Thomas Zeiske

TARANAKITE : THE MINERAL WITH THE LONGEST CRYSTALLOGRAPHIC AXIS

For the first time large single-crystals of taranakite K3Al5(HPO4)6(PO4)2·18H2O were grown by gel crystallization. An X-ray structure determination showed taranakite to be the mineral with the longest crystallographic axis described hitherto. X-ray crystal data are: space group R3c, a = 870.25 (11), c = 9505(1)_pm, Z = 6, Rg = 0.028. A neutron scattering experiment with subsequent Rietveld refinement of the powder pattern of deuterated taranakite elucidated the D-atom positions. Neutron crystal data are: a=868.82(3), c=9498(2) pm, Rwp=0.061. Taranakite is a layer structure mineral having six layers of composition [K3Al5(HPO4)6(PO4)2(H2O)12] separated from each other by water layers. The rigid layer is formed by columns of corner sharing hydrogen phosphate tetrahedra and Al octahedra which are interconnected by additional six-coordinated Al ions. In trigonal holes of the layer orthophosphate ions are situated. Non-exchangeable K ions are trapped within the layers. Hydrogen bonds in taranakite are formed between building units within the rigid layer, within the water interlayer, and between layer and interlayer. Similarities and differences between the coordination of interlayer water in taranakite and ice Ih are discussed. 31P solid-state MAS NMR spectra show two isotropic lines at σ180 = −17.2 and 5.2 ppm. The intensity ratio 3:1 of the two sideband systems corresponds to the molar ratio of HPO42− and PO43− units in the structure of taranakite. The P(1) atom of hydrogen phosphate has a larger anisotropy in agreement with the more different PO bond lengths of HPO42− compared to PO43−. The 31P nuclear magnetic shielding tensor of HPO42− is characterized by larger shielding values σ22 and σ33.

Variation of magnetism and structure on mechanically alloyed Fe40Co10Cu50 powder with process time and heat treatment

The effects of milling time and heat treatment on local structural changes during mechanical alloying Fe40Co10Cu50 have been investigated by means of x-ray diffraction (XRD), neutron diffraction technique, vibrating sample magnetometer, and Mossbauer spectroscopy. Mossbauer spectroscopy showed that two kinds of magnetic phases coexist in the mechanically alloyed powder, which is in agreement with XRD and neutron diffraction study. One is corresponding to α-Fe and the other is a new magnetic phase formed during mechanical alloying. With increasing processing time, the new magnetic phase becomes more ordered and dominant. Mossbauer spectrum from the annealed sample consists of ferromagnetic component with a hyperfine field of 36.2 T and a paramagnetic component.

The structure of monoclinic NaNiO 2 as determined by powder X-ray and neutron scattering

The crystal structure of low temperature NaNiO 2 has been refined by Rietveld methods using powder X-ray diffraction and neutron scattering data. The starting model was based on parameters that had been obtained earlier by X-ray film methods. At room temperature NaNiO 2 is monoclinic, C 2/ m , a =0.53192(2), b =0.28451(1), c =0.55826(4) nm, β=110.449(2)°. NaNiO 2 has a layered structure. The Ni–O layer is formed by edge sharing of Jahn–Teller elonganted NiO 6 octahedra with Ni–O distances of 0.1911(2) nm and 0.2144(4) nm. The Na ions between these layers also exhibit a distorted octahedral coordination with Na–O distances of 0.2328(2) nm and 0.2369(4) nm. The final R values were R wp =0.069, R I =0.059, R exp =0.059 for the neutron and R wp =0.032, R I =0.034, R exp =0.017 for the X-ray data.

Francoanellit K3Al5HPO4)6(PO4)2 · 12H2O: Struktur und Synthese durch topochemische Entwässerung von Taranakit / Francoanellite K3Al5HPO4)6(PO4)2 · 12H2O: Structure and Synthesis by Topochemical Dehydration of Taranakite

Abstract Single crystals of synthetic francoanellite K3Al5(HPO4)6(PO4)2-12H2O could be obtained for the first time by topochemical dehydration of taranakite crystals. An X-ray structure determination showed francoanellite to be the mineral with the second longest crystallographic axis described hitherto. Crystal data: space group R3c, a = 869.0(2), c = 8227(1)pm, Z = 6 , Rg = 0.042. Francoanellite is a layer structure mineral having six layers of composition [K3Al5(HPO4)6(PO4)2(H2O)12], connected by hydrogen bonds. The rigid layer is formed by columns of corner sharing hydrogen phosphate tetrahedra and AIO6-octahedra which are inter connected by additional six-coordinated Al ions. In trigonal holes of the layer orthophosphate ions are situated. The structure of francoanellite is very similar to the structure of taranakite K3H6Al5(PO4) 8 · 18H2O which has planar water interlayers between the Al-phosphate layers. A neutron scattering experiment with subsequent Rietveld refinement of the powder pattern gave the H-atom positions. Hydrogen bonds in francoanellite are formed within the rigid layers and between them. During the reaction taranakite → francoanellite crystals in an intermediate stage of dehydration could be obtained. From the c-axis of 8858 pm and one-dimensional electron density projections it can be proposed that in these crystals every second water interlayer was lost and a first order staging product of the deintercalation of water from taranakite was formed.

