Volume 226, Issue 1 -

The applications of DMA methods for the investigation of the dynamical mechanical properties of materials are reviewed. Experimental details of this method are described including the combination of DMA with X-ray rocking curve measurements to investigate the details of the domain patterns. Emphasis is given to the effect of phase transitions, in particular in ferroelastic and co-elastic systems where the ‘easy’ movement of domain boundaries, twin walls and other interfaces lead to super-elastic softening of the material. Thermodynamic constraints, such as in the adiabatic-isothermal crossover are discussed. Smooth, ballistic propagation of domain walls are contrasted with the jerky behaviour of interfaces when phase transitions of first order are investigated. It is shown that for the ballistic behaviour, momentum driven wall movements are common and that their representation in Cole-Cole plots leads to depressed semi-circles, similar to methods used in dielectric spectroscopy. In jerky elastic responses the energy dissipation follows power law dependences. Domain wall pinning and domain wall freezing is discussed in some detail.

From the standpoint of designing microporous frameworks with desired pore diameter and dimensionality, and, especially, the optimization of crystal habit, the crystal engineering of new zeolites must be based on concepts/procedures different from those appropriate for the design of organic crystals. This is because the structure building units proposed by Barrer and co-workers are probably not instrumental for framework construction, thus eliminating the important ‘synthon’ approach used for the construction of innovative organic solids. With some variant of the Flanigen model for crystal growth via one SiO 2 unit at a time, the best approach to zeolite crytal growth engineering appears to occur indirectly via structure directing agents that can also be modified to influence the emergent crystal habit. Prospects for identifying frameworks emerging from synthesis gels are also discussed in this review, revealing that the use of radial distribution functions is less informative than for the analysis of silicate glasses.

This paper describes an approach to the deduction and labeling of crystallographic point groups in n-dimensional spaces where n is an odd number. It shows that point groups in such spaces may be formed from the generators of rotational groups and a single inversion operation characteristic of the odd dimension. Results are given for 188 of the 955 crystallographic point groups in a five dimensional space and the extension to the remainder of the groups is made clear. Since 3 is an odd number, the 32 classical point groups are used to illustrate the use of generators for this purpose. Further extensions to seven dimensions and to even dimensions are then discussed.

New polyhedra are derived with symmetry codes and variables.

We derive many generalized polyhedra of dodecahedron type with analytic mathematics. The findings are compared with the structures of the Adeno Herpes series of viruses.

The structural changes of hydrothermally synthesized cubic Li 2 TiO 3 were studied in-situ during heating and cooling at the neutron powder diffractometer SPODI at FRM II, Garching. The initially cubic ( Fm -3 m ) phase shows a slight Li deficit, but becomes stoichiometric with increasing temperature. It transforms to a monoclinic ( C 2/ c ) structure above 300 °C and back to the same cubic ( Fm -3 m ) structure at 1155 °C. A two-phase region is observed between 300 and 500 °C, indicating a first order character of the transformation. Below 1000 °C, during the first heating, the monoclinic Li 2 TiO 3 phase is disordered due to stacking faults. Between 1160 and 1140 °C, the cubic phase becomes monoclinic again upon cooling and stays well ordered down to room temperature. Anomalies in the thermal expansion of the monoclinic modification occur at around 600 °C for both, heating and cooling. These are assigned to strains building up during the first phase transformation upon heating and the confinement in the compacted ceramic sample during cooling, respectively.

The skutterudite-derivative phase RhGe 1.5- x Se 1.5+ x has been synthesized and structurally characterized by metallography and powder X-ray diffraction data. At x = 0, RhGe 1.5 Se 1.5 displays trigonal sym metry, space group R -3, with unit cell parameters a = 12.0601(8) Å, c = 14.826(1) Å, V = 1867.5(4) Å 3 , and Z = 24. The crystal structure can be viewed as a modification of the cubic structure of the skutterudite CoAs 3 , where Ge and Se anions order in layers perpendicular to the [111] direction of the cubic skutterudite unit cell. Electrical resistivity, Seebeck coefficient and thermal conductivity of the phase RhGe 1.5– x Se 1.5+ x ( p -type semiconductor) have been investigated for different x values in the temperature range from 3 to 640 K.

