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Chemistry of materials 28(9), 2941 - 2948(2016). doi:10.1021/acs.chemmater.5b04719, Significant developments of Li-ion batteries will be necessary to cope with the growing demands in electromobility or home storage of (sustainable) electrical energy. A detailed knowledge on the microscopic processes during battery cycling will be increasingly crucial for improvements. Involved phase changes at ambient temperature often involve metastable intermediate states, making both experimental observation and theoretical prediction of process pathways difficult. Here we describe an in situ high energy X-ray diffraction study following the initial alloying and dealloying of Li with an Au thin-film model anode using ionic liquid electrolyte. Six different crystalline alloy phases were observed to be involved in the cyclic phase transitions. Apart from the highest lithiated phase determined in this study, Li3Au, none of the observed phases could be related to known, thermodynamically stable Li–Au phases. Structural search calculations following the minima hopping method (MHM) allowed the assignment of these phases to distinct metastable Au–Li alloy unit cells., Published by American Chemical Society, Washington, DC

We report a new multiferroic family of $(R,\mathrm{Y}){\mathrm{CrO}}_{4}(R=\mathrm{Sm},\mathrm{Gd},\text{and} \mathrm{Ho})$. The ferroelectric ordering temperatures are observed at much higher temperatures than their corresponding magnetic ordering temperatures for all the members. The values of electric polarization ($P$) are significantly high, which vary from $\ensuremath{\sim}217$ to $\ensuremath{\sim}640\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}{\mathrm{C}/\mathrm{m}}^{2}$ for a 4 kV/cm poling field. The large value of $P\phantom{\rule{0.16em}{0ex}}(\ensuremath{\sim}590\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}{\mathrm{C}/\mathrm{m}}^{2})$ for ${\mathrm{YCrO}}_{4}$ compared to the smallest $P$ value in the series for ${\mathrm{GdCrO}}_{4}$ signifies that the magnetic rare-earth element does not directly influence the $P$ value. Our careful analysis of the low-temperature synchrotron diffraction studies confirms that the distortion of ${\mathrm{CrO}}_{4}$ tetrahedra directs the $P$ value. The structural transformation from the centrosymmetric $I{4}_{1}/amd$ structure to a noncentrosymmetric $I\overline{4}2d$ space group at the onset of polar order is found to be correlated with the appearance of spontaneous electric polarization.

Physical review / B 96(19), 195433 (2017). doi:10.1103/PhysRevB.96.195433, High-energy surface-sensitive x-ray diffraction (HESXRD) is a powerful high-energy photon technique (E>70 keV) that has in recent years proven to allow a fast data acquisition for the 3D structure determination of surfaces and nanoparticles under in situ and operando conditions. The use of a large-area detector facilitates thedirect collection of nearly distortion-free diffraction patterns over a wide q range, including crystal truncation rods perpendicular to the surface and large-area reciprocal space maps from epitaxial nanoparticles, which is not possible in the conventional low-photon energy approach (E=10−20 keV). Here, we present a comprehensivemathematical approach, explaining the working principle of HESXRD for both single-crystal surfaces and epitaxial nanostructures on single-crystal supports. The angular calculations used in conventional crystal truncation rod measurements at low-photon energies are adopted for the high-photon-energy regime, illustratingwhy and to which extent large reciprocal-space areas can be probed in stationary geometry with fixed sample rotation. We discuss how imperfections such as mosaicity and finite domain size aid in sampling a substantial part of reciprocal space without the need of rotating the sample. An exact account is given of the area probed inreciprocal space using such a stationary mode, which is essential for in situ or operando time-resolved experiments on surfaces and nanostructures., Published by Inst., Woodbury, NY

We report significant correlation of the multiferroic order and ferroelectric polarization to the orthorhombic structural distortion for ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{BaCo}}_{4}{\mathrm{O}}_{7}$ ($x\ensuremath{\le}$ 0.05). Analysis of the synchrotron diffraction studies reveal that La doping increases considerable structural distortion, which is associated with the increase of multiferroic ordering temperature and electric polarization. Intriguingly, the value of polarization increases remarkably to $\ensuremath{\approx}385\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{C}/{\mathrm{m}}^{2}$ ($x=0.05$) from $\ensuremath{\approx}150\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{C}/{\mathrm{m}}^{2}$ ($x=0$) for a 3 kV/cm poling field. Synchrotron diffraction studies in magnetic field provides an important clue, where structural distortion provides more impact on the polarization value than the contribution from the change in unit cell volume. Geometric magnetic frustration holds the key for the occurrence of the structural distortions, around which multiferroic ordering takes place for ${\mathrm{CaBaCo}}_{4}{\mathrm{O}}_{7}$. Our work thus highlights crystal structural distortion as a rich playground for tuning multiferroic order as well as polarization value.

