Tomoyuki Hikita

Phase Transitions in Mixed Crystal System K1-x(NH4)xH2PO4

Mixed crystals of potassium dihydrogen phosphate and ammonium dihydrogen phosphate (K 1- x (NH 4 ) x H 2 PO 4 ) have been investigated. Dielectric constants e 11 and e 33 and elastic compliance ( s 45°- Z ) on the 45°– Z -cut bars were measured. The crystals with \(0{\leqq}x{\lesssim}0.17\) show temperature dependence of the dielectric constant e 33 and elastic compliance s 45°- Z similar to that of pure KH 2 PO 4 . Occurrence of the spontaneous polarization in these crystals was observed by pyroelectric charge method. The crystals with \(0.85{\lesssim}x{\leqq}1\) show dielectric and elastic properties similar to those of pure NH 4 H 2 PO 4 , except for weak rounding-offs in the anomalies of dielectric constant for smaller x . It was confirmed by X-ray diffraction that the crystal with x =0.85 undergoes an antiferroelectric phase transition. The crystal with x =0.25 shows broad and round-off anomalies in e 33 and s 45°- Z similar to the temperature dependence of e 33 in Rb 0.66 (NH 4 ) 0.34 H 2 PO 4 cryst...

Phase Transitions in Some Langbeinite-Type Crystals

Phase transitions were searched in five langbeinite-type crystals in the temperature range 20∼-196°C. Dielectric and elastic anomalies were found at -72°C in a single crystal of K 2 Mn 2 (SO 4 ) 3 . Pyroelectric measurements showed that the low temperature phase of K 2 Mn 2 (SO 4 ) 3 is not polar. Dielectric and pyroelectric measurements were made on the polycrystalline samples of the solidified molten K 2 Zn 2 (SO 4 ) 3 and K 2 Cd 2 (SO 4 ) 3 . It was found that K 2 Zn 2 (SO 4 ) 3 undergoes pyroelectric phase transition at -137°C, but K 2 Cd 2 (SO 4 ) 3 does not show any phase transition. It was also found that K 2 Mg 2 (SO 4 ) 3 and Rb 2 Mg 2 (SO 4 ) 3 of pressed powder crystals do not show any phase transition signal.

Anomalous x dependence of Tc and possibility of low-temperature structural phase transition in La2−xSrxCu0.99M0.01O4 (M Ni, Zn, Ga)

Abstract Effects of the 1% substitution of Ni, Zn and Ga for Cu on the x dependence of T c have been investigated in La 2−x Sr x CuO 4 . The T c increases by the Cu-site substitution in all compositions of x. The local minimum of T c as a function of x, appearing slightly near x = 0.115 for La 2−x Sr x CuO 4 , becomes conspicuous with decreasing T c . However, structural phase transition from the orthorhombic mid-temperature phase to the tetragonal low-temperature phase has not been detected from powder X-ray diffraction measurements at low temperatures down to 12 K. The local minimum of T c is attributable to the fluctuation of the undiscovered structural phase transition, which will occur at a low temperature below 12 K as long as samples are in the normal-conducting state. In the 1% Ga-substituted system for Cu, the local minimum clearly shifts from x = 0.115 to 0.125. This suggests that the undiscovered low-temperature structural phase transition is due to the optimum hole concentration of 0.115 per Cu atom, analogously to La 2−x Ba x CuO 4 .

Effect of Hydrostatic Pressure on the Phase Transitions of Langbeinite-Type Crystals

Effect of hydrostatic pressure up to 9 kbar has been studied by dielectric measurements on the six langbeinite-type crystals, Tl 2 Cd 2 (SO 4 ) 3 , Rb 2 Cd 2 (SO 4 ) 3 , K 2 Zn 2 (SO 4 ) 3 , K 2 Mn 2 (SO 4 ) 3 , Tl 2 Mn 2 (SO 4 ) 3 and (NH 4 ) 2 Mn 2 (SO 4 ) 3 . The pressure-temperature phase diagrams were deternained. In the former two crystals, Tl 2 Cd 2 (SO 4 ) 3 and Rb 2 Cd 2 (SO 4 ) 3 , a new phase was found between the P1 and P2 1 2 1 2 1 phases at high pressure. In the crystal K 2 Zn 2 (SO 4 ) 3 another phase appeared between the P2 1 3 and P2 1 phases, and the behavior is similar to that in (NH 4 ) 2 Cd 2 (SO 4 ) 3 . The P2 1 3→P2 1 2 1 2 1 transition temperature in K 2 Mn 2 (SO 4 ) 3 linearly rises with increasing pressure. The last two crystals, Tl 2 Mn 2 (SO 4 ) 3 and (NH 4 ) 2 Mn 2 (SO 4 ) 3 do not show any transition down to -196°C even under high pressure studied.

