Experimental evidence for an intermediate phase in the multiferroic YMnO3

We have studied YMnO$_{3}$ by high-temperature synchrotron X-ray powder diffraction, and have carried out differential thermal analysis and dilatometry on a single crystal sample. These experiments show two phase transitions at about 1100K and 1350K, respectively. This demonstrates the existence of an intermediate phase between the room temperature ferroelectric and the high temperature centrosymmetric phase. This study identifies for the first time the different high-temperature phase transitions in YMnO$_{3}$.Comment: 10 pages 5 figures. New version, Additional data, Journal of Physics: Condensed Matter, in Pres

Magnetic and crystallographic order in alpha-manganese

We have made time-of-flight neutron diffraction measurements on {alpha}-manganese metal Powder diffraction measurements were made at 14 temperatures between 15 and 305K, and single crystal measurements were made at 15 and 300K. We found that the crystal structure of {alpha}-Mn is tetragonal below is Neel point of 100K, with crystal symmetry 142m and magnetic (Shubnikov) symmetry P{sub 1}42{sub 1}c. In agreement with the earlier results of Yamada et al., there are six independent magnetic atoms, and we found that their moments are weakly temperature dependent. The onset of magnetic order causes slight changes in the atomic positions and in the average atomic elastic constant

Properties of Binary Transition-Metal Arsenides (TAs)

We present thermodynamic and transport properties of transition-metal (T) arsenides, TAs with T = Sc to Ni (3d), Zr, Nb, Ru (4d), Hf and Ta (5d). Characterization of these binaries is made with powder X-ray diffraction, temperature and field-dependent magnetization and resistivity, temperature-dependent heat capacity, Seebeck coefficient, and thermal conductivity. All binaries show metallic behavior except TaAs and RuAs. TaAs, NbAs, ScAs and ZrAs are diamagnetic, while CoAs, VAs, TiAs, NiAs and RuAs show approximately Pauli paramagnetic behavior. FeAs and CrAs undergo antiferromagnetic order below TN = 71 K and TN \approx 260 K, respectively. MnAs is a ferromagnet below TC = 317 K and undergoes hexagonal-orthorhombic-hexagonal transitions at TS = 317 K and 384 K, respectively. For TAs, Seebeck coefficients vary between + 40 uV/K and - 40 uV/K in the 2 K to 300 K range, whereas thermal conductivity values stay below 18 W/(m K). The Sommerfeld-coefficient {\gamma} are less than 10 mJ/(K2mol). At room temperature with application of 8 Tesla magnetic field, large positive magnetoresistance is found for TaAs (~25%), MnAs (~90%) and for NbAs (~75%).Comment: 7 figures; Will be published in the upcoming focus issue in Superconductor Science and Technolog

Melting of tantalum at high pressure determined by angle dispersive x-ray diffraction in a double-sided laser-heated diamond-anvil cell

The high pressure and high temperature phase diagram of Ta has been studied in a laser-heated diamond-anvil cell (DAC) using x-ray diffraction measurements up to 52 GPa and 3800 K. The melting was observed at nine different pressures, being the melting temperature in good agreement with previous laser-heated DAC experiments, but in contradiction with several theoretical calculations and previous piston-cylinder apparatus experiments. A small slope for the melting curve of Ta is estimated (dTm/dP = 24 K/GPa at 1 bar) and a possible explanation for this behaviour is given. Finally, a P-V-T equation of states is obtained, being the temperature dependence of the thermal expansion coefficient and the bulk modulus estimated.Comment: 31 pages, 8 figures, to appear in J.Phys.:Cond.Matte

Powder diffraction studies on proteins: An overview of data collection approaches

Following the seminal work of Von Dreele, high quality powder X-ray diffraction studies on proteins are being established as a valuable complementary technique to single-crystal measurements. Several studies using a variety of experiments approaches have been reported in the literature, including high-resolution studies employing parallel beam geometry and high intensity measurements using position sensitive detectors. The choice of the optimum instrumental configuration depends on a number of competing factors such as the amount of sample available, its radiation sensitivity, and the quality of the data required for data analysis, e.g. angular resolution, the extent of the data in d-spacing, or the number of patterns required to explore the protein’s behaviour at different temperatures, or under different crystallisation conditions, etc. Here we discuss several advantages and disadvantages of different data collection methods followed for selected examples of small proteins

Modeling of non-structural contribution to the pattern

The Rietveld refinement technique requires that sensible models be chosen for both the Bragg reflection profiles and intensities as well as the background contribution. The reflection profiles contain an instrumental part and a sample dependent part that is affected by particle size and strain. The intensities are affected by geometric factors peculiar to the diffraction method and sample dependent effects such as absorption, extinction and preferred orientation. The various functions used for these effects in Rietveld refinement will be discussed along with the possible interpretation of their coefficients. 19 refs., 3 figs