While the incommensurability in melilites is well documented, the underlying atomic configurations and the composition-dependent phase behavior are not yet clear. We have studied the transition from the incommensurate phase to the high-temperature normal phase (IC-N), and to the low-temperature commensurate phase (IC-C) of selected members of the Ca(2)Co(1 - x)Zn(x)Si(2)O(7) system using X-ray and single-crystal electron diffraction, as well as calorimetric measurements. The space group of the unmodulated normal phase and of the basic structure of the incommensurate phase is P42(1)m; the commensurate lock-in superstructure was refined as a pseudomerohedral twin in the orthorhombic space group P2(1)2(1)2. We found that the commensurate modulation is mainly connected with a sawtooth-like periodicity of rotations of the T(1) tetrahedra in the 3 x 3 superstructure. In this structure, the clustering of the low-coordinated Ca(2+) ions is not complete so that only imperfect octagons were detected. Generally, the effect of increasing substitution of Co by Zn was a continuous reduction of the IC-N and IC-C transition temperatures.

Complex spin glass behavior is found to be present in a spinel system Ga0.8Fe0.2NiCrO4 where the cations occupy two different sites, namely the tetrahedrally coordinated A site and the octahedrally coordinated B site. The complex findings include anomalous time (t) variations of isothermal remanent magnetization (IRM), thermoremanent magnetization (TRM), zero field cooled magnetization [M(ZFC)], and field cooled magnetization [M(FC)]. Surprising results, like M(FC) < M(ZFC), M(FC) decreasing and changing faster than a slow increasing M(ZFC) with t, the M(FC), M(ZFC) not moving (i.e., not changing with t) toward a common equilibrium value, M-eq, and the IRM, TRM increasing with t, also exist for certain H (external magnetic field), T (temperature), t values. We have tried to understand the above and other observed results on the basis of the possible separate time behaviors of the A- and B-site magnetizations in the system.

CaCuGe2O6 shows a strongly distorted clinopyroxene-type structure with P2(1)/c symmetry at 298 K. The Cu2+ ion at the M1 site is coordinated by six O atoms forming an octahedron, which deviates significantly from ideal geometry. Individual M1 sites are connected via common edges to form an infinite zigzag chain parallel to the crystallographic c axis. The Ca2+ ion at M2 shows a sevenfold coordination. M2 sites are connected to the M1 chain via three common edges, thereby forming a metal layer within the bc plane. Besides the strong Jahn-Teller distortion of the Cu site, the structure of the title compound differs from ;normal' clinopyroxenes by a distortion of alternate layers of Ge sites. While the Ge(A) site is fourfold coordinated by O atoms, forming infinite chains of corner-sharing chains parallel to the c axis, the Ge(B) site exhibits a fivefold coordination, thereby forming a true two-dimensional layer of edge-sharing GeO5 bipyramids. Decreasing the temperature causes a magnetic phase transition at 40 K, as monitored by a broad maximum in the magnetic susceptibility and by discontinuities in the lattice parameters. Increasing the temperature causes variations in bond lengths, edge lengths and bond angles. Most prominent is the increase of one bond length of the Ge(B) site and the increase of the tetrahedral bridging angle of the Ge(A) site. At 660 K a crystallographic phase transition is observed where the symmetry changes from P2(1)/c to C2/c. The transition is accompanied by large changes in the lattice parameters which are indicative of distinct topological changes of several structural building units. The high-temperature C2/c structure is similar to that of the germanate clinopyroxene CaMgGe2O6.

Electron microscope studies are reported of MWCNTs without and with metallic encapsulations prepared by pyrolysis of organo-metallic precursors such as iron(II)phthalocyanine and ferrocene/anthracene mixtures. For straight and well-ordered MWCNTs, we obtained clear evidence of a scroll type structure with uniform chirality. Conical growth implicated the opening of the tube walls by termination of the graphene layers at the wall surfaces. Evidence is provided of the participation of iron carbide as an intermediate phase during graphite formation. Coercivities of the nanowire material up to 2550 Oe at 5 K, and constant saturation magnetization were measured.

