Kazuo Gesi

Dielectric properties and structural phase transitions in TMMnC (N(CH3)4MnCl3) type crystals

Abstract Dielectric properties have been studied for single crystals of various TMMnC (N(CH3)4MnCl3)-type compounds; TMMnC, TMMnB, TMNiC, TMNiB, TMCdC, TMCdB, and TMCuC, Dielectric constants show stepwise anomalies at the structural phase transitions of these compounds. In TMCdB, ferroelectricity is found below the Curie temperature of about 156 K (on cooling). For other compounds, no ferroelectricity nor pyroelectricity has been detected in the low temperature phases.

Effect of hydrostatic pressure on the structural phase transitions in CH3NH3PbX3 (X = Cl, Br, I)

Abstract The dielectric properties and the effect of hydrostatic pressure on the successive phase transitions have been studied for single crystal CH3NH3PbX3 (X: Cl, Br, I). Anisotropic anomalies in dielectric constant at phase transitions are obtained. The pressure (p)-temperature (T) phase diagrams are shown for the three CH3NH3PbX3 crystals. The intermediate tetragonal phase of CH3NH3PbCl3 and the lower-temperature tetragonal phase (space group: P4/mmm) in CH3NH3PbBr3 become narrower as pressure increases and disappear at around 100 MPa. There appears a pressure-induced phase above about 360 MPa and 90 MPa in CH3NH3PbBr3 and CH3NH3PbI3, respectively. On the other hand, CH3NH3PbCl3 does not show a pressure-induced phase up to about 700 MPa and the p-T phase diagram is different from those of the Br- and I-salts. A common reduced p-T phase diagram is proposed for CH3NH3PbBr3 and CH3NH3PbI3 and discussed.

Temperature dependence of lattice constants of Rb3H(SeO4)2, a ‘zero-dimensional’ h-bonded crystal

Abstract In view of the interest in the ‘zero-dimensional’ H-bond network and low transition temperature in M3H(XO4)2-type crystals, lattice constants of Rb3H(SeO4)2 have been measured in a temperature range between room temperature and 20 K using an automatic X-ray 4-circle diffractometer with a closed-cycle refrigerator in order to confirm whether it does not exhibit any phase transitions. With lowering temperature, a-, b-, and c-axes in monoclinic cell showed a monotonous decrease, while β decreased very little, with no remarkable changes suggesting phase transitions. These results raise a fundamental problem, i.e. why Rb3H(SeO4)2 does not exhibit phase transition although the crystallographically symmetric H-bond of 2.514(7) A lies in a range where disordering of the protons will almost certainly occur.

Effect of hydrostatic pressure on the phase transitions in tetraethylammonium tetrahalogenometallic compounds

Abstract The effect of hydrostatic pressure on the structural phase transitions has been studied for the tetraethylammonium tetrahalogenometallic compounds, {N(C2H5)4}2-XY4 (XY4 = ZnCl4, ZnBr4, CoCl4, CoBr4, MnBr4, and CuBr4) by dielectric measurements. The transition temperatures increase linearly with increasing pressure. The observed pressure coefficients of the transition temperatures are ranging from 0.17 K/MPa to 0.34 K/MPa, which are comparable with those reported previously for the ferroelastic tetramethylammonium tetrahalogenometallic compounds, {N(CH3)4}2XY4.

Low temperature phase transition in K2ZnCL4 at 146 K

Abstract The heat capacity of K 2 ZnCl 4 has been measured between 10 and 300 K using an adiabatic calorimeter. The low temperature phase transition is strongly influenced by the small amount of water remaining in the crystals which were grown from an aqueous solution. A sharp heat capacity anomaly is observed at 146.24 K for the sample dried under vacuum above 100°C for 30 h, and the enthalpy and entropy of transition are 57.16J mol −1 and 0.387 J K −1 mol −1 , respectively.

