Present status of polytypism in MX2 compounds

The theoretical and experimental progress made in the last couple of years in our understanding and consequent control of the growth of polytypes of MX₂ compounds has been briefly reviewed. The theoretical advances include the development of ANNNI model, based on short range competing interactions between Ising spins, and the use of Landau theory for explaining the formation of both ordered and disordered polytypes. The experimental evidence favours the existence of a nucleation and growth mechanism of stacking faults during phase transformations. A comparison is drawn with the experimental results in other (non-MX₂) polytypic compounds.

Advancements in the knowledge of dependence of polytype formation on growth conditions have been surveyed. The growth conditions include the factors of temperature of crystallization, rate of crystallization, nature of solvent, impurities, deviations from stoichiometry and external electric and magnetic fields. Some recent concrete information on the role of impurities has been highlighted. The limited success achieved so far in gaining control on the growth of desired polytypes of a compound is outlined. It is emphasized that the hitherto gained cumulative knowledge has greatly helped in rendering the nature of polytypism much clearer than before.

Attention is drawn to the Brillouin and Raman scattering and X-ray diffraction studies carried out in some MX₂-compounds, towards better knowledge of both interlayer and intralayer couplings, which hold positive promise for clarifying our concepts of creation of polytypes of these and other compounds. A short account of the investigation of electrical and electronic properties of the MX₂-compounds in relation to their structural characteristics has been given. The optical, electron microscopic and X-ray diffraction studies made on mixed crystals and intercalated compounds have been briefly reported. The new polytypes and polytypic substances reported in nearly last one decade have been listed.     

Pressure-induced change of the interlayer exchange interaction from ferromagnetic to antiferromagnetic in CS2CuF4 observed by neutron scattering experiments up to 2 GPa

Abstract

Neutron scattering experiments on the two-dimensional Heisenberg ferromagnets Cs₂ CuF₄ and K₂CuF₄ have been performed around 2 ～ 3 GPa over 1·4–15 K. At ambient pressure both the intralayer and the interlayer exchange interactions in these two compounds are ferromagnetic. At about 2 GPa, the interlayer exchange coupling in Cs₂ CuF₄ is found to change from ferromagnetic to antiferromagnetic, while the ferromagnetic intralayer exchange interaction is maintained. Contrary to Cs₂CuF₄, the ferromagnetism in K₂Cuf₄ is not destroyed by pressure up to 9 GPa, that was confirmed in the early study of the magnetic susceptibility measurements.     

Regularities of the formation of polytype structures in layered metal dichalcogenides

Possible sequences of alternating triple layers X-M-X in MX₂ dichalcogenides (where M is a metal and X is a chalcogen) that are consistent with the close packing conditions are considered, and the structure and symmetry of the unit cells of the main polytypic modifications of these compounds are determined. Reasoning from the molecular structure of the crystals and the fact that their smallest structural unit is an MX₂ molecule, the notation of each triple layer can be represented in a reduced form in which only the position of the central metal atom and the orientation of the molecule are specified. In the proposed notation, allowances are made for two types of coordination of the metal atom in the molecule, namely, the octahedral coordination, in which the nearest X layers occupy different positions and form close packing, and the trigonal prismatic coordination, in which both X layers occupy identical positions. The use of the reduced (molecular) form of the notation significantly simplifies the analysis of the polytype structures and makes it possible to distinguish several structural groups differing in structure and symmetry among the crystals with unit cells including two, three, and four layers.     

Vibrational spectroscopy at high pressures: Part 32 pressure-dependencies of the Raman active modes of CdX2 and PbFX type layer materials

The variation of Raman-active phonons of CdX₂, (X = Cl, Br and I), and of PbFX, (X = Cl and Br), has been investigated to about 40 kbar. Rigid layer shear modes were found for CdBr₂ and CdI₂ and shown to have much greater Gruneisen constants than intralayer modes.     

Valence bands of layer structure transition metal chalcogenides

The valence band structure of representative MX₂ layer structure compounds has been obtained by X-ray photoemission with monochromatized radiation. Chalcogen s and p and metal d band are identified and their width and position obtained. The results are compared with u.v. and He II photoemission and with recent band structure calculations.     

