Small Single Crystals Research Articles

Resonant magnetic X-ray scattering: a new probe for actinides

The magnetic cross-section for photons is small and difficult to observe; however, when the X-ray energy is tuned to certain absorption edges an enormous enhancement of the magnetic scattering cross-section is observed. This process is now understood in terms of atomic resonances. By chance, the largest enhancements, and most convenient for diffraction experiments, occur at the M edges (3.5–4.5 keV) in the actinides. We shall give some examples of the work performed at the NSLS, Brookhaven National Laboratory. Our recent experiments have been on small single crystals of NpAs. The higher resolution available with X-ray synchrotron beams, compared with that at neutron sources, has allowed us to determine new features in both the magnetic structure and the critical fluctuations just above T N. Because of the high absorption of the X-rays, the technique is extremely sensitive to the near-surface region, and the results often exhibit significant differences to those determined from neutron diffraction, which examines the bulk of the material. Thus, the two techniques are complementary. Very small samples are all that is required for the synchrotron experiments; we estimate that the beam illuminates no more than 1 μg of NpAs in this study, so that studies of the magnetic properties of transcurium materials become possible for the first time.

Dynamic behavior of Josephson-coupled layered structures.

We have investigated Josephson effects in stacks of both artificial and natural Josephson junctions. The measurements have been performed on Nb/Al-${\mathrm{AlO}}_{\mathit{x}}$/Nb multilayers and on small single crystals of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$. Both systems exhibit multiple branched I-V characteristics in zero magnetic field. In finite magnetic fields coupling via currents flowing along the superconducting layers is essential, since the layers are thinner than the London penetration depth. All observations are in good agreement with numerical simulations of stacks of coupled Josephson junctions. These simulations predict that a large number of junctions can be phase locked in large magnetic fields via Fiske resonances excited in all junctions.

Structure determination with Laue diffraction data – including refinement when anomalous scatterers are present

The structure of a small single crystal of the organometallic compound [AuOs 3 H(CO) 8 {Ph 2 PCH 2 P(Ph)C 6 H 4 }(PPh 3 )]PF 6 .0.5C 6 H 5 Cl, whose chemical constitution was only partially known, has been determined and refined. Synchrotron radiation with wavelengths in the range 0.24-0.65 A was used for recording the Laue diffraction pattems from which the reflection intensities were measured. Data processing and initial structure determination followed established procedures

Microscopic single-crystal refractometry as a function of wavelength

The refractive indices of crystal fragments 50–200 μm in size can be measured for light wavelengths between 365 and 1100 nm with a spindle-stage refractometer. Established methods from optical crystallograpy are used to orient a crystal on the microscope spindle stage and then to match its refractive index to an immersion fluid. The refractive index of the fluid for the wavelength of light and matching temperature is determined by comparison of a reference crystal on a second spindle axis with the fluid under the match conditions. Investigations of new nonlinear-optical crystals admirably demonstrate the advantages of measuring the refractive index to ±0.0004 in small single crystals.

Crystallization studies of the human immunodeficiency virus (HIV-1) Tat protein and its trans-activation response element (TAR) RNA.

Small single crystals are reported of a complex between a small peptide fragment of the HIV-1 Tat protein and a fragment of the RNA to which it binds. Tat is responsible for enhancing the level of expression of the human immunodeficiency virus type 1 (HIV-1) and is a logical target for AIDS therapy. Tat may function to increase the level of transcription initiation or to prevent premature termination of transcripts. In vitro, Tat binds through its basic domain (two Lys and six Arg in nine residues) to a three-nucleotide bulge of a stem-loop RNA structure called TAR. Complex formation between Tat and TAR is necessary for Tat activity. Peptides which contain the basic region of Tat also bind to TAR RNA. We have carried out crystallization experiments on a 27-nucleotide fragment of TAR RNA and on complexes between two Tat peptides and TAR.

Triple-axis instrument with doubly focusing (“zoom”) monochromator and horizontally focusing analyser: seven years experience

The experience gained with a triple-axis spectrometer equipped with a doubly focusing monochromator with variable horizontal and vertical curvature and with a horizontally focusing analyser is presented. The performance of the instrument is demonstrated by comparing one-phonon maxima, measured with different instrumental configurations on a small copper single crystal. Intensity gains, peak positions and peak half widths are discussed and compared with resolution calculations.

Computer Simulation of Bending Plastic Deformation and Creation of Dislocations in Copper Thin Films

AbstractA small single crystal of copper with a notch has been bent by use of the molecular dynamics method. The bend axis was [110]. Dislocations were created near the tip of the notch and moved on (111) slip plane. Pulling a copper single crystal, half dislocations were created in such a way that the bending was compensated.

A new method to attach electrical leads on a single crystal of a submillimeter size

A new method was developed to prepare a small single crystal sample for electrical conductivity measurement. The method was based on the use of commercially available XYZ mechanical stages as micromanipulators, and its accuracy was high enough to attach successfully four gold wires for four-probe measurement on a needlelike crystal 0.2 mm long.

