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

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

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

Rietveld refinement of magnetic structures from pulsed-neutron-source powder-diffraction data

The General Structure Analysis System, GSAS, has recently been modified to include magnetic neutron-scattering cross-sections. Low-temperature diffraction data have been taken on the hexagonal noncollinear antiferromagnet UPdSn on both the HIPD and the NPD powder diffractometers at LANSCE. The low-resolution data reveal that the magnetic structure has orthorhombic symmetry (magnetic space group P[sub c]112[sub 1]) between 25K and 40K, and monoclinic symmetry (magnetic space group P[sub c]l12[sub 1]) below 25K. The high-resolution data reveal that there are structural distortions with corresponding symmetry changes in each of these phases, to give chemical space groups Cmc2[sub 1] and P2[sub 1] respectively, while the paramagnetic phase above 40K has space group P6[sub 3]mc. Using GSAS, we have refined data sets from both diffractometers simultaneously, including both magnetic and structural cross-sections. Magnetoelastic coefficients for the distortions have been extracted and we have determined the sip of the coupling between the structural monoclinicity and the magnetic monoclinicity. The magnetic results from Rietveld refinement are in good agreement with model fitting to the integrated intensities of seven independent magnetic reflections and these, in turn, agree with measurements made on the same sample using the constant-wavelength reactor technique. Our results therefore validate, to some level, both the technique of using spallation sources for complicated magnetic structures and the specifics of the GSAS Rietveld code

Rietveld refinement of magnetic structures from pulsed-neutron-source powder-diffraction data

The General Structure Analysis System, GSAS, has recently been modified to include magnetic neutron-scattering cross-sections. Low-temperature diffraction data have been taken on the hexagonal noncollinear antiferromagnet UPdSn on both the HIPD and the NPD powder diffractometers at LANSCE. The low-resolution data reveal that the magnetic structure has orthorhombic symmetry (magnetic space group P{sub c}112{sub 1}) between 25K and 40K, and monoclinic symmetry (magnetic space group P{sub c}l12{sub 1}) below 25K. The high-resolution data reveal that there are structural distortions with corresponding symmetry changes in each of these phases, to give chemical space groups Cmc2{sub 1} and P2{sub 1} respectively, while the paramagnetic phase above 40K has space group P6{sub 3}mc. Using GSAS, we have refined data sets from both diffractometers simultaneously, including both magnetic and structural cross-sections. Magnetoelastic coefficients for the distortions have been extracted and we have determined the sip of the coupling between the structural monoclinicity and the magnetic monoclinicity. The magnetic results from Rietveld refinement are in good agreement with model fitting to the integrated intensities of seven independent magnetic reflections and these, in turn, agree with measurements made on the same sample using the constant-wavelength reactor technique. Our results therefore validate, to some level, both the technique of using spallation sources for complicated magnetic structures and the specifics of the GSAS Rietveld code

Continuous first order orbital order-disorder transition in Nd1−xCaxMnO3Nd_{1-x}Ca_xMnO_3

The nature of the cooperative Jahn-Teller (JT) transition accompanied by orbital order-disorder in Nd1-xCaxMnO3 has been studied by high temperature (300 - 1200K) synchrotron and laboratory X-ray powder diffraction in the low doping region (0<x<0.1). For very low doping a large temperature range of phase coexistence associated with a first-order transition has been observed, resembling a martensitic transformation. The transition appears continuous because of a gradual evolution of the volume fractions of the phases over a broad temperature interval. The first-order nature of the transition and the phase coexistence is suppressed with increasing of doping.Comment: 14 pages 4 figure

A study of low-energy guest phonon modes in clathrate-II NaxSi136 (x = 3, 23, and 24)

Single-crystal x-ray diffraction from clathrate-II NaxSi136 (x = 24) prepared by a new technique reveals the exceptionally large Na@Si-28 atomic displacement parameter (U-eq) is strongly temperature dependent, and can be attributed to low-energy rattling modes associated with the Na guest. Inelastic neutron scattering (INS) spectra collected from NaxSi136 powder specimens (x = 3, 23) confirm the presence of low-energy guest-derived phonon modes for Na@Si-28 and Na@Si-20. The lower energy Na@Si-28 rattler mode falls in the frequency range of the silicon host acoustic phonons, indicating the possibility for interaction with these phonons. The presence of these low-energy modes combined with the ability to controllably vary the guest content presents a unique opportunity for exploring the influence of guest-framework interactions on the lattice dynamics in intermetallic clathrates