Sequence of transformations during ordering of nonstoichiometric compounds with the formation of M 3 X 2-type superstructures

A symmetry analysis of monoclinic, orthorhombic, and trigonal M ₃ X ₂-type superstructures, which can be formed in highly nonstoichiometric MX _y compounds with B1 structure, has been performed. Channels of disorder-order transitions MX _y → M ₃ X ₂ have been determined. It has been shown that two physically allowed sequences of transformations associated with the formation of M ₃ X ₂ superstructures are possible in nonstoichiometric MX _y compounds of Group IV transition metals with decreasing temperature, i.e., “cubic (space group \(Fm\bar 3m\) ) disordered MX _y phase → orthorhombic (space group Immm) ordered M ₃ X ₂ phase → orthorhombic (space group C222₁) ordered M ₃ X ₂ phase” and “cubic (space group \(Fm\bar 3m\) ) disordered MX _y phase → orthorhombic (space group Immm) ordered M ₃ X ₂ phase → monoclinic (space group C2) ordered M ₃ X ₂ phase.”     

The stable CaBe2Ge2 structures in antimonide compounds ATM2Sb2 (A = Ca, Sr, Ba; TM = Fe, Co, Ni, Cu): A first-principles study

We have investigated the structural properties of 3d transition metal antimonide compounds ATM₂Sb₂ by first-principles calculations. We find that the calcium-based CaNi₂Sb₂, the strontium-based SrNi₂Sb₂, SrCu₂Sb₂, and the barium-based BaCu₂Sb₂ are stable in the CaBe₂Ge₂-type structure. All the other compounds are stable in the ThCr₂Si₂-type structure. SrCo₂Sb₂ in the ThCr₂Si₂-type structure has specifically ferromagnetic preference. The stable compounds in the CaBe₂Ge₂-type structure have strong interlayer interactions. Although the stable stacking structures are different, all the Fe-based compounds have the stripe-like antiferromagnetic ground states. The Co-based compounds have the ferromagnetic ground states. The Ni-based and Cu-based compounds have the nonmagnetic ground states.     

Magnetic properties and structure of some ternary chromium chalcogenides with thallium and silver

The structure and magnetic properties of TlCrX₂ (X= S, Se, Te), Tl₃CrS₃, Tl₃Cr₃S₅, TlCr₃S₅, TlCl₅S₈ and AgCrO₂ have been investigated. TlCrS₂, TlCrSe₂ and Tl₃CrS₃ order ferromagnetically with Curie temperatures in the range 60–140 K. The other compounds are antiferromagnetics with negative paramagnetic Curie temperatures, whereas TlCrTe₂ is antiferromagnetic with a positive paramagnetic Curie temperature. In the case of the layered hexagonal TlCrS₂ and TlCrSe₂ owing to the great radius of the Tl-ion and the large Cr-Cr-distance the intralayer exchange interaction is positive giving rise to the ferromagnetic order. For AgCrO₂ the smaller intralayer Cr-Cr-distance gives rise to negative exchange interaction and antiferromagnetism.     

Raman and Infrared Spectra of Crystals with the Cadmium Iodide Structure

Abstract

Raman and far infrared spectra (in the frequency range 20–360 cm^?1) have been recorded for polycrystalline samples of six crystals having the cadmium iodide layered structure. The four fundamental zone-center vibrational frequencies are assigned for CoBr₂, FeBr₂, MgBr₂, MnBr₂, CdI₂ and MgI₂. Values of the principal interlayer force constants are deduced from a simple linear chain dynamical model, and comparisons are made with recent results from spectroscopic studies of crystals with the related cadmium chloride structure.     

Pressure-induced weak moment in the two-dimensional antiferromagnet (C2H5NH3)2CuCl4

Abstract

The two-dimensional Heisenberg antiferromagnet (C₂H₅NH₃)₂CuCl₄ has the ferromagnetic intralayer exchange interaction, while the extremely weak interlayer exchange interaction is antiferromagnetic. Neutron scattering experiments under high pressures have been performed on this compound. We confirm that the spin structure changes around 1～2 GPa from the collinear alignment along the a-axis to a spin-canting one. The weak moment due to the canting is parallel to the c-axis. The results indicate that the ferromagnetic intralayer and the antiferromagnetic interlayer exchange interactions are maintained up to 1～2 GPa. Why the weak ferromagnetic moment along the c-axis occurs is due to a lowering of crystal symmetry by pressure.     

