Yasusada Yamada

A Microscopic Theory on the Phase Transitions in NH4Br –An Ising Spin Phonon Coupled System–

CsCl type NH 4 Br crystal has been treated as an Ising spin-phonon coupled system to explain the phase transitions associated with the orientational order of NH 4 + ions. The appearance of two ordered phases is explained as essentially due to the fact that the direct interaction between NH 4 + ions stabilizes parallel ordering while the indirect interaction through phonons stabilizes the antiparallel ordering. Based on a microscopic hamiltonian describing the spin-phonon coupled system, several experimental results have been analized such as (i) anomalous lattice expansions (ii) orientational order parameters (iii) spontaneous displacement of Br - ions (iv) P - T phase diagram (v) X-ray critical diffuse scattering. Semiquantitative agreements are obtained. Especially the property of correlated fluctuations of displacement of Br - ions seems to give a support on the validity of the microscopic model used.

Neutron Scattering and Nature of the Soft Optical Phonon in SrTiO3

Measurements of neutron scattering from an SrTiO 3 crystal have been carried out in the temperature range from 4.5°K to 300°K. The integrated intensity of inelastically scattered neutrons due to excitation of the soft optical phonon was observed at various reciprocal lattice points at room temperature. The structure factors deduced from the observed intensities are in qualitative agreement with Slaters model of ionic movement. A strong anisotropy of the dispersion surface of the soft mode in reciprocal space was observed around q =0, the direction of the lowest frequency being along [100]. The intrinsic energy width of the soft mode shows a gradual in crease with increasing temperature from 0.22MeV (20°K) to 0.85MeV (250°K). The experimental results indicate that in SrTiO 3 , the polarization fluctuation is entirely due to the excitation of optical phonons and the phonon system can be treated as a quasi-harmonic system over the temperature range from 4.5°K to 300°K.

Soft phonon mode at the martensitic phase transformation of AuCuZn2

Abstract The lattice dynamical property of AuCuZn2 has been investigated by means of inelastic neutron scattering technique in connection with its martensitic phase transformation. The temperature dependent soft phonon was observed transformation. The temperature dependent soft phonon was observed around q = 2 3 [110] of TA2 phonon branch. We have also found a premartensitic metastable phase, in which new satellite reflections at (H± 2 3 , K ∓ 2 3 , 0) have been observed around fundamental reflections with H + K = 4n. The atomic displacements of the soft phonon mode correspond to the atomic arrangement of the premartensitic phase.

Inter-Dipolar Interaction in NaNO2

The X-ray diffuse scattering from a crystal of NaNO 2 at temperatures above the Neel temperature is analysed to determine the nature of the inter-dipolar interaction in this substance. By taking the Ising model, the space correlation is calculated with the random phase approximation leaving the coefficients of the short range interaction as parameters, and is compared with the observed intensity distribution of the diffuse scattering. It is found that a large part of the inter-dipolar interaction is of the electrostatic nature, and in addition to it, there are an antiferroelectric coupling between the neighbours along the a -axis and a ferroelectric coupling between the neighbours of the corner- and body-centre molecules. The coefficients of the short range interaction decrease with temperature more rapidly than that of the electrostatic interaction. The short range interaction may come from the covalent bonding between N and O and also from the Pauli repulsive interaction between the molecules.

X-Ray Scattering and the Phase Transition of KMnF3 at 184°K

The phase transition that KMnF 3 exhibits at T c =184°K has been studied in detail by X-ray scattering. The anomalous scattering that accompanies the transition near T c is found to be anisotropic for q ∼0.05A -1 . For these wave vectors therefore, the Ornstein-Zernike approximation for the density-density correlation function cannot describe the critical properties. The temperature dependence of the square of the frequency of the soft \(\varGamma_{25}\) mode at R is found to be reasonably well described by a Curie-law divergence. The space group of the condensed phase of KMnF 3 has been reexamined. The space group is found to be the tetragonal I 4/ m c m ( D 4 h 18 ) rather than the orthorhombic P b n m ( D 2 h 16 ) proposed by Beckman and Knox. The space groups of the condensed phases of KMnF 3 and SrTiO 3 are therefore identical.

