Abstract: Half antiperovskites were investigated within the series Sn2-xInxCo3S2 by X-ray diffraction, thermal analysis, and DFT band structure calculations. The shandite type structure was confirmed from powder and single crystal diffraction for the studied compounds. Co orders like Ni in the previously studied InNi3/2S. SnCo3/2S is metrically pseudo cubic and can easily be deduced from the antiperovskite MgCo3C. The Curie temperature T-c = -97 degrees C of SnCo3/2S was confirmed by calorimetric data. Its electronic structure indicates a spin (1)/(2) type I-A half metallic ground state.
Abstract: Half antiperovskites were investigated within the series Sn2-xInxCo3S2 by X-ray diffraction, thermal analysis, and DFT band structure calculations. The shandite type structure was confirmed from powder and single crystal diffraction for the studied compounds. Co orders like Ni in the previously studied InNi3/2S. SnCo3/2S is metrically pseudo cubic and can easily be deduced from the antiperovskite MgCo3C. The Curie temperature T-c = -97 degrees C of SnCo3/2S was confirmed by calorimetric data. Its electronic structure indicates a spin (1)/(2) type I-A half metallic ground state.
Abstract: The crystallographic and electronic structures of the ternary shandite type sulfides M(3)A(2)S(2) (M = Co, Ni; A = In, Sri) were investigated by X-ray diffraction, as well as density functional theory (DFT) band structure calculations with respect to superstructure and type-antitype relations. The crystal structure of Ni3In2S2 (space group R3m, a = 5.371 angstrom, c = 13.563 angstrom) was determined from a single crystal. The shandites show type-antitype relations to oxostannates(II) M2Sn2O3 (M = K, Rb) analogously to perovskite and antiperovskite. With a perovskite superstructure a group-subgroup relation is given to antiperowskites like Ni3MgC. Because of the ordered occupation of half of the M-positions the title compounds are described as half-antiperowskites M(3/2)AS. The occupation scheme causes the formation of Kagome-nets. From bond distances covalent Ni-S bonds (< 2.20 angstrom), ionic Sn-S and In-S-interactions, as well as metallic In-Ni and Sn-Ni are concluded. The electronic band structures of the shandites show metallic characteristics similar to Ni3MgC. A band filling scheme explains the stability and properties from Co3In2S2 to Ni3Sn2S2. The highest partly occupied bands are formed by a strong mixing of Co(Ni)-3d, ln(Sn)-5p and S-3p states. A bonding description [M3S2](n-)[A2](n+) accounts for the structural and electronic properties. Band gaps for Ni3In2S2 and Co3Sn2S2 indicate metal-insulator and magnetic phase transitions.
Abstract: The electronic band structures of orthorhombic (oP28) and monoclinic (mC28) MnSb2S4 were investigated with ab initio calculations in the local spin density approximation to the density functional theory. An analysis of the electronic properties and of the chemical bonding is provided using the augmented spherical wave method considering nonmagnetic, ferromagnetic, ferrimagnetic, and antiferromagnetic model orderings. In agreement with experimental results both modifications of MnSb2S4 are predicted to be antiferromagnetic. While the experimental band gap is missed for the monoclinic polymorph, the calculated band gap for orthorhombic MnSb2S4 is close to the experimental one.
Abstract: An analysis of the electronic and magnetic properties of Ca2MnO4 and Ca2MnO3.5 is carried out within local spin density functional theory using the augmented spherical wave method. From energy differences between the hypothetic magnetic configurations both systems are found to be insulating antiferromagnets in the ground state with a similar to1 eV gap. However we identify an intermediate half metallic ferromagnetic state with the Hund's rule expected moments for Mn-IV (3 mu(B)) and Mn-III (4 mu(B), high spin HS configuration), respectively. The latter result of moment magnitude finds support in recent experimental evidence of Mn-III bismuth oxide as a ferromagnet in its ground state. This is characterized by a small density of states (DOS) magnitude of itinerant states in spin (dagger) channel pointing to a metallic-like behavior as it is experimentally evidenced. For both Ca2MnO4 and Ca2MnO3.5 the chemical bonding characteristics are resolved for the two spin channels. Relationship to colossal magnetoresistive compounds is proposed. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: The electronic band structures of orthorhombic (oP28) and monoclinic (mC28) MnSb2S4 were investigated with ab initio calculations in the local spin density approximation to the density functional theory. An analysis of the electronic properties and of the chemical bonding is provided using the augmented spherical wave method considering nonmagnetic, ferromagnetic, ferrimagnetic, and antiferromagnetic model orderings. In agreement with experimental results both modifications of MnSb2S4 are predicted to be antiferromagnetic. While the experimental band gap is missed for the monoclinic polymorph, the calculated band gap for orthorhombic MnSb2S4 is close to the experimental one.
