Y. Noda

Successive Jahn-Teller transitions and the canted pseudospin state in K2PbCu(NO2)6

Abstract The successive phase transitions in K2PbCu(NO2)6 has been studied by precise X-ray measurements. The unit cell in Phase III is doubled along each of the cubic principal axes. The experimental results can be interpreted as sequencial cooperative Jahn-Teller transitions where the higher transition is due to the coupling with the uniform distortion, while the lower transition is caused by the coupling with the uniform distortion, while the lower transition is caused by the coupling with the zone boundary phonon mode with k 0 = ( 1 2 , 1 2 , 1 2 . The electronic state in Phase III is characterized as the “canted pseudospin” state.

‘Modulated lattice relaxation’ in β-based premartensitic phase

Recent X-ray studies on a TiNi(Fe) alloy by Shapiroet al. show that the incommensurability of satellite reflections changes from Brillouin zone to Brillouin zone, giving an overall swirl-like shift pattern in the reciprocal space, which precludes any simple incommensurate wave model. We have succeeded in reproducing the intriguing shift pattern observed, based on two simple assumptions: (i) existence of a dip in TA phonon dispersion atq = 1/3[110] and (ii) existence of embryos of low temperature martensite phase. The proposed features of the premartensitic phase suggest a possible microscopic mechanism of martensite transformation in general as well as of w-phase formation. fa]This paper is based on a presentation made in the symposium “Pretransformation Behavior Related to Displacive Transformations in Alloys” presented at the 1986 annual AIME meeting in New Orleans, March 2–6, 1986, under the auspices of the ASM-MSD Structures Committee.

Successive jahn-teller phase transitions in Cs2PbCu(NO2)6

Abstract The sequential Jahn-Teller phase transitions in Cs 2 PbCu(NO 2 ) 6 were studied by neutron and X-ray diffraction technique. In each phase we observed satellite reflections. The position of satellites in the reciprocal space is commonly given by k 0 = ( 1 2 1 2 ζ) with 0⪕ζ⪕0.5. In Phase II and Phase IV, the values of ζ are definitely determined as ζ = 0 and ζ = 0.5 respectively. In Phase III, ζ is not definite within 0.20⪕ζ⪕0.25 depending on samples, thermal cycling,etc. Phase III may be an incommensurate phase although there is the tendency that the value of ζ is locked in either at 0.20 ( 1 5 ) or at 0.25 ( 1 4 ).

Incommensurate Jahn-Teller transition in K2PbCu (NO2)6

Abstract We report the results of a structural investigation of the intermediate phase of K2PbCu(NO2)6 by neutron scattering technique. We have observed satellite reflections at (H±0.416, K±0.430, 0) around each fundamental reflections. This clearly indicates that the crystal has the “incommensurate” structure in this phase. Our tentative model structure in this phase is that the sinusoidally modulated local Jahn-Teller distortion propagates along [110] with the wave vector |k| ≈ 0.42. In terms of pseudospin description, this phase is characterized as a “fan pseudospin” structure.

Neutron diffuse scattering in Fe3O4 due to molecular polarons

Abstract The results of a precise re-examination of neutron diffuse scattering above the Verwey transition of Fe3O4 are reported. A model to describe the properties of valence fluctuations at the B-site Fe-ions are proposed, which assumes the existance of “molecular polarons”. The experimental results of anisotropic distribution of diffuse scattering seem to give an evidence for the feasibility of the proposed model.

Characteristics of Chromium Oxycarbide Thin Film Prepared by Plasma‐Assisted CVD Method

The authors describe thin films containing chromium compound prepared onto SUS 304 stainless steel and slide glass substrates by plasma decomposition of hexacarbonylchromium with hydrogen carrier gas. The chemical composition of the films was identified as chromium oxycarbide (Cr/sub 2/CO) by XPS measurements, and the structure of Cr/sub 2/CO was revealed as a face-centered cubic structure by x-ray diffraction experiments. Vickers hardness and the anodic polarization current density of the films were measured in order to investigate the properties of corrosion resistance. As a result, the Cr/sub 2/CO film was found to have one of the best characteristics for coating the material to resist surface corrosion.

Critical behavior and scaling law in the kinetics of ordering process in Cu3Au alloy

Abstract The kinetics of ordering process in Cu 3 Au in the vicinity of its order-disorder transition point has been studied with x-ray scattering technique. The time variation of the line profile of a super structure reflection was observed by changing the annealing temperature, or the degree of thermodynamical instability of the initial state relative to the final state, which we call the degree of metastability. It was observed that the critical cluster size at which the ordered cluster starts to grow diverges as the degree of metastability is decreased towards zero. It is also shown that when the time is properly scaled depending on the degree of metastability the plotts of the integrated intensities versus the scaled time fall on a single universal curve, which may indicate the existence of a scaling law in the time development of the ordering process from a metastable state to the stable state.

Structural and Mechanical Properties of Cr2CO Films

Thin films of chromium oxycarbide(Cr2CO) were prepared on SUS 304 stainless steel and slide glass by plasma decomposition of hexacarbonylchromium with hydrogen carrier gas. The structure of the film was investigated in detail by X-ray diffraction. It was found that the structure of Cr2CO was fcc and that the (111) plane was highly oriented on the substrate surface. The characteristics of Vickers hardness, electric conductivity and deposition rate strongly correlated to the X-ray intensity of the particular Bragg scattering.

The Remarkable Incubation Time on the Order-Disorder Phase Transition of Mg3In

In a first order phase transition, the system transforms from a meta-stabl estate to a stable state, where the probability distribution function of the system in the phase space must ride over the barrier of the local free energy potential between the stable state and the metastable state associated with thermodynamical fluctuations. This stochastic process is of-ten expressed by anucleation-growth picture in a real space.

Quenching and the Ordering Process of Dipole Moments in K2Ba(N02)4

The dynamical properties of phase transitions of various kinds are subjected to theoretical and experimental studies during the past decades. The typical situation widely studied is that a system is rapidly quenched from a disordered phase to an ordered phase through over the first-order phase transition point1. In such a quenched system of the first-order transition, the system transforms from a metastable state to a stable state by riding over the barrier of thermodynamical potential between two local minima. In addition to the above nonlinearity of the potential in the system, the slow diffusion of atoms sometimes plays an important role to really observe the transition process by experiments, especially seen in the metal alloy system such as CU3Au 2–4 Ni3Mn 5 etc.