Masashi Iizumi

Neutron Scattering Study on the Incommensurate Phases in Ferroelectric Rb2ZnCl4 and K2ZnCl4

The satellite reflections at ( h , 0, l ±(1/3-δ)) in Rb 2 ZnCl 4 and K 2 ZnCl 4 have been studied by neutron scattering. Incommensurate structures (δ≠0) were observed in temrerature ranges of +30^° C ∼-81^° C for Rb 2 ZnCl 4 and of +130^° C ∼+280^° C for K 2 ZnCl 4 . The temperature dependence of the parameter δ was obtained for the two compounds and compared with the results of Rb 2 ZnBr 4 .

Phase Transition in SnTe with Low Carrier Concentration

The phase transition in SnTe was investigated on a single crystal with low carrier concentration (0.88×10 20 cm -3 ) by the neutron diffraction method. A second-order phase transition takes place a...

Neutron Scattering Study on the Incommensurate Phases in Rb2ZnBr4

The satellite scatterings at ( h , 0, l ±(1/3-δ)) in Rb 2 ZnBr 4 have been studied by neutron scattering. Two incommensurate phases (δ≠0) were observed above the ferroelectric Curie point T c =-83^° C in temperature ranges of +50^° C ∼ -74^° C and -74^° C ∼ T c in a cooling process. The parameter δ is 0.04 and almost temperature independent in the upper phase, and it decreases rapidly with decreasing temperature in the lower phase, then, vanishes discontinuously at T c .

Neutron Diffraction Study on the Incommensurate Phase in {N(CH3)4}2CuCl4

An incommensurate phase was found in {N(CH 3 ) 4 } 2 CuCl 4 in the temperature range of 20.5°C∼27°C where a satellite reflection was observed by neutron diffraction at (2,0, 1-(1/3 -δ)) with δ-0.007. The satellite exists just at the commensurate position (2, 0, 1-1/3) between 20.5°C and -11°C in the monoclinic frame with the unique axis being parallel to the b -axis. The satellite disappears in the lower-temperature monoclinic phase below -11°C.

Lattice Instability in Cubic Cu69.2Al25.4Ni5.4 Related to Martensitic Phase Transition

The dispersion relation of [110] TA 1 phonon ( e //[1\bar10]) of β 1 -phase Cu 69.2 Al 25.4 Ni 5.4 was measured extensively at 261, 295 and 372 K by neutron scattering technique. It was found that the dispersion surface of the transverse acoustic mode has a pronounced valley along the [110] axis. The temperature dependence of the phonon energy shows a partial softening of the phonon mode. An anomalous dip of the dispersion curve was observed around q =(2/3) q max on the [110] axis. Interplaner force-constant analysis revealed that the lattice dynamics of the crystal indicates dual instabilities both to the 2H and 18R type martensites. Quasielastic scattering was investigated on and in the vicinity of the [110] axis. Peaks attributable to the 2H and 18R martensites were observed as functions of temperature, which also indicates that 2H and 18R are competing.

Neutron Scattering Study on the Incommensurate-Commensurate Phase Transition in Ferroelectric {N(CH3)4}2ZnCl4

Incommensurate satellite reflections at ( h ,0, l ±(1/3-δ)) were measured by neutron scattering for ferroelectric {N(CH 3 ) 4 } 2 ZnCl 4 . In phase II (7.5∼23.6°C) an incommensurate structure (δ≠0) was observed. The parameter δ is about -0.086 at the II-I transition temperature, and the absolute value of δ slightly decreases with decreasing temperature. At the ferroelectric Curie temperature (7.5°C) it tends discontinuously to zero. The results confirm that the ferroelectricity in this material is caused by the incommensurate-to-commensurate phase transition as in the case of K 2 SeO 4 .

Incommensurate Satellite Reflections in the Ferroelectric Phase of Deuterated Ammonium Rochelle Salt

Ferroelectric phase of NaND 4 C 4 H 2 D 2 O 6 ·4D 2 O exhibits incommensurate satellite reflections described by h ± n δ k l or \(h{+}\frac{1}{2}{\pm}n{\delta}\) k l with \(\delta{\cong}0.08\) in addition to the known superlattice reflections with index \(h{+}\frac{1}{2}\) k l . Satellites with the order index n up to 4 are observable, of which second-order ones are predominant. With the parameter δ increasing linearly with decreasing temperature, no transition to a commensurate phase is likely to occur.

Study on the Incommensurate Nature of the Intermediate Phase of RbD3(SeO3)2 by Neutron Diffraction

Search for an intermediate phase between the para- and ferro-electric phases of RbD 3 (SeO 3 ) 2 was carried out by neutron diffraction. The superlattice reflection at (2, 0, 0.5) in the ferroelectric phase was found to shift to an incommensurate position, (2, 0, 0.5+δ), above the Curie point though the magnitude of the parameter δ is very small. It increases with increasing temperature and reaches to 0.003 at the paraelectric-to-intermediate transition point which is about 3 K above the Curie point.

Incommensurate-Commensurate Phase Transition in Rb2ZnBr4

The existence of the successive long-period superstructures, or the “devils staircase”, in the temperature evolution of the incommensurately modulated structure has been examined in Rb 2 ZnBr 4 by the neutron diffraction method. Detailed measurements of the third order satellite scattering in the temperature region close to the incommensurate-to-commensurate phase transition have shown that the temperature evolution takes place in more complicated manners than previously reported for this compound. At least three different modulations with distinct wave numbers separated by finite gaps coexist and each of them changes continuously with temperature accompanying the change in the population of the domains corresponding to the respective wave numbers. Assignment of some part of the wave number vs temperature curves to the stairs with fractional numbers afforded no positive evidence of the “devils staircase”.

Lattice Dynamics and Invar Properties in f.c.c. FeMn Alloy

Using inelastic neutron scattering the phonon dispersion relations in a single crystal of face centered cubic Fe 0.7 Mn 0.3 have been determined at several temperatures above and below the Neel temperature of 412 K. The shifts of the frequencies of the [110] T 1 mode below T N have been observed, which eventually gives the elastic softening similar to the previous observation in the typical ferromagnetic Invar of Fe 0.65 Ni 0.35 or Fe 0.72 Pt 0.28 . Lattice dynamical features in this material are discussed with respect to the Invar characters in the f.c.c. FeMn alloys.