ALBERT

Description: High-pressure technique is a powerful tool for physical property measurements and structural analyses as well as other external conditions, such as magnetic fields and temperatures. In the field of neutron experiments, measurements under high-pressure conditions are also useful and attractive; because pressure is one of important thermodynamic parameter that can be used to tune magnetic property, crystal field, and other parameters to obtain insight into the microscopic physics of many phenomena. SENJU have been constructed for a single crystal time-of-flight neutron Laue diffractometer at beamline 18 (BL18) of Materials and Life Science Experimental Facility (MLF) at Japan Proton Accelerator Research Complex (J-PARC) [1], designed in consideration of precise crystal and magnetic structure analyses for small size single crystals, 1 mm3 or less in volume: and also taking account of the neutron diffraction measurements under multiple extreme conditions. In this research we are planning to introduce high-pressure sample environments into SENJU. Two types of compact high-pressure cells have been prepared, one is clamp type piston-cylinder cell made of copper-beryllium alloy (〈 2 GPa), and the other is clamp type opposite anvil cell, can be expected to reach maximum pressure of 10 GPa. A taurine single crystal (3 mm3) was enclosed in the piston-cylinder pressure cell together with deuterated glycerol (pressure transmitting medium) and pressurized up to 1 GPa. Accelerator power of J-PARC was 300 kW and the exposure time was 6 hours. We can observe many distinct Bragg reflections from the sample crystal (taurine) even through the pressure cell body, as shown in the figure. In this presentation, we will show more details and current status of high-pressure sample environments in SENJU.

Description: SENJU, a TOF-Laue single crystal neutron diffractometer at the BL18 of MLF/J-PARC, was designed for precise crystal and magnetic structure analyses under multiple extreme environments such as low-temperature, high-pressure and high-magnetic field, and also capable of taking diffraction measurements of small single crystals, less than 1.0 mm3 in volume [1]. Just after the launch of SENJU in March 2012, we newly installed and/or upgraded some sample environment devices. SENJU has a vacuum sample chamber and 37 two-dimensional scintillation detectors. Wavelength of incident neutron is 0.3 - 4.4 Å for the 1st frame and 4.6 - 8.8 Å for the 2nd frame. Because the short wavelength neutron is available and the sample position is covered by large solid angle of the detectors, wide reciprocal space within 30 Å-1 can be measured simultaneously by one measurement. As sample environment devices, 4K cryostat with 2-axes goniometer, longitudinal magnet, high-pressure cell, high temperature furnace and other devices are available or in commissioning. The most popular sample environment device on SENJU is the 4 K cryostat with a fixed-chi type 2-axes goniometer. We adopted piezo-rotators to rotate the sample crystal under vacuumed and cryo conditions. The 2-axes goniometer works stably even at 4 K and the time for cooling was 4.5 hours. A longitudinal magnet was recently installed on SENJU. The lowest temperature was 1.42 K and the maximum magnetic field was 6.85 T. A test diffraction measurement of a CeCoGe3 single crystal (1.5 x 1.5 x 3.0 mm) under 1.5 K and 0.5 T showed that Bragg reflections from the sample was clearly observed and the Bragg peaks of the sample crystal were much higher than the peaks from the magnet itself as shown in the figure. In this presentation, we will show the current status of sample environment devices for SENJU such as cryostat, magnet and other devices.

Description: The materials represented as M3H(XO4)2 (M = alkaline metal, X = S or Se) are known to exhibit high protonic conductivities at moderately high temperature. The high protonic conductivity emerges upon a structural phase transition and hydrogen bonds become directionally disordered. The protonic conduction is presumably realized through the disordered hydrogen bonds, but no experimental evidence has been reported. Meanwhile, although the mechanism of the protonic conduction is considered to be the same among this group of materials, the transition temperature (Tc) varies depending on the elements of M and X. For example, the material with M = Rb and X = Se undergoes the transition at 440 K while with M = K and X = Se the transition occurs at 390 K. Since the chemical characteristics of Rb and K are, as a principal, the same, some structural features may play crucial roles in triggering the phase transition. In order to clarify the mechanism of the proton conduction in the superprotonic phase and the relation between the crystal structure and Tc, structural studies on Rb3H(SeO4)2 at high temperature and solid solutions of Rb3H(SeO4)2 and K3H(SeO4)2 (Rb3-xKxH(SeO4)2, x=0,1,2,3) were conducted by means of single crystal neutron diffraction at FONDER at JRR-3M and SENJU at J-PARC/MLF. The proton density distribution map obtained from the high temperature neutron diffraction experiment clearly demonstrates 2-dimensional continuous spread of the proton distribution, which is considered to be the proton conduction path (figure). The structure analyses of Rb3-xKxH(SeO4)2 revealed that K ions tend to occupy one of two possible sites. As the concentration of K ion increases, the distortion of SeO4 appears to be enhanced. The variation of the distortion is consistent with the variation of the transition temperature, suggesting the close relationship between the distortion and the phase transition temperature.