High pressure X-ray diffraction study of ReS2

The high-pressure behavior of rhenium disulfide (ReS₂) has been investigated to 51.0 GPa by in situ synchrotron X-ray diffraction in a diamond anvil cell at room temperature. The results demonstrate that the ReS₂ triclinic phase is stable up to 11.3 GPa, at which pressure the ReS₂ transforms to a new high-pressure phase, which is tentatively identified with a hexagonal lattice in space group P6?m2. The high-pressure phase is stable up to the highest pressure in this study (51.0 GPa) and not quenchable upon decompression to ambient pressure. The compressibility of the triclinic phase exhibits anisotropy, meaning that it is more compressive along interlayer directions than intralayer directions, which demonstrates the properties of the weak interlayer van der Waals interactions and the strong intralayer covalent bonds. The largest change in the unit cell angles with increasing pressures is the increase of β, which indicates a rotation of the sulfur atoms around the rhenium atoms during the compression. Fitting the experimental data of the triclinic phase to the third-order Birch-Murnaghan EOS yields a bulk modulus of K_OT=23±4 GPa with its pressure derivative K_OT′= 29±8, and the second-order yields K_OT=49±3 GPa.     

Valence band photoemission spectroscopy of a ternary layered semiconductor: ZnIn2S4

UV photoemission spectroscopy (UPS) experiments have been carried out on the layer compound ZnIn₂S₄ employing several different photon energies in the range $\text{h}\text{?}{}_{\mathrm{\omega }}\text{= 9.5?21.2}\text{eV}$. The energy distribution curves (EDC's) exhibit four valence band density of states structures besides the Zn 3d peak. These five peaks appear 0.90 eV, 1.6 eV, 4.3 eV, 5.8 eV and 8.7 eV respectively below the top of the valence band, E_v. The atomic orbital character of the shallowest peak A appears different from that of the three deeper valence band peaks B, C and D and this is discussed in terms of the more or less pronounced ionic character of the intralayer chemical bonds. These results demonstrate that an overall understanding of the electronic states in complex structures can be achieved by an approach based on photoemission experiments and chemical bonding considerations which has been widely used in the past to study simple binary layer compounds.     

Structural studies of rhodium doped Sr2RuO4

The influence of Rh doping on the structure of Sr₂RuO₄ has been investigated using neutron powder diffraction methods. The metallic Ru rich compounds adopt a regular K₂NiF₄-type structure, space group I4/mmm, with Ru-O-Ru bond angles of 180°. The structures of the nonmetallic Rh rich compounds crystallise in space group I4/acd and are characterised by tilting of the MO₆ octahedra reducing the Ru-O-Ru angle to about 160°. Irrespective of Rh content the MO₆ polyhedra are not regular octahedra but are elongated along the c direction. The temperature dependence of the structure of Sr₂Ru_0.9Rh_0.1O₄ was investigated and revealed this elongation to be weakly temperature dependent.     

Shubnikov-de Haas oscillations in a new dual-layered quasi-two-dimensional organic metal (BETS)4CoBr4(C6H4Cl2)

The interlayer and intralayer resistances and Shubnikov-de Haas oscillations in a new dual-layered quasi-two-dimensional organic metal (BETS)₄CoBr₄(C₆H₄Cl₂) with a periodically varying structure of cation layers have been studied. It has been shown that the interlayer resistivity corresponds to an incoherent or weakly incoherent transport regime. The oscillations of the magnetoresistance have been described by a model of a chain of coherent magnetic breakdown orbits taking into account the quantum interference effect. The behavior of the interlayer transport, as well as quantum oscillations, is in good agreement with the theoretical calculations of the band structure.     

Bond charge parameters from integrated infrared intensities

A procedure is outlined to analyze the infrared absorption intensities of fundamental vibration bands in terms of “bond charge parameters”. The method is illustrated for some small C_2v- and C_3v-type molecules: SO₂, NF₃, and PF₃. The values obtained for the “bond charge reorganizations” and “effective bond charges” for SO, NF, and PF bonds are discussed. The method offers the possibility of calculating rotational contributions without the use of a reference molecule.     

On the theory of layered high-temperature superconductors

Assuming that charge carriers form a Fermi liquid state, we study a model for layered high-temperature superconductors with unretarded intralayer and interlayer pairing. Guided by band structure calculations and inverse photoemission experiments, we adopt a tight binding band with nearest and next-nearest neighbors hopping within the sheets and weak interlayer hopping. The gap equations are solved numerically, without imposing a cutoff energy, characteristic to phonon mediated superconductivity. On this basis we calculate the gap parameters,T _c , the tunneling conductance, infrared absorption and the coherence length for various band fillings =1/2–x by introducing excess holes of concentrationx. Assuming the interlayer coupling strength to be smaller than the intralayer one, our main results are as follows:T _c is dominated by the intralayer properties, reaching a maximum atx0.3, where strong coupling features appear. In the presence of interlayer pairing, the gap becomes anisotropic perpendicular to the layers, and standard BCS-behavior is modified. In particular the BCS-square root singularity in the density of states and in the tunneling conductance is replaced by van Hove singularities characterizing the anisotropic gap. In particular, we investigate the anisotropy of the tunneling conductance for specular and diffuse tunneling for a junction parallel or perpendicular to the layers, infrared absorption, as well as the coherence length, parallel and perpendicular to the layers.     

