Phase transitions in Sr 0.61 Ba 0.39 Nb 2 O 6 :Ce 3+ : I. Susceptibility of clusters and domains

The complex dielectric susceptibility of Sr _0.61 Ba _0.39 Nb ₂ O ₆ :Ce ³⁺ (SBN61:Ce) has been measured at frequencies and temperatures before and after poling. The relaxor behaviour with large polydispersivity observed above the ferroelectric phase transition temperature, T _c = 360 and 340 K for x (Ce) = 0 and 0.0066, respectively, is perfectly modeled within the framework of Chamberlin's dynamically correlated domain approach. Below T _c the dynamic nanodomain state crosses over into a ferroelectric state with polydispersive domain wall dynamics at very low frequencies. Presumably SBN61:Ce belongs to the three-dimensional random field Ising rather than to the dipole glass universality class. Received 1 October 1999     

Non-cubic Ag− centers in mixed crystals of alkali halides

The optical properties of tetragonal Ag _¯B centers forming (AgK₄Na₂)⁵⁺ complexes in KCl∶Na are investigated. They are similar to those of Ag _ā centers, which consist of (AgK₅Na)⁵⁺ complexes. The additional Na⁺ ion in the Ag _¯B center causes an increase of the crystal field splitting of the excited states. In CsBr∶K the symmetry of Ag _ā centers proves to be trigonal as expected from the lattice structure of CsBr. This becomes evident from the dichroic absorption bands after polarized irradiation and from the angular dependence of the polarized emission. The energy levels of the new centers are calculated by use of Kojima's theory of the Ag _ā center.     

Optical transitions of Ag− centers in alkali halides

Ag^? centers in alkali halides give rise to a strong absorption band in the 300 nm region (formerly called “B band”). Its resolved triplet structure in CsCl suggests that it corresponds to the C band of the isoelectronic In⁺ center. Two very weak bands are found in several alkali halides in the 400 nm region. These new bands are assigned to the A and B transitions of the In⁺-type centers. This is supported by the doublet structure in the A band, and by the temperature dependence of the oscillator strength of the B band. In KCl∶Ag^? the ratios of the oscillator strengths are found to bef _c /f _A =610 andf _c /f _B =3,400 at low temperatures. The energy parameters of Ag^? centers are computed and compared with those of others ²-type centers. The electron-lattice coupling parameters are estimated from the Jahn-Teller splitting of the C band in CsCl and of the A band in KC1. The temperature dependence of the lifetime of the visible fluorescence suggests that a metastable state is involved in the emission process after a C band excitation.     

Polarization-based adjustable memory behavior in relaxor ferroelectrics

The irreversible decay of the spontaneous polarization above the phase-transition temperature is a limiting factor in any application of ferroelectric crystals. Here we show that electric fields applied at high temperatures induce a preferred direction in the crystal which is stable even after repeated heating and cooling through the phase transition. This preference in direction leads to a reorientation of domains in the ferroelectric phase. We use pyroelectric measurements to show that the directional preference originates from internal charge carriers interacting with domain walls.     

A New Method of Preparation of Ifosfamide and Cyclophosphamide; Synthesis of Side Products

Abstract

This study focusses on the preparation of ifosfamide (1; R¹=CH₂CH₂Cl, R²=NHCH₂CH₂Cl) and cyclophosphamide (2 R¹=H, R²=N(CH₂CH₂Cl)₂), standard drugs in tumor therapy, in order to avoid the alkylating educts like 2-chloroethylamine by introducing chlorine in the final reaction step. The reaction of the trimethylsilyl compounds (3; R¹=CH₂CH₂Cl, R²=NHCH₂CH₂0SiMe₃) and (4; R¹=H, R²=N(CH₂CH₂0SiMe₃)₂), respectively, with 2-chloro- 1,3,5-trimethyl-1,3,5-triaza-σ³λ₃-2-phosphoM-4,6-dione, followed by chlorination of the resulting product with sulphuryl chloride, furnished the cytotoxic drugs (1) and (2) [l].     

Fabrication of blazed gratings by area-coded effective medium structures

Area-coded effective medium structures (ACES) are a recently presented novel type of diffractive structure. Because of their higher stability compared to 2D binary blazed gratings, they have the potential of a broader use in micro-optics applications. The first fabrication with electron-beam lithography validate the theoretical model of blazed ACES. The measured diffraction efficiencies are in very good agreement with the values obtained from rigorous electromagnetic analysis.