Jan Rothballer

First principles calculations on structure, bonding, and vibrational frequencies of SiP2

Pyrite type SiP(2) is reinvestigated by first principles calculations on various levels of functionals including local density approximation, generalized gradient approximation, Becke-Lee-Yang-Parr hybrid functional, and the Hartree-Fock method. SiP(2) is seen as a model compound with molecular [P-P] entities and [SiP(6)] octahedra. Structure and bonding are addressed by electronic structure calculations. Special attention is spent on P-P and Si-P bonds in terms of bond lengths and respective stretching modes from simulated Raman spectra. The electronic structure is analyzed in both direct and momentum space by the electron localization function and site projected density of states. The main goals of this work are to understand the nature of chemical bonding in SiP(2) and to compare and contrast the different methods of calculation.

Origin and effect of In–Sn ordering in InSnCo3S2: a neutron diffraction and DFT study

The solid solution In2−xSnxCo3S2 is attractive due to a variety of interesting properties depending on the In/Sn content, i.e. half metal ferromagnetic Sn2Co3S2, low dimensional metal In2Co3S2, and semiconducting thermoelectric InSnCo3S2. For the latter, crystal structure effects and a metal to insulator transition are not only related to electron counting but also to ordering of In and Sn within and between Co Kagome nets. These observations have not been adequately understood to date. The degree of ordering is now evaluated from neutron diffraction data to distinguish In and Sn. The origin and effects on crystal and electronic structures are studied by DFT calculations on a superstructure model. Relations of local bonding (electron localization function ELF and Baders AIM theory), In/Sn site preference, crystal structure distortions, and the opening of the gap are explored. Results are generalised from predictions on isoelectronic compounds.

Cd4Cu7As, The First Representative of a Fully Ordered, Orthorhombically Distorted MgCu2 Laves Phase

The ternary Laves phase Cd(4)Cu(7)As is the first intermetallic compound in the system Cu-Cd-As and a representative of a new substitution variant for Laves phases. It crystallizes orthorhombically in the space group Pnnm (No. 58) with lattice parameters a = 9.8833(7) Å; b = 7.1251(3) Å; c = 5.0895(4) Å. All sites are fully occupied within the standard deviations. The structure can be described as typical Laves phase, where Cu and As are forming vertex-linked tetrahedra and Cd adopts the structure motive of a distorted diamond network. Cd(4)Cu(7)As was prepared from stoichiometric mixtures of the elements in a solid state reaction at 1000 °C. Magnetic measurements are showing a Pauli paramagnetic behavior. During our systematical investigations within the ternary phase triangle Cd-Cu-As the cubic C15-type Laves phase Cd(4)Cu(6.9(1))As(1.1(1)) was structurally characterized. It crystallizes cubic in the space group Fd3m with lattice parameter a = 7.0779(8) Å. Typically for quasi-binary Laves phases Cu and As are both occupying the 16c site. Chemical bonding, charge transfer and atomic properties of Cd(4)Cu(7)As were analyzed by band structure, ELF, and AIM calculations. On the basis of the general formula for Laves phases AB(2), Cd is slightly positively charged forming the A substructure, whereas Cu and As represent the negatively charged B substructure in both cases. The crystal structure distortion is thus related to local effects caused by Arsenic that exhibits a larger atomic volume (18 Å(3) compared to 13 Å(3) for Cu) and higher ionicity in bonding.

Phase stabilities at a glance: stability diagrams of nickel dipnictides.

In the course of the recent advances in chemical structure prediction, a straightforward type of diagram to evaluate phase stabilities is presented based on an expedient example. Crystal structures and energetic stabilities of dipnictides NiPn2 (Pn = N, P, As, Sb, Bi) are systematically investigated by first principles calculations within the framework of density functional theory using the generalized gradient approximation to treat exchange and correlation. These dipnictides show remarkable polymorphism that is not yet understood systematically and offers room for the discovery of new phases. Relationships between the concerned structures including the marcasite, the pyrite, the arsenopyrite/CoSb2, and the NiAs2 types are highlighted by means of common structural fragments. Electronic stabilities of experimentally known and related AB2 structure types are presented graphically in so-called stability diagrams. Additionally, competing binary phases are taken into consideration in the diagrams to evaluate the stabilities of the title compounds with respect to decomposition. The main purpose of the stability diagrams is the introduction of an image that enables the estimation of phase stabilities at a single glance. Beyond that, some of the energetically favored structure types can be identified as potential new phases.

