Minoru Sumita

New Phase Transitions of Monoclinic RbD_2PO_4 and Its Forced Ttansition to the Ferroelectric State

Dielectric and dilatometric properties of monoclinic RbD 2 PO 4 have been studied. The crystal undergoes successive phase transitions accompanied with a large peak of the dielectric constant along the unique b axis e b at 44°C ( T II ), and a change in temperature gradient of e b -1 at 104°C ( T I ) respectively. The phase below T II is antiferroelectric or approximately antiferroelectric. The forced transition to a ferroelectric phase is observed below T II as a double hysteresis loop. The critical electric field to cause the forced transition and spontaneous polarization of a sublattice are about 12 kV/cm and 1.8 µC/cm 2 at 30°C respectively.

Dielectric Relaxation in Ferroelectric MASD, CH3NH3Al(SO4)2·12H2O

The complex dielectric constant of methylammonium aluminum sulfate dodecahydrate, CH 3 NH 3 Al(SO 4 ) 2 ·12H 2 O, was measured at various frequencies in a frequency range between 1×10 6 and 1×10 9 Hz in its para- and ferroelectric temperature regions. The experimental result was analyzed for the paraelectric phase. The crystal exhibits critical slowing-down process of polarization. The observed temperature and frequency dependences of the complex dielectric constant can be described to good approximation in terms of a single relaxation time τ( T ) of Debye type. Above the Curie point ( T c ), the static dielectric constant follows the Curie-Weiss law (( e 0 - e ∞ )∼( T - T 0 ) -1 ), while τ( T ) is described well by the relation τ( T )=α·( T - T 0 ) -1 ·exp ( Δ U / k B T ) with the potential barrier height Δ U =(2.18±0.06) kcal·mol -1 and α=(0.92±0.14) ( h / k B ).

X-Ray Diffraction Study of Monoclinic RbD2PO4

The crystal structure of monoclinic RbD 2 PO 4 at room temperature was determined by use of an automatic four-circle X-ray diffractometer, and the super-lattice structure was examined. The crystal,...

Ferroelectricity and Dielectric Relaxation in NH4Al(SO4)2·12H2O

The pyroelectric charge of NH 4 Al(SO 4 ) 2 ·12H 2 O was measured after poling the crystal. It was found that the crystal becomes ferroelectric below the Curie temperature of T c =-197°C on warming and has spontaneous polarization of 0.01 µC/cm 2 at -240°C. The complex dielectric constant was measured at various frequencies in a frequency range between 330 Hz and 1×10 6 Hz. The crystal exhibited dielectric relaxation and showed a critical slowing down process of polarization. The observed temperature and frequency dependences of the complex dielectric constant can be well approximated in terms of single relaxation time τ( T ) of the Debye-type. Above the Curie temperature, τ( T ) was well described by the relation τ( T )=( h / k B )·( T - T 0 ) -1 ·exp ( Δ F / k B T ) with the activation free energy Δ F = Δ F 0 [1-α( T - T c )]. The values of Δ F 0 and α were estimated to be 1.49×10 -20 and 0.0028 deg -1 , respectively.

Isotope Effect of Monoclinic Rb(D1-xHx)2PO4 on the Phase Transitions

Dielectric constant along the b axis of crystals in monoclinic Rb(D 1- x H x ) 2 PO 4 system was measured in a temperature range from -190°C to 160°C. Phase diagram of the system obtained in a range between x =0 and 0.8 shows a relatively large isotope effect on the III-II phase transition with a shift of its transition temperature to higher temperature for the decrease of x , and a small isotope effect on the II-I phase transition but with a shift of its transition temperature to lower temperature for the decrease of x .

ESR Study of Manganese Complexes in Heavily Doped CaF2 Crystals

In heavily manganese doped CaF 2 , three kinds of Mn 2+ centers and exchange coupled pairs other than the usual Mn 2+ centers in cubic crystal fields were found at room temperature. The spin Hamiltonian parameters are (I) g =2.00, | D |=740(G), E =36(G), A =80(G), a s =12(G) and a p =2∼3(G); (II) g =2.00, | D |=938(G), E =20(G), A =67(G), a s =11.2(G) and a p =1∼2(G); (III) g =2.00, | D |=1250(G), E =0(G), A =64(G) and a [111] =15.1(G), respectively.