Spin wave stiffness constant on the amorphous and nanocrystalline state of Fe/sub 76/Cu/sub 1/Nb/sub 3/Si/sub 14/B/sub 6/ alloys

We report here on the effect of annealing temperatures on the spin wave excitations for the amorphous and nanocrystalline state of Fe/sub 76/Cu/sub 1/Nb/sub 3/Si/sub 14/B/sub 6/ alloys, observed from the low temperature magnetization curves and the spectroscopic splitting g factors. The temperature dependence of saturation magnetization was measured with a vibrating sample magnetometer and the SQUID magnetometer. Spectroscopic splitting g-values were estimated from ferromagnetic resonance experiments (FMR). The thermo-magnetization curve is found to obey Blochs law, M/sub s/(T)=M/sub s/(0)(1-BT/sup 3/2/-CT/sup 5/2/). The spin wave stiffness constants were also observed from the preliminary results of the neutron inelastic scattering measurements. The spin wave stiffness constants obtained from two different experimental methods on the Fe/sub 76/Cu/sub 1/Nb/sub 3/Si/sub 14/B/sub 6/ alloys agree fairly well.

Inelastic Neutron Scattering Studies on the Amorphous and Nanocrystalline Fe76Cu1Nb3Si14B6

Inelastic neutron scattering measurement has been used to study the temperature and wave-vector dependence of the spin wave excitation in the amorphous and nanocrystalline state of ferromagnetic Fe76Cu1Nb3Si14B6 alloy. For the amorphous alloy the spin waves were observed over the accessible wave-vector range of 0.055 A-1 ≤q ≤0.105 A-1, for temperature between 300 K (0.49 T c) and 547 K (0.90 T c). The magnon dispersion curves exhibit the conventional quadratic relationship E = D(T)q2 + Δ, typical of an isotropic ferromagnet, where the small energy gap Δ≈0.05 meV is attributed primarily to the dipole-dipole interaction. An estimate of the T = 0 value of D(0) ≈153 meV A2 was obtained from a plot of D(T) vs T extrapolated to low temperatures, while the Curie temperature of 610 K was obtained from the extrapolation of D(T) to zero at high temperatures. There is the increase of the spin wave stiffness constant D with the transformation of the alloy from amorphous to nanocrystalline state.

Aluminiumphosphate mit nichtzentrosymmetrischen Schicht-und Raumnetzstrukturen aus topologisch verwandten Motiven: 1. KAl2(PO4)2(OH)·4H2O/Aluminum Phosphates with Non-Centrosym metric Layer-and Framework-Structures of Topologically Related Motifs 1. KAl2(PO4)2(OH)·4H2O

Abstract KAl2(PO4)2(OH)·4H2O could be obtained by the reaction of gibbsite with a potassium-phosphate solution of pH = 5.5 at 333 K. A single crystal X-ray structure analysis showed that KAl2(PO4)2(OH)·4H2O is isotypic with the mineral minyulite. Crystal data: orthorhombic space group Pba2, a = 934.7(1), b = 982, 1 (1), c = 551,0(1) pm, Z = 2. It possesses dimeric units of Al-octahedra which are bridged by an OH-and two bidentate phosphate groups. The dimers are linked by phosphate to plane polar layers. Cavities in the layers are occupied by K ions. One corner of each phosphate tetrahedron is not connected to Al but accepts four hydrogen bonds from water molecules bound to Al of the adjacent layer. A powder neutron scattering experiment with KAl2(PO4)2(OD)·4D2O showed that four D atoms and P coordinate the hydrogen bond accepting O in KAl2(PO4)2(OD)·4D2O like a square pyramid. Analogous dimeric building units are found in the Al phosphate minerals minyulite and morinite; similar dimers of Fe octahedra occur in the mineral copiapite, a number of Fe containing enzymes and numerous synthetic iron complexes. High-resolution 31P solid-state NMR spectra show only one isotropic line at δiso = -9.6 ppm and confirm the presence of one symmetry independent phosphorus site in the unit cell. In the 1H-MAS-NMR spectra the water molecules and the bridging OH groups show signals at 5.4 and 1.4 ppm, respectively

Phonon anomalies in copper due to a strong magnetic field observed by inelastic neutron scattering

The influence of a strong magnetic field (B = 6.1 T) perpendicular to the wave vector of longitudinal acoustic phonons in [0 0 1] direction of copper was investigated by inelastic neutron scattering. Small shifts in phonon energies and line widths are attributed to the interaction with Landau levels of nearly free electrons.