Elastic constants,acoustic mode vibrational anharmonicity and Grüneisen parameters of hexahalometallate crystals

The room-temperature elastic constants of a number of hexahalometallate A₂MX₆ single crystals [K₂SnCl₆, K₂ReCl₆, (NH₄)₂SnCl₆, (NH₄)₂SnBr₆, (NH₄)₂SiF₆, Rb₂SnBr₆, K₂SeBr₆, (NH₄)₂TeBr₆, K₂PtBr₆ and (NH₄)2PtBr₆] have been measured either by Brillouin scattering or by the ultrasonic pulse echo overlap technique. Refractive indices have also been determined. These antifluorite structure compounds contain large MX^2?₆ ions and the interionic spacings are much greater than those of the alkaline-earth fluorite structure halides: their elastic stiffnesses are correspondingly smaller. Hydrostatic pressure derivatives of the elastic stiffness constants have been measured for K₂SnCl₆, (NH₄)₂SnBr₆ and (NH₄)₂SnCl₆ and are found to be positive; there is no marked softening of the long-wavelength acoustic-phonon modes at room temperature. The vibrational anharmonicities of these long-wavelength modes are discussed in terms of the acousticmode Grüneisen parameters, which are compared with the thermal Grüneisen parameters. For K₂SnCl₆ a mean of optic- and acoustic-mode Grüneisen parameters is shown to correlate well with the thermal Grüneisen parameter.     

A neutron powder diffraction study of the helimagnetic structure of TlCo2Se2−xSx

The magnetic layer structure of TlCo₂Se_2−xS_x has been thoroughly re-investigated with neutron powder diffraction. The cobalt magnetic moments are ferromagnetically arranged within the layers, but the interlayer coupling differs profoundly with varying composition (x): the spins in TlCo₂Se₂ form a helix along the c-axis with a turning-angle of ∼119° at 1.4 K. This kind of helical structure prevails for 0≤x≤1.5 with a gradual decrease of the angle with increasing sulphur content, down to 34°, showing an almost linear relationship with the interlayer distance of Co-Co. For x≥1.75 the interlayer coupling changes to ferromagnetic. Unexpectedly, two helices were found to coexist at x=0.5 and x=1.0. The interaction between adjacent cobalt layers is there characterized by an incommensurate angle (106°, resp., 73°) together with a commensurate angle of 90°. The magnetic structures have been refined as two magnetic phases, each having a characteristic wave vector. A tentative model where the symmetry of the structure and the interlayer distance compete is considered for explaining the simultaneous occurrence of the two kinds of diffraction profile satellites.     

Distinct electronic and transport properties between 1T-HfSe2 and 1T-PtSe2

In this study, we employed the first-principles calculation to investigate the structural, electronic and transport properties of 1T-HfSe₂ and 1T-PtSe₂ transition metal dichalcogenides, and further explain why they share the same 1T (octahedral) layered structure but exhibit very different electronic and transport properties. There are two underlying concepts: the degree of interlayer bond ionicity and the number of 5d valence electrons of transition metal. The high degree of Hf-Se bond ionicity not only gives rise to the indirect energy gap of HfSe₂ bulk and thin films, but also results in the weak Se-Se vdW interlayer coupling to further restrict the electron transport only within a HfSe₂ layer. On the other hand, the modulation of metallic/semiconducting property of PtSe₂ bulk and thin films can be understood by the significant vdW interlayer coupling, which induces charge redistribution of Se atom and allows electrons to transport within a PtSe₂ layer as well as cross neighboring layers. Finally, our transport calculation for 1T-HfSe₂/1T-PtSe₂ bulks and monolayers suggests the great electron transport within Hf-Se/Pt-Se layer but suppresses/allows electron from neighboring layers. The robust two-dimensional characteristic of 1T-HfSe₂ and the metal-to-semiconductor transition of 1T-PtSe₂ may provide more knowledge for future application in nanoelectronic and optoelectronic devices.     

Plasmon modes of double-layer graphene at finite temperature

We calculate the dynamical dielectric function of doped double-layer graphene (DLG), made of two parallel graphene monolayers with carrier densities n₁ and n₂, and an interlayer separation of d at finite temperature. The results are used to find the dispersion of plasmon modes and loss functions of DLG for several interlayer separations and layer densities. We show that in the case of n₂=0, the finite-temperature plasmon modes are dramatically different from the zero-temperature ones.