Rj Haange

MISFIT LAYER COMPOUNDS (MS)NTS2 (M = SN, PB, BI, RARE-EARTH ELEMENTS - T = NB, TA - N = 1.08 - 1.19), A NEW CLASS OF LAYER COMPOUNDS

Abstract A study by X-ray diffraction and electron microscopy is reported of the type of compounds that is usually designated as ternary sulphides “MTS 3 ” ( M = Sn, Pb, Bi, rare earth metals; T = Nb, Ta). They are misfit layer compounds built of alternating double layers of MS with M in distorted square pyramidal coordination by sulphur and sandwiches TS 2 with T is distorted trigonal-prismatic coordination by sulphur. One intralayer lattice constant of MS is incommensurate with that of TS 2 . Recent progress in the understanding of the crystallographic description is discussed and a unifying view based on about ten compounds is presented. Results of electrical transport measurements are briefly summarized.

STRUCTURE AND PHYSICAL-PROPERTIES OF (SNS)1.18NBS2,SNNBS3, A COMPOUND WITH MISFIT LAYER STRUCTURE

Abstract (SnS) 1.18 NbS 2 , “SnNbS 3 ”, is a compound with misfit layer structure consisting of two-atom thick layers of SnS with distorted NaCl type structure and three-atom thick sandwiches of NbS 2 as in 2H-NbS 2 , which alternate along the c axis. The lattices of SnS and NbS 2 both have C -centered orthorhombic unit cells which match along the b and c axes (respectively 5.751 A and 11.761 A) but not along the a axes, 5.673 A for SnS and 3.321 A for NbS 2 . The composition of the compound, (SnS) 1.18 NbS 2 , is determined by the mismatch along the a axes. The physical properties measured on single crystals (resistivity, Hall coefficient) and powder compacts (magnetic susceptibility and Seebeck coefficient) resemble strongly those of 2H-NbS 2 and intercalated 2H-NbS 2 : The Hall coefficient is positive, the thermoelectric power negative; the magnetic susceptibility is Pauli-paramagnetic.

CRSE2, A NEW LAYERED DICHALCOGENIDE

Abstract CrSe2 was prepared by oxidation of KCrSe2 with I2 in acetonitrile at ambient temperature. It has the Cd(OH)2 structure with a = 3.399, c = 5.915 A at 300 K; the compound is metastable and transforms into Cr2Se3 and Se around 600 K. Between ≈ 180 and ≈ 160 K a (reversible) phase transition occurs with an abrupt decrease of ≈ 1% of the c-axis and the appearance of some weak reflections. This paper reports the first physical properties obtained on powders and compacts by X-ray diffraction, thermal analysis, transport (resistivity, Hall effect, thermoelectric power) and magnetic susceptibility measurements. These data, especially the lattice parameters, show a remarkable behaviour with temperature and suggest that CrSe2 has itinerant d electrons.

CRYSTAL-GROWTH, STRUCTURE AND SOME PROPERTIES OF PBNBS3, SNSNBS3, LANBS3 AND RELATED MISFIT LAYER COMPOUNDS

Abstract Compounds with the assumed composition PbMS3, SnMS3 (M=Ti, V, Nb, Ta), known for about 20 years, were investigated by single crystal and powder X-ray diffraction. Since crystals were grown by vapour transport using Cl2, from (NH4)2PbCl6, as transport agent. The compounds may be classified as misfit layer compounds, built of alternately two-atom thick layers of SnS or PbS with distorted NaCl type structure and sandwiches MS2. This was proved by single crystal X-ray diffraction for “SnNbS3” which turned out to have the composition (SnS1.17NbS2; the NbS2 sandwiches have niobium atoms in trigonal prismatic coordination by sulfur like 2H-NbS2. Single crystals “LaNbS3” show a similar diffraction pattern which indicates a layered structure with units LaS and NbS2. The electrical transport properties of (PbS)1.14NbS2 resemble strongly those of 2H-NbS2. Crystals PbHfS3 and SnHfS3 obtained by vapour transport have the NH4CdCl3 structure.

SYNTHESIS AND STRUCTURES OF COMPOUNDS NAXVSE2

Abstract Compounds NaxVSe2, prepared from the elements, crystallize in two different forms : type I for 0.5 ⩽ x ⩽ 0.7 and type II for x = 1. The structures of Na0.6VSe2 and NaVSe2 were refined from neutron-diffraction powder data. Both compounds have vanadium in trigonally distorted octahedra which are elongated in the direction of the trigonal axis for type I ( Vse = 2.516 A ) and compressed for type II ( Vse = 2.588 A ). Na is in trigonal-prismatic coordination for type I ( Nase = 2.972 A ) and in trigonally distorted octahedra for type II ( Nase = 2.947 A ). The bonding is discussed. NaVSe2 is an ionic compound analogous to NaCrSe2.