Description: Supramolecular ferroelectric cocrystal of phenazine (Phz) with chloranilic acid (H2ca), which exhibits three successive phase transitions, have been characterized by the interplay between their structural transformations and solid-state acid–base (proton transfer) reactions (Figure) [1]. This material undergoes a ferroelectric phase (FE-I phase) transition of displacive-type at 253 K followed by successive phase transitions to the lattice modulated phases with incommensurate periodicities and with commensurate 2-fold periodicity (FE-II phase) at lower temperature [2]. To elucidate the origin of the ferroelectricity in the FE-I phase, it is crucial to study the crystal structure using single crystals. The synchrotron x-ray diffraction experiment was carried out on the imaging-plate diffractometer at BL-8A of Photon Factory in KEK. Superstructure reflections with the modulation wave vector q=(1/2 1/2 1/2) were clearly observed below 103 K. Considering the preserved 2/m Laue symmetry, the lattice can be transformed to a C-centered monoclinic lattice, which is related by (-2a, -2b, a + c) or (2a, -2b, -a - c) with the FE-I structure. Although the lattice distortion and the intensities of the superlattice reflections are consistent with the 2/m Laue symmetry, the space group C1 is deduced from the polar nature and a subgroup symmetry of the FE-I structure. Moreover, we performed single-crystal neutron diffraction experiments at SENJU of MLF/J-PARC in order to determine the displacement of the hydrogen atom. The crystal structure analysis at 10 K was carried out using the reflections measured in a half-sphere of reciprocal space at d 〉 0.4. The structure analysis was performed on the basis of the space group C1, where four Phz and four H2ca become crystallographically inequivalent. Finally, all the structural parameters including all hydrogen atoms were successfully refined. In the FE-II phase, the neutral and ionic molecules alternately align along the π-molecular stack.

Description: "SENJU" is a newly built pulsed neutron single crystal diffractometer at J-PARC/MLF for structural research of inorganic and organic materials with relatively small cell sizes under multiple extreme environments, such as low temperature and a high magnetic field. Since the launch of the instrument in 2012, SENJU has been commissioned and now tuned to be capable of crystal and magnetic structure analyses with a sample as tiny as 1mm cube or less. SENJU has the total of 37 two-dimensional scintillation detectors installed. Groups of 3 detectors are accommodated in detector banks and 12 detector banks are placed so as to cylindrically surround the sample chamber (Figure), and additional one detector is settled at the bottom. The instrumental parameters including the positions of the detectors and the neutron flight path length were determined in order to obtain accurate lattice parameters of samples. Since the instrumental parameters correlate with each other, series of different measurements were needed in order to obtain unique values for each parameter. As the first step of the procedure, a powder diffraction pattern of diamond was measured in order to determine the scattering angle of 90 [deg] utilizing the nature that a Bragg reflection vertically lines up at 90 [deg]. Simultaneously, we determined the neutron path length L1 from the neutron source to the sample position. As a next step, a Bragg reflection was repeatedly measured as the sample crystal was rotated with small steps. From this data, the equatorial plane on the detectors and the distance between the sample and the detectors L2 were determined. As a third step, many Bragg reflections from a sample with known lattice constants were measured and from the positions of the reflections the positions of each detector were determined. As a result, the lattice parameters can be obtained with the accuracy of about 0.05 % using the determined instrumental parameters.