Dielectric properties of N(CH3)4CuCl3 (TMCuC)

Abstract Single crystals of N(CH3)4CuCl3 (TMCuC) were grown, and the dielectric properties have been studied in a temperature range below about 320 K. There are three phase transitions at 313 K, 260 K, and 200 K. The dielectric constant along the pseudo-hexagonal c-direction shows a λ-type maximum at the 200 K transition. The crystal shows pyroelectricity along the c-direction below 313 K. The pyroelectric charge is partially reversed by reversing the polarizing electric field of 5.6 × 105 V/m suggesting that the crystal is ferroelectric below the 313 K transition.

Effect of hydrostatic pressure on the phase transitions in {N(CH3)4}2ZnI4, {N(CH3)4}2CdI4, {N(CH3)4}2HgCl4, AND {N(CD3)4}2MnCl4

Abstract The effect of hydrostatic pressure on the phase transitions in {N(CH3)4}2XY4 (XY4 = ZnI4, CdI4, HgCl4) and in a deuterated compound of {N(CD3)4}2MnCl4 have been studied. The intermediate monoclinic phase in the -ZnI4 compound narrows as pressure increases, and disappears at 176 MPa. The transition temperatures in the -CdI4 and -HgCl4 compounds increase linearly with increasing pressure with the rates of 0.22 K/MPa and 0.20 K/MPa, respectively. The pressure-temperature phase diagram of {N(CD3)4}2MnCl4 is similar to that of the hydrogen compound. The appearance of the dielectric-constant peak in the incommensurate phase shifts towards about 35 MPa higher pressure by deuteration.

Effect of Hydrostatic Pressure on the Phase Transitions in [N(C 2 H 5 ) 4 ] 2 CuCl 4

The phase transitions in tetraethylammonium tetrachlorocuprate, [N(C 2 H 5 } 4 ] 2 CuCl 4, have been investigated by dielectric measurements. At normal pressure the I-II transition at 259 K is of the first order. On the other hand, at the II-III transition (at 199 K) the dielectric constant shows a diffuse step-like anomaly and no discontinuous jump is observed. The pressure ( p )--temperature ( T ) phase diagram was determined. Both the I-II and the II-III transition temperatures nonlinearly increase with increasing pressure. The initial pressure coefficients of the I-II and II-III transition temperatures are 0.11 K/MPa and 0.15 K/MPa, respectively. The II-III transition has been reported to be isomorphous transition, but the critical point is not induced by application of pressure. The nature of the transitions at high pressure is discussed.

Study of phase transitions in some TMMnC-type crystals by AC calorimetry

Abstract Phase transitions have been studied by AC calorimetric measurements for N(CH3)4MnBr3 (TMMnB), N(CH3)4NiCl3 (TMNiC), and N(CH3)4NiBr3 (TMNiB), which commonly have the hexagonal P6 3 m structure (Z = 2) at room temperature. Phase transitions accompanied with λ-type anomalies of heat capacity are observed at 384 K and 141 K for TMMnB, 433 K and 164 K for TMNiC, and 430 K, 135 K, and 97 K for TMNiB. The phase transitions above room temperature are newly detected for these crystals. The present study confirms the existence of an extra phase transition at 97 K in TMNiB which has been suggested previously by dielectric-constant measurements.

Effect of hydrostatic pressure on the phase transitions and dielectric properties of {(CH3)2NH2}5CD3CL11

Abstract The effect of hydrostatic pressure has been studied for the phase transition and dielectric properties of {(CH3)2NH2}5Cd3Cl11. The maximum value of the dielectric constant peak, which exists at T 1 = 280 K at 0 pressure, decreases with increasing pressure obeying a Curie-Weiss like law with the Curie-Weiss pressure of about −860 MPa. This implies that the material becomes ferroelectric at this hypothetical negative pressure. As pressure increases the first order phase transition point Tc , which exist at 127 K at 0 MPa, first decreases with dTc /dp = − 0.064 K/MPa, and then it turns to increase with dTc /dp = + 0.25 K/MPa above about 100 MPa. This indicates that there is a pressure-induced low-temperature phase above about 100 MPa.