Structural Aspects of Fe–Mg Artificial Superstructure Films Studied by X-Ray Diffraction

In the [Fe(15 A)|Mg(30 A)] 60 film with its superstructure period , Λ=45 A, the significant peak-broadening and intensity-reduction are observed in Bragg reflections from the bulk structures within the (110)Fe/(00.1)Mg-textured layers. Such a diffraction profile has been well explained by an extended step model, which takes into account a random distribution of the superstructure period, varying according to a gaussian distribution with the width ΔΛ/Λ=5.4%. This value corresponds to about one monolayer thickness. A loss of the texture has been also observed in the Fe layer of other two ASFs with their Fe-layer thickness less than 15 A, which seems to be a critical value for the formation of the textured Fe layer.

Symmetry of High-Piezoelectric Pb-Based Complex Perovskites at the Morphotropic Phase Boundary: I. Neutron Diffraction Study on Pb(Zn1/3Nb2/3)O3–9%PbTiO3

The symmetry was examined using neutron diffraction method on Pb(Zn 1/3 Nb 2/3 )O 3 –9%PbTiO 3 (PZN/9PT) which has a composition at the morphotropic phase boundary between Pb(Zn 1/3 Nb 2/3 )O 3 and PbTiO 3 . The results were compared with those of other specimens with same composition but with different prehistory. The equilibrium state of all examined specimens is not the mixture of rhombohedral and tetragonal phases of the end members but exists in a new polarization rotation line Mc# (orthorhombic-monoclinic line). Among examined specimens, one exhibited tetragonal symmetry at room temperature but recovered monoclinic phase after a cooling and heating cycle.The symmetry was examined using neutron diffraction method on Pb(Zn 1/3 Nb 2/3 )O 3 –9%PbTiO 3 (PZN/9PT) which has a composition at the morphotropic phase boundary between Pb(Zn 1/3 Nb 2/3 )O 3 and PbTiO 3 . The results were compared with those of other specimens with same composition but with different prehistory. The equilibrium state of all examined specimens is not the mixture of rhombohedral and tetragonal phases of the end members but exists in a new polarization rotation line Mc# (orthorhombic-monoclinic line). Among examined specimens, one exhibited tetragonal symmetry at room temperature but recovered monoclinic phase after a cooling and heating cycle.

Optical Observation of Heterophase and Domain Structures in Relaxor Ferroelectrics Pb(Zn1/3Nb2/3)O3/9%PbTiO3

Phase boundary and domain structure in relaxor ferroelectrics Pb(Zn1/3Nb2/3)O3/9%PbTiO3 were observed using a (111) plate under a polarization microscope. The dielectric constant was measured simultaneously to determine exact transition temperatures and to connect the dielectric behavior with phase boundary movements. Movement of the phase boundary between cubic and tetragonal phases was observed at the temperature Tc where the dielectric constant changed abruptly. Below Tc, tetragonal and rhombohedral phases coexisted over a wide temperature range. The complicated domain/phase patterns are classified into two blocks according to extinction position. The difference in extinction positions between the two blocks deviated from 30? and manifested temperature dependence. This fact indicates that the symmetry-lowering occurs in the heterophase. A small, but the distinct temperature hysteresis of the dielectric constant was related to the nucleation and growth of one phase into another phase.

X-Ray Critical Scattering in Ferroelectric Tri-Glycine Sulphate

The x-ray critical scattering due to the polarization fluctuation in Tri-Glycine Sulphate in the vicinity of the ferroelectric Curie point ( T C =48°C) has been observed in order to clarify the nature of inter-dipolar interaction to cause its phase transition. The observed critical scattering intensity shows a strong anisotropy around the reciprocal lattice points, with very narrow width in the direction of the polar axis. Based on the Ising model, the result was analyzed and it was found that the observed anisotropy was due to the long range electrostatic interaction between constituent microscopic dipoles. The critical behavior of the inverse correlation range, expressed as κ= A ( T / T C -1) ν , is investigated. From the temperature dependence of the critical scattering, the following values are obtained in the a -direction: A =0.18 6 A -1 , ν=0.50 2 . The value of the critical exponent ν is in good agreement with that obtained in the molecular field approximation.

The Nearly Commensurate Phase and Effect of Harmonics on the Successive Phase Transition in 1T-TaS2

The phase transitions of the charge density wave (CDW) state in 1T-TaS 2 are studied both theoretically and experimentally. The positions and intensities of the fundamental and third-order harmonic...