Review of scientific instruments 88(10), 104102 (2017). doi:10.1063/1.4999688, A new versatile and easy-to-use remote-controlled reactor setup aimed at the analysis of chemical reactions under solvothermal conditions has been constructed. The reactor includes a heating system that can precisely control the temperature inside the reaction vessels in a range between ambient temperature and 180 °C. As reaction vessels, two sizes of commercially available borosilicate vessels (V$_{max}$ = 5 and 11 ml) can be used. The setup furthermore includes the option of stirring and injecting of up to two liquid additives or one solid during the reaction to initiate very fast reactions, quench reactions, or alter chemical parameters. In addition to a detailed description of the general setup and its functionality, three examples of studies conducted using this setup are presented., Published by American Institute of Physics, [S.l.]

Alloy nanoparticles on oxide supports are widely used as heterogeneous catalysts in reactions involving oxygen. Here we discuss the oxidation behavior of Pd–Rh alloy nanoparticles on MgAl2O4(001) supports with a particle diameter from 6–11 nm. As an In situ tool, we employed high energy grazing incidence X-ray diffraction at a photon energy of 85 keV. We find that physical vapor deposited Pd–Rh nanoparticles grow epitaxially on MgAl2O4(001) with a truncated octahedral shape over the whole concentration range. During our systematic oxidation experiments performed at 670 K in the pressure range from 10−3 to 0.1 mbar, we observe for Rh containing nanoparticles the formation of two different Rh oxide phases, namely RhO2 and a spinel-like Rh3O4 phase. PdO formation is only observed for pure Pd nanoparticles. This oxidation induced segregation behavior is also reflected in the oxidation induced enlargement of the average nanoparticle lattice parameter towards to value for pure Pd. Our results have ramifications for the phase separation behavior of alloy nanocatalysts under varying reducing and oxidizing environments.

The application of large magnetic fields ($B \sim B_{c2}$) to layered cuprates suppresses their high temperature superconducting behaviour and reveals competing ground states. In the widely-studied material YBa$_2$Cu$_3$O$_{6+x}$ (YBCO), underdoped ($p \sim 1/8$) samples show signatures of field-induced electronic and structural changes at low temperatures. However, the microscopic nature of the field-induced reconstruction and the high-field state are unclear. Here we report an x-ray study of the high-field charge density wave (CDW) in YBCO, for doping, $0.1 \lesssim p \lesssim 0.13$. For $p \sim 0.123$, we find that a field ($B \sim 10$~T) induces new CDW correlations along the CuO chain ($b$) direction only, leading to a 3-D ordered state along this direction at $B \sim 15$~T. The CDW signal along the $a$-direction is also enhanced by field, but does not develop a new pattern of correlations. We find that field modifies the coupling between the CuO$_2$ bilayers in the YBCO structure, and causes the sudden appearance of 3D CDW order. The mirror symmetry of individual bilayers is broken by the CDW at low and high fields, allowing recently suggested Fermi surface reconstruction., Comment: 4 pages 5 figures

We report ferroelectricity around ∼88 K that appears well below T N (∼25 K), unlike other members of RCrO3 series. A synchrotron diffraction study suggests that the occurrence of ferroelectricity in NdCrO3 is coupled to the structural transformation from centrosymmetric Pnma to a non-centrosymmetric Pna21 space group. A strong magnetoelectric coupling is observed in the electric polarization [P(T)]. This coupling is significantly influenced by the magnetic field cooling effect, suggesting an exchange bias effect in P(T). This exchange bias effect is also revealed by the systematic shift of the magnetic hysteresis loops below T(N). The rare occurrence of an exchange bias effect in both the magnetic and electric polarizations associated with a strong magnetoelectric coupling is of fundamental interest, as well as being attractive for technological applications close to liquid nitrogen temperature.

