Srinivasa Thimmaiah

Structure and Properties of γ-Brass-Type Pt2Zn11—δ (0.2 < δ < 0.3)†

The γ-brass type phase Pt2Zn11—δ (0.2 2σ (Io) for 293 symmetrically independent intensi ties and 19 variables. The structure consists of a 26 atom cluster which is comprised of four crystallographically distinct atoms. The atoms Zn(1), Pt(1), Zn(2) and Zn(3) form an inner tetrahedron IT, an outer tetrahedron OT, an octahedron OH, and a distorted cuboctahedron CO respectively. About 14 % of the Zn(1) sites are unoccupied. Pt2Zn10.73 melts at 1136(2) K. It is a moderate metallic conductor (ρ298 = 0.2—0.9 mΩ cm) whose magnetic properties (χmol = —4.6 10—10 to —5.4 10—10 m3 mol—1) are dominated by the core diamagnetism of its components. Struktur und Eigenschaften von Pt2Zn11—δ (0.2 < δ < 0.3) vom γ-Messing-Typ Die Phase Pt2Zn11-δ (0.2 2σ (Io), 293 symmetrisch unabhangigen Reflexen und 19 Variablen. Charakteristische Baugruppe der Struktur ist ein 26-Atom-Cluster, der aus vier kristallographisch unterschiedlichen Atomen aufgebaut ist. Die Atome Zn(1), Pt(1), Zn(2) und Zn(3) bilden ein inneres Tetraeder IT, ein auseres Tetraeder OT, ein Oktaeder OH beziehungsweise ein verzerrtes Kuboktaeder CO. Die Zn(1)-Lage ist zu etwa 14 % unterbesetzt. Pt2Zn10.73 schmilzt bei 1136(2) K. Die Verbindung ist ein masiger metallischer Leiter (ρ298 = 0.2—0.9 mΩ cm), dessen magnetische Eigenschaften (χmol = —4.6 10—10 to —5.4 10—10 m3 mol—1) vom Diamagnetismus der Atomrumpfe der Komponenten bestimmt werden.

β-Mn-Type Co8+xZn12–x as a Defect Cubic Laves Phase: Site Preferences, Magnetism, and Electronic Structure

The results of crystallographic analysis, magnetic characterization, and theoretical assessment of β-Mn-type Co-Zn intermetallics prepared using high-temperature methods are presented. These β-Mn Co-Zn phases crystallize in the space group P4(1)32 [Pearson symbol cP20; a = 6.3555(7)-6.3220(7)], and their stoichiometry may be expressed as Co(8+x)Zn(12-x) [1.7(2) < x < 2.2(2)]. According to a combination of single-crystal X-ray diffraction, neutron powder diffraction, and scanning electron microscopy, atomic site occupancies establish clear preferences for Co atoms in the 8c sites and Zn atoms in the 12d sites, with all additional Co atoms replacing some Zn atoms, a result that can be rationalized by electronic structure calculations. Magnetic measurements and neutron powder diffraction of an equimolar Co:Zn sample confirm ferromagnetism in this phase with a Curie temperature of ∼420 K. Neutron powder diffraction and electronic structure calculations using the local spin density approximation indicate that the spontaneous magnetization of this phase arises exclusively from local moments at the Co atoms. Inspection of the atomic arrangements of Co(8+x)Zn(12-x) reveals that the β-Mn aristotype may be derived from an ordered defect, cubic Laves phase (MgCu2-type) structure. Structural optimization procedures using the Vienna ab initio simulation package (VASP) and starting from the undistorted, defect Laves phase structure achieved energy minimization at the observed β-Mn structure type, a result that offers greater insight into the β-Mn structure type and establishes a closer relationship with the corresponding α-Mn structure (cI58).