Abstract: The crystallographic and electronic structures of the ternary shandite type sulfides M(3)A(2)S(2) (M = Co, Ni; A = In, Sri) were investigated by X-ray diffraction, as well as density functional theory (DFT) band structure calculations with respect to superstructure and type-antitype relations. The crystal structure of Ni3In2S2 (space group R3m, a = 5.371 angstrom, c = 13.563 angstrom) was determined from a single crystal. The shandites show type-antitype relations to oxostannates(II) M2Sn2O3 (M = K, Rb) analogously to perovskite and antiperovskite. With a perovskite superstructure a group-subgroup relation is given to antiperowskites like Ni3MgC. Because of the ordered occupation of half of the M-positions the title compounds are described as half-antiperowskites M(3/2)AS. The occupation scheme causes the formation of Kagome-nets. From bond distances covalent Ni-S bonds (< 2.20 angstrom), ionic Sn-S and In-S-interactions, as well as metallic In-Ni and Sn-Ni are concluded. The electronic band structures of the shandites show metallic characteristics similar to Ni3MgC. A band filling scheme explains the stability and properties from Co3In2S2 to Ni3Sn2S2. The highest partly occupied bands are formed by a strong mixing of Co(Ni)-3d, ln(Sn)-5p and S-3p states. A bonding description [M3S2](n-)[A2](n+) accounts for the structural and electronic properties. Band gaps for Ni3In2S2 and Co3Sn2S2 indicate metal-insulator and magnetic phase transitions.
Abstract: An analysis of the electronic and magnetic properties of Ca2MnO4 and Ca2MnO3.5 is carried out within local spin density functional theory using the augmented spherical wave method. From energy differences between the hypothetic magnetic configurations both systems are found to be insulating antiferromagnets in the ground state with a similar to1 eV gap. However we identify an intermediate half metallic ferromagnetic state with the Hund's rule expected moments for Mn-IV (3 mu(B)) and Mn-III (4 mu(B), high spin HS configuration), respectively. The latter result of moment magnitude finds support in recent experimental evidence of Mn-III bismuth oxide as a ferromagnet in its ground state. This is characterized by a small density of states (DOS) magnitude of itinerant states in spin (dagger) channel pointing to a metallic-like behavior as it is experimentally evidenced. For both Ca2MnO4 and Ca2MnO3.5 the chemical bonding characteristics are resolved for the two spin channels. Relationship to colossal magnetoresistive compounds is proposed. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: The crystallographic and electronic structures of PtSnS, PtSnSe and PtSnTe were investigated by X-ray structure analysis and density functional theory (DFT) calculations. Conductivity measurements and diffraction patterns show semiconducting ordered pyrite type related compounds containing SnX (X = S, Se, Te) entities. A scheme is presented to model ordered variants according to the relative orientation of the XY dumbbells. It represents the ullmannite, the cobaltite and a new rhombohedral structure type. The scheme allows for a systematic investigation of ordering preferences from first principles. According to the total electronic energy PtSnTe and PtSnSe prefer the cobaltite, PtSnS the rhombohedral structure type. The structural and electronic properties agree with experimental results. The three compounds are predicted to be narrow gap indirect semiconductors from conductivity measurements and band structure calculations. (C) 2004 Elsevier Inc. All rights reserved.
Abstract: The electronic structures of CuTe2 and Cu7Te4 Were determined from first principles. The band structures, densities of states and projected contributions of atomic states were calculated with DFT ASW- and FP-LAPW codes. Both compounds stabilize by establishing metallic instead of ionic systems. This behaviour is explained in terms of partly occupied valence bands that result from a range of Cu-Te, Cu-Cu and Te-Te bonding. As a consequence the Cu-d states show contributions to the valence states, while their maxima lie at -2 eV below the Fermi energy. In CuTe2 the bonding of Te-Te-pi* and Cu-d states leads to an overlap of valence and conduction bands. Thus the character of the valence band is of Cu-e(g) and Te-p character. (C) 2003 Elsevier SAS. All rights reserved.
Abstract: Inelastic neutron scattering spectra of the 9-hydroxyphenalenone and ring deuterated analogue reveal an intense band at 91 cm(-1) with all characteristics anticipated for a tunnelling the transition of a (quasi) symmetric double minimum potential for proton transfer along the intramolecular hydrogen bond. This frequency is compared to those previously reported for similar systems. (C) 2003 Elsevier B.V. All rights reserved.