Physical chemistry, chemical physics 20(37), 24494 - 24495 (2018). doi:10.1039/C8CP91851A, Surface induced smectic order was found for the ionic liquid 1-methyl-3-docosylimidazolium bis(trifluoromethlysulfonyl)imide by X-ray reflectivity and grazing incidence scattering experiments. Near the free liquid surface, an ordered structure of alternating layers composed of polar and non-polar moieties is observed. This leads to an oscillatory interfacial profile perpendicular to the liquid surface with a periodicity of 3.7 nm. Small angle X-ray scattering and polarized light microscopy measurements suggest that the observed surface structure is related to fluctuations into a metastable liquid crystalline SmA2 phase that was found by supercooling the bulk liquid. The observed surface ordering persists up to 157 °C, i.e. more than 88 K above the bulk melting temperature of 68.1 °C. Close to the bulk melting point, we find a thickness of the ordered layer of L = 30 nm. The dependency of L(τ) = Λ ln(τ/τ1) vs. reduced temperature τ follows a logarithmic growth law. In agreement with theory, the pre-factor Λ is governed by the correlation length of the isotropic bulk phase., Published by RSC Publ., Cambridge

Rare-earth ultraphosphate glasses with nominal R2O3 fractions of 10 and 15 mol% and small ionic radius (large atomic number) R3+ ions (R = Tb, Tm, Lu) are measured by X-ray diffraction at a synchrotron with photons of 119 keV (maximum scattering vector 260 nm− 1). The total correlation functions T(r) show well-resolved R–O and O–O first-neighbor peaks. In contrast to all ultraphosphate RP5O14 crystals and the ultraphosphate glasses of larger R3+ ions, where RO8 polyhedra (mean R–O coordination numbers of ~ 8 for the glasses) are observed, the R3+ ions in glasses with R = Lu, Tm, Tb have mean R–O coordination numbers of ~ 7.5. The R–O first-neighbor peaks extracted from the T(r) functions are compared with those obtained from atomic coordinates of related RP5O14 and RP3O9 crystals. The R–O distances of the ultraphosphate glasses studied are found to fall between those of the two crystals but with tails to the side of longer bonds.

We report appearance of polar order with a ordering temperature $({T}_{C})$ at 28 K, which is well below the ferrimagnetic order at 225 K $({T}_{N})$ for ${\mathrm{CoCr}}_{2}{\mathrm{S}}_{4}$. Intriguingly, the value of spontaneous electric polarization $(P)$ is $\ensuremath{\sim}122 \ensuremath{\mu}\mathrm{C}/{\mathrm{m}}^{2}$ at 15 K, which is the second largest value in Cr octahedra-based spinels after ${\mathrm{CdCr}}_{2}{\mathrm{S}}_{4}$. Incidentally, the $P$ value is $\ensuremath{\sim}60$ times larger than the value of $P$ for the oxide counterpart ${\mathrm{CoCr}}_{2}{\mathrm{O}}_{4}$. The significant magnetoelectric coupling is verified from the magnetodielectric response and magnetic field dependent enhancement of $P$. We note that the field-dependent dielectric permittivity scales linearly to the squared magnetization in the low field regime below $\ensuremath{\sim}10$ kOe as described by the Ginzburg-Landau theory. Synchrotron diffraction studies over a wide temperature range, $15\text{--}300$ K, illustrate strong magnetoelastic coupling at ${T}_{N}$ and isostructural distortion at ${T}_{C}$. Analyses of the diffraction patterns reveal that the occurrence of polar order involves expansion of Co tetrahedra and contraction of Cr octahedra of the spinel structure and these distortions are further enhanced driven by the magnetic field. The delicate interplay between magnetoelastic, magnetoelectric, and electroelastic couplings in ${\mathrm{CoCr}}_{2}{\mathrm{S}}_{4}$ proposes the system as a potential candidate in multiferroics.