Discovery of ferromagnetism with large magnetic anisotropy in ZrMnP and HfMnP

ZrMnP and HfMnP single crystals are grown by a self-flux growth technique, and structural as well as temperature dependent magnetic and transport properties are studied. Both compounds have an orthorhombic crystal structure. ZrMnP and HfMnP are ferromagnetic with Curie temperatures around 370 K and 320 K, respectively. The spontaneous magnetizations of ZrMnP and HfMnP are determined to be 1.9 μB/f.u. and 2.1 μB/f.u., respectively, at 50 K. The magnetocaloric effect of ZrMnP in terms of entropy change (ΔS) is estimated to be −6.7 kJ m−3 K−1 around 369 K. The easy axis of magnetization is [100] for both compounds, with a small anisotropy relative to the [010] axis. At 50 K, the anisotropy field along the [001] axis is ∼4.6 T for ZrMnP and ∼10 T for HfMnP. Such large magnetic anisotropy is remarkable considering the absence of rare-earth elements in these compounds. The first principle calculation correctly predicts the magnetization and hard axis orientation for both compounds, and predicts the experimental...

Zn13(CrxAl1−x)27 (x = 0.34–0.37): a new intermetallic phase containing icosahedra as building units

Abstract The title compounds Zn13(CrxAl1–x)27 (x = 0.34–0.37) were obtained by melting the pure elements at 923 K, and followed by a heat treatment at 723 K in a tantalum container. According to single crystal structural analysis, the title compounds crystallize in the rhombohedral system, adopting a new structure type (R-3m, a = 7.5971(8), c = 36.816(6), for crystal I). Single crystal X-ray structural analysis reveals a statistical mixing of Cr/Al in their crystallographic positions. Single crystal and powder X-ray diffraction as well as energy dispersive X-ray analyses suggested the title phase to have a narrow homogeneity range. The substructure of Zn13(CrxAl1–x)27 shows close resemblance with the Mn3Al10 structure type. A bonding analysis, through crystal orbital Hamiltonian populations (COHPs), of “Cr9Al18Zn13” as a representative composition indicated that both homo- and heteronuclear interactions are important for the stability of this new phase.

Crystal structure, homogeneity range and electronic structure of rhombohedral γ-Mn5Al8

Abstract The γ-region of the Mn–Al phase diagram between 45 and 70 at.% Al was re-investigated by a combination of powder and single crystal X-ray diffraction as well as EDS analysis to establish the distribution of Mn and Al atoms. Single crystals of γ-Mn5–x Al8+x were grown using Sn-flux at 650 °C. The crystal structure, atomic coordinates and site occupancy parameters of γ-Mn5−x Al8+x phases were refined from single crystal X-ray data. The γ-Mn5-x Al8+x phase adopts the rhombohedral Cr5Al8-type structure rather than a cubic γ-brass structure. The refined compositions from two crystals extracted from the Al-rich and Mn-rich sides are, respectively, Mn4.76Al8.24(2) (I) and Mn6.32Al6.68(2) (II). The structure was refined in the acentric R3m space group (No.160, Z=6), in order to compare with other reported rhombohedral γ-brasses. In addition, according to X-ray powder diffraction analysis, at the Al-rich side the γ-phase coexists with LT–Mn4Al11 and, at the Mn-rich side, with a hitherto unknown phase. The refined lattice parameters from powder patterns fall in the range a=12.6814(7)−12.6012(5) Å and c=7.9444(2)−7.9311(2) Å from Al-rich to Mn-rich loadings, and the corresponding rhombohedral angles distorted from a pseudo-cubic cell were found to be 89.1(1)°−88.9(1)°. Magnetic susceptibility and magnetization studies of Mn4.92Al8.08(2) are consistent with moment bearing Mn and suggest a spin glass state below 27 K. Tight-binding electronic structure calculations (LMTO-ASA with LSDA) showed that the calculated Fermi level for γ-“Mn5Al8” falls within a pseudogap of the density of states, a result which is in accordance with a Hume-Rothery stabilization mechanism γ-brass type phases.