Structural Study of Phase Transition in K2Co2(SO4)3 Crystals

Crystal structure of K 2 Co 2 (SO 4 ) 3 was determined at nine temperatures in the high-temperature cubic phase. In contrast to the early structural study, the split-atom calculation concludes that the SO 4 ion has a number of equilibrium arrangements. Mean-square atomic displacements become large with approaching the phase transition temperature despite no soft mode has been found. Atomic distances between Co and O are fairly shorter than the normal distance in the average structure. We explain these results by assuming that the nuclei of the low-temperature phase have already been generated in the cubic phase.

Glassy dielectric behavior in the mixed crystal system K1-x(NH4)xH2PO4

Abstract Low frequency dispersion of the dielectric constants in the mixed crystals K1-x(NH4)xH2PO4 [x=0.17, 0.23 and 0.85] was measured. Glassy behavior which is similar to that in the rubidium equivalent was found in the crystal:x=0.23.

Rayleigh-Brillouin Scattering in NH4Cl under Hydrostatic Pressure

Rayleigh-Brillouin scattering in NH 4 Cl has been studied under hydrostatic pressure. Brillouin shifts of the longitudinal phonons propagating along the and directions and of the transverse phonons propagating along the direction are measured as a function of temperature at four fixed pressures up to 2.3 kbar. All components increase almost linearly with decreasing temperature in the disordered phase and show incremental increases similar to the temperature dependence of the long range order parameter in the ordered phase. It was found that the lonsitudinal modes exhibit large despersion effect even at high pressures. On the other hand, the transverse mode appears to give no dispersion effect. The intensity of the Rayleigh component in the VV polarization condition increases as the temperature approaches the transition point, while that in the VH polarization does not show such anomalous increase.

Brillouin Scattering Study of Ferroelectric (NH4)2BeF4

Brillouin scattering study was made on the ferroelectric (NH 4 ) 2 BeF 4 over the temperature range –150°C∼20°C, and the temperature dependence of elastic constants c 44 , c 55 and c 66 was determined.

Pressure-induced quantum ferroelectricity in tris-sarcosine calcium bromide ((CH3NHCH2COOH)3CaBr2)

Dielectric properties of tris-sarcosine bromide ((CH 3 NHCH 2 COOH) 3 CaBr 2 ) which is isomorphous to the ferroelectric crystal tris-sarcosine calcium chloride ((CH 3 NHCH 2 COOH) 3 CaCl 2 , TSCC) have been investigated under high pressure down to 4.2 K. The hydrostatic pressure of excellent quality has been generated using helium gas as a pressure transmitting medium. The ferroelectric activity has been confirmed by the dielectric hysteresis loops. The pressure-temperature phase diagram of the ferroelectric phase transition is determined from the peak of the dielectric constant of the b -plate in various isobaric measurements. The transition temperature is well represented by an equation T C ∝( p - p C ) 1/2 which is a well known relationship for quantum ferroelectrics.

Mössbauer Spectroscopy of Langbeinite-Type K2Fe2(SO4)3

The langbeinite-type K 2 Fe 2 (SO 4 ) 3 was synthesized. The temperature dependence of the dielectric constant shows a clear anomaly at about 130 K. Mossbauer spectrum of 57 Fe was recorded in the temperature range from 15 to 290 K. The spectrum at all temperatures consists of two doublets which are analyzed with isomer shifts and quadrupole couplings. The quadrupole splitting of one site is 3.00 mm/s at 290 K and shows only 10% increase down to 15 K. The other one is 0.82 mm/s at 290 K and shows a drastic increase, resulting at 15 K in 2.5 times that at 290 K. The two 57 Fe sites for the electric field gradient were assigned to the two crystallographically inequivalent iron sites compairing the two octahedral oxygen environments of the iron ions. The broad background spectrum is interpreted as the distribution of the electric field gradient due to the multi-positional distribution in SO 4 groups proposed by Itoh et al.