{4 + 2x} S _x I ₄ (where x = 0.95 to 1.10). It may include a heterogeneous phase having a crystal structure such as TlI. However, the single phase is preferable.

(purity)
The infrared ray transmitting material preferably has a purity of 2N or more, more preferably 4N or more, and particularly preferably 6N or more.
In order to increase the purity of the infrared transmitting material as described above and reduce the impurity concentration, for example, as described later, in the zone melting purification, the ampoule is made an inert gas atmosphere and the heating temperature is set as low as possible. Specifically, by suppressing the evaporation of thallium iodide at a temperature of 440 to 450 ° C., after performing the number of purifications at least 50 times or more, only the high-purity tip is taken out and put in another ampule again, It is preferable to carry out purification 50 times or more. However, it is not limited to this method.

<Characteristics of this infrared transmitting material>
The present infrared transmitting material has excellent infrared region transmission characteristics at λ = 0.78 μm to 37 μm, and the refractive index at λ = 0.78 μm to 14 μm is 2.0 to 4.0. In addition, it has excellent moisture resistance that does not deteriorate infrared transmission due to moisture absorption.
Therefore, it is useful as an infrared transmitting material used in temperature measurement, fire detection, human body detection, thermography, infrared camera lens, infrared laser system, optical communication field, night vision field, composition imaging system field, high sensitivity gas sensing field, etc. It is.

Moreover, since this infrared transmissive material has a melting point of 390 to 450 ° C., it can be molded.
Since molding is possible, optical elements such as a spherical lens, an aspherical lens, a lens array, a microlens array, and a diffraction grating can be easily manufactured.

<Manufacturing method>
As an example of the method for producing the infrared transmitting material, a predetermined amount of TlI powder and a predetermined amount of Tl ₂ S powder are mixed, the mixture is sealed in a glass tube, heated and vacuum-melted, and Tl-SI The synthesized compound is subjected to predetermined purification, and the purified crystal obtained is enclosed in a glass tube again and subjected to zone melt purification (band purification). A method for obtaining an infrared transmitting material (“the present infrared transmitting polycrystalline body”) can be mentioned.
Then, if necessary, the infrared transmission polycrystalline material thus obtained is crystal-grown, polished as necessary, and the infrared transmission material made of a single crystal (“infrared transmission material”). Single crystal ") can be obtained.

As raw materials, single-phase TlI and single-phase Tl ₂ S are preferably manufactured or purchased and prepared, and these are preferably used as raw materials.
At this time, if either TlI or Tl ₂ S has a different phase, the single crystal produced is likely to have a different phase, which is not preferable.

  The atmosphere in the sealed tube when synthesizing the Tl-SI compound is preferably an inert gas atmosphere such as argon, and the heating temperature at that time is preferably equal to or higher than the melting points of the raw material and the product. From this viewpoint, it is particularly preferably 440 ° C. or higher, particularly 500 ° C. or higher, and particularly preferably 550 ° C. or higher. The upper limit temperature varies depending on the amount of the apparatus and raw materials, but as a guideline, it is more preferably 700 ° C. or lower, particularly 650 ° C. or lower. The heating time is preferably several minutes or longer, preferably 6 hours or longer, and appropriately adjusted depending on the heating temperature.

  As a purification method, a Tl-S-I compound is put in an ampoule and sealed as an inert atmosphere such as argon, and then the zone melt purification (zone purification) in which the ampoule is heated from the surrounding with a moving heater is repeatedly performed. Is preferred.

In zone melt refining (zone refining), the temperature during the zone melting refining is set as low as possible, specifically 440 to 450 ° C., and the number of refining is reduced at least 50 times while suppressing evaporation of thallium iodide. After the above, it is preferable to take out only the high-purity tip and place it again in another ampoule and further refine it 50 times or more. In addition, since the refinement | purification effect will become difficult to be acquired when the temperature in the zone melting refinement is lower than 440 degreeC, and it may isolate | separate when it is higher than 450 degreeC, about 440-450 degreeC is preferable.
Thereby, evaporation of thallium iodide can be suppressed, and furthermore, remixing of evaporated thallium iodide can be suppressed.

  The crystal growth method is arbitrary as long as it can grow a single crystal. Examples include the Czochralski method (CZ method), the slow cooling method (GE method), the horizontal Bridgman method (HB method), the vertical Bridgman method (VB method), and the traveling heater method (THM method).

  The polishing method is also arbitrary. For example, polishing with abrasive paper or wet polishing may be employed as appropriate.