Diluted magnetic semiconductor materials, e.g. (GaMn)As, based on III/V materials have attracted increasing interest recently due to the possibility of combining magnetic properties with existing electronics and optics based on GaAs. These materials have been grown by MOVPE (metal organic vapour phase epitaxy) using triethylgallium and te r t ia rybu ty la rs ine as more e f f i c ien t g roup I I I and group V precursors . As Mn source bis(methylcyclopentadienyl)manganese has been applied. Under certain growth conditions the formation of (MnGa)As clusters in the GaAs matrix is observed. Hexagonal (MnGa)As clusters are formed when the substrate temperature is chosen between 500 and 600°C and the Mn/Ga ratio in the gas phase during growth is high. These clusters are responsible for the ferromagnetic coupling in the samples up to temperatures exceeding room temperature. Depending on the growth temperature and the annealing as well as overgrowth conditions, different magnetic characteristics of the (MnGa)As cluster containing films have been found. These magnetic properties can be correlated to the structure and shape of the (MnGa)As clusters and their heteroepitaxial relationship to the GaAs matrix. High resolution transmission electron microscopy investigations show that clusters, which have been grown at high temperatures have only one distinct heteroepitaxial relationship of the clusters with respect to the matrix – in contrast to those grown at lower temperatures. This results in an anisotropy of the ferromagnetic coupling, which might be very important for device applications. Furthermore, these clusters can be overgrown heteroepitaxially with (AlGa)As. A model correlating the structure of the clusters with the magnetic measurements will be presented. These investigations show that it is possible to grow (MnGa)As by MOVPE which fulfills important prerequisites for device applications, namely the possibility to be overgrown and a single heteroepitaxial relationship to the substrate. 10th European Workshop on MOVPE, Lecce (Italy) 8−11 June 2003 PS.IV.15

We have investigated surface morphologies and cluster formation in Mn-incorporated (GaIn)As layers grown by metal-organic vapor phase epitaxy (MOVPE) on InP (1 0 0) substrates. Whisker growth occurs on the layer surfaces under low V/III ratios and low growth temperature conditions. For temperatures above 500°C, MnAs-based cluster structures are formed near the (GaIn)As layer surfaces. The MnAs-based clusters show an in-plane anisotropy in the magnetic characteristics. During the overgrowth of the MnAs-based clusters by undoped InP, a change in cluster composition to MnP presumably occurs as indicated by the change in the observed Curie temperature.

Hybrid structures consisting of ferromagnetic Mn(Ga)As clusters, which are embedded defect-free in a p-GaAs:Mn matrix, are realized by epitaxial growth using metalorganic vapour-phase epitaxy (MOVPE). The successful Te-co-doping of the GaAs:Mn matrix leads to a change from p- to n-type carrier transport. This behaviour enables the growth of structures for studying electron spin-injection effects. First laser devices including a co-doped hybrid structure in the n-type region of the contact of the device are realized. Investigations by SQUID magnetometer show that the ferromagnetic properties of the Mn(Ga)As clusters are not influenced by the Te co-doping of the surrounding GaAs matrix.

Single crystals of Ca2Co1−xZnxSi2O7 solid solutions with 0≤x≤1 were synthesized by the floating zone melting technique. A small melting length and a growing speed of 1.0mm/h were suitable for obtaining high quality crystals. Two-dimensional modulation phenomena were observed in the whole range of x. The modulation amplitude was proved to weaken unsteadily with increasing Zn content. Octagonal clustering as the fundamental mechanism of the modulation formation is described as well as variations in the microdomain pattern according to varying degrees of internal stress accommodation.

We present the results of the epitaxial overgrowth of magnetic (MnGa)As-cluster structures with GaAs, (AlGa)As and AlAs using metal organic vapor phase epitaxy (MOVPE). The structural differences in the overgrowth are investigated by means of atomic force microscopy (AFM) combined with transmission electron microscopy (TEM), in particular, to proof the successful overgrowth of the cluster layers with AlAs. Out of these experiments a first model for the overgrowth is developed. Measurements using a SQUID-magnetometer confirm the existence of ferromagnetism above room temperature in the cluster layers after overgrowth; however, other magnetic properties as the coercitive field are influenced by the overgrowth process.