The synthesis, structure and physical properties of a non-stoichiometric sodium chromium selenide, Na0.34Cr1.15Se2

Abstract Non-stoichiometric Na0.34Cr1.15Se2 was prepared from the elements. It is rhombohedral, space group R3m with a= 3.617, c= 38.99 A. It is a layer compound consisting of sandwiches CrSe2 with chromium in octahedral coordination. The gaps between these sandwiches are alternatingly occupied by the extra (2 × 0.15) chromium in octahedral coordination, and sodium (2 × 0.34) in trigonal-prismatic coordination. The structural, magnetic and transport properties indicate in essence the itineracy of the chromium d electrons, strongly influenced by electron correlation and structural randomness.

Irreversible antiferromagnetic to ferromagnetic transition in the TlCu2−xFexSe2 system (0<x≤0.5)

Abstract Preliminary magnetic data of TlCu2-xFexSe2 (x=0.25 and 0.5) are presented. Antiferromagnetic behaviour is observed for x=0.5. For x=0.25 antiferromagnetism occurs up to 30 K. Between 30 and 68 K ferromagnetism is observed. Below 30 K the application of external fields brings about an irreversible transition from antiferromagnetic to ferromagnetic.

Kinetic properties of intercalates AgxTaS2

Abstract The diffusion of the mobile silver ions in intercalates Ag x TaS 2 with electronic as well as ionic conduction was studied at temperatures above 440 K in a three electrode cell with AgI as solid electrolyte. Use was made of ac response and the galvanostatic intermittent current titration technique (GITT). Experiments were performed with samples prepared from the elements. ac response and GITT showed superionic conduction of silver ions in stage-1 Ag x TaS 2 (0.55⩽ x ⩽0.68) and stage-2 Ag x TaS 2 (0.22⩽ x ⩽0.35). The chemical diffusion coefficient, D , of silver for stage 1 from ac response is 7.5×10 −5 cm 2 s −1 at 500 K; the activation energy of D is 0.13 eV. D of the stage-2 phase, about 10 −6 cm 2 s −1 at 500 K, is smaller than in stage 1. The kinetics of intercalation in the stage-2 phase is slow, also because of the formation of a reaction layer at the interface with AgI. An island type domain structure for the stage-2 phase in which diffusion is accompanied by shear transformation might explain the much slower diffusion of silver in the stage-2 phase compared to diffusion in stage 1.

Lattice distortion and electronic structure of La2CuO4 as host lattice for its high-Tc superconducting derivatives

Abstract Results of electrical transport and magnetic measurements obtained on cold-pressed compacts of La 2 CuO 4 are reported. The p-type metallic and Pauli paramagnetic behaviour between 300 and ∼ 85 K is attributed to the existence of low density-of-states oxygen-2p valuence band holes induced by ground state monovalent copper [Cu(I), 3d 10 , S = O]. La 2 CuO 4 exhibits an electronic phase transition at ∼ 85 K and transforms to a p-type polaron-mediated incommensurate CDW state. A valence band structure with split-off antibonding O-2p x,y (σ∗) branch is depicted, both for “native”, copper deficient, metallic La 2 Cu 1− δ O 4 and “ideal” stoichiometric insulating La 2 CuO 4 . The corrugated (almost) square oxygen network of native La 2 Cu 1− δ O 4 ( δ = 0.01) is explained by an attractive interaction between on the average O −3 2− ions inside the Cu(I)-mediated basal [Cu(I)O 2 ] 2− layers: a resonating valence bond (RVB) state.

PREPARATION AND PHYSICAL-PROPERTIES OF THE COPPER MANGANESE TELLURIDE CU5MNTE4

Abstract The ternary phase Cu5MnTe4 with a rhombohedral, but pseudo-f.c.c. structure at room temperature, was characterized by various physical measurements. Copper is present in the monovalent Cu(I;d10) state, as indicated by the transport properties: there is essentially one hole (effective mass 1.3m0) per formula unit in the Te 5p valence band. In addition to p-type metallic conduction, the delocalized holes give rise to an exchange-enhanced Pauli paramagnetic term which is swamped by diamagnetism and contributions to the magnetic susceptibility χ from localized manganese(II) S = 5 2 spins. χ versusT follows a Curie-Weiss law with a negative asymptotic temperature demonstrating large antiferromagnetic exchange interactions; however, no transition to a three-dimensional magnetic ordering is found down to 10 K.