<Explanation of words>
In the present invention, “infrared rays” means a near-infrared (0.78 to 2.0 μm), mid-infrared (2.0 to 4.0 μm), and far-infrared (wavelength) distributed in a wavelength range of 0.78 to 1000 μm. 4.0 to 1000 μm).
In the present invention, when expressed as “X to Y” (X and Y are arbitrary numbers), “X is preferably greater than X” or “preferably greater than Y” with the meaning of “X to Y” unless otherwise specified. The meaning of “small” is also included.
In addition, when expressed as “X or more” (X is an arbitrary number) or “Y or less” (Y is an arbitrary number), it is “preferably greater than X” or “preferably less than Y”. Includes intentions.

  Hereinafter, the present invention will be described in more detail based on examples.

[Example 1]
<Preparation of sample>
1) A mixture of 37.5 TlI single-phase (4N) powder and 12.5 g of Tl ₂ S single-phase (4N) powder was placed in a glass tube, and then the glass tube was used as an argon atmosphere of 0.5 atm. Sealed.
2) The glass tube sealed in 1) was heated at 600 ° C. for 6 hours to synthesize a Tl-SI compound in the glass tube.
3) The Tl-SI compound synthesized in 2) was repeatedly subjected to zone melt purification (zone purification) at 440 ° C. using a moving heater 50 times, and the purified part was taken out to obtain a crystal. .
4) The crystal obtained in 3) was pulverized in an agate mortar, and powder X-ray diffraction measurement (XRD measurement) of the obtained powder was performed by RINT-TTR III (manufactured by Rigaku Corporation). CuKα rays were used for the measurement, the acceleration voltage was 50 kV, and the applied current was 300 mA. The evaluation results are shown in FIG. From the measurement results, it was confirmed that the powder showed a peak of Tl ₆ SI ₄ structure.
5) After putting 10.0 g of the powder obtained in 4), that is, Tl ₆ SI ₄ powder, in a glass tube, the glass tube was sealed in an argon atmosphere of 0.5 atm.
6) The glass tube sealed in 5) was repeatedly subjected to zone melt purification (zone purification) at 440 ° C. 50 times using a moving heater, and only a high purity tip was taken out to obtain a purified crystal (sample). .

<Evaluation of sample (purified crystal)>
After the purified crystal obtained by the above-described method was polished with a lapping film having a particle size of 0.3 μm, single crystal X-ray diffraction, infrared transmittance, refractive index and Vickers hardness were evaluated as follows. Further, the crystal was pulverized in an agate mortar, and powder X-ray diffraction measurement (XRD measurement) was performed. The purity of the obtained purified crystal was 6N.

(Single crystal X-ray diffraction)
Single crystal X-ray diffraction was performed using Smart Lab X-RAY DIFFRACT METER (manufactured by Rigaku Corporation). CuKα rays were used for the measurement, the acceleration voltage was 40 kV, and the applied current was 30 mA.
The X-ray diffraction pattern is shown in FIG. From the measurement results, a profile in which streaks were observed was confirmed, and it was found that the purified crystal obtained above was a polycrystal.

(Refractive index)
The refractive index was derived by evaluating in the visible wavelength range using ESM-300 (manufactured by JAWoolam) and extrapolating to the infrared wavelength range.
As a result of derivation, the extrapolated value of refractive index at a wavelength of 10 μm was 2.37.

(Infrared external transmittance)
The infrared external transmittance was measured using Vertex70v (manufactured by Bruker), and the measurement results are shown in FIG. From this measurement result, it was found that the purified crystal had excellent infrared transmission characteristics in the measurement wavelength range of 2.5 μm to 25 μm.

The measurement result of the infrared region external transmittance was converted to the infrared region internal transmittance by the following method, and the magnitude of the transmission loss inside the crystal was evaluated.
First, the reflectance R on the crystal surface was calculated using the Fresnel equation (1). Here, n represents a refractive index.

(1) ... R = {(n-1) / (n + 1)} ²

Then, by considering the multiple reflections occurring at the surface and the back surface of the crystal, the reflection loss ratio R _loss of the entire crystal was calculated, further, the internal transmittance% T _i and the external transmittance% T _o relation (2 ) The external transmittance was converted to the internal transmittance.

