M. Reehuis

Neutron diffraction study of YVO3, NdVO3, and TbVO3

The structural and magnetic properties of YVO3, NdVO3 and TbVO3 were investigated by single-crystal and powder neutron diffraction. YVO3 shows a structural phase transition at 200 K from an orthorhombic structure with the space group Pbnm to a monoclinic one with the space group P21 /b. But supplementary highresolution synchrotron diffraction experiments showed that the monoclinic distortion is extremely small. A group theoretical analysis shows that this magnetic state in the monoclinic phase is incompatible with the lattice structure, unless terms of higher than bilinear order in the spin operators are incorporated in the spin Hamiltonian. This observation is discussed in the light of recent theories invoking unusual many-body correlations between the vanadium t2g orbitals. A structural phase transition back to the orthorhombic space group Pbnm is observed upon cooling below 77 K. This transition is accompanied by a rearrangement of the magnetic structure into a mode compatible with the lattice structure. The crystal structures of NdVO3 and TbVO3 are closely similar to that of YVO3. However, only a single magnetic phase transition was found in the vanadium sublattice down to 9.5 K. Below 60 K the magnetic moments of the Nd 3+ - and Tb 3+ -ions are gradually polarized by the ordered vanadium moments. Below 11 K, we found a noncollinear order of the terbium moments.

Crystal structure and high-field magnetism of La2CuO4

Neutron diffraction was used to determine the crystal structure and magnetic ordering pattern of a La2CuO4 single crystal, with and without applied magnetic field. A previously unreported, subtle monoclinic distortion of the crystal structure away from the orthorhombic space group Bmab was detected. The distortion is also present in lightly Sr-doped crystals. A refinement of the crystal structure shows that the deviation from orthorhombic symmetry is predominantly determined to drive a continuous reorientation of the copper spins from the orthorhombic b axis to the c axis, directly confirming predictions based on prior magnetoresistance and Raman scattering experiments. A spin-flop transition induced by a c-axis oriented field previously reported for nonstoichiometric La2CuO4 is also observed, but the transition field 11.5 T is significantly larger than that in the previous work.

Single-crystal growth, crystallography, magnetic susceptibility, heat capacity, and thermal expansion of the antiferromagnetic S=1 chain compound CaV2O4

The compound CaV{sub 2}O{sub 4} contains V{sup +3} cations with spin S=1 and has an orthorhombic structure at room temperature containing zigzag chains of V atoms running along the c axis. We have grown single crystals of CaV{sub 2}O{sub 4} and report crystallography, static magnetization, magnetic susceptibility x, ac magnetic susceptibility, heat capacity C{sub p}, and thermal expansion measurements in the temperature T range of 1.8--350 K on the single crystals and on polycrystalline samples. An orthorhombic-to-monoclinic structural distortion and a long-range antiferromagnetic (AF) transition were found at sample-dependent temperatures T{sub S}{approx}108--145 K and T{sub N}{approx}51--76 K, respectively. In two annealed single crystals, another transition was found at {approx}200 K. In one of the crystals, this transition is mostly due to V{sub 2}O{sub 3} impurity phase that grows coherently in the crystals during annealing. However, in the other crystal the origin of this transition at 200 K is unknown. The x(T) shows a broad maximum at {approx}300 K associated with short-range AF ordering and the anisotropy of x above T{sub N} is small. The anisotropic x(T{yields}0) data below T{sub N} show that the (average) easy axis of the AF magnetic structure is the b axis. The C{sub p}(T) data indicatemorexa0» strong short-range AF ordering above T{sub N}, consistent with the x(T) data. We fitted our x data by a J{sub 1}-J{sub 2} S=1 Heisenberg chain model, where J{sub 1}(J{sub 2}) is the (next)-nearest-neighbor exchange interaction. We find J{sub 1}{approx}230 K and surprisingly, J{sub 2}/J{sub 1}{approx}0 (or J{sub 1}/J{sub 2}{approx}0). The interaction J{sub {perpendicular}} between these S=1 chains leading to long-range AF ordering at T{sub N} is estimated to be J{sub {perpendicular}}/J{sub 1}{approx_equal}0.04.«xa0less

