The Layered Structures of Ternary Chalcogenides with Alkali and Transition Metals

Abstract

Layered Structures of Binary Metal Chalcogenides as a Basis for the Construction of Ternary CompoundsThe structure of binary fluorides and oxides with alkali and alkaline-earth metals can be largely understood by means of the supposition of electrostatic binding forces. Thus the sequence of structure types with changing co-ordination numbers for cations and anions depending on their radial quotients, can be nearly quantitatively predicted by a simple model using electrostatically attracting and repelling forces. Moreover, the lattice energies calculated by means of the electrostatic model agree satisfactorily with those which can be experimentally determined by means of the Born-Haber cycle. The dominant role of the electrostatic binding forces decreases considerably if one changes over to the fluorides and oxides of the remaining main group and transition metals, especially when the electropositive atoms are in higher oxidation states. The binary chlorides and sulphides, bromides and selenides, as well as the iodides and tellurides of the alkali and alkaline-earth metals exhibit progressively increasing covalent binding character, especially as the atomic weight of the anionic partner increases. Analogous compounds with the other main group metals continue this tendency and the corresponding phases with the transition metals can already assume metallic properties. Above all, the systems with sulphur, selenium and tellurium, which will be discussed here in greater detail, can often be recognized as intermetallic systems. Here the existence of many phases, mostly having wide ranges of homogeneity, can be observed.The transition from the more salt-like structured chalcogenides to those already having metallic properties can be seen clearly from the example of the largely stoichiometrically composed mono-and dichalcogenides -MX and MX 2 -of the metals of the 4th period (see Tables I and II). Included in the tables are the hitherto known high pressure and high temperature phases [1].In the range of the MX compounds with the NaCl structure type, in which the cations occupy all the octahedral sites in a cubic close-packed structure of anions, mainly ionic bonds are found. Exceptions are the scandium chalcogenides, in which for scandium, the oxidation state + 3 must be assumed, the electrons not required for the inert gas configuration of the X 2 -giving rise to metallic properties. In the MX com-

First-principles study of layered antiferromagnetic CuCrX2(X = S, Se and Te)

“…Ionic and lattice relaxations were carried out to optimize the lattice parameters using variable-cell relaxation; see table 1. The optimized lattice parameters were found to agree within 2% with the experimental data for X = S and Se (to the best of our knowledge the CuCrTe 2 compound has not been synthesized yet) [3][4][5][6]; the relaxed crystal structures were thus employed in further electronic structure calculations. In all calculations, the plane wave (kinetic energy) cut off of 60 Ryd was used.…”

“…In most of these compounds the magnetic susceptibility follows the Curie-Weiss law at high temperatures, suggesting localized fluctuations of the 3d 3 electrons on trivalent Cr cations and the quenching of the orbital moment due to the crystal field. In spite of the 2D nature of the Cr lattice, the behavior of the paramagnetic susceptibility is not 2D-like, and all the compounds show a 3D magnetic order at low temperatures due to weak interlayer coupling [3][4][5][6][7].…”

“…In CuCrO 2 , spin-driven ferroelectricity was recently discovered in the magnetic ground state [2]. The corresponding sulfide and selenide compounds of chromium, ACrX 2 (A = Li, Na, K, Cu or Ag; X = S, Se), on the other hand show antiferromagnetic ordering at low temperatures from 19 to 55 K [3][4][5][6][7]. The rhombohedral crystal structure of these compounds is based on a slightly distorted cubic close packing of the anions, where the trivalent Cr cations occupy octahedral positions in X-Cr-X layers and the monovalent A cations are located in octahedral or tetrahedral interstitial sites between the layers.…”

Dichalcogenide Bonding in Seven Alkali-Metal Actinide Chalcogenides of the KTh2Se6 Structure Type

“…The occurrence of disorder depends on the radius ratio between the eight-coordinate actinide ( R An ) and the alkali-metal ( R A ) ions. A similar trend of the cation radius ratio determines whether members of the ALnS 2 (Ln = La−Lu) family of compounds adopt the NaCl or the α-NaFeO 2 structure type . As shown in Table , which also includes the only nonactinide members of the KTh 2 Se 6 structure type (K 1.10 Zr 2 Se 6 and Rb 0.86 Zr 2 Se 6 ), compounds with smaller values of R An / R A exhibit disorder.…”

Electronic and magnetic properties of ternary sulfide Rb2Mn3S4

“…The experimental lattice parameter of Rb2Mn3S4 are a = 5.84 Å, b = 11.21 Å, c = 13.66 Å and angles α = β = γ = 90 • , respectively. The atomic Wyckoff positions are [0.268, 0.119, 0.0] for Rb, [0.0, 0.276, 0.25] for Mn(I), [0.0, 0.0, 0.25] for Mn(II), [0.2183, 0.133, 0.344] for S[17].…”