Frederik Bachhuber

Black Arsenic–Phosphorus: Layered Anisotropic Infrared Semiconductors with Highly Tunable Compositions and Properties

New layered anisotropic infrared semiconductors, black arsenic-phosphorus (b-AsP), with highly tunable chemical compositions and electronic and optical properties are introduced. Transport and infrared absorption studies demonstrate the semiconducting nature of b-AsP with tunable bandgaps, ranging from 0.3 to 0.15 eV. These bandgaps fall into the long-wavelength infrared regime and cannot be readily reached by other layered materials.

The extended stability range of phosphorus allotropes.

Phosphorus displays fascinating structural diversity and the discovery of new modifications continues to attract attention. In this work, a complete stability range of known and novel crystalline allotropes of phosphorus is described for the first time. This includes recently discovered tubular modifications and the prediction of not-yet-known crystal structures of [P12] nanorods and not-yet-isolated [P14] nanorods. Despite significant structural differences, all P allotropes consist of covalent substructures, which are held together by van der Waals interactions. Their correct reproduction by ab initio calculations is a core issue of current research. While some predictions with the established DFT functionals GGA and LDA differ significantly from experimental data in the description of the P allotropes, consistently excellent agreement with the GGA-D2 approach is used to predict the solid structures of the P nanorods.

Synthesis and Identification of Metastable Compounds: Black Arsenic—Science or Fiction?†

Back in black: All metastable and stable phases can be identified for the solid solution arsenic/phosphorus by a combination of quantum-chemical calculations and investigations of the phase formation. Reaction paths for phase formations and transitions in situ were also evaluated. The results show that orthorhombic black arsenic (o-As) is metastable in pure form and has only been previously obtained by stabilizing impurities.

Van der Waals interactions in selected allotropes of phosphorus

Abstract Selected allotropes of phosphorus are investigated by different levels of density functional theory (DFT) calculations to evaluate the relative stability orders with a special focus on the role of van der Waals interactions. Phosphorus is an excellent reference system with a large number of allotropes. Starting from low-dimensional molecular (0D, white P) and polymer structures (1D, P nanorods) to layered (2D, black P) and tubular structures (2D and 3D, crystalline forms of red P), covalent structure motifs are interconnected by van der Waals interactions. They are a key factor for the correct energetic description of all P allotropes. A comparative study is carried out within the local density approximation (LDA) and the generalized gradient approximation (GGA), with and without implementation of a dispersion correction by Grimme (GGA-D2). Our intention is to achieve a reasonable agreement of our calculations with experimental data, the plausibility of energy values, and the treatment of long-range interactions. The effect of van der Waals interactions is exemplified for the interlayer distances of black phosphorous and its electronic structure.

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.

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.

Low-activated Li-ion mobility and metal to semiconductor transition in CdP2@Li phases

Solids with high ion mobility are of broad interest for energy storage applications. New systems featuring low-activated ion mobility are important to improve the performance in such systems. Herein we report on a model system dealing with such improved properties. Li0.2CdP2 was synthesized from the elements, lithium as structure stabilizer and CdI2 as reaction promoters in sealed silica ampoules at 823 K. It crystallizes tetragonal, in space group I4122 (α-CdAs2 structure type), with lattice parameters a = 7.6691(8) A, c = 4.4467(4) A and V = 261.53(4) A3. After 24 h of storage in air lithium ions can be removed in a spontaneous delithiation reaction resulting in Li(OH)·H2O formation on the surface of the crystals. Formed α′-CdP2 adopts the α-CdAs2 structure type. Both compounds consist of isolated cadmium atoms and helical 1∞[P−]-chains generating empty channels suitable to accommodate Li ions. The heavy atom structure was determined by X-ray diffraction methods while a full model including lithium was derived from a combined solid state NMR and quantum chemical calculation approach. An low activation barrier range in the order of 0.1 to 0.2 eV was determined by NMR spectroscopy pointing towards an extraordinary high Li mobility in Li0.2CdP2. Of course a Cd-based solid will have certain disadvantages like toxicity and mass for storage applications but substitution of Cd by suitable lighter elements can solve this issue.

Element allotropes and polyanion compounds of pnicogenes and chalcogenes: stability, mechanisms of formation, controlled synthesis and characterization

Abstract The application of the EnPhaSyn (theoretical Energy diagrams, experimental Phase formation, Synthesis and characterisation) concept is reviewed with respect to prediction of structures and stability of element allotropes and compound polymorphs, their phase formation and transition processes, and their directed synthesis, respectively. Therein, the relative energetical stability (En) of target compounds and possible decomposition are determined from quantum chemical DFT calculations. Phase formation and transition (Pha) is probed by a gas balance method, developed as high temperature gas balance concept. It helped to study the synthesis and stability range of several compounds experimentally. Applications of the concept and synthesis principles (Syn) of non-equilibrium phases are presented for allotropes of P, As, P1-xAsx, as well as binary and ternary compounds including the Zintl and Laves like phases IrPTe, NiP2, CoSbS, NiBiSe, Li0.2CdP2, Cu3CdCuP10, and Cd4Cu7As.