Maria A. Kirsanova

Semiclathrates of the Ge–P–Te System: Synthesis and Crystal Structures

Novel compounds [Ge(46-x) P(x) ]Te(y) (13.9≤x≤15.6, 5.92≤y≤7.75) with clathrate-like structures have been prepared and structurally characterized. They crystallize in the space group Fm ̅3 with the unit cell parameter changing from 20.544(2) to 20.698(2) Å (Z=8) on going from x=13.9 to x=15.6. Their crystal structure is composed of a covalently bonded Ge-P framework that hosts tellurium atoms in the guest positions and can be viewed as a peculiar variant of the type I clathrate superstructure. In contrast to the conventional type I clathrates, [Ge(46-x) P(x) ]Te(y) contain tricoordinated (3b) atoms and no vacancies in the framework positions. As a consequence of the transformation of the framework, the majority of the guest tellurium atoms form a single covalent bond with the host framework and thus the title compounds are the first representative of semiclathrates with covalent bonding. A comparison is made with silicon clathrates and the evolution of the crystal structure upon changing the tellurium content is discussed.

Synthesis, Structure, and Transport Properties of Type‑I Derived Clathrate Ge46−xPxSe8−y (x = 15.4(1); y = 0−2.65) with Diverse Host− Guest Bonding

A first clathrate compound with selenium guest atoms, [Ge(46-x)P(x)]Se(8-y)□(y) (x = 15.4(1); y = 0-2.65; □ denotes a vacancy), was synthesized as a single-phase and structurally characterized. It crystallizes in the space group Fm3 with the unit cell parameter a varying from 20.310(2) to 20.406(2) Å and corresponding to a 2 × 2 × 2 supercell of a usual clathrate-I structure. The superstructure is formed due to the symmetrical arrangement of the three-bonded framework atoms appearing as a result of the framework transformation of the parent clathrate-I structure. Selenium guest atoms occupy two types of polyhedral cages inside the positively charged framework; all selenium atoms in the larger cages form a single covalent bond with the framework atoms, relating the title compounds to a scanty family of semiclathrates. According to the measurements of electrical resistivity and Seebeck coefficient, [Ge(46-x)P(x)]Se(8-y)□(y) is an n-type semiconductor with E(g) = 0.41 eV for x = 15.4(1) and y = 0; it demonstrates the maximal thermoelectric power factor of 2.3 × 10(-5) W K(-2) m(-1) at 660 K.

Clathrates and semiclathrates of Type-I: crystal structure and superstructures

Abstract The review surveys the crystal chemistry of inorganic compounds belonging to the structure type of clathrate-I. The compounds of this family exhibit an unexpected variety of both chemical composition and features of the crystal structure. The basic crystal structure of clathrate-I and different variants of its modification, such as siting, position splitting, and superstructure formation including transformation to semiclathrates, are considered in terms of the group-subgroup relationship.

New Fe-based layered telluride Fe3−δAs1−yTe2: synthesis, crystal structure and physical properties

A new ternary telluride, Fe3-δAs1-yTe2, was synthesized from elements at 600 °C. It crystallizes in the hexagonal P63/mmc space group with the unit cell parameters a = 3.85091(9) Å and c = 17.1367(4) Å for δ = 0.3 and y = 0.04. Its layered crystal structure contains partially occupied intralayer and interlayer Fe positions, which give rise to significant nonstoichiometry: Fe3-δAs1-yTe2 was found to possess the homogeneity range of 0.25 < δ < 0.45 and y = 0.04. Regions of local vacancy ordering alternate with regions of randomly distributed vacancies, so that the ordering of Fe atoms and vacancies is not complete in the average structure. Clear evidence of the magnetic phase transition is obtained by thermodynamic measurements, Mössbauer spectroscopy, and neutron powder diffraction. Magnetic susceptibility measurements reveal weak ferromagnetism below TC = 123 K with a net moment of MS∼ 0.1μB/Fe at T = 2 K. This transition is confirmed by differential scanning calorimetry. Additionally, neutron powder diffraction indicates the onset of a complex AFM-like magnetic ordering below 100 K.

On the crystal structure of the germanium-based cationic clathrates [Ge38.3Sb7.7]I7.44, [Ge38.1P7.9]I8, and [Ge30.5Sn7.7P7.75]I7.88

Compounds [Ge38.3Sb7.7]I7.44, [Ge38.1P7.9]I8, and [Ge30.5Sn7.7P7.75]I7.88 with the clathrate type-I structure were synthesized. They crystallize in the cubic space group

Bi{sub 6}(SeO{sub 3}){sub 3}O{sub 5}Br{sub 2}: A new bismuth oxo-selenite bromide

Pm\bar 3n

Intermetallic solid solution Fe1−xCoxGa3: Synthesis, structure, NQR study and electronic band structure calculations

with the unit cell parameter a = 10.8592(9), 10.4983(12), and 10.7210(10) Å (Z = 1), respectively. Their crystal structure represents the germanium(tin)-pnictogen framework, capturing the guest iodine anions in its cavities. All compounds have no vacancies in the host substructure; however, two of them show vacancies in the guest positions. The atomic distribution over the framework sites is of the most interest as it follows trends associated with the relative electronegativities of the atoms composing the framework. The results of the band structure calculations and application of the Zintl counting scheme are also discussed in relation to potential thermoelectric properties.

Low-temperature structure and lattice dynamics of the thermoelectric clathrate Sn24P19.3I8

A first germanium-based cationic clathrate of type-III, Ge(129.3)P(42.7)Te(21.53), was synthesized and structurally characterized (space group P4(2)/mnm, a = 19.948(3) Å, c = 10.440(2) Å, Z = 1). In its crystal structure, germanium and phosphorus atoms form three types of polyhedral cages centered with Te atoms. The polyhedra share pentagonal and hexagonal faces to form a 3D framework. Despite the complexity of the crystal structure, the Ge(129.3)P(42.7)Te(21.53) composition corresponds to the Zintl counting scheme with a good accuracy. Ge(129.3)P(42.7)Te(21.53) demonstrates semiconducting/insulating behavior of electric resistivity, high positive Seebeck coefficient (500 μV K(-1) at 300 K), and low thermal conductivity (<0.92 W m(-1) K(-1)) within the measured temperature range.