Magnetic structure and interactions in the quasi-one-dimensional antiferromagnet CaV 2 O 4

CaV2O4 is a spin-1 antiferromagnet, where the magnetic vanadium ions have an orbital degree of freedom and are arranged on quasi-one-dimensional zig-zag chains. The first- and second-neighbor vanadium separations are approximately equal suggesting frustrated antiferromagnetic exchange interactions. High-temperature susceptibility and single-crystal neutron diffraction measurements are used to deduce the dominant exchange paths and orbital configurations. The results suggest that at high temperatures CaV2O4 behaves as a Haldane chain, but at low temperatures, it is a spin-1 ladder. These two magnetic structures are explained by different orbital configurations and show how orbital ordering can drive a system from one exotic spin Hamiltonian to another.

Influence of Sample Preparation Technology and Treatment on Magnetism and Superconductivity of UCoGe

Ferromagnetic superconductor UCoGe is reported to be a week ferromagnet with Curie phase transition temperature T C ≈2.5 K. The magnetic order coexists with superconductivity that appears below critical temperature T SC ≈0.7 K. Nevertheless, the exotic coexistence of the mentioned physical phenomena is strongly sample dependent and only few samples can take pride in thereby. We have prepared a series of polycrystalline samples and single crystals using Czochralski and floating zone method. The samples were treated by a standard annealing under the vacuum or refined using solid state electrotransport. All samples were carefully characterized by X-ray and neutron diffraction methods and by electron-dispersive X-ray analysis in a scanning electron microscope. They were subsequently also studied with respect to magnetism and superconductivity by electrical resistivity, magnetization, AC susceptibility and specific heat measurements. The results have been analyzed and the relations between the sample quality, ...

Erratum: Sr{sub 2}CrOsO{sub 6}: End point of a spin-polarized metal-insulator transition by 5d band filling [Phys. Rev. B 75, 020404(R) (2007)]

Y. Krockenberger, K. Mogare, M. Reehuis, 3 M. Tovar, M. Jansen, G. Vaitheeswaran, 4 V. Kanchana, 4 F. Bultmark, A. Delin, F. Wilhelm, A. Rogalev, A. Winkler, and L. Alff ∗ Darmstadt University of Technology, Petersenstr. 23, 64287 Darmstadt, Germany Max-Planck-Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany Hahn-Meitner-Institut (HMI), 14109 Berlin, Germany Department of Materials Science and Engineering, Royal Institute of Technology (KTH), 10044 Stockholm, Sweden Department of Physics, University of Uppsala Box 530, 75121 Uppsala, Sweden European Synchrotron Radiation Facility (ESRF), 6 Rue Jules Horowitz, BP 220, 38043 Grenoble Cedex 9, France (Dated: Received 13 December 2006)

Electron density distribution in paramagnetic chromium: A γ -ray diffraction study

