Experimental studies of structurally incommensurate crystal phases

Measurements were made of the dielectric constant along the ferroelectric axis e a , the spontaneous polarization P s and the heat capacity c p of Rb 2 ZnCl 4 crystals in the temperature range from -150^° C to 150°C. The normal-incommensurate transition point determined as the peak position of c p lies at 30.1°C, which is about 6°C lower than the temperature for the maximum of e a . It was found that e a and P s show a characteristic thermal hysteresis over a wide temperature range including the incommensurate-commensurate trarasition point (-80^° C ). The thermal hysteresis occurs even when heating and cooling runs are cycled within the incommensurate phase. The observed thermal hysteresis is understood if we assume that defects such as impurities act as obtacles to diffusion of discommensurations and prevent the crystal from reaching thermal equilibrium.

Critical Phenomena and Anomalous Thermal Hysteresis Accompanying the Normal-Incommensurate-Commensurate Phase Transitions in Rb 2 ZnCl 4

Control of the ferroelectric properties of Dy Mn 2 O 5 by magnetic fields

Electron paramagnetic resonance (EPR) is a well-established technique for investigating the properties of metal ions in crystals, chemical complexes and biological molecules. It provides information about electronic structure and the site symmetry or nature of the environment of metal ions. The authors review the historical development of low-symmetry ideas and their theoretical basis as its major uses have been restricted to metal ions in sites of high symmetry. Major experimental results are tabulated and the discussion includes some of the attempts to interpret low-symmetry EPR data using crystal field and molecular orbital models. It is concluded that the role of excited-state orbitals, under the influence of low-symmetry fields, is largely responsible for the different orientations of the principal directions of the various interactions experienced by a paramagnetic ion.

http://iopscience.iop.org/article/10.1088/0034-4885/43/4/002/pdf

Low-symmetry effects in electron paramagnetic resonance

In the ferroelectric phase below T 3 (93 K) of K 2 SeO 4 a new soft-mode, belonging to the B 2 representation, has been observed by means of Raman scattering experiment. This mode is different from the soft A 1 mode previously reported to soften as T 2 (129.5 K) was approached from below. The frequency of this newly observed mode is abotut 13.2 cm -1 at 9.8 K, and decreases as the temperature increases. The observed result is discussed in connection with the successive phase transitions in the K 2 SeO 4 crystal.

The Lower Frequency Soft-Mode in the Ferroelectric Phase of K2SeO4

The behaviours of the various dielectric properties and specific heat of potassium selenate, K 2 SeO 4 , have been studied with particular reference to the transitions at 129.5°K and 93°K. Below 93°K the crystal is ferroelectric along the c axis. The spontaneous polarization and coercive field determined from the hysteresis loop are 6.5×10 -8 coul/cm 2 and 60 V/cm respectively at 80°K. The curve of dielectric constant vs temperature obeys the Curie-Weiss law above the Curie point and the Curie constants for warming and cooling processes are 27°K and 30°K respectively. The Curie temperature varies almost linearly with the biasing field with the coefficient of 0.19°/kV/cm. Effects of the γ-radiation on the dielectric behaviour are observed. Discontinuity of the specific heat of about 2.3 cal/mol·deg has found at the transition of 129.5°K. Anomaly at the transition of 93°K is very small. The transition energy and the entropy change are estimated to be of the order of 0.5 cal/mol and 5×10 -3 cal/mol·deg of le...

Dielectric and Thermal Study of K2SeO4 Transitions

EPR spectra of KDP (KH 2 PO 4 ):Cr 3+ have been studied to reveal mech-anism of the ferroelectric and ferroelectric phase transition. There are two kinds of Cr 3+ ion sites in the para phase and these sites are divided into two slightly different kinds of sites in the ferro phase which reflects the domain structure. An EPR sample holder with a uniaxial stress squeezer is designed; the domain switching by electric field and by uniaxial stress is observed. The Cr 3+ ions of site I substitutionally take the position of K + ions and the spin Hamiltonian parameters are g X = g Y = g Z =1.97, | D |=0.455 cm -1 , | E |=0.143 cm -1 , and D · E >0. The transition at site I is related to the second-order transition, on the other hand, the one at site II, the first-order transition. Therefore the first and second order transitions, which are anharmonically coupled to each other, are there. Dielectric, thermal and optical properties are also studied.

Tadashi Kobayashi

Papers

Dielectric and Thermal Study of K2SeO4 Transitions

Electron Paramagnetic Resonance Study of Potassium Di-Hydrogen Phosphate Doped with Cr 3+ Ions

The Electron Paramagnetic Resonance of K 2 SeO 4

Effects of Uniaxial Stress and Electric Field on Paramagnetic Spectra of Cr 3+ in KH 2 PO 4