Crystal Structures, Electronic Structures, and Physical Properties of Tl₄MQ₄ (M = Zr or Hf; Q = S or Se)

Abstract

The ternary thallium chalcogenides of the general formula Tl(4)MQ(4) (M = Zr or Hf; Q = S or Se) were obtained from high-temperature reactions without air. These sulfides and selenides are isostructural, crystallizing in the triclinic system with space group P1 and Z = 5, in contrast to Tl(4)MTe(4) compounds that adopt space group R3. The unit cell parameters for Tl(4)ZrS(4) are as follows: a = 9.0370(5) Å, b = 9.0375(5) Å, c = 15.4946(9) Å, α = 103.871(1)°, β = 105.028(1)°, γ = 90.138(1)°, and V = 1183.7(1) Å(3). In contrast to the corresponding tellurides, the sulfides and selenides exhibit edge-shared MQ(6) octahedra, propagating along the c axis in a zigzag manner. All elements occur in the most common oxidation states, according to the formulation (Tl(+))(4)M(4+)(Q(2-))(4). Electronic structure calculations predict energy band gaps of 1.7 eV for Tl(4)ZrS(4) and 1.3 eV for Tl(4)ZrSe(4), which are in accordance with the large resistivity values observed experimentally.

Structures and properties of the ternary thallium chalcogenides Tl2MQ3 (M = Zr, Hf; Q = S, Se)

“…28 The slightly smaller volume of the Hf compounds can be attributed to the slightly smaller size of Hf (1.44 Å) compared to Zr (1.45 Å). This observation is consistent with the cases of Tl 4 MS 4 15 and Tl 4 MTe 4 . 16 Here, we discuss the structural properties of Tl 2 ZrS 3 as a representative of the newly-discovered Tl 2 MQ 3 family.…”

“…These distances are much longer than the Tl-Tl interactions <3.5 Å observed in Tl 5 Te 3 34 and Tl 4 ZrTe 4 , 16 while being comparable to those in Tl 4 ZrS 4 . 15 The electronic structures of Tl 2 ZrS 3 and Tl 2 HfSe 3 exhibit (calculated) band gaps of 1.13 and 0.57 eV, respectively (Fig. 3).…”

“…Most of these distances are comparable to the corresponding ones in ZrS 2 (2.56 Å) 29 and Tl 4 ZrS 4 (2.52-2.64 Å). 15 The shortest Zr-S distance of 2.46 Å connects the central Zr atom to one of the apical S atoms (labelled S1) of the ZrS 6 octahedron. The other apical S atom (labelled S3) is involved in the longest Zr-S bond (2.68 Å).…”

“…13 Our recent investigations in the Tl-M-Q systems (M = Zr, Hf; Q = S, Se, Te) has unveiled new compounds such as Tl 2 ZrTe 3 14 and Tl 4 MQ 4 . 15,16 We have studied their structure, bonding, electrical and thermal transport properties in order to determine their potential as thermoelectric materials. Tl 4 MTe 4 exhibits facesharing MTe 6 octahedra, forming linear M 3 Te 12 trimeric units that run along the c axis of the trigonal unit cell with space group R3 ˉ.…”

Oxygen, sulfur, selenium, tellurium and polonium

“…19 The isostructural ternary chalcogenides Tl 4 ME 4 (M = Zr or Hf; E = S or Se) feature edge-shared ME 6 octahedra which propagate along the c axis. 20 The mixed-valence compound CsTe 2 O 6 (rhombohedral-pyrochlore) displays order of Te(IV) and Te(VI) positions; oxygen vacancies develop above 600 1C leading to a cubic pyrochlore phase with disorder of the Te sites and electrical conductivity. However, CsTe 2 O 6-x (x = 0.25-1.15) obtained at 500 1C features orthorhombic symmetry.…”