We report a high-pressure investigation of the relaxor ferroelectric lead zinc niobate PbZn{sub 1/3}Nb{sub 2/3}O{sub 3} (PZN) up to 46 GPa, which is the highest pressure yet attained in the study of relaxors. The evolution of both Raman and x-ray scattering with pressure gives evidence for important pressure instabilities, which find its expression in three successive phase transitions. The observed pressure-induced suppression of diffuse scattering above 5 GPa is similar to recent reports and supports the hypothesis that this is a general feature in relaxors at high pressures.Stable pairing states of superfluid {sup 3}He in aerogel are examined in the case with a global uniaxial anisotropy which may be created by applying a uniaxial stress to the aerogel. Due to such a global anisotropy, the stability region of an Anderson-Brinkman-Morel (ABM) pairing state becomes wider. In a uniaxially stretched aerogel, the pure polar pairing state with a horizontal line node is predicted to occur, as a three-dimensional superfluid phase, over a measurable width just below the superfluid transition at T{sub c}(P). A possible relevance of the present results to the case with no global anisotropy is also discussed.A series of Ca{sub x}CoO{sub 2} (0.15{<=}x{<=}0.40) materials have been prepared by means of an ion exchange reaction from Na{sub x}CoO{sub 2}. Transmission electron microscopy (TEM) measurements revealed a rich variety of structural phenomena resulting from cation ordering, structural distortion, and twinning. Systematic structural analysis, in combination with the experimental data of Na{sub x}CoO{sub 2} (0.15{<=}x{<=}0.8) and Sr{sub x}CoO{sub 2} (1.5{<=}x{<=}0.4) systems, suggests that there are two common well-defined cation ordered states corresponding, respectively, to the orthorhombic superstructure at around x=1/2 and the 3{sup 1/2}ax3{sup 1/2}a superstructure at around x=1/3 in this kind of system. Multiple ordered states, phase separation, and incommensurate structural modulations commonly appear in the materials with 0.33<x<0.5. The TEM observations also reveal an additional periodic structural distortion with q{sub 2}=a{sup *}/2 in materials for x{<=}0.35. This structural modulation also appears in the remarkable superconducting phase Na{sub 0.33}CoO{sub 2}{center_dot}1.3H{sub 2}O.Electrical resistance, thermoelectric power, dc magnetization, ac susceptibility, and electron spin resonance (ESR) are investigated for the polycrystalline Nd{sub 1-x}Sr{sub 1+x}CoO{sub 4} (x=0.25, 0.33, and 0.60). Powder x-ray diffraction (XRD) confirms that these compounds crystallize in K{sub 2}NiF{sub 4}-type structure with space group I4/mmm. The specimens exhibit ferromagnetic and semiconducting behaviors. With Sr doping, the lattice parameter c increases, the cusp intensity related to spin-glass states weakens, and the ferromagnetic property intensifies. The transport mechanisms in high temperature range obey Arrhenius law and might be understood by small polaron models. The magnetic properties present spin-glass states at {approx}18 K and Griffiths singularity at {approx}210 K.In this work we report on a low-energy electron diffraction (LEED) study of MnO(100) thick films grown on Ag(100) in order to determine their surface geometry. The LEED results indicate a topmost layer rumple of (4.8{+-}2.0)% with the oxygen ions moving towards the vacuum side. These results are in line with other surface structure determinations carried out on the (100) surface of different oxides having rock-salt structure but are in disagreement with MEIS results reported in the literature for the MnO(100) using a MnO single crystal.The structural and magnetic properties of YVO{sub 3}, NdVO{sub 3} and TbVO{sub 3} were investigated by single-crystal and powder neutron diffraction. YVO{sub 3} shows a structural phase transition at 200 K from an orthorhombic structure with the space group Pbnm to a monoclinic one with the space group P2{sub 1}/b. But supplementary high-resolution synchrotron diffraction experiments showed that the monoclinic distortion is extremely small. A group theoretical analysis shows that this magnetic state in the monoclinic phase is incompatible with the lattice structure, unless terms of higher than bilinear order in the spin operators are incorporated in the spin Hamiltonian. This observation is discussed in the light of recent theories invoking unusual many-body correlations between the vanadium t{sub 2g} orbitals. A structural phase transition back to the orthorhombic space group Pbnm is observed upon cooling below 77 K. This transition is accompanied by a rearrangement of the magnetic structure into a mode compatible with the lattice structure. The crystal structures of NdVO{sub 3} and TbVO{sub 3} are closely similar to that of YVO{sub 3}. However, only a single magnetic phase transition was found in the vanadium sublattice down to 9.5 K. Below 60 K the magnetic moments of the Nd{supmorexa0» 3+}- and Tb{sup 3+}-ions are gradually polarized by the ordered vanadium moments. Below 11 K, we found a noncollinear order of the terbium moments.