H. Arend

Dynamics of the n‐decylammonium chains in the perovskite‐type layer structure compound (C10H21NH3)2CdCl4

The two structural phase transitions in the perovskite layer structure compound (C10H21NH3)2CdCl4 at Tc1=35 °C and Tc2=39 °C have been studied by x‐ray diffractional, calorimetric and dielectric measurements, proton NMR spin–lattice relaxation and second moment investigations, 35Cl and 14N quadrupole resonance spectroscopy as well as group theoretical considerations. The results show that the transition (Tc1) from the low temperature phase (P21/n) to the intermediate temperature phase (Pmnn) is basically a dynamic order–disorder transition of rigid alkyl chains, whereas the transition (Tc2) from the intermediate to the high temperature phase (Amaa) is connected with a melting of the alkyl chains. In the intermediate phase the rigid alkyl chains are flipping around their long axes between two equivalent orientations separated by 90°. The two transitions are somewhat analogous to the ones found in the lipid bilayer membranes and can be described by order parameters used for smectic liquid crystals.

2‐cyclooctylamino‐5‐nitropyridine, a new nonlinear optical crystal with orthorhombic symmetry

A new nonlinear optical crystal 2‐cyclooctylamino‐5‐nitropyridine (COANP) with point group symmetry mm2 has been grown by a temperature difference solution growth technique. Its linear and nonlinear optical properties have been investigated. The refractive indices na =1.702, nb =1.847, and nc =1.681 (λ=550 nm) and the nonlinear optical susceptibilities d31 and d33 (λω =1.06 μm) have been determined. Type I phase matching for Nd:YAG lasers (λ=1.06 μm) has been demonstrated for the configuration using a combination of d31 and d32 by angle tuning. A peak efficiency η=P2ω/Pω =3.6% has been observed with a 0.90‐mm‐thick crystal and a fundamental power Pω =560 W.

A new family of ferroelectric materials with composition A2BMO3F3 (A, B = K, Rb, Cs, for rA+ ⩾ rB+ and M = Mo, W)

Abstract The dielectric, pyroelectric, calorimetric and crystallographic properties of materials with formulas K3M03F3, Rb3M03F3, Cs3M03F3, Rb2KM03F3 and Cs2RbM03F3 (M = Mo, W) are shown to be consistent with ferroelectric behavior. The first seven of these materials exhibit two transitions, the latter three a single transition, above 78 K. A maximum in dielectric constant is associated only with the higher temperature (T2) phase transition. The structure is paraelectric above T2 in point group m3m and ferroelectric below in point group 3. The probable point group below the lower transition temperature is 3 or 1.

Progress in KNbO3 crystal growth

Abstract Known methods for the growth of KNbO 3 crystals are reviewed. Improvements in the growth of crystals by top-seeded pulling are based on: (1) the use of a melt composition as close as possible to the peritectic point; (2) the application of accelerated crucible rotation; (3) a [110] seed orientation and refined growth parameters (e.g. temperature field and rotation rates); (4) a modification of the cooling process. The crystals obtained are characterized by a list of dielectric, elastic and optical properties.

Proton NMR study of the structural phase transitions in perovskite layer compounds: (CnH2n+1NH3)2CdCl4 and (NH3–(CH2)n–NH3) CdCl4

The temperature dependences of the second moments M2 of the proton magnetic resonance absorption spectra and of the proton spin–lattice relaxation times T1 of perovskite layer compounds (CnH2n+1NH3)2CdCl4 with n=1–3 and (NH3– (CH2)n–NH3) CdCl4 with n=2−5 have been studied together with the frequency dispersion of T1. The structural phase transitions in these compounds were found to be connected with a change in the state of motion of the alkyl or alkylene groups, i.e., by an interplay of transitions between different N–H−−−Cl hydrogen bonding schemes and the excitation of hindered rotations of the hydrocarbon chains.

Study of phase transitions in A3MO3F3 compounds (A = K, Rb, Cs; M = Mo, W)

Abstract A comparative study of phase transition in six compounds: K 3 MoO 3 F 3 , K 3 WO 3 F 3 , Rb 3 MoO 3 F 3 , Rb 3 WO 3 F 3 , Cs 3 MoO 3 F 3 and Cs 3 WO 3 F 3 was performed. All of them exhibit two transitions. One, at lower temperatures is possibly due to short range order-disorder phenomena in the anion octahedra, the second one is of a ferroelectric-paraelectric type.

Optical Second-Harmonic Generation in Single Crystals of Thiosemicarbazide Cadmium Bromide Hydrate (Cd(NH2NHCSNH2)Br2?H2O)

A new nonlinear optical organic-inorganic crystal, thiosemicarbazide cadmium bromide hydrate (TSCCB), with crystal class m has been grown by a temperature-lowering technique (TLT) and a temperature-difference technique (TDT). The crystal structure of TSCCB is reported. Its linear and nonlinear optical properties have been investigated. The highest measured nonlinear optical susceptibility is dmax = 2.8 pm/V.

Proton-14N double resonance study of the structural phase transitions in the perovskite type layer compound (CH3NH3)2CdCl4

Using a proton-nitrogen double resonance technique we have determined the quadrupole coupling of14N in the room temperature orthorhombic (Cmca), the low temperature tetragonal (P42/ncm), and the monoclinic low temperature (P21/c) phases of (CH3NH3)2CdCl4. In all these phases all nitrogens are chemically equivalent demonstrating that the disorder in the orientations and H-bonding arrangements of the CH3-NH3 groups in theC m c a andP42/ncm phases is indeed dynamic and not static. In the monoclinic phase the14N quadrupole coupling constant equalse2qQ/h=880 kHz and the asymmetry parameter isη=0.20, wherease2qQ/h=790 kHz,η=0.1 in the tetragonal low temperature phase ande2qQ/h=726 kHz,η=0.21 in the room temperature orthorhombic phase. The observed increase in the14N quadrupole coupling constant on going from the orthorhombic phase to the tetragonal low temperature phase which is coupled with a simultaneous decrease in the asymmetry parameter can be understood in terms of a partial freezing in of the dynamic disorder in the C-N bond directions whereas the14N quadrupole coupling tensor in the monoclinic phase is characteristic of a frozen in C-N bond in a deformed lattice, where the N-H — Cl bonds are of different length.

Pure and doped barium titanate. Crystal growth and chemical composition

Abstract Barium titanate single crystals are grwon in two ways, i.e. by the Remeika and the Czochralski method. Doping ions (FeCoCu) are introduced into Remeika crystals and determined by a spectro photometric method. Impurity ions (FKPtSb) are determined in both kinds of crystals; pulled crystals are purer than Remeika crystals. The stoichiometry Ti/Ba + Sr is excellent in pulled crystals but the Ti, Ba, Sr, K concentrations in Cu-doped crystals show ion size effects in the compensation mechanism.

Dielectric properties of (CnH2n+1NH3)2CdCl4 and (NH3(CH2)nNH3)CdCl4 perovskite layer compounds

Abstract The dielectric properties of (CH3NH3)2CdCl4, NH3(CH2)3NH3.CdCl4 and NH3(CH2)5NH3.CdCl4 single crystals as well as of (CnH2n+1NH3)2CdCl4 (n = 1, 2, 3) and (NH3(CH2)nNH3)CdCl4 (n = 2,3,4,5) pressed powder pellets were investigated as a function of temperature between 77 and 373°K and significant dielectric anomalies were found to occur at all structural phase transitions. The dialectric constant in the alkyl-ammonium as well as alkylene-diammonium crystals is found to be much higher parallel to the octahedral planes than in a direction perpendicular to these layers.