The crystal and molecular structures of three racemic erythro- ( R *, S *)-(±)- α -2-piperidinyl-2,8-bis(trifluoromethyl)-4-quinolinemethanol n -nitrobenzoates, n = 2, 3, and 4, are described. The cations uniformly adopt an L-shaped geometry with protonation leading to a piperidin-1-ium ring in each case. A non-planar arrangement is found for the anion in ( 1 ; n = 2), owing to steric congestion, but almost planar arrangements are found in each of the anions in ( 2 ; n = 3) and ( 3 ; n = 4). Supramolecular chains are found in each of the crystal structures but these are sustained by different hydrogen bonding synthons. More efficient crystal packing in ( 3 ; n = 4) is correlated with a single synthon as opposed to two distinct synthons in each of ( 1 ; n = 2) and ( 2 ; n = 3). Crystal data for ( 1 ; n = 2): monoclinic, P 2 1 / n, a = 13.2144(6), b = 8.1028(2), c = 22.9724(9) Å, β = 105.919(1)°, V = 2365.41(15) Å 3 , Z = 4, R = 0.053. Crystal data for ( 2 ; n = 3): monoclinic, P 2 1 / n, a = 6.8389(2), b = 12.8432(4), c = 26.7910(9) Å, β = 90.868(2)°, V = 2352.87(13) Å 3 , Z = 4, R = 0.044. Crystal data for ( 3 ; n = 4): monoclinic, P 2 1 / n, a = 16.9854(8), b = 6.5069(2), c = 20.2083(9) Å, β = 92.116(2)°, V = 2231.94(16) Å 3 , Z = 4, R = 0.065.

The structure of (TMA) 2 [TiF 6 ] was determined to be rhombohedral (pseudo-cubic) at room temperature and has been refined to wR = 7.6%. It is isostructural with (TMA) 2 [OsF 6 ]. Crystals grown from aqueous solution have pseudo-octahedron and pseudo-cube faces and exhibit twinning along pseudo-cubic {110} and {100} planes. Ferroelastic switching of the lattice orientation of a single crystal has been achieved by application of shear stress.

The ground-state structural, electronic and optical properties of the Al 1– x Ga x Sb compound in the zinc-blende phase for x = 0.125, 0.25 and 0.50 have been stud ied employing the full potential linearized augmented plane wave plus local orbitals method based on the density functional theory. For the exchange-correlation effects we have adopted the GGA approach. In order to model Al 1– x Ga x Sb zincblende alloys, we employ a 16-atoms supercell of the type 2 × 2 × 2. The lattice structures of Al 0.875 Ga 0.125 Sb, Al 0.75 Ga 0.25 Sb and Al 0.50 Ga 0.50 Sb are obtained by replac ing one, two and four Al atoms, respectively, with an equal number of Ga atoms in the crystal lattice of AlSb. The variation of energy band gap of Al 1– x Ga x Sb versus the value of ‘ x ’ reveals that the band gap bowing para m eter has strong composition dependence for small concentrations of Ga. The DOS plots and the various calculated optical properties are in accordance with the calculated electronic properties. The characteristic properties of Al 1– x Ga x Sb are dominated by the Sb 5 p electrons. Linear optical properties such as the dynamic dielectric function, static refractive index, reflectivity and energy loss function for an energy range of 0 to 13.50 eV have been studied. The static dielectric constant increases with the increase in Ga concentration in Al 1– x Ga x Sb. The bulk plasma frequency ω p for this material corresponds to 13.19 eV.

An in-situ optical spectroscopy investigation at high temperatures is reported into the defect chemistry and cation kinetics of cobalt-containing olivines. The equilibrium defect optical absorption is studied as a function of oxygen activity at high temperatures. The kinetics of cation redistribution between the two non-equivalent sites in the olivine crystal structure is investigated by temperature-jump relaxation experiments. It is found that both the compositional dependence of kinetics of the cation redistribution process and the dependence of point defect concentration on oxygen activity favor a defect model involving cation vacancies and electron holes (Co 3+ ).

The crystal structure of Nd 2 BC, synthesized by arc melting of the element mixture, repeated in a high frequency furnace and subsequent annealing at 1070 K, has been determined from single crystal X-ray diffraction data. It crystallizes in a new structure type in the monoclinic space group C 2/ m ( a = 12.732(2) Å, b = 3.6848(4) Å, c = 9.398(2) Å, β = 130.43(1)°, R 1 = 0.038 ( wR 2 = 0.091) for 596 reflections with I o > 2 σ ( I o )). Characteristic building units are C—B—B—C entities with the distances d C—B = 1.51(2) Å and d B—B = 1.65(2) Å. These units are embedded in a framework of neodymium atoms resulting from the stacking of slightly corrugated two-dimensional square nets of Nd. Within the ionic limit Nd 2 BC can be described as (Nd 2+ ) 4 [C—B—B—C] 8– or (Nd 3+ ) 4 [CBBC] 8– · 4 e – with [C=B—B=C] 8– units, which are isoelectronic to 1,3-butadiene. The measured magnetic moment of 3.65 μ B per Nd atom in Nd 2