The magnetic excitations in the cuprate superconductors might be essential for an understanding of high-temperature superconductivity. In these cuprate superconductors the magnetic excitation spectrum resembles an hour-glass and certain resonant magnetic excitations within are believed to be connected to the pairing mechanism, which is corroborated by the observation of a universal linear scaling of superconducting gap and magnetic resonance energy. So far, charge stripes are widely believed to be involved in the physics of hour-glass spectra. Here we study an isostructural cobaltate that also exhibits an hour-glass magnetic spectrum. Instead of the expected charge stripe order we observe nano phase separation and unravel a microscopically split origin of hour-glass spectra on the nano scale pointing to a connection between the magnetic resonance peak and the spin gap originating in islands of the antiferromagnetic parent insulator. Our findings open new ways to theories of magnetic excitations and superconductivity in cuprate superconductors.

The Basic Energy Sciences Synchrotron Radiation Center Collaborative Access Team has built three independent beamlines, which simultaneously utilize the X-ray radiation from an elliptical multipole wiggler, located at Sector 11 of the Advanced Photon Source. This insertion device produces circularly polarized X-rays on-axis and linearly polarized X-rays above and below the ring plane. The lower linearly polarized radiation is used in the monochromatic 11ID-D station for scattering and spectroscopy experiments in the 5–40 keV range. The on-axis circularly polarized photons are used for magnetic Compton scattering experiments in the 11ID-B station. The upper linearly polarized radiation is utilized by the high-energy diffraction station, 11ID-C. We report here on the beamline optics and experimental station equipment.

The Basic Energy Sciences Synchrotron Radiation Center (BESSRC) has designed and built a diffractometer specialized for high energy synchrotron radiation ( E >60 keV) at the Advanced Photon Source (APS). The diffractometer, which is installed at the elliptical multipole wiggler, uses linearly polarized light (U. Rutt et al., Proc. SPIE 3348 (1998) 132.). The instrument is a triple-axis diffractometer allowing high resolution measurement in two dimensions of the reciprocal space. As opposed to the other diffractometers for high photon energies at HASYLAB (Germany) and ESRF (France) (R. Bouchard et al., 5 (1998) 90; K.-D. Liss et al., J. Synchrotron Rad. 5 (1998) 82), this diffractometer utilizes the vertical scattering plane to take full advantage of the small vertical divergence of the beam and to allow horizontal focusing of the broad beam from the wiggler without disturbing the resolution of the instrument. The instrument is designed to carry heavy sample equipment up to a weight of 200 kg, while maintaining high resolution and low background. First tests have been done to check the overall performance, stability, background and resolution of the diffractometer.

The Basic Energy Sciences Synchrotron Radiation Center Collaborative Access Team (BESSRC-CAT) has built a bending magnet beamline for spectroscopy and scattering experiments at the Advanced Photon Source (APS). The windowless beamline uses a water cooled, double crystal, fixed exit monochromator with Si 1 1 1 crystals as the first optical element. The monochromator is capable of operations from 55° to 3° allowing experiments from approximately 2.5–30 keV. A monochromatc double mirror system located in the white-beam enclosure focuses the beam in the experimental station. When the first mirror is removed energies above the mirror cutoff (∼22 keV) can be used. The 12BM-B user station is equipped for scattering (Huber 6-circle diffractometer) and spectroscopy (optical table, ion chambers and solid state detectors) experiments.

The magnetic spin structure factor of ${\mathrm{FeF}}_{2}$ has been directly determined from high energy magnetic x-ray diffraction at 115 keV photon energy. A pure spin moment of $\ensuremath{\mu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}4.01(5){\ensuremath{\mu}}_{B}$ was observed, which agrees very well with the spin moment of the free ${\mathrm{Fe}}^{2+}$ ion and differs significantly from values of the total magnetic moment obtained by other methods. The magnetic phase transition of ${\mathrm{FeF}}_{2}$ has been carefully investigated and values for the critical exponent of the order parameter obtained by other techniques have been confirmed.

The temperature dependence of the spontaneous strain in has been studied using triple-axis diffractometers of high momentum-space resolution for synchrotron radiation with energies above 100 keV. In the temperature range from 40 to 80 K the experimental data are in excellent agreement with a Landau theory taking into account the quantum mechanical zero-point fluctuations. At temperatures below 40 K a difference between experiment and theory of about 4% occurs which is considered as an indication for a novel phase in the quantum paraelectric regime. The difference between the fitted theory and the experimental data is nearly the same for all of the samples investigated; the biggest difference is observed in the fit parameter describing the theoretical saturation value of the spontaneous strain at zero temperature. In absolute numbers the measured spontaneous strain is strongly sample dependent, i.e. flux-grown, float-zone-grown and Verneuil-grown crystals show differences in the spontaneous strain of about 30% at 10 K. On the other hand, only small variations are observed in samples which were subjected to different heat treatments, which determine the number of oxygen vacancies.