Abstract: The crystallographic and electronic structures of PtSnS, PtSnSe and PtSnTe were investigated by X-ray structure analysis and density functional theory (DFT) calculations. Conductivity measurements and diffraction patterns show semiconducting ordered pyrite type related compounds containing SnX (X = S, Se, Te) entities. A scheme is presented to model ordered variants according to the relative orientation of the XY dumbbells. It represents the ullmannite, the cobaltite and a new rhombohedral structure type. The scheme allows for a systematic investigation of ordering preferences from first principles. According to the total electronic energy PtSnTe and PtSnSe prefer the cobaltite, PtSnS the rhombohedral structure type. The structural and electronic properties agree with experimental results. The three compounds are predicted to be narrow gap indirect semiconductors from conductivity measurements and band structure calculations. (C) 2004 Elsevier Inc. All rights reserved.
Abstract: Starting from formerly investigated graphitic like C3N4, selective substitution of nitrogen with boron led to model structures for the experimentally observed BC3N3 stoichiometry. Similar investigations were extended to the 2nd- and 3rd-period elements Al and Ga. Geometry optimisation and studies of the electronic properties were carried out using the pseudo-potential (VASP) method in the framework of the local density functional theory for the two and three dimensional structures (2D and 3D). They respectively lead to propose a precursor (2D), a beta-structure and new ultra hard materials (3D), with hardness (B-0 similar to 358 GPa) for BC3N3 and (B-0 similar to 325 GPa) for AlC3N3 for the high-pressure phases. The chemical role of the IIIrd column substituting element is discussed. (C) 2004 Academie des sciences. Published by Elsevier SAS. All rights reserved.
Abstract: The changes in electronic structure and hardness as inferred from the bulk modulus are investigated for model structures of ternary compounds XC3N3 (X = B, Al, P, As, Ga) within the framework of density functional theory (DFT). The optimisations of the proposed two- (2D) and three-dimensional (3D) structures and the calculations of the bulk moduli are performed by a pseudo potential method. The electronic structures are calculated with the augmented sphere wave method (ASW). The obtained hardness for 2D BC3N3 system (B-0 similar to 220 GPa) points to a magnitude close to that of graphitic C3N4. For heavier X atoms it decreases rapidly. This is equally observed for the 3D systems examined in the beta-C3N4 structure for which B-0 (beta-BC3N3) amounts to similar to330 GPa. Within the magnitude of the well known hard material cubic BN, the BC3N3 phases can be predicted as candidates for ultra hard materials. The electronic effect induced by the chemical nature of the X substitutional was examined according to its position in the periodic table i.e. X-III or X-V. Both, band structures and the electron localisation function (ELF) were used for this analysis. The ELF plots show a decreasing covalency with heavier X-atoms. Potential applications of the devised systems are proposed such as dopings with atoms (Li, rare gas) and molecules (N-2).
Abstract: The changes in electronic structure and hardness as inferred from the bulk modulus are investigated for model structures of ternary compounds XC3N3 (X = B, Al, P, As, Ga) within the framework of density functional theory (DFT). The optimisations of the proposed two- (2D) and three-dimensional (3D) structures and the calculations of the bulk moduli are performed by a pseudo potential method. The electronic structures are calculated with the augmented sphere wave method (ASW). The obtained hardness for 2D BC3N3 system (B-0 similar to 220 GPa) points to a magnitude close to that of graphitic C3N4. For heavier X atoms it decreases rapidly. This is equally observed for the 3D systems examined in the beta-C3N4 structure for which B-0 (beta-BC3N3) amounts to similar to330 GPa. Within the magnitude of the well known hard material cubic BN, the BC3N3 phases can be predicted as candidates for ultra hard materials. The electronic effect induced by the chemical nature of the X substitutional was examined according to its position in the periodic table i.e. X-III or X-V. Both, band structures and the electron localisation function (ELF) were used for this analysis. The ELF plots show a decreasing covalency with heavier X-atoms. Potential applications of the devised systems are proposed such as dopings with atoms (Li, rare gas) and molecules (N-2).
Abstract: Starting from formerly investigated graphitic like C3N4, selective substitution of nitrogen with boron led to model structures for the experimentally observed BC3N3 stoichiometry. Similar investigations were extended to the 2nd- and 3rd-period elements Al and Ga. Geometry optimisation and studies of the electronic properties were carried out using the pseudo-potential (VASP) method in the framework of the local density functional theory for the two and three dimensional structures (2D and 3D). They respectively lead to propose a precursor (2D), a beta-structure and new ultra hard materials (3D), with hardness (B-0 similar to 358 GPa) for BC3N3 and (B-0 similar to 325 GPa) for AlC3N3 for the high-pressure phases. The chemical role of the IIIrd column substituting element is discussed. (C) 2004 Academie des sciences. Published by Elsevier SAS. All rights reserved.