Half antiperovskites: V. Systematics in ordering and group-subgroup relations for Pb2Pd3Se2, Bi2Pd3Se2, and Bi2Pd3S2

Abstract The crystal structures of Pb2Pd3Se2 and superconducting Bi2Pd3Se2 are investigated from single crystal X-ray diffraction. Thus, an enhanced view on structure trends can be given for the series Bi2M3Ch2 and Pb2M3Ch2 (M = Ni, Rh, Pd, Ch = S, Se) including structure-property relations to superconducting parkerite, Bi2Ni3S2, non superconducting Bi2Pd3S2, and trigonal shandite, Pb2Ni3S2. Systematics and deviations of the structure types are pointed out by group-subgroup relations from a common aristotype perovskite superstructure. Found structures and atomic site occupations are differenciated by means of Bärnighausen trees, M site ordering, coordination spheres, and polyhedra sharing [Ch2A4] as proposed by the scheme of ordered half antiperovskites A2M3Ch2 = AM3/2Ch (A = In, Sn, Tl, Pb, Bi; M = Co, Ni, Rh, Pd, and Ch = S, Se). Low dimensional partial structures with short M—M distances are analyzed with respect to the occurrence of superconductivity in parkerites.

In Search for Novel Sn2Co3S2-based Half-metal Ferromagnets

Substitution effects on magnetism of shandite-type compounds have been studied by density functional theory. The decrease of the Fermi level in the novel half-metallic ferromagnet Sn2Co3S2 to higher maxima of the density of states was modeled for substitutions on the Co site by the 3d metals Fe, Mn and Cr due to a rigid band scheme. Spin-polarized energy hyper surfaces and densities of states are calculated for Sn2Co3S2, and experimentally not yet known Sn2Fe3S2, Sn2Mn3S2 and Sn2Cr3S2 with shandite-type structure. The stability of half-metallic ferromagnetic characteristics, Slater-Pauling behavior, and alternative metastable spin states are discussed. Graphical Abstract In Search for Novel Sn2Co3S2-based Half-metal Ferromagnets

Mercury-ordering in Hg3S2Cl2 polymorphs

There is a big variety of structure types for MPn2 (Pn = N, P, As, Sb, Bi) compounds with promising properties. Pyrite-type SiP2 served as a model compound for DFT calculations on electronic structure in both direct and momentum space as well as IRand Raman spectra [1] [2]. The calculations were extended to the system SiP2-xAsx where P was successively substituted by As [3]. An ordering scheme derived from the pyrite structure type according to [4] resulted in heteroand homoatomic dumbbells with the first clearly preferred over the latter due to dipole momentums from the charges of P (-0.8 e) und As (-0.3 e). Here we present an extension of the calculations to different structure types not only with dumbbell but also with strand-like structures. In addition to the system SiP2-xAsx, related compounds with either different metal or different pnictogen atoms were taken into consideration. New ordering schemes could be derived leading to the occurrence of distinct preferences and tendencies within the 5th main group. With a special focus on the 1:1:1 stoichiometry, the examined compounds were evaluated in terms of thermodynamic stability and hence the possibility of being capable of synthesis.

Extending the concept of half antiperovskites

C645 also carried out a detailed analysis of the crystal structures at different temperatures, where we found trapped HS and LS complexes or spincrossover materials. Additionally, this kind of complexes also shows a light induced excited spin state trapping (LIESST). The structural features of these excited states were also studied in the solid state by light irradiation at very low temperatures.