Results of high-energy non-resonant magnetic X-ray diffraction experiments performed on the model system MnF 2 at a photon energy of 80keV are presented. A surprisingly high peak intensity of the magnetic 300 reflection of 13 000 photonss -1 in the three-crystal mode and 19 000 photons s -1 in the two-crystal mode, with a peak-to-background ratio of 230 :1 and 10 :1, respectively, has been achieved. At 80keV, the penetration depth is 7mm. When the path length of the beam through the crystal is varied, the effect of volume enhancement of the intensity diffracted by magnetic reflections is demonstrated. The Q dependence of the magnetic and the charge Bragg reflections has been measured and agrees well with theory. The measurement of the temperature dependence of the sublattice magnetization allows a very accurate determination of the critical exponent β = 0.333 (3) and the Neel temperature T N = 67.713 (2) K. Finally, the multiple charge scattering is discussed, which is very pronounced for the magnetic reflections of MnF 2 .

Superstructure and temperature dependence of the critical scattering and the spontaneous strain in SrTiO3 and YBa2Cu3O6+x have been studied using a new 3-crystal diffractometer of high-momentum space resolution for synchrotron radiation with energies ⩾80 keV. The temperature dependence of the bulk critical scattering from a highly perfect SrTiO3 crystal above the critical temperature of the cubic to tetragonal phase transition at Tc∼100 K is in excellent agreement with neutron data taken at Brookhaven with the same sample as before although the q-space resolution was 27 times better. The sharp quasi-Bragg peak discovered by means of high resolution X-ray diffraction was found only in a near surface volume, indicating that the two-length scales behaviour postulated for this phase transition is related to a skin layer at the sample surface. A low temperature study of the spontaneous strain in pure and Ca doped SrTiO3 provided further indications for the existence of a novel phase in the quantum par...

High-q-resolution diffraction experiments performed with 100 keV synchrotron radiation on the identical ${\mathrm{SrTiO}}_{3}$ crystal used in neutron and x-ray-scattering experiments before, unambiguously show that the bulk critical scattering above ${\mathit{T}}_{\mathit{c}}$=99.5 K does not exhibit a sharp component, which had been found in recent x-ray-diffraction experiments. The observed temperature dependence of the inverse correlation length is in excellent agreement with the neutron data. However, scattering on two length scales was observed in a near-surface volume element of the same crystal.

The k-space resolution function of a triple-crystal diffractometer is calculated for an arrangement of three perfect silicon single crystals Bragg diffracting in nondispersive Laue geometry. A comparison is made with the results of measurements using synchrotron radiation in the energy range from 80 to 150 keV. In this case, absorption is very weak and according to dynamical theory the width of the diffraction pattern of thick perfect single crystals is proportional to the wavelength λ, whereas its Lorentzian tails are proportional to λ2. Together with the fact that the Bragg angles are only of the order of 2°, this leads to a concentration of the starlike k-space resolution function into a narrow band parallel to the reciprocal-lattice vector G. For diffraction of 80 keV synchrotron radiation at the silicon 111 reflection, the full width at half-maximum (FWHM) of the intensity distribution in the scattering plane is 1.1 × 10−5 A−1 perpendicular to G and 2.2 × 10−4 A−1 parallel to G. The observed differences in the contributions from monochromator and analyzer crystal to the resolution function are explained by the finite width of the electronic window of the detector counting chain and the non-Bragg scattering contribution from the crystals. If annealed Czochralski-grown silicon single crystals with a mosaicity of ~3′′ are used as monochromator and analyzer, the resolution is reduced by one order of magnitude, but for studies of imperfect samples or of diffuse scattering large gains in intensity can be accomplished this way.

At the high energy synchrotron radiation beamline BW5 at DORIS III at DESY a new monochromator providing broad energy bandwidth and high reflectivity is in use. On a small 10×10×5 mm3 silicon crystal scattering at the (311) reflection a thermal gradient is applied, which tunes the scattered energy bandwidth. The (311) reflection strongly suppresses the higher harmonics allowing the use of an image plate detector for crystallography. The monochromator can be used at photon energies above 60 keV.