We provide here clear evidence of transverse freezing in the anisotropic spin glass Fe2TiO5 from remanent magnetization studies. In addition, we find anomalous time dependences for zero field cooled and field cooled magnetizations below the transverse freezing temperature. An attempt is made to understand these results on the basis of possible separate time behaviors of f- and c-site magnetizations in the system.

We present the results of the successful epitaxial growth of magnetic MnGaAs-cluster structures using metal organic vapour phase epitaxy (MOVPE). Investigations by SQUID-magnetometer established ferromagnetism with Curie temperature as high as 320 K, clear in-plane anisotropic magnetic behavior is detected. The structural properties of the epitaxial layers are investigated by means of atomic force microscopy (AFM) and in particular transmission electron microscopy (TEM). A statistical analysis of the cluster sizes shows, that the clusters expand strongly with increasing deposition temperature, whereas the part of the surface, covered with clusters, stays constant at 25 +/-5 %. Preliminary experiments show that the MnGaAs clusters can be overgrown and embedded defect-free in host (AlGa)As layer structures.

Single crystals of the following phosphates were grown by the floating zone technique using a mirror furnace and their crystal structures refined (0.02 < R-1 < 0.04 and 0,04 < wR(2) < 0,10, resp.): Ba2Mn(PO4)(2) (a=531.1(1), b = 896.8(1), c = 1625.6(3) pm, beta = 90,26(1)degrees), Ba2Co(POS)p (a = 529.8(1), b = 884.4(1), c = 1614.4(3) pm, beta = 90.68(2)degrees) and BaNi2(PO4)(2) (a = 480.0(1), c = 2327.3(5)pm, Z = 3, space group R (3) over bar). Both compounds Ba2MII(PO4)(2) crystallize with Z = 4 in space group P2(1)/n of the monoclinic Ba2Ni(PO4)(2) type; BaNi2(PO4)(2) has the hexagonal-rhombohedral structure of the BaNi2(AsO4)(2) type. Magnetic measurements of powders of Ba2Mn(PO4)(2) and Ba2Co(PO4)(2) yielded room temperature moments of mu (eff) = 5.73 and 4.93 mu (B), resp., but only the manganese compound obeys the Curie-Weiss law down to flow temperatures. Weak antiferromagnetic interactions at both compounds only near T-M = 5 K lead to a reciprocal susceptibility minimum.

We report the successful growth of magnetic Ga1−xMnxAs layers on (100) GaAs substrates by metal-organic vapour-phase epitaxy. Depending on the growth parameters, two different magnetic phases of Ga1−xMnxAs can be grown. (i) At low Mn-concentrations, Ga1−xMnxAs alloys are obtained. These alloys exhibit a paramagnetic behaviour with a strong exchange interaction between the localised magnetic moments of the Mn2+ions and the extended excitonic states. (ii) At high Mn-concentrations, Mn(Ga)As clusters are formed within a Ga1−xMnxAs host. The samples are ferromagnetic even above room temperature. The ferromagnetism has been investigated by SQUID and ESR measurements. The s–d and p–d exchange integrals have been determined independently by combining photoluminescence excitation and spin-flip Raman spectroscopy. A reversal of sign of the valence band exchange integral has been detected along with the transition from the paramagnetic to the ferromagnetic phase.

