Masaru Kawaminami

Neutron Diffraction Study of Fe7Se8

Ferrimagnetic orderings of the moments of iron atoms in Fe 7 Se 8 are confirmed by neutron diffraction method; the moment is parallel to that of each atom on the same atomic layer parallel to the (001), while it is antriparallel to that on the next layer. When the temperature of the crystal is lowered from the Neel temperature (455°K), the direction of the moment, which is at first on the (001), changes towards the [001]. The behaviour of this “stand up” process depends on the type of the crystallographic superstructure of the specimen: gradually in the 4 c type crystal and abruptly in the 3 c type one. The ionic models of the magnetic structure, used in the case of pyrrhotite, for example, are not consistent with the result of the present experiment. A model with a more metallic character seems to be acceptable, which is supported by the Mossbauer effect experiment.

Accurate Measurement of Lattice Constants in a Wide Range of Temperature: Use of White X-Rays and Double-Crystal Diffractometry

The use of double-crystal diffractometry using white incident X-rays is proposed for measurements of lattice constants of common, not perfect crystals in a wide range of temperatures. A detailed experimental procedure is presented. Measurements with the full width at half maximum of 0.1° in Bragg peaks can be made at θ=88° without any particular difficulty for many crystals mounted in a cryostat (or thermostat). The accuracy of the change of the lattice constant determined by these measurements is about 10-5 which is much better than that obtained by the conventional diffraction methods. Applications to the measurement of anomalies at the phase transition are emphasized. As an example the results on KNiF3 are shown.

Condensed Modes of WO_3 Represented by the Normal Modes in the Space Group D^7_

The prototypic phase of WO 3 is considered to be the tetragonal phase with the space group D 4 h 7 , that is the highest temperature phase, to explain systematically the displasive successive phase transitions of WO 3 . The normal modes at the points \( \varGamma \), Z , M and A in the tetragonal Brillouin zone are studied with the help of the group theory and are compared with the normal modes in the cubic WO 3 that is the hypothetic prototype with the space group O h 1 . The condensed modes in each phase of WO 3 are described by the irreducible representations of the normal modes in the tetragonal WO 3 , referring to the results of X-ray and neutron diffraction studies.

A Method of the Determination of Lattice Constants from Single-Crystal X-ray Photographs

A graphical extrapolation method from single-crystal X-ray photographs was used by Farquhar and Lipson to determine accurate lattice constants of an orthorhombic crystal. An extension of the above method to the case using shorter wavelength radiations such as MoKα is discussed. The difficulties due to the use of such radiations, i.e. low intensities of the Bragg reflections at high angles and a high background, are obviated with the help of a simple attachment to an ordinary single crystal oscillation camera. This technique is also useful for longer wavelength radiations such as CuKα. The application is made to an orthorhombic crystal, sodium nickel double fluoride NaNiF3. The present method will be particularly effective for the crystal with small lattice constants and/or with low symmetry. The method is compared with that using a back-reflection Weissenberg goniometer.

Thermal Anomalies in Bi2WO6 and EPR of Doped Gd3+ Ions

The dielectric constant e ′ (ω, T ), e ′′ (ω, T ) and the electric conductivity σ(ω, T ) of Bi 2- x Gd x WO 6 single crystals have been measured in the temperature range from 350 K to 4.2 K and at frequencies from 20 Hz to 1 MHz. EPR measurements with an X-band spectrometer have been made on Gd 3+ -doped Bi 2 WO 6 crystals. Two kinds of centres observed are ascribed to Gd(1) and Gd(2) ions substituted for Bi(1) and Bi(2) ions, respectively. Fine structure parameters obtained are | b 2 0 |=1367, 1138, and | b 2 2 |=1060, 384 in units of 10 -4 cm -1 . Temperature dependence of resonant fields for the Gd(1) centre shows a marked difference from that for the Gd(2) centre. This is due to the difference between the coupling of the two Gd ions to phonon modes.

EPR Study of V4+ Ions in Tungsten Trioxide WO3

The EPR spectra from V 4+ ions in WO 3 single crystal were studied in the temperature range from 300 K to 77 K by using a K-band spectrometer. At 77 K, the g values and hf parameters A of V 4+ ions were strong c -axis anistropy, so that g X =1.928, g Y =1.945, g Z =1.815, (Eulerian angles; θ g =3.9°, φ g =45.8°, ψ g =-0.2°), and A X =-60.5 G, A Y =-55.0 G, A Z =-190.4 G, (θ A =4.4°, φ A =43.6°, ψ A =1.5°). The ground state of 3 d 1 electron of V 4+ ion was determined to be | x y > state by hf structure analysis. WO 3 is determined to be a ferroelectric material with the spontaneous polarization along the c -axis connected to the condensed ferroelectric \(\varGamma_{15}^{z}\) phonon modes of W 6+ ions in the monoclinic II phase at 77 K. At the relations of g av vs A iso for V 4+ and VO 2+ ions in some host crystals, WO 3 is shown to be a remarkable material.

Acoustic Instability near the Monoclinic-Orthorhombic Phase Transition in WO3

The X-ray scattering profiles from WO 3 single crystal are carefully examined to investigate the acoustic instability near the monoclinic-orthorhombic phase transition temperature T c =310°C. The spontaneous shear strain e zx is 0.016 at 25°C, decreases with increasing temperature and becomes zero at T c . The temperature dependence of X-ray diffuse scattering intensity indicates that the elastic stiffness consant C 55 becomes vanishingly small just above T c .

X-Ray Monochromators for High-Angle Double-Crystal Diffraction

The use of a monochromator crystal in high-angle double-crystal diffraction is proposed in connection with lattice-constant measurements with a relative accuracy of 10-6. Requirements for monochromator crystals are that they should have high reflectivity and that effects of X-ray irradiation on the relevant lattice spacing should be negligible during the period of an experiment at least. The temperature coefficient of the lattice spacing should be small unless the temperature of the crystal is controlled very precisely. The results of some preliminary examinations are given on crystals of LiF, Si and GGG (gadolinium gallium garnet)