(2)...% T _i =% T _o / (1−R _loss )

  Further, the obtained internal transmittance was converted to a value at a thickness of 2 mm using Lambert Beer's equation (3). Here, κ is an absorption coefficient, and t is a thickness. As a result of estimating the internal transmittance, the internal transmittance was calculated to be 96.4% at a wavelength of 10 μm, and it was found that the transmission loss inside the crystal could be suppressed.

(3)...% T _i = exp (−κt)

(Vickers hardness)
Vickers hardness was measured using MICRO HARDNESS TESTER MHT2 (Matsuzawa).
As a result of measurement, it was found that the purified crystal had a hardness of Hv = 61 kg / mm ² .

(Powder XRD measurement)
For powder XRD measurement, RINT-TTR III (manufactured by Rigaku Corporation) was used. CuKα rays were used for the measurement, the acceleration voltage was 50 kV, and the applied current was 300 mA. The X-ray profile is shown in FIG.
As a result of XRD measurement, it was found to be single phase Tl ₆ SI ₄ .

[Example 2]
<Preparation of sample>
1) After putting 10.0 g of Tl ₆ SI ₄ powder obtained in 4) of Example 1 into a glass tube, the glass tube was sealed in an argon atmosphere of 0.5 atm.
2) The glass tube sealed in 1) was repeatedly subjected to zone melt purification (zone purification) at 440 ° C. 50 times using a moving heater, and the purified portion was taken out to obtain a purified crystal.
3) The purified crystal obtained in 2) was grown by a traveling heater method (THM method) at a heating temperature of 440 ° C. and a growth rate of 5 mm / hour to obtain a single crystal (sample).

<Evaluation of sample (single crystal)>
After the single crystal obtained by the above method was polished with a lapping film having a particle size of 0.3 μm, single crystal X-ray diffraction, infrared transmittance, refractive index and Vickers hardness were evaluated in the same manner as in Example 1. . Further, the single crystal was pulverized in an agate mortar, and powder X-ray diffraction measurement (XRD measurement) was performed.
The purity of the obtained single crystal was 6N.

Single crystal X-ray diffraction was performed in the same manner as in Example 1, and the X-ray diffraction pattern is shown in FIG. From this measurement result, a spot was confirmed, and it was found to be a single crystal.
The refractive index was measured in the same manner as in Example 1, and as a result of derivation, the extrapolated refractive index at a wavelength of 10 μm was 2.37.
The infrared external transmittance was measured in the same manner as in Example 1, and the measurement results are shown in FIG. From this measurement result, it was found that the crystal had excellent infrared transmission characteristics at a measurement wavelength of 2.5 μm to 25 μm. Further, as a result of measuring the infrared region external transmittance, it was converted into the infrared region internal transmittance in the same manner as in Example 1, and the magnitude of the transmission loss inside the crystal was evaluated. As a result, the internal transmittance at a wavelength of 10 μm was 99. It was calculated to be 8%, and it was found that transmission loss inside the crystal could be suppressed.
As a result of measuring the Vickers hardness in the same manner as in Example 1, it was found that the hardness was Hv = 61 kg / mm ² .
As a result of powder XRD measurement as in Example 1, the X-ray profile of the single crystal obtained in Example 2 is shown in FIG. As a result of XRD measurement in this manner, it was found to be a single-phase Tl ₆ SI ₄ .

[Examples 3 and 4]
<Preparation of sample>
In Example 3, purified crystals were obtained in the same manner as in Example 1 except that the amount of the raw material was changed to 37.9 g of TlI single-phase (4N) powder and 12.1 g of Tl ₂ S single-phase (4N) powder. Got the body.
In Example 4, purified crystals were obtained in the same manner as in Example 1 except that the amount of the raw material was changed to 36.8 g of TlI single phase (4N) powder and 13.2 g of Tl ₂ S single phase (4N) powder. A body (sample) was obtained.

<Evaluation of sample (purified crystal)>
After the purified crystal (sample) obtained in Examples 3 and 4 was polished with a lapping film having a particle size of 0.3 μm, the infrared transmittance and refractive index were evaluated in the same manner as in Example 1. Further, the purified crystal was pulverized in an agate mortar, and powder X-ray diffraction measurement (XRD measurement) was performed in the same manner as in Example 1. Further, the composition of the powder was analyzed by ICP emission analysis.
The purity of the purified crystals obtained in Examples 3 and 4 was 6N.

  In the ICP emission analysis method, a sample was dissolved in a mixture of hydrochloric acid and nitric acid, diluted appropriately, and then Tl, S, and I in the liquid were quantified using an ICP emission analyzer. The Tl, S, and I composition (% by weight) of the powder was calculated from the weight of the powder and the dilution rate.