Single crystals of Li-aegirine LiFe3+Si2O6 were synthesized at 1573 K and 3 GPa, and a polycrystalline sample suitable for neutron diffraction was produced by ceramic sintering at 1223 K. LiFe3+Si2O6 is monoclinic, space group C2/c, a=9.6641(2) Å, b= 8.6612(3) Å, c=5.2924(2) Å, β=110.12(1)∘ at 300 K as refined from powder neutron data. At 229 K Li-aegirine undergoes a phase transition from C2/c to P21/c. This is indicated by strong discontinuities in the temperature variation of the lattice parameters, especially for the monoclinic angle β and by the appearance of Bragg reflections (hkl) with h+k≠2n. In the low-temperature form two non-equivalent Si-sites with 〈SiA–O〉=1.622 Å and 〈SiB–O〉=1.624 Å at 100 K are present. The bridging angles of the SiO4 tetrahedra O3–O3–O3 are 192.55(8)° and 160.02(9)° at 100 K in the two independent tetrahedral chains in space group P21/c, whereas it is 180.83(9)° at 300 K in the high-temperature C2/c phase, i.e. the chains are nearly fully expanded. Upon the phase transition the Li-coordination changes from six to five. At 100 K four Li–O bond lengths lie within 2.072(4)–2.172(3) Å, the fifth Li–O bond length is 2.356(4) Å, whereas the Li–O3 A bond lengths amount to 2.796(4) Å. From 57Fe Mössbauer spectroscopic measurements between 80 and 500 K the structural phase transition is characterized by a small discontinuity of the quadrupole splitting. Temperature-dependent neutron powder diffraction experiments show first occurrence of magnetic reflections at 16.5 K in good agreement with the point of inflection in the temperature-dependent magnetization of LiFe3+Si2O6. Distinct preordering phenomena can be observed up to 35 K. At the magnetic phase transition the unit cell parameters exhibit a pronounced magneto-striction of the lattice. Below T N Li-aegirine shows a collinear antiferromagnetic structure. From our neutron powder diffraction experiments we extract a collinear antiferromagnetic spin arrangement within the a–c plane.

AbstractFor Abstract see ChemInform Abstract in Full Text.

This paper deals with transmission electron microscope experiments of Ca,Sr-åkermanite solid solutions at temperatures between 100 K and 375 K. The aim of the investigations was to study the compositional and temperature dependence of phase transitions from the normal to the incommensurately modulated structure of (Ca1-xSrx)2MgSi2O7 single crystals observed by electron diffraction. With increasing Sr-content the normal to incommensurate phase transition temperature decreases linearly from ca. 350 K (x = 0.04) to 295 K (x = 0.22). Depending on temperature and composition, the incommensurate phase is structurally not uniform, its stability field can be divided into two regions. The transition between the two regions can be recognized by the additional appearance of higher order satellite reflections in the diffraction pattern. Additionally, the transition is characterized by an abrupt change of the modulation wavelength. Within the high-temperature, low ordered region, the modulation wavelength varies between 18.5 Å and 20.5 Å, whereas no changes of the wavelength have been found in the low-temperature, higher ordering region. With increasing Sr-content the high-temperature, low ordered phase covers a steadily increasing temperature range.

Clinopyroxenes along the solid solution hedenbergite-aegirine M2[Ca2+1-xNa+xM1{Fe2+1-xFe3+x}Si2O6 were synthesized using hydrothermal techniques at 4 kbar. Different temperatures and redox conditions were used to determine optimum synthesis conditions and the stability range of individual compositions in the T - log fO2 field. Synthesized samples were characterized using microprobe analysis, X-ray powder diffraction and Mossbauer spectroscopy at 298 K and 80 K. The structure was refined in the C 2/ c space group by means of the Rietveld method.Along the solid-solution series between hedenbergite (a = 9.8448(6) A, b = 9.0296(6) A, c = 5.2452(4) A, β = 104.813) and aegirine endmembers (a = 9.6547(6) A, b = 8.7941(8) A, c = 5.2944(4) A, β = 107.398), the changes in unit cell dimensions show significant deviations from linearity. Mean and individual M1-O distances decrease linearly from hedenbergite to aegirine; mean M2-O and T-O distances do not change significantly, whereas individual length may vary. While in hedenbergite the coordination of the M2 site is 6+2, it is 4+4 in aegirine. The Mossbauer spectra of the solid-solution endmembers display narrowly split resonance absorption lines with hyperfine parameters typical for Fe2+ (δ = 1.18 mm/s, Δ = 2.25 mm/s at 298 K) and Fe3+ (δ = 0.38 mm/s, Δ = 0.30 mm/s at 298 K). Fe occupies only the M1 site. The Fe2+ resonance absorption is somewhat broadened in the 80 K spectra of the solid solution, which is due to a distribution of quadrupole splittings.