The doping dependence of short-range lattice superstructures in (Y,Ca)Ba2Cu3O6+x has been studied with high-energy x-ray scattering. We observe diffuse features with a well defined periodicity which depend on the oxygen concentration but not on the charge carrier concentration. In addition, we find that diffuse scattering is absent in underdoped YBa2Cu4O8, which does not sustain oxygen defects. Our combined data highlight that the diffuse scattering arises from short-range oxygen ordering and associated lattice distortions. Signatures of stripe ordering or fluctuations are not seen and therefore must be much weaker.

We present an overview of recent results from nonresonant magnetic diffraction experiments on the antiferromagnetic compounds ${\mathrm{MnF}}_{2},$ ${\mathrm{FeF}}_{2},$ ${\mathrm{CoF}}_{2},$ and ${\mathrm{NiF}}_{2}$ using high-energy synchrotron radiation of photon energies above 100 keV. New results are presented on the determination of the spin and of the $L/S$ ratio for ${\mathrm{CoF}}_{2}$ and ${\mathrm{NiF}}_{2}.$ For ${\mathrm{CoF}}_{2},$ the saturation value of the long-range-ordered pure spin ${S}_{z}$ component ${S}_{z}=1.11(1)$ is considerably lower than the value ${S}_{z}=3/2$ for the free ${\mathrm{Co}}^{2+}$ ion. This is in contrast to our results for ${\mathrm{NiF}}_{2},$ where the full spin of the free transition-metal ion was found, ${S}_{z}=0.98(1).$ The temperature dependence of the magnetization in the critical region as well as in the low-temperature region is also presented. For all compounds, Ising behavior is found in the critical regime, whereas the crossover to the low-temperature spin-wave behavior varies. We attribute this to different anisotropies in this series of compounds.

The elliptical multipole wiggler located at Sector 11 of the Advanced Photon Source has a maximum critical energy of 32 keV and a total power output of ∼ 8 kW. The device produces circularly polarized X‐rays on‐axis and linearly polarized X‐rays above and below the ring plane. A double crystal monochromator intercepts the lower linearly polarized radiation to be used in the 11ID‐D experimental station for scattering and spectroscopy experiments in the 5 to 40 keV energy range. The on‐axis circularly polarized photons and upper linearly polarized photons are monochromatized by one crystal horizontally deflecting monochromators at fixed Bragg angles of θ = 3.8° and θ = 1.9° respectively. We will present here the designs and detail the performance of Laue and Bragg monochromator designs for these two beamlines which operate at fixed energies from ∼60 keV to 125 keV. For the high‐energy diffraction station, 11ID‐C, which utilizes the upper linearly polarized radiation, the Laue monochromator design allows rapid interchange of crystals and provides a photon flux of 2 × 1012 photons/sec in a 2.5 × 3.5 mm spot at 98 keV using annealed Silicon crystals (Si(220)). The Bragg monochromator design provides focusing and a smaller bandwidth while retaining a high photon flux. The high heatloads generated by the EMPW present design challenges for both monochromator designs.

The doping dependence of short-range lattice superstructures in (Y,Ca)Ba2Cu3O6+x has been studied with high-energy x-ray scattering. We observe diffuse features with a well defined periodicity which depend on the oxygen concentration but not on the charge carrier concentration. In addition, we find that diffuse scattering is absent in underdoped YBa2Cu4O8, which does not sustain oxygen defects. Our combined data highlight that the diffuse scattering arises from short-range oxygen ordering and associated lattice distortions. Signatures of stripe ordering or fluctuations are not seen and therefore must be much weaker.

