New Quaternary Chalcogenides Tl₂M^IIM^IV₃Se₈ and Tl₂M^IIM^IVX₄ ^†

Abstract

New quaternary thallium-containing chalcogenides Tl₂M^IIM^IV₃X₈ and Tl₂M^IIM^IVX₄ were synthesized, and their crystal structure was determined by XRD. Three Tl₂M^IIM^IV₃X₈ chalcogenides crystallize in orthorhombic symmetry (S.G. P2₁2₁2₁; Tl₂CdGe₃Se₈ lattice parameters a = 0.76023(9), b = 1.2071(2), c = 1.7474(2) nm), eight isostructural Tl₂B^IID^IVX₄ compounds crystallize in tetragonal symmetry, S.G. I-42m. These compounds form in the quasi-ternary systems Tl₂X–M^IIX–M^IVX₂ (X–S, Se, Te) at the component ratio 1:1:1 and 1:1:3 at the sections Tl₂M^IVX₃–B^IIX and Tl₂M^IIM^IVX₄–M^IVX₂, respectively. The composition of the Tl₂CdGe₃Se₈ compound was additionally confirmed by SEM and EDS.

1. Introduction

The formation of 12 new quaternary compounds, Tl₂HgSi(Ge,Sn)S₄, Tl₂PbSi(Ge)S₄, Tl₂CdSi(Ge,Sn)Se₄, Tl₂HgSi(Ge,Sn)Se₄, Tl₂PbSi(Ge)Se₄, was found in the investigation of sulfur- and selenium-containing quasi-ternary systems Tl₂X–M^IIX–M^IVX₂ by XRD and differential thermal analysis (DTA) along the Tl₂M^IVX₃–M^IIX sections. The structure of six of them (Tl₂HgSi(Ge, Sn)Se₄, Tl₂HgSnS₄, Tl₂CdGe(Sn)Se₄) and their two analogs Tl₂Cd(Hg)SiTe₄ was determined in the isotropic approximation within the Tl₂HgGeTe₄ structure (S.G. I-42m) as a model. Along with five other tellurides Tl₂М^IIМ^IVTe₄ (М^II—Mn, Cd, Hg; М^IV—Si, Ge, Sn), these were reported in [1]. Pb-containing compounds Tl₂PbSiS₄ and Tl₂PbGeS₄ are isostructural and crystallize in the monoclinic structure, S.G. Р2₁/а).

A large number of the compounds of this type with Cu, Ag, and alkali metals are known, e.g., Li₂CdGeSe₄, Li₂CdSnSe₄, Cu₂CdSnS₄, Cu₂CdGeSe₄, Ag₂FeSnS₄. They belong to diamond-like semiconductors and have already found applications in non-linear optics and other fields of semiconductor technology, high thermal stability, and other important optical and thermoelectric properties [1,2,3].

The compounds of the 2-1-3-8 composition are known with alkali metals and Сu. They crystallize in the orthorhombic (S.G. P2₁2₁2₁: та Cs₂ZnGe₃Te₈, Cs₂CdGe₃S₈, Cs₂CdGe₃Se₈ [4]), monoclinic (S.G. Р2₁/а: Cs₂ZnGe₃S₈, α-K₂ZnSn₃S₈, [4,5,6]), and tetragonal structure (S.G. I4₁/a: Cu₂FeSn₃S₈, Cu₂CdSn₃S₈ [7]).

2. Materials and Methods

Three new quaternary selenides of the general formula Tl₂M^IIGe₃Se₈ (M^II = Zn, Cd, Hg) were obtained by direct high-temperature synthesis. The method consisted of co-melting high-purity elements thallium, zinc, cadmium, germanium, and selenium (at least 99.99 wt.% purity) and mercury selenide in evacuated to 1 × 10⁻³ torr and soldered quartz ampoules. The synthesis involved heating to 673 K at the rate of 20 K/h, 12 h exposure; heating to 1333 K at the rate of 10 K/h, 7 h exposure; cooling to 773 K at the rate of 6 K/h; homogenizing annealing at this temperature for 350 h. Finally, the ampoules were quenched into 20% aqueous saline solution.

Powder patterns for the determination of the phase composition of the synthesized samples Tl₂ZnGe₃Se_8, Tl₂CdGe₃Se₈, and Tl₂HgGe₃Se₈ were recorded at a DRON 4–13 diffractometer, Cu Kα radiation, 2θ range 10° ≤ 2θ ≤ 80°, scan step 0.05°, 5 s exposure in each point. Datasets for structure computation were recorded in the 2θ range of 10° ≤ 2θ ≤ 100°, scan step 0.05°, 20 s exposure in each point. The crystal structure of new quaternary chalcogenides was determined by Rietveld method realized in WinCSD software package [8]. Visualization of the crystal structure elements utilized Diamond software.

