M. Hidaka

Structural phase transitions of RbCaF3

Abstract Specific heat measurements, dielectric measurements and X-ray diffraction of RbCaF3 show two first-order transitions at 193 K and 41 K. Structural information about these transitions has been obtained using energy-dispersive X-ray diffraction. It was found that successive transitions are Oh 1(Pm3m) → D4h 18(14/mcm) → D2h 13(Pmmn) on cooling. These results were interpreted by successive condensation of an R25 z and an M3 x + y soft phonon mode from the ideal cubic phase.

X-ray diffraction study of the crystal structures of K2CuF4 and K2CuxZn1-xF4

The investigation of the crystal structures of K2CuF4 and its diluted system K2CuxZn1-xF4 has been made by means of an X-ray camera and computer method. Both the pure (x = 1) and the diluted compounds were found to have two kinds of structures, an ordered and a disordered type. The ordered K2CuF4 generally shows a multi-domain structure, which means that a single domain sample is orthorhombic, D182h-Bbcm. On the contrary, the ordered K2CuxZn1-xF4 with x < ≈0.9 has a tetragonal symmetry due to the absence of a correlation between the displacement of F- ions around the body-centered Cu2+ ion with respect to those in the basal plane. Some comments concerning the magnetic structures of the pure and the diluted systems are given on the basis of the present crystallographic analyses.

Re-examination of the room temperature crystal structure of by x-ray diffraction experiments: observation of new superlattice reflections

A recent electron paramagnetic resonance (EPR) study of revealed the existence of an antisymmetric exchange interaction, , between Cu spins on the c-axis in this compound (Yamada I, Nishi M and Akimitsu J 1996 J. Phys.: Condens. Matter 8 2625). To allow this interaction, the crystal structure must have no inversion centre halfway between relevant Cu sites. However, the structure with the space group Pbmm , which was proposed for this compound by Vollenkle et al and has been accepted so far, does not allow this interaction because it has an inversion centre at the midpoint between nearest-neighbour Cu sites on the c-axis. This conflict was pointed out in the EPR study and was taken to indicate the inconsistency of the structure with Pbmm. To find the correct crystal structure of , x-ray diffraction experiments are performed at room temperature on samples which are markedly improved in their quality because they are produced by the annealing and slow cooling of single crystals grown by the floating-zone method. As a result, new superlattice reflections which had not been reported so far are detected. The space group and the unit cell are determined to be and , respectively, where is the pseudo-unit cell proposed by Vollenkle et al. The newly found structure allows the antisymmetric interaction mentioned above. Possible connections between the new structure and the question of why the structural transition, which brings about the dimerization of Cu spins, occurs in are discussed.

Structural phase transitions in KCdF3

Abstract The successive phase transitions in KCdF 3 have been studied by using X-ray diffraction and differential type thermal analysis. Three transitions are observed at 485, 471 and 243 K; these are of 1st-, 2nd- and 2nd-order respectively. The space groups of the four phases are found, from the high temperature side, to be Pm 3 m , P 4/ mbm , Pbnm and Pbnm respectively. The first and second transitions can respectively be explained in terms of the condensation of soft-phonon modes at the M and R points in the cubic Brillouin zone. The third transition may be caused by instability due to titlings of CdF 6 octahedra.

A comment on the phase transitions in KMnF3

Abstract The structural and magnetic phase transitions in KMnF 3 have been examined by means of neutron diffraction. The structural transition at 91 K accompanied with the M 3 soft-mode phonon condensation shows typical second-order character. The transition temperature and characterization of other structural and magnetic transitions are also given.

Piezoelectric resonance study of structural anomalies in BaMnF4

Abstract Structural anomalies in BaMnF4 are studied via piezoelectric resonance and a.c. thermal diffusivity techniques. The present results show the existence of anomalies at seven different temperatures: 248, 234, 226, 220, 120, 43, and 30 K. It is not clear that all of these temperatures represent true phase transitions; some may mark the onset of only short-range ordering. In any case, however, the results suffice to reconcile existing discrepancies in the literature, where some transitions are observed via piezoelectric techniques (at 226 and 235 K) whereas others are found by neutron or light scattering (e.g. 248 K). The present study confirms the transitions at 248 and 226 K via thermal diffusivity and piezoelectric resonance; only the latter technique reveals anomalies at 234, 220, and lower temperatures.

Diffractometer for neutron crystallography in biology

Abstract We have constructed a dedicated diffractometer for neutron crystallography in biology (BIX) at JRR-3M in Japan Atomic Energy Research Institute. The diffraction intensity from a protein crystal is weaker than that from most of the inorganic materials. In order to overcome the intensity problem, an elastically bent silicon monochromator and a large area detector system have been specially designed. A preliminary result of a diffraction experiment using BIX is reported.

Energy dispersive X-ray diffraction study of structural phase transitions in BaMnF4

Abstract An energy dispersive (15–30KeV) X-ray diffraction study of BaMnF4 was carried out from 200 to 300 K. The temperature dependences of scattering intensities for superlattice reflections around (±0.2, 0, 0) and (±0.4, 1 2 , 1 2 ) are interpreted as indicative of two structural phase transitions near 222 and 231 K in the specimens used. These transitions are also confirmed via dielectric measurements along the b-axis and by piezoelectric resonance measurements. Even above the transitions at room temperature there are two sets of diffuse reflections: one at (±0.4, 1 2 , 1 2 ) , related to fluctuations associated with the structural phase transition(s); and a second at (h, 0, 0) with h odd, which is not permitted for the space group A21am accepted as the room-temperature structure. We interpret the latter as originating from a particular stacking fault.

Magnetic measurement of rare earth ferromagnet gadolinium under high pressure

Abstract Magnetic property of rare earth ferromagnet gadolinium(Gd: T c =293 K ) has been observed under pressures up to 15 GPa , using a miniature diamond anvil cell (DAC) installed in a SQUID system. Pressurization suppresses both the ferromagnetic moment and the transition temperature with increasing pressure. In the pressure region above 6 GPa , the ferromagnetic signal has disappeared. The structural analysis, however, suggests that the crystal symmetry does not change in the pressure region up to 9 GPa . The results will be discussed on the exchange interaction among the isotropic 4f-metallic system.

Antiferroelectric short-range order of BaMnF4

Micro-structural modulation in the A21am phase of BaMnF4 is studied by means of the energy-dispersive X-ray diffraction and piezoelectric resonance. The results show that the antiferroelectric short-range order exists up to 800 K, and that the antiferroelectric modulation has anomalies at about 350 and 650 K. The structural phase transitions in the region from 220 to 250 K are interpreted in terms of increasingly long-range order among the chain-like modulation of the MnF6 octahedra in the cp planes.