Abstract: Inelastic neutron scattering spectra of the 9-hydroxyphenalenone and ring deuterated analogue reveal an intense band at 91 cm(-1) with all characteristics anticipated for a tunnelling the transition of a (quasi) symmetric double minimum potential for proton transfer along the intramolecular hydrogen bond. This frequency is compared to those previously reported for similar systems. (C) 2003 Elsevier B.V. All rights reserved.
Abstract: The electronic structures of CuTe2 and Cu7Te4 Were determined from first principles. The band structures, densities of states and projected contributions of atomic states were calculated with DFT ASW- and FP-LAPW codes. Both compounds stabilize by establishing metallic instead of ionic systems. This behaviour is explained in terms of partly occupied valence bands that result from a range of Cu-Te, Cu-Cu and Te-Te bonding. As a consequence the Cu-d states show contributions to the valence states, while their maxima lie at -2 eV below the Fermi energy. In CuTe2 the bonding of Te-Te-pi* and Cu-d states leads to an overlap of valence and conduction bands. Thus the character of the valence band is of Cu-e(g) and Te-p character. (C) 2003 Elsevier SAS. All rights reserved.
Abstract: The dinitrides CN2, SiN2, and GeN2 in assumed pyrite-type structures are studied by means of density functional theory using both ultrasoft pseudopotentials and the augmented spherical wave (ASW) method. The former two materials constitute the large-x limit of the broader class of CNx and SiNx compounds, which are well known for their interesting mechanical and electronic properties. For CN2 a large bulk modulus B-0 of 405 GPa was determined. While SiN2 is found to be a wide band gap compound, the calculated gaps of CN2 and GeN2 are considerably smaller. The trends in structural and electronic properties, as e.g., bond lengths, band gaps and covalency are well understood in terms of the interplay of different types of bonding. (C) 2003 Elsevier Science B.V. All rights reserved.
Abstract: A hypothetical CN2 structure was investigated as a model to study the release of N-2 from the octahedral hole of 3D carbon based ultra hard compounds, which is the most important drawback in the attempts to synthesize ultra hard compounds like C3N4 and C11N4. Full structure relaxations using DFT methods led to a structure at the energy minimum showing a significantly enlarged N-N distance of 1.34 A compared to the molecular N-2 (1.09 Angstrom). While for small volume changes a high hardness for CN2 of 405 GPa is calculated, we found that enlargements of the cell constant lead to the release of N-2 that could be followed calculating the ELF and the charge transfer within the AIM theory. The whole procedure simulates an inverted "harpoon mechanism". (C) 2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved.
Abstract: Starting from formerly investigated graphitic like C3N4, selective substitution of nitrogen with boron led to model structures for the experimentally observed BC3N3 stoichiometry. Investigations of the geometry optimisation and of the electronic properties were carried out using pseudo potential and full potential computations in the framework of the local density functional theory for the two and three dimensional structures (2D and 3D). They lead to propose a precursor (2D), a beta-structure and a new ultra hard rhombohedral compound with a hardness (B-0 similar to 358 GPa) that reaches the range of formerly studied BC2N structures built from hexagonal and cubic diamond. (C) 2003 Elsevier Ltd. All rights reserved.
Abstract: Starting from formerly investigated graphitic like C3N4, selective substitution of nitrogen with boron led to model structures for the experimentally observed BC3N3 stoichiometry. Investigations of the geometry optimisation and of the electronic properties were carried out using pseudo potential and full potential computations in the framework of the local density functional theory for the two and three dimensional structures (2D and 3D). They lead to propose a precursor (2D), a beta-structure and a new ultra hard rhombohedral compound with a hardness (B-0 similar to 358 GPa) that reaches the range of formerly studied BC2N structures built from hexagonal and cubic diamond. (C) 2003 Elsevier Ltd. All rights reserved.
Abstract: In the present study the synthesis of metastable alloys of the noble metal Au and the semimetal Sb is realized by using high pressure techniques. The influence of An on bulk Sb is shown experimentally by stepwise substitution. In addition to the effect of the most electronegative noble metal on Pauling's scale in the binary Au-Sb phases, some ternary compounds with In, Sn, As, and Te in addition to An and Sb are discussed. The experiments are planned to obtain a better knowledge on the reasons for building up a simple cubic Sb partial lattice, and their crystallographic results are used to construct model structures for new electronic structure calculations from first principles on the metastable pi-phases. Using the LCAO-CO ansatz and density-functional methods, we study total energies, band structures, densities of states and charge transfer properties according to Bader's method by integrating zero flux surfaces. Finally, from electronic band structure analysis, the quantities "chemical potential" mu, and "chemical hardness", eta, are derived according to the original ideas of Pearson and Parr and are applied to solid state problems using special points of the Brillouin zones within a band structure approximation for the first time. The results are shown to support the experimental findings about the substitution path within the system Au-Sb.
