Phase equilibria in the Tl2S–PbS–GeS2 system and crystal structure of Tl0.5Pb1.75GeS4

Abstract

The phase equilibria in the quasi-ternary system Tl 2 S-PbS-GeS 2 at 520 K were investigated by XRD methods. Three intermediate quaternary phases were discovered, Tl 2 PbGeS 4 , ~Tl 2 PbGe 3 S 8 and Tl 0.5 Pb 1.75 GeS 4. The crystal structure of the quaternary phase Tl 0.5 Pb 1.75 GeS 4 was determined by powder XRD in the non-centrosymmetric space group I-43d, Z = 16 and a = 1.420082(6) nm (Ag 0.5 Pb 1.75 GeS 4 structure type). Atomic parameters were refined in the isotropic approximation (R I = 0.0496, R p = 0.0944). The phase diagram of the Tl 2 GeS 3-PbS section was investigated. The section is quasi-binary, of the eutectic type, with formation of the equimolar compound Tl 2 PbGeS 4 , which melts incongruently at 781 K and undergoes a polymorphous transformation at 688 K. The НТ-modification crystallizes with a monoclinic structure (S.G. P12/a1, а = 0.89079(7), b = 0.90951(7), c = 1.04772(7) nm, β = 94.116(7)°).

Electronic and optical features of the mixed crystals Ag0.5Pb1.75Ge(S1–xSex)4

“…Compounds possessing the general formula A 0.5 M 1.75 GeX 4 (A ¼ Ag, Cu, Na, Tl; M ¼ Pb, Eu; X ¼ S, Se) crystallize in a noncentrosymmetric structure [1][2][3] and are promising non-linear optical materials. The best studied of this series of compounds is Ag 0.5 Pb 1.75 GeS 4 , which possesses promising non-linear optical properties such as second harmonic generation, two-photon absorption, etc.…”

Insights from Experiment and Theory on Peculiarities of the Electronic Structure and Optical Properties of the Tl2HgGeSe4 Crystal

“…The crystal growth conditions were chosen from the analysis of the T−x diagram of the Tl 2 GeSe 3 −HgSe system. 46 The process of growth of single crystals was carried out using a special furnace consisting of two temperature zones with independent temperature regulation. The temperature gradient in the crystallization area was set to be within 4−6 K/mm, the temperature of the upper "hot" zone of the growth furnace was chosen to be 900 ± 20 K, the lower "cold" zone was 500 ± 20 K, while the growth rate was 0.2 mm/h.…”

“…48 All the above-mentioned thallium-containing quaternary sulfides and selenides Tl 2 B II D IV X 4 (where B II stands for Cd and Hg; while D IV is Si, Ge, and Se; whereas X denotes S and Se) isostructurally crystallize in a noncentrosymmetric tetragonal space group I4̅ 2m. 45−48 Due to the results of the studies of phase equilibrium in the near-ternary system Tl 2 Se−HgGe− GeSe 2 , 46 in which the quaternary selenide under consideration is formed, Tl 2 HgGeSe 4 melts congruently at 764 K. After synthesis, the Tl 2 HgGeSe 4 compound keeps its crystal structure and physicochemical properties under ambient conditions.…”

“…The problem of determination of the centrosymmetric/noncentrosymmetric crystal structure belonging to the A 2 B II D IV X 4 -type compounds (A = Cu and Tl; B II = Zn, Cd, and Hg; D IV = Si, Ge, and Sn; and X = S, Se, and Te) was known long ago. There exist a number of publications where a noncentrosymmetric SG I4̅ 2m of the structure is accepted for these compounds.95−100 Previously, this space group was already attributed to the compound Tl 2 HgGeSe 4 based on the powder XRD pattern of the Tl 2 HgGeSe 4 alloy 46. In the present work, we have performed structure refinement for the Tl 2 HgGeSe 4 crystal aiming only to identify that the crystal is a single-phase material with the crystal structure belonging to Tl 2 CdGeTe 4 type (SG I4̅ 2m) as it is suggested previously for many other A 2 B II D IV X 4 -type quaternary chalcogenides, 94−100 including Tl 2 HgGeSe 4 46.…”

“…There exist a number of publications where a noncentrosymmetric SG I4̅ 2m of the structure is accepted for these compounds.95−100 Previously, this space group was already attributed to the compound Tl 2 HgGeSe 4 based on the powder XRD pattern of the Tl 2 HgGeSe 4 alloy 46. In the present work, we have performed structure refinement for the Tl 2 HgGeSe 4 crystal aiming only to identify that the crystal is a single-phase material with the crystal structure belonging to Tl 2 CdGeTe 4 type (SG I4̅ 2m) as it is suggested previously for many other A 2 B II D IV X 4 -type quaternary chalcogenides, 94−100 including Tl 2 HgGeSe 4 46. Assuming the structure belonging to SG I4̅ 2m, we have performed DFT calculations of the electronic structure and optical properties of Tl 2 HgGeSe 4 , and the theoretical results are found to be in excellent agreement with the experimental measurements carried out for the Tl 2 HgGeSe 4 crystal.…”