The structural phase transition in the orthovanadates ${\mathrm{LaVO}}_{3}$ and ${\mathrm{CeVO}}_{3}$ has been studied with high energy synchrotron x-ray diffraction. ${\mathrm{LaVO}}_{3}$ undergoes a second order phase transition at ${T}_{N}=143\mathrm{K}$ and a first order transition at ${T}_{t}=141\mathrm{K},$ while in CeVO3 there are phase transitions occurring at ${T}_{0}=154\mathrm{K}$ of second order and at ${T}_{N}=134\mathrm{K}$ of first order. These phase transitions are confirmed by specific heat measurements. The phase transition at ${T}_{t}$ in ${\mathrm{LaVO}}_{3}$ or ${T}_{0}$ in ${\mathrm{CeVO}}_{3}$ is due to a G-type orbital ordering which lowers the structure symmetry from orthorhombic $\mathrm{Pbnm}$ to monoclinic ${P2}_{1}/b11.$ The structure change at ${T}_{N}$ in ${\mathrm{CeVO}}_{3}$ is ascribed to an orbital ordering enhanced magnetostrictive distortion, while that at ${T}_{N}$ in ${\mathrm{LaVO}}_{3}$ is most probably due to an ordered occupation of the vanadium $3d$ ${t}_{2g}$ orbitals associated with an antiferromagnetic ordering. We propose that the first order phase transition at ${T}_{t}$ in ${\mathrm{LaVO}}_{3}$ should be associated with a sudden change of both spin and orbital configurations, similar to the phase transition at ${T}_{s}=77\mathrm{K}$ in ${\mathrm{YVO}}_{3}$ [Ren et al., Nature (London) 396, 441 (1998)], causing a reversal of the net magnetization. However, the ordered state above ${T}_{t}$ in ${\mathrm{LaVO}}_{3}$ is identical to that below ${T}_{s}$ in ${\mathrm{YVO}}_{3}.$ It is found that, with increasing lanthanide ionic radius, the N\'eel temperature ${T}_{N}$ increases while the orbital ordering onset temperature decreases in these orthovanadates.

The temperature dependence of the sharp and the broad component of the critical scattering above the cubic-to-tetragonal phase transition of ${\mathrm{SrTiO}}_{3}$ has been studied by means of high-resolution triple-crystal diffractometry using 100--200 keV synchrotron radiation in five samples differing with respect to growth technique and oxygen vacancy concentrations. Emphasis is on changes in the critical behavior, the critical temperature, and the strain fields at the transition from bulk to surface. The sharp component was observed only in surface near regions of highly perfect crystals and is coupled to the occurrence of a long-range strain gradient that was identified by an exponential increase of mosaicity, lattice parameter fluctuations, and Bragg-peak intensity when approaching the surface from the bulk of the sample. Vanishing of the sharp component was observed at the polished/etched surface of a platelet cut off the large perfect crystal after release of strain due to free bending of the platelet. The values of the critical temperature observed in the bulk of the different samples vary between $98.7l~{T}_{c}l~105.8 \mathrm{K}.$ In the surface near regions of a highly perfect float-zone grown crystal a variation of ${T}_{c}$ of about 0.5 K has been found. Concerning the broad component the critical exponent describing the temperature dependence of the inverse correlation length ${\ensuremath{\kappa}}_{b}$ varies between $0.73l~{\ensuremath{\nu}}_{b}l~1.19,$ the exponent for the susceptibility between $1.49l~{\ensuremath{\chi}}_{b}l~2.9,$ however, the ratio of the two exponents is almost sample independent and equal to ${\ensuremath{\chi}}_{b}/{\ensuremath{\nu}}_{b}=2.1$ with a variance of 0.2, in good agreement with the theoretical value of 1.97 obtained by LeGouillou and Zinn-Justin [Phys. Rev. B 21, 3976 (1980)]. The occurrence of the sharp component did not affect significantly the critical exponents for the underlying broad component of the critical scattering. The exponents for the sharp component observed in surface near layers of about 100 $\ensuremath{\mu}\mathrm{m}$ thickness at the highly perfect float zone and flux grown crystals varied between $0.58l~{\ensuremath{\nu}}_{s}l~1.30,$ the values for the ratio varied between $3.3l~{\ensuremath{\chi}}_{s}/{\ensuremath{\nu}}_{s}l~4.6.$ The average value of ${\ensuremath{\chi}}_{s}/{\ensuremath{\nu}}_{s}$ is 3.9 with a variance of 0.5, and is about twice the ratio ${\ensuremath{\chi}}_{b}/{\ensuremath{\nu}}_{b}$ for the broad component as suggested by Harris et al. [Phys. Rev. B 52, 14 420 (1995)].