As a result of the above evaluation, the refractive index was derived as 2.37 at the wavelength of 10 μm for the samples of Examples 3 and 4 from the refractive index evaluation results.
From the evaluation results of the infrared region external transmittance, it was confirmed that both the samples of Examples 3 and 4 have excellent infrared region transmission characteristics at a measurement wavelength of 2.5 μm to 25 μm.
As a result of calculating the infrared region internal transmittance, the infrared internal transmittance at a wavelength of 10 μm was calculated to be 95.8% in Example 3 and 96.1% in Example 4. For this reason, it was found that any of the samples of Examples 3 and 4 can suppress transmission loss inside the crystal.
From the results of powder X-ray diffraction measurement (XRD measurement), it was found that all the samples of Examples 3 and 4 were single-phase Tl ₆ SI ₄ .

Further, as a result of ICP emission analysis, _{x in the} formula: Tl _{4 + 2x} S _x I ₄ was calculated from the respective Tl, S, and I composition values. As a result, in Example 3, x = 0.96. 4, it was confirmed that x = 1.08.

Results of producing and evaluating polycrystals and single crystals by shifting the composition from the stoichiometric composition ratio of Tl ₆ SI ₄ to the Tl ₂ S rich side and Tl ₂ S poor side as in Examples 3 and 4 It was found that at least a single crystal and a polycrystal represented by the composition formula: Tl _{4 + 2x Sx} I ₄ (0.95 ≦ x ≦ 1.10) are effective as a far-infrared transmitting material. .

[Reference test]
Accelerated deterioration test shown in the following steps 1) and 2) regarding the moisture resistance of KRS-5 (product number S5 / 13-2 manufactured by Pier Optics) and the Tl ₆ SI ₄ polycrystal obtained in Example 1 It was evaluated by.

1) The sample (KRS-5) and the polycrystal of Tl ₆ SI ₄ obtained in Example 1 were each formed into a 1 cm square and a thickness of about 2 mm, and immersed in a beaker filled with 50 mL of water, Ultrasound was irradiated for 15 minutes.
2) The infrared external transmittance of the sample after the accelerated deterioration test of 1) was evaluated, and the change in infrared transmission characteristics before and after the test was evaluated.
Infrared external transmittance was measured using Vertex70v (manufactured by Bruker).

FIG. 6 shows the measurement result of KRS-5. From the measurement results, it was found that the transmittance of KRS-5 significantly decreased at a wavelength of 2.5 μm to 14 μm due to the execution of the moisture resistance test.
On the other hand, FIG. 7 shows the measurement results of the Tl ₆ SI ₄ polycrystal obtained in Example 1. From this measurement result, no decrease in transmittance was observed, and it was confirmed that the Tl ₆ SI ₄ polycrystal was superior in moisture resistance compared to KRS-5.
The deterioration rate was confirmed to be 20% or more for KRS-5 and 2% or less for Tl ₆ SI ₄ polycrystal in the wavelength range of 8 to 14 μm.
Here, the deterioration rate was calculated using the following equation.
Degradation rate (%) = [1− (external transmittance after moisture resistance test / external transmittance before moisture resistance test)] × 100

[Additional tests]
With respect to the purified crystal obtained in Example 1, that is, the Tl ₆ SI ₄ polycrystal, the transmission characteristics in the visible light region to the mid-infrared region were evaluated using U-4100 (manufactured by Hitachi High-Technologies Corporation).
FIG. 8 shows the measurement results of the transmission characteristics in the visible light region to the mid-infrared region. From this measurement result, the material has an absorption edge in the vicinity of the wavelength of 0.6 μm, while in the near infrared region to the mid infrared region from the wavelength of 0.78 μm to 2.5 μm, it is far from the mid infrared region shown in FIG. It was confirmed that the transmittance was as high as that in the infrared region.
From the above evaluation results, it was confirmed that Tl ₆ SI ₄ is effective as a near-infrared transmitting material.

Claims (3)

  An infrared transmitting material comprising a single crystal or a polycrystal containing Tl, S and I. The infrared transmitting material according to claim 1, comprising a single crystal or a polycrystal that can be represented by the formula: Tl _{4 + 2x} S _x I ₄ (wherein x = 0.95 to 1.10). The infrared transmitting material according to claim 1, wherein the infrared transmitting material is produced using single-phase TlI and single-phase Tl ₂ S as raw materials.