«xa0lessWe report the observation of Co{sup 3+}/Co{sup 4+} short-range charge ordering in 10% Ho-doped SrCoO{sub 3-x} by means of high resolution neutron powder diffraction. The associated one-dimensional commensurate modulation, which can be described with the propagation vector q{sub CO}=(0 0 1/2) with respect to the cubic perovskite cell Pm3m, occurs for compositions close to x=0.20, corresponding to a 1:1 Co{sup 3+}/Co{sup 4+} ratio and extends over clusters of finite size (D{approx}250 A). The bond valence sums for the Co{sup 3+} and Co{sup 4+} sites are +3.07(7) and +3.95(11) (x=0.19), very close to their nominal values +3 and +4. We attribute this astonishing observation to the one-dimensional (1D) character of the associated distortion pattern, whose elastic stabilization is eased with respect to the 3D arrays reported for other charge-ordered perovskite oxides.The compounds RNi{sub 2}Mn (R=Tb, Dy, Ho, and Er) with a MgCu{sub 2}-type structure have been synthesized. The R to transition metal atom ratio is confirmed to be 1:3 using the energy dispersive spectroscopy. The structural and magnetic properties have been investigated by various experimental methods. The x-ray diffraction patterns (XRD) can be well indexed with a cubic Laves cell and space group Fd3m. The refinement results of the XRD patterns show the presence of vacancies in the crystallographic structure. The ordering temperatures T{sub C} have been derived to be 131, 94, 75, and 50 K for R=Tb, Dy, Ho and Er, respectively, which are much higher than those of their corresponding RNi{sub 2} and RMn{sub 2} compounds. A large difference of M-T curves between zero-field-cooling and field-cooling magnetization for all samples at a certain temperature range is observed in a low field, which can be understood in the terms of narrow-domain-wall pinning and a sensitive temperature dependence of coercivity.The structure of liquid CdTe was investigated at pressures up to 23.5 GPa using synchrotron x-ray diffraction. The structure factor, S(Q), and the pair distribution function, g(r), drastically change in two pressure regions, 1.8-3.0 and 7.0-9.0 GPa, accompanied with marked increase in the average coordination number. These findings suggest that there exists at least three stable liquid forms below 23.5 GPa. The pressure interval of the structural change is much smaller compared to other liquids of tetrahedrally bonded materials. Comparing the shapes of S(Q) and g(r) and other structural parameters with the respective data for the reference materials reveals that the lowest- and intermediate-pressure forms have the same local structures as the crystalline counterpart (zinc-blende-like local structure and a NaCl-like local structure), while the highest-pressure form has a different local structure from that in the crystalline form.The charge distributions of slow atomic particles that are singly scattered, multiply scattered, recoiled, and sputtered from metal surfaces are analyzed in terms of both nonadiabatic particle-substrate electron transfer and electron transfer from electronically excited substrates. The results are compared to experimental data for 50 eV Na{sup +} ions scattered from Cu(001), and Al atoms sputtered and recoiled from Al(100). The comparison allows for a quantitative determination of the role of substrate excitations in surface charge exchange. In addition, an analysis of kinetic electron emission (KEE) is carried out using similar low-energy metal projectile-metal substrate systems. Contributions to KEE from various nonadiabatic processes are quantitatively evaluated, including the same process that is responsible for charge formation in single-scattering experiments. The results are compared to experimental KEE data induced by Na{sup +} impinging on Ru(0001). The contributions of nonadiabatic one-electron processes are shown to be small when realistic particle-substrate parameters are used. Many-electron interactions are assumed to play an important role in explaining KEE and, as an illustration, a simplified hot-spot model is outlined.Neutron powder diffraction and transport measurements have been used to investigate the