Lattice dynamics and Raman spectroscopy of protoenstatite Mg2Si2O6

Enstatites (Mg2Si2O6) are important rock forming silicates of the pyroxene group whose structures are characterised by double MgO6 octahedral bands and single silicate chains. Orthoenstatite transforms to protoenstatite above 1273 K with a doubling of the a axis and a rearrangement of the silicate chains with respect to the Mg2+ ions. Lattice dynamical calculations based on a rigid-ion model in the quasi-harmonic approximation provide theoretical estimates of elastic constants, long wavelength phonon modes, phonon dispersion relations, total and partial density of states and inelastic neutron scattering cross-sections of protoenstatite. The computed elastic constants are in good agreement with experimental data. The computed density of states of a chain silicate such as protoenstatite is distinct from that of olivines (forsterite, Mg2SiO4 and fayalite, Fe2-SiO4) with isolated silicate tetrahedra. The band gaps in the density of states in forsterite are largely due to the separation in the frequency ranges of the external and internal vibrations of the isolated silicate group, whereas in protoenstatite these gaps are filled by the vibrations of the bridging oxygens of the silicate chain. The computed density of states is used to calculate the specific heat, the mean square atomic displacements and temperature factors. Validity of these calculations are supported by Raman scattering measurements. Polarised and unpolarised Raman spectra are obtained from small single crystals of protoenstatite (Li,Sc)0.6Mg1.4Si2O6 stable at room temperature using the 488 nm or 514.5 nm lines of an Ar+ ion laser and a micro-Raman spectrometer with backscattering geometry. The Raman spectra were analysed and interpreted based on the lattice dynamical model. The experimental Raman frequencies and mode assignments (based on polarised single crystal spectra) are in good agreement with those obtained from lattice dynamical calculations.

Concerning the components contributing to Bragg reflection profile shapes in synchrotron-radiation studies of small single crystals

Certain basic matters in a recent synchrotron-radiation study [Rossmanith (1993). Acta Cryst. A49, 80–91] and an allied study which developed a new peak-width formula [Rossmanith (1992). Acta Cryst. A48, 596–610] are questioned. These matters concern the mode of combination of certain components which determine the one-dimensional profile shape of Bragg reflections and the functional form of the wavelength dispersion dependence on the Bragg angle of the sample crystal and that of a monochromator crystal where the respective crystal axes are parallel.

Crystallization and preliminary X-ray analysis of nonglycosylated tissue inhibitor of metalloproteinases-1, N30QN78Q TIMP-1.

A nonglycosylated (N30QN78Q) form of the human tissue inhibitor of metalloproteinases, TIMP-1, has been prepared and crystallized in a form suitable for X-ray diffraction analysis. Small single crystals have been grown using sodium tartrate as a precipitant. The crystals are in space group P2(1), with cell dimensions a = 35.28, b = 53.95, c = 48.56, and beta = 96.0 degrees. There is a single molecule of TIMP-1 in the asymmetric unit. The crystals diffract to at least 2.3 A resolution. Complete data have been collected to 2.9 A and a search for heavy-metal derivatives is in progress.

Tunnel diode oscillator application to high sensitivity de Haas–van Alphen and superconducting critical field studies of anisotropic organic conductors

We report the first application of a tunnel diode circuit to investigate the de Haas–van Alphen effect and superconducting critical field values in small single crystal organic conductors. The materials of interest are in the family of charge transfer salts (BEDT-TTF)2X, where X is one of a number of anion structures. Small (≤1 mm3) oriented single crystal samples were placed inside the inductance coil of the LC tank circuit of the oscillator, and measurements were carried out at low temperatures and in high magnetic fields. We observe a marked improvement over previous magnetic methods for the determination of both de Haas–val Alphen signals and for critical field studies. The method is applicable where such information is required for very small samples.

Determination of the crystal orientation of CuInSe2 by Raman spectroscopy

We have developed a new method for the determination of the crystal orientation of CuInSe2 by Raman spectroscopy. The single crystal of CuInSe2 has a (112) natural surface. On this surface we often observe slip lines, which construct a triangular shape. However, it is difficult to identify which corner is the projection of the c axis. We established a new method to identify the c direction by using Raman spectroscopy. This method is also effective for small single crystals down to the size of 1 μm.

On the modelling of neutron diffraction reflections from small single crystals

For the case where the rotation axis of the monochromator crystal and that of the small specimen crystal are parallel, i.e. (+, −) or (−, +) configuration, the apparatus function in two-dimensional Δω, Δ2θ space is associated with the source, S, the monochromator crystal, M, and an idealized specimen crystal, c, which is vanishingly small and has zero mosaic spread. For any value of t (= tan θc/tan θM), the apparatus function is a product of the distributions (with their respective loci of translation) of: (i) the emissivity of S; (ii) the reflectivity over the length of M; (iii) the mosaic spread of M; and (iv) the wavelength band arising from the vector addition in Δω, Δ2θ space of the wavelength dispersion of M and of c. To combine the apparatus function with other components such as the mosaic spread of a real specimen crystal, its physical dimension, the size of the aperture in front of a quantum detector or the point-spread function of a position-sensitive detector, the appropriate mathematical operation in Δω, Δ2θ space is sequential convolution. Examples are given, for t = 0 (0.25) 1.0 (0.5) 2.0, of synthetic apparatus functions based on typical dimensions appropriate to neutron diffraction experimental arrangements. These are presented in Δω, Δ2θ(0) space, which corresponds to ω-scan data collection. The advantage of modifying these by affine transformation to Δω, Δ2θ(2) space or, equivalently, to correspond to ω–2θ-scan data collection, is demonstrated.