Electron spin resonance study of laser-annealed (Zn,Mn)O ceramics

Investigations have been made on the effects of pulse laser annealing on the ceramics (Zn,Mn)O by means of electron spin resonance (ESR) measurements. A remarkable change in the ESR spectrum was observed after annealing by a Nd:YAG pulse laser (λ=1.064 μm, 5–7 J/cm2). It suggests that segregated Mn2+ precipitates at grain boundaries in ZnO are diffused into crystal grains by preferential heat conduction through the boundaries. The scanning electron microscope observation verified enough melting features within the ceramics.

ERR of Off-Center Eu2+ Ions with Resolved Structures in RbCl

In RbCl: Eu 2+ crystals, there are found three groups of EPR spectra with an intensity ratio of 2: 1: 1, which are attributed to off-center displacements of Eu 2+ ions along [111] axis from normal lattice sites. The spectra of the off-center Eu 2+ ions can be described by combining the spin Hamiltonian in an orthorhombic field and the perturbation terms depending on the magnetic field as follows; \(\mathscr{H}{=}g\beta\textbf{\itshape HS}+\sum_{l{=}1}^{3}\sum_{m{=}0}^{l}\text{B\)_2l^2m O _2l^2m\(}+\textbf{\itshape AIS}+u\beta[S_{x}^{3}H_{x}+S_{y}^{3}H_{y}+S_{z}^{3}H_{z}-\frac{1}{5}(\textbf{\itshape SH})[3S(S+1)-1]]+U[S_{x}^{3}I_{x}+S_{y}^{3}I_{y}+S_{z}^{3}I_{z}-\frac{1}{5}(\textbf{\itshape SI})[3S(S+1)-1]]\), where g =1.995, b 2 0 =294.6, b 2 2 =-139.8, A 151 =-31.3, u =-0.2∼0.9, U =0∼-0.05 (10 -4 cm -1 ) at room temperature. The displacement is estimated from the angles between the principal axes and [110] directions to be 0.082 a ( a : anion to cation distance), and the induced dipole moment to be 2.57 ...

ESR study of europium ions in hydrolyzed CaF2: Eu crystals

Abstract Oxidation and reduction processes of europium ions in CaF 2 were studied by optical and ESR methods. Europium ions can be readily oxidized from the divalent to the trivalent states by heat-treatment above 600°C in a moist air atmosphere. The conversion can be well approximated by first order kinetics with an activation energy of 0.75 eV. The reverse conversion from the trivalent to the divalent states can be accomplished by exposure to ionizing X-ray radiation. However, those reduced europium ions are thermally unstable and bleached completely by heating to 200°C. The decay process by low temperature annealing also follows first order kinetics with an activation energy of 0.60 eV. The conversions between the two states are explained by electron transfer.

Electron spin resonance studies on locally additive low symmetry distortions in the mixed-crystal Sr1 − xBaxF2 system monitored with Eu2+ dopants

Abstract Local distortions in mixed crystals or solid solutions Sr 1 − x Ba x F 2 have been investigated by monitoring a doping Eu 2+ ion by means of electron spin resonance measurement. With increasing mixture ratios x near 0, the resolved hyperfine linewidths broaden in sequence from larger | M | spin magnetic quantum numbers to smaller values for the M → M − 1 transition. This tendency also holds on decreasing the mixture ratio x near 1. As a source of these characteristic broadening effects, distortion compensation for lattice misfit has been considered to be made at local sites in the mixed crystal by adding the second-order low symmetry crystal fields of B 2 0 followed by B 2 2 . It was found that the observed line broadening effects would be reasonable if the sum of the B 2 0 term and the second-order contribution term of B 2 2 was distributed over the region from −2.75 to 1.75 G, in which it corresponded to the displacements of fluorine ions to be detectable with the high sensitivity below 0.4% of the lattice parameter of SrF 2 .