The magnetic spin structure factor of FeF2 has been directly determined from high energy magnetic x-ray diffraction at 115 keV photon energy. A pure spin moment of mu = 4.01(5)mu(B) was observed, which agrees very well with the spin moment of the free Fe2+ ion and differs significantly from values of the total magnetic moment obtained by other methods. The magnetic phase transition of FeF2 has been carefully investigated and values for the critical exponent of the order parameter obtained by other techniques have been confirmed.

The sharp component observed in the critical scattering above the cubic-to-tetragonal phase transition in SrTiO3 is now generally considered as a surface-related phenomenon. In the present paper, for the first time, the structural difference between the bulk and a skin layer of a highly perfect crystal has been shown quantitatively by diffraction methods. High-resolution measurements have been performed with highly penetrating 121 keV synchrotron radiation X-rays. The lattice parameter d, its fluctuations Δd/d, the crystal mosaicity and the critical scattering were found to be depth dependent. The two-length-scale behaviour in the critical fluctuations is confined to a skin layer approximately 100 μm thick and related to an increase in strain. Approaching the crystal surface the lattice parameter decreases by 6.2·10−5 A, Δd/d increases from the bulk value of 5.2·10−5 to 13·10−5, the mosaicity increases from 1.0 to 8.2 seconds of arc. In addition the critical temperature determined from the temperature dependence of the 1/2(511) reflection at the surface and 150 μm below, is smaller than the bulk value by 1.7 and 2.4 K, respectively.

The triple-crystal diffractometer installed at HASYLAB beamline BW5 with a high-field wiggler of critical energy 27 keV for DORIS III, operated at 4.5 GeV electron energy, is described. Samples can be mounted in large cryostats or furnaces normally used in neutron scattering experiments. The instrument has been successfully applied to measure structure factors S(Q) in liquids and amorphous materials, to collect full data sets of highly accurate structure factors for charge-density work, to measure the spin component of the ground-state magnetization in transition-metal and rare-earth compounds, to study the diffuse scattering from stacking faults and dislocation loops in Si single crystals, and for the investigation of various aspects of structural phase transitions: critical scattering in SrTiO3, oxygen order and stripe order in high-To materials. A crossed-beam technique allows for local studies of texture, internal strain and phase changes in the bulk of materials.

Structural parameters of bulk crystalline materials can be accessed in a damage-free manner by high-energy x-ray diffraction. Specific parts of the sample can be characterized if they are distinguished by the lattice spacings. This technique is applied for the first time to massive Si1-xGex crystals with locally varying concentrations x. The data reveal e.g. an unusual lattice distortion with an in-plane expansion and an out-of-plane contraction if the samples are deposited at high temperature (1350 K). This is opposite to a distortion inferred from the difference in the lattice constants of Si and Ge.

Single crystals of the in situ composite TaSi2-Si are candidates for wide bandpass monochromators and analyzers for x-ray experiments. Two large crystals (32 × 39 × 7 mm3 and 32 x 39 x 30 mm3) were studied in detail using 115 keV synchrotron radiation at the BESSRC beamline at the APS. For transmission geometry, the Si(111) reflection and 39 mm crystal thickness, rocking curve widths of 95 arc sec with reflectivities of 50% were measured over the entire crystal. Turning the crystal to an angle of 45° to the incident beam and translating it through the beam allowed investigation of the influence of crystal thickness on diffracted intensity. For sample thicknesses ranging between 5 mm and 12.5 mm, the full width at half maximum (FWHM) of the rocking curve increased linearly from 29 arc sec to 53 arc sec. The greatest gain in integrated intensity was obtained for 8-9 mm thickness where FWHM = 40 arc sec. If this crystal were used as a monochromator for synchrotron radiation, it would provide 40 times more intensity than a perfect silicon crystal and an energy bandwidth of 1 keV at 115 keV photon energy. With the increase of the rocking curve FWHM, a plateau across the peak maximum developed; its width increased from 5 arc sec to 25 arc sec when the crystal thickness increased from 5 mm to 12.5 mm, respectively. This plateau allows these crystals to be used as analyzer crystals in diffraction experiments, i.e., as very narrow slits to suppress background. A plateau width of about 5-20 arc sec for the analyzer crystal is needed to insure that the entire intensity scattered from a sample is delivered to the detector. The simple expedient of changing the crystal thickness alters the plateau width of the analyzer crystal and tunes its acceptance to the needs of the sample under study.