Structure determination from small crystals of two aluminophosphates CrAPO-14 and SAPO-43

Small single crystals, ca 10 5 μm 3 , of two alumino-phosphates have been used to determine their structures. Both compounds were synthesized with the aim of incorporating chromium into the framework, and for CrAPO-14 the X-ray diffraction results show that this has been achieved. For CrAPO-14, Al 3.96 Cr 0.04 (PO 4 ) 4 (OH)-C 3 H 10 N-H 2 O, data recorded on a Rigaku AFC-SR diffractometer (Cu Kα) and, independently, on an Enraf-Nonius FAST diffractometer (Mo Kα), showed the structure to be like that of GaPO-14 [Parise (1986). Acta Cryst. C42, 670-673]; aluminium atoms occupy four-, five- and six-coordinated sites, and 45% of the aluminium in the six-coordinate site is replaced by chromium

Temperature dependencies of the lattice constants and thermal expansion coefficients of Sm3Fe5O12 and Er3Fe5O12 single crystals

The temperature dependencies of the lattice parameters of erbium and samarium iron garnets (ErIG and SmIG) are refined from precise x-ray diffraction data obtained on small spherical single crystals in the 20–300 K temperature range; the variations of the thermal linear expansion coefficients are immediately deduced. For ErIG, the lattice constants and the expansion coefficients present anomalies observed at 20, 59, 75, and 105 K, respectively, whereas for SmIG singularities are situated at 40 and 68 K. These anomalies are interpreted as originating from spin-reorientation transition and from the onset of noncollinear magnetic structure of the rare-earth magnetic moments.

Doping of BiSrCaCuO compounds with (V+Y), As, Sb, Pb, Cr and Ge

The effect of various doping elements such as (V+Y), As, Sb, Pb, Ag and Ge in range of 3–5 atom% on the phase stability of the high temperature superconducting (HTS) Bi - Sr - Ca - Cu - O system was investigated by preparing (1) small single crystals from flux solvents, and (2) thick films by doctor blade casting. It was found that the Bi 2Sr 2CaCu 2O x (2212) is the predominant phase in all doped samples Examples of HTS critical temperature Tc results measured by microwave absorption are 94 K for (V+Y), 78 K for As, 74 K for Ge. For the Sb doped compound a two phase BSCCO material was obtained with two T c of 90 and 78°K in the proportion of 20 to 80% respectively.

Growth of KTiOAsO4 (KTA) and CsTiOAsO4 (CTA) crystals from new molten salt system

A new molten salt system has been used for the growth of KTA and CTA. Large transparent crystals of KTA with a size of up to 47×28×10 mm have been grown by flux seeding with slow cooling, but they showed ferroelectric domains which decrease the efficiency of nonlinear optical conversion. Small single crystals of KTA and CTA up to 4 and 6 mm 3 in size have been grown. respectively, from the new melt by spontaneous crystallization. The results of powder second harmonic generation proved to be good. The lattice parameters of KTA and CTA were measured by X-ray powder diffraction analysis

Measurement of magnetic forces on small high Tc superconductor single crystals in magnetic field gradients

In this paper a brief account is given of a new instrument which allows quantitative measurements of magnetic forces in very small single crystals. The crucial element of the instrument is a capacitor with plane and parallel plates, one of which can be elastically displaced to change the distance between them. Forces in the range of 10 −9 N can be measured with this set-up. Good quality single crystals of Nd 1.85Ce 0.85CuO 4, placed in low magnetic field gradients, have been studied at different fields and temperatures. To prove the capability of the method for studying magnetic relaxation phenomena, results are presented of d M/dln t for this material at several low magnetic fields, H c1 < H < H ∗ (where H ∗ is the field for complete field penetration) at 4.2 K.

Experimental determination of the atomic quadrupole moment of graphite

The symmetry of the graphite crystal implies that each carbon atom has an electric quadrupole moment associated with it. This atomic moment may have a significant influence on the location of adsorbed molecules on a graphite surface and on the adsorption energy. A novel experiment, involving the measurement of the effect of a laboratory electric field gradient on the period of oscillation of a small single crystal of graphite mounted on a glass torsion fibre, has been carried out. The quadrupole moment of a carbon atom within the sample is found to be (–3.03 ± 0.10)× 10–40 C m2; however, this value may include a contribution from surface effects.