Keisuke Tanaka

Roles of Cation and Anion Molecules in Ferridistortive Phase Transition in [A(CH_3)_4]_2XBr_4 Type Crystals

In order to clarify the roles of cation and anion molecules in the ferridistortive phase transition, four kinds of single crystals of [A(CH 3 ) 4 + ] 2 XBr 4 2- type, where cat-ion sites are occupied either by tetramethylphosphonium ion P(CH 3 ) 4 + or by tetramethylammonium ion N(CH 3 ) 4 + , and anion site either by tetrabromocobaltate ion CoBr 4 2- or by tetrabromomanganate ion MnBr 4 2- , have been grown. The temperature dependence of the spontaneous shear strain x 5 s ( T )=(π/ 180° )(β( T )-90° ), where β( T ) is the monoclinic angle, has been measured from 420 K down to 20 K. The substitution for cations is found to cause a major change in the temperature dependence of x 5 s ( T ), while the substitution for anion only a minor one. The results are explained on the basis of a ferridistortive model whose spontaneous strain consists of two kinds of sublattice strains. It is concluded that cations A(CH 3 ) 4 + play a primary role in the ferridistortive phase transition in [A(CH 3 ) 4 + ] 2 XBr 4 2- typ...

Ferridistortive Phase Transitionin [N(CH3)4]2CoBr4 Crystals

The orthorhombic-monoclinic phase transition in [N(CH 3 ) 4 ] 2 CoBr 4 crystals has been studied by measuring static dielectric constants along the a, b and c axes in the wide range of temperature from 370 K down to 10 K including the transition temperature T c =287.0 K. Anomalous parts of dielectric constants due to the phase transition are found to show a broad maximum around 70 K for b plate samples, while a shallow minimum around 120K for c plate samples.The phase transition is regarded as the ferridistorsive phase transitionThe orthorhombic-monoclinic phase transition in [N(CH 3 ) 4 ] 2 CoBr 4 crystals has been studied by measuring static dielectric constants along the a , b and c axes in the wide range of temperature from 370 K down to 10 K including the transition temperature T c =287.0 K. Anomalous parts of dielectric constants due to the phase transition are found to show a broad maximum around 70 K for b plate samples, while a shallow minimum around 120 K for c plate samples. The phase transition in [N(CH 3 ) 4 ] 2 CoBr 4 is regarded as the ferridistortive phase transition in which the spontaneous shear strain consists of two kinds of sublattice strains with opposite signs and different magnitudes.

Domain structures at ferridistortive phase transitions in [N(CH3)4]2ZnBr4 and related crystals

Abstract Ferridistortive phase transition in [N(CH3)4]2ZnBr4 crystal has been studied by a polarized microscope. Ordinary domain structures with a-domains which appear below Tc=287 K in the absence of external stress have been found not to vanish at the compensation temperature Tz=123 K where the monoclinic angle β happens to be 90°, but remain the same as those above and below Tz. Square domains surrounded with two kinds of walls which are parallel to the a-axis and the c-axis can be created below Tc by external stress. Furthermore the extinction condition of a-wall of square domains has been found to show a distinct change just at Tz. This is the first observation of the compensation temperature Tz by a polarized microscope.

Optical and X-ray studies of ferridistortive phase transition in [N(CH3)4]2ZnBr4 crystal

Abstract The orthorhombic-monoclinic phase transition at Tc=287K in [N(CH3)4]2 ZnBr4 crystals has been studied with an X-ray diffractometer and an optical polarizing microscope. Temperature dependence of the spontaneous shear strain x5s=(π/180)(β-90°) has been measured in the wide range of temperature from Tc down to 44K, and found to take positive values just below Tc, and after taking a maximum value gradually decrease to zero at Tz=123K, and take negative values below Tz. The rotation angle Φ(T) of optical indicatrix around the b-axis rapidly increases just below Tc, and shows a change of slope around 230K. The rotation angle does not vanish, but takes a finite value of about 33° at the compensation temperature Tz. It is shown that the results can be explained on the basis of a ferridistortive model in which spontaneous strain consists of two kinds of sublattice strains with opposite signs and different magnitudes.

Dielectric study of ferridistortive phase transition in [N(CH3)4]2MnBr4 crystals

Abstract The orthorhombic - monoclinic phase transition at Tc=277. 1K in [N(CH3)4]2MnBr4 crystals have been studied by measuring dielectric constant along the a, b, and c-axes in the wide range of temperature from 350K to 10K. The anomalous parts of the dielectric constants due to the phase transition are found to show very anomalous temperature dependence. In particular, the anomalous part of the dielectric constant along the c-axis shows broad maximum around 190K and shallow minimum around 120K. The results can be explained on the basis of the two sublattice ferridistortive model. [N(CH3)4]2MnBr4 is regard as ferridistortive crystal whose spontaneous shear strain consists of two kinds of sublattice strains with the opposite signs and the different magnitude.

Ferroelasticity in [P(CH3)4]2CoBr4 Single Crystals

The phase transition at 369.4 K in [P(CH 3 ) 4 ] 2 CoBr 4 crystals has been studied using a polarizing microscope and by dielectric measurement. Domain patterns are observed on b -plate samples and are found to move under an external stress at room temperature. This shows that [P(CH 3 ) 4 ] 2 CoBr 4 crystal is ferroelastic. The phase transition is found to be of the first-order, since the phase front between the high- and low-temperature phases is observed exactly at T c . Values of the rotation angle φ( T ) of the optical indicatrix from the a -axis about the monoclinic b -axis are almost constant at 44° from room temperature to near T c , but discontinuously drop to zero at T c . As temperature is lowered, the dielectric constant e a ′ ( T ) along the a -axis shows a discontinuous drop at T c , and then a gradual decrease below T c . The phase transition in [P(CH 3 ) 4 ] 2 CoBr 4 crystal is discussed in comparison with that in [N(CH 3 ) 4 ] 2 CoBr 4 crystal.

Optical study of strain distribution in the vicinity of ferroelastic domain walls in Pb3(PO4)2 crystals

Abstract Intensity distribution of the transmitted light in Pb3(PO4) crystals has been observed by CCD camera and analyzed by an image processor. It is found that there exist small regions with the stronger intensity in the vicinity of domain walls of needle shaped domains.

Ferroelastic Phase Transition in [P(CH 3 ) 4 ] 2 MnBr 4 by EPR of Mn 2+ Ions

This paper presents temperature changes of the EPR lines in [P(CH 3 ) 4 ] 2 MnBr 4 in the vicinity of the ferroelastic phase transition temperature T c =376.5K. Significant changes in the EPR spectrum have been observed in the temperature range of 370-380K. It is suggested that the changes observed above T c are caused by rotations of the MnBr 4 groups loosely situated in the crystal lattice, whereas changes below T c are a direct result of rebuilding of the ferroelastic domain structure.

Optical study of domain structures in ferrielastics

Abstract Using an image analysis system we have analyzed square domains observed in ferrielastic [N(CH3)4]2ZnBr4 crystal whose spontaneous strain X5s =(π/180)(βbT-90) changes sign at the compensation temperature Tz. It is found that the intensity distributions around the a-wall surrounding square domains change as the temperature changes from above the compensation temperature Tz to below Tz.

Ferroelastic phase transition in [A(CH3)4]2 MBr4 crystals by EPR

Abstract Single crystal of tetramethylophosphonium tetrabromomanganate [P(CH3)2]4MnBr4 was studied by EPR Two inequivalent Mn2+ centers are observed in the paraelastic phase. Below Tc = 376.5 K the EPR spectrum consists of three unresolved lines reflecting the domain structure of the crystal.