PrBaCo{sub 2}O{sub 5.48} compound between room temperature and 820 K. A structural phase transition, involving a rearrangement of oxygen vacancies, was found at T{sub OD}=776 K. Across the transition the perovskite structure loses its vacancy ordering, and the crystal symmetry changes from orthorhombic Pmmm to tetragonal P4/mmm. The resistivity measurements for temperatures above {approx}350 K yield high values of {rho}, indicating that the compound is rather semiconducting than metallic as usually accepted. A model in terms of thermally activated hole (polaronic) hopping is proposed.Granular films composed of well defined nanometric Co particles embedded in an insulating ZrO{sub 2} matrix were prepared by pulsed laser deposition in a wide range of Co volume concentrations (0.15<x<0.43). High-resolution transmission electron microscopy (TEM) showed very sharp interfaces between the crystalline particles and the amorphous matrix. Narrow particle size distributions were determined from TEM and by fitting the low-field magnetic susceptibility and isothermal magnetization in the paramagnetic regime to a distribution of Langevin functions. The magnetic particle size varies little for Co volume concentrations x<0.32 and increases as the percolation limit is approached. The tunneling magnetoresistance (TMR) was successfully reproduced using the Inoue-Maekawa model. The maximum value of TMR was temperature-independent within 50-300 K, and largely increased at low T, suggesting the occurrence of higher-order tunneling processes. Consequently, the tunneling conductance and TMR in clean granular metals are dominated by the Coulomb gap and the inherent particle size distribution.Zr-rich, Nb-doped lead zirconate titanate ceramic and powder samples with composition near Pb{sub 0.99}(Zr{sub 0.95}Ti{sub 0.05}){sub 0.98}Nb{sub 0.02}O{sub 3} [PZT95/5(2Nb)] have been studied in the range of hydrostatic pressure 0-6.2 kbar and temperature 12-295 K by time-of-flight neutron powder diffraction and dielectric measurements. The combination of the two techniques has led to further insights into the properties and pressure-induced ferroelectric rhombohedral R3c (F{sub R(LT)}) to antiferroelectric orthorhombic Pbam (A{sub O}) phase transition in this material, and the diffraction results have provided a detailed view of the ionic displacements induced by changes in pressure and temperature as well as the displacements accompanying the transition. At 295 K the diffraction results revealed a sharp transition at 2.1 kbar; at 200 K this transition occurs at 1.1 kbar. The transformation is incomplete: after the initial sharp drop in the F{sub R(LT)} content at the transition, 20 wt % of the sample remains in the low-pressure F{sub R(LT)} phase. Above the transition, the fraction of F{sub R(LT)}, which exists as a minority phase in the high-pressure A{sub O} phase, continues to decrease, but even at our highest pressure of 6.2 kbar, {approx}8 wt % of the sample remains in the F{sub R(LT)} phase. Themorexa0» volume contraction at the F{sub R(LT)}-to-A{sub O} transition unexpectedly results in the retained minority F{sub R(LT)} being anisotropically clamped, with its a axis slightly expanded and c axis contracted at the transition. On pressure release to 1 bar at 295 K, only 26% of the F{sub R(LT)} phase is recovered, and this remains in the clamped state because of the surrounding majority A{sub O} phase. Heating the sample above 350 K at 1 bar followed by cooling to room temperature results in full recovery of the F{sub R(LT)} phase. The spontaneous polarization (P{sub S}) of the F{sub R(LT)} phase and its pressure and temperature dependences were determined from the ionic displacements. At 295 K, P{sub S}=38 {mu}C/cm{sup 2}--a value greater than the 31-32 {mu}C/cm{sup 2} commonly observed on ceramic PZT95/5(2Nb) samples. The difference is undoubtedly related to residual porosity in ceramic samples as well as the inability of the poling electric field to align all the polar domains. P{sub S} increases monotonically with decreasing temperature, reaching a value of {approx}44 {mu}C/cm{sup 2} at 12 K.