The investigation of the composition of the Tl₂CdGe₃Se₈ compound was additionally confirmed by SEM and EDS at a Tescan Vega 3 LMU scanning microscope (Tescan Brno s.r.o., Brno, Czech Republic) equipped with Oxford Instruments Aztec ONE X-ray microanalyzer with X-Max^N20 detector (accelerating voltage 25 kV; K-, L- and M-lines of the spectrum; magnification ×1000).

3. Results and Discussion

3.1. Phase Equilibria in the Tl₂Se–CdSe–GeSe₂ System

Isothermal sections of 14 quasi-ternary systems Tl₂X–M^IIX–M^IVX₂ (X–S, Se) at 570 K were plotted from the X-ray phase analysis results. Twelve compounds of the 2-1-1-4 type were found, Tl₂HgSi(Ge,Sn)S₄, Tl₂PbSi(Ge)S₄, Tl₂CdGe(Sn)Se₄, Tl₂HgSi(Ge,Sn)Se₄, Tl₂PbSi(Ge)Se₄. According to DTA results, they all form incongruently (formation temperatures are listed in

Table 1. Peritectic formation temperatures of the 2-1-1-4 compounds.

Compound	Temperature, K	Compound	Temperature, K
Tl2HgSiS4	654	Tl2PbSiS4	818
Tl2HgSiSe4	703	Tl2PbSiSe4	788
Tl2HgGeS4	698	Tl2HgSnS4	718
Tl2HgGeSe4	764 (congruent)	Tl2HgSnSe4	883
Tl2PbGeS4	781	Tl2CdGeSe4	809
Tl2PbGeSe4	710	Tl2CdSnSe4	860

). Additionally, seven compounds of the 2-1-3-8 composition were found, Tl₂CdGe₃Se₈, Tl₂HgSi(Ge)₃S_8, Tl₂HgSi(Ge)₃Se₈ Tl₂PbSi(Ge)₃S₈. Two analogous quaternary tellurides Tl₂Cd(Hg)SiTe₄ were also obtained.

Isothermal section of the Tl₂Se–CdSe–GeSe₂ system at 570 K is shown in Figure 1. The system at the annealing temperature features in the state of thermodynamic equilibrium nine single-phase, 17 two-phase, and nine three-phase fields. Like other thallium-containing systems, two sections are quasi-binary in the entire temperature and concentration range—Tl₂GeSe₃–CdSe where the quaternary compound Tl₂СdGeSe₄ forms, and Tl₄GeSe₄–CdSe where no new compounds were found. Investigation of the vertical section Tl₂CdGeSe₄–GeSe₂ found the formation of a new quaternary compound of approximate composition Tl₂CdGe₃Se_8. According to DTA data, its melting point is 835 K as seen in the respective endothermal effect (Figure 2).

3.2. EDS Analysis

The chemical composition of the quaternary compound Tl₂CdGe₃Se₈ that forms at the Tl₂CdGeSe₄–GeSe₂ section (1:2) was confirmed by SEM/EDS analysis of the surface of the studied sample (Figure 3). Electron photograph of the crystal chip that was used for quantitative elemental analysis is shown in Figure 3a, and EDS results are shown in Figure 3b–d. Averaged formula of the investigation of six probes is Tl_1.79Cd_1.00Ge_2.99Se_7.83, which indicates the uniformity of the sample over its surface and the composition close to Tl₂CdGe₃Se₈ (

Table 2. SEM/EDS results of the Tl₂CdGe₃Se₈ sample.

Parameter	Atom	Number_2	Number_3	Number_4	Number_5	Number_6	Number_7	Number_1_SUM
	Tl	26.8	27.3	27.8	28.7	29	27.6	27.8667
Wt.%	Cd	8.2	8.8	7.7	9.5	8.3	9.1	8.6000
	Ge	16.8	15.8	17	16.9	15.9	16.7	16.5167
	Se	48.2	48.1	47.5	44.9	46.7	46.6	47.0000
	Tl	1.7975	1.7063	1.9857	1.6616	1.9216	1.6682	1.7901
At.%, n	Cd	1.0000	1.0001	1.0000	1.0000	0.9999	1.0000	1.0000
	Ge	3.1717	2.7798	3.4179	2.7541	2.9656	2.8412	2.9884
	Se	8.3679	7.7820	8.7820	6.7287	8.0097	7.2906	7.8268

). The red square in Figure 3a shows the region where the formation of the layered structure is observed. The sample was cleaved along the с axis.

Unit cell parameters of the Tl₂B^IID^IVX₄ compounds (S.G. I-42m) on the whole agree with well-known trends and depend on the nature of constituent atoms. In the majority of cases, the increase in the atomic number and, consequently, the mass of the compound components is accompanied by the increase in atom size and compound density. The calculated density increases substantially with the molar mass in all cases of the substitution of either a two-, four-, or six-valent element.