«xa0lessThe five independent elastic moduli of single-crystalline hexagonal boron nitride (h-BN) are determined using inelastic x-ray scattering. At room temperature the elastic moduli are in units of GPa C{sub 11}=811, C{sub 12}=169, C{sub 13}=0, C{sub 33}=27.0, and C{sub 44}=7.7. Our experimental results are compared with predictions of ab initio calculations and previously reported incomplete datasets. These results provide solid background for further theoretical advances and quantitative input to model elasticity in boron nitride (BN) nanotubes.I argue that certain bosonic insulator-superfluid phase transitions as an interaction constant varies are driven by emergent geometric properties of insulating states. I examine the renormalized chemical potential and population of disordered bosons at different energy levels. These quantities define the geometric aspect of an effective low energy Hamiltonian which I employ to investigate various resonating states and quantum phase transitions. In a mean field approximation, I also demonstrate that the quantum phase transitions are in the universality class of a percolation problem.The electronic structure and physical properties of {gamma}-Sn{sub 3}N{sub 4} in the spinel structure are investigated by first-principles calculations. The calculated band structure, electronic bonding, and optical properties are compared with two well-studied spinel nitrides {gamma}-Si{sub 3}N{sub 4} and {gamma}-Ge{sub 3}N{sub 4}. {gamma}-Sn{sub 3}N{sub 4} is a semiconductor with a direct band gap of 1.40 eV and an attractive small electron effective mass of 0.17. Its optical properties are different from that of {gamma}-Si{sub 3}N{sub 4} and {gamma}-Ge{sub 3}N{sub 4} because of the difference in the conduction band minimum. The Sn K, Sn L{sub 3}, Sn M{sub 5}, and N K edges of the x-ray-absorption near-edge structure spectra in {gamma}-Sn{sub 3}N{sub 4} are calculated using a supercell approach and are found to be rich in structures. These spectra are discussed in the context of the electronic structure of the unoccupied conduction band in the presence of the electron core-hole interaction. These calculated spectra can be used for the characterization of this novel compound.The structure of the incommensurate phase of Rb{sub 2}ZnCl{sub 4} has been determined at 194 K (2 K above the lock-in transition) within the soliton regime using satellites up to fifth order. The rather anharmonic modulation functions agree with the expected steplike functions supported by theoretical arguments. In addition, the constancy of the ratio between the amplitudes of the fifth-order and first-order harmonics, a relation predicted by theory, indicate the correctness of the model and imply a value of 0.4 for the soliton density n{sub s}. A symmetry mode analysis shows that the incommensurate structure is consistent with the one of the lock-in phase in the sense that the displacement pattern of every symmetry mode remains unaltered in the transition except for a global change in the amplitudes.X-ray diffraction of SnO{sub 2} (cassiterite) at high pressures and temperatures demonstrates the existence of four phase transitions to 117 GPa. The observed sequence of phases for SnO{sub 2} is rutile-type (P4{sub 2}/mnm){yields}CaCl{sub 2}-type(Pnnm){yields}pyrite-type(Pa3){yields}ZrO{sub 2} orthorhombic phase I (Pbca){yields}cotunnite-type (Pnam). Our observations of the first three phases are generally in agreement with earlier studies. The orthorhombic phase I and cotunnite-type structure (orthorhombic phase II) were observed in SnO{sub 2} for the first time. The Pbca phase is found at 50-74 GPa during room-temperature compression. The cotunnite-type structure was synthesized when SnO{sub 2} was compressed to 74 GPa and heated at 1200 K. The cotunnite-type form was observed during compression between 54-117 GPa with additional laser heating carried out at 91 and 111 GPa. Fitting the pressure-volume data for the high-pressure phases to the second-order Birch-Murnaghan equation of state yields a bulk modulus of 259(26) GPa for the Pbca phase and 417(7) GPa for the cotunnite-type phase.We report x-ray photoelectron spectroscopy (XPS) study of Na and K adlayers on icosahedral Al{sub 70.5}Pd{sub 21}Mn{sub 8.5} (i-Al-Pd-Mn) quasicrystal. The Na 1s core-level exhibits a continuous linear shift of 0.8 eV towards lower binding energies (BE) with increasing coverage up to one monolayer (ML) saturation coverage. In the case of K/i-Al-Pd-Mn, a similar linear shift in the K 2p spectra towards lower BE is observed. In both cases, the plasmon related loss features are observed only above 1 ML. The substrate core-level peaks, such as Al 2p, do not exhibit any shift with the adlayer deposition up to the highest coverage. Based on these experimental observations and previous studies of alkali metal growth on metals, we conclude that below 1 ML, both Na and K form a dispersed phase on i-Al-Pd-Mn and there is hardly any charge transfer to the substrate. The variation of the adlayer and substrate core-level intensities with coverage indicates layer by layer growth.Using neutron elastic and inelastic scattering and high-energy x-ray diffraction, we present a comparison of 40% Pb(Mg{sub 1/3}Nb{sub 2/3})O{sub 3}-60% PbTiO{sub 3} (PMN-60PT) with pure Pb(Mg{sub 1/3}Nb{sub 2/3})O{sub 3} (PMN) and PbTiO{sub 3} (PT). We measure the structural properties of PMN-60PT to be identical to pure PT, however, the lattice dynamics are exactly that previously found in relaxors PMN and Pb(Zn{sub 1/3}Nb{sub 2/3})O{sub 3} (PZN). PMN-60PT displays a well-defined macroscopic structural transition from a cubic to tetragonal unit cell at 550 K. The diffuse scattering is shown to be weak indicating that the structural distortion is long-range in PMN-60PT and short-range polar correlations (polar nanoregions) are not present. Even though polar nanoregions are absent, the soft optic mode is short-lived for wave vectors near the zone center. Therefore PMN-60PT displays the same waterfall effect as prototypical relaxors PMN and PZN. We conclude that it is random fields resulting from the intrinsic chemical disorder which is the reason for the broad transverse optic mode observed in PMN and PMN-60PT near the zone center and not due to the formation of short-ranged polar correlations. Through our comparison of PMN, PMN-60PT, and pure PT, we interpret the dynamic and static properties of themorexa0» PMN-xPT system in terms of a random field model in which the cubic anisotropy term dominates with increasing doping of PbTiO{sub 3}.«xa0lessPolarized and unpolarized neutron-diffraction studies have been carried out on single crystals of the coupled spin tetrahedra systems Cu{sub 2}Te{sub 2}O{sub 5}X{sub 2} (X=Cl,Br). A model of the magnetic structure associated with the propagation vectors k{sup }{sub Cl}{approx_equal}(-0.150,0.422,(1/2)) and k{sup }{sub Br}{approx_equal}(-0.172,0.356,(1/2)) and stable below T{sub N}=18 K for X=Cl and T{sub N}=11 K for X=Br is proposed. A feature of the model, common to both the bromide and chloride, is a canted coplanar motif for the four Cu{sup 2+} spins on each tetrahedron which rotates on a helix from cell to cell following the propagation vector. The Cu{sup 2+} magnetic moment determined for X=Br,0.395(5){mu}{sub B}, is significantly less than for X=Cl,0.88(1){mu}{sub B} at 2 K. The magnetic structure of the chloride associated with the wave vector k{sup } differs from that determined previously for the wave vector k{approx_equal}(0.150,0.422,(1/2)) [O. Zaharko et al., Phys. Rev. Lett. 93, 217206(E) (2004)].We report the magnetic properties of the ZnL{sub 2}S{sub 4} (L=Er,Tm,Yb) olivines, in which the magnetic lanthanide ions are in a potentially frustrated geometry consisting of sawtooth chains of corner-sharing triangles. Fits to the high-temperature magnetic susceptibility yielded Curie-Weiss temperatures of {theta}{sub W}{approx_equal}-4, -13, and -75 K for the Er, Tm, and Yb compounds, respectively. None of the compounds displayed magnetic long-range order above T=1.8 K. The lack of ordering at temperatures near {theta}{sub W} may be attributed to either the low dimensionality of the structure or the frustrating effect of the triangular geometry.We have investigated the Jahn-Teller transition accompanied by orbital order-disorder transition in La{sub 1-x}Ca{sub x}MnO{sub 3} by high temperature x-ray powder diffraction with synchrotron radiation and resistivity measurements. The unit cell volume of LaMnO{sub 3} decreases with increasing temperature in a narrow temperature range below T{sub JT}{approx_equal}750 K and then undergoes a volume collapse at T{sub JT}. The transition is first order. Similar behavior is also obtained in Ca-doped La{sub 1-x}Ca{sub x}MnO{sub 3} for x=0.025 and 0.075. The amount of volume collapse, however, decreases with the doping and also the first order discontinuous transition crosses over to a quasi-continuous transition with doping. We interpret the volume contraction at the transition is due to a more efficient packing of the MnO{sub 6} octahedra in the orbitally liquid state and the crossover from the discontinuous to the quasi-continuous transition is due to the change in the anharmonic coupling parameter with the hole doping. The resistivity of LaMnO{sub 3} decreases as a function of temperature and then shows abrupt drop at T{sub JT} becoming almost temperature independent at higher temperature. The resistivity of La{sub 1-x}Ca{sub x}MnO{sub 3} also decreases at T{sub JT} but the abrupt drop becomes smeared out at higher doping. Themorexa0» similar behavior of the unit cell volume and the resistivity at the Jahn-Teller transition suggests that the volume contraction at T{sub JT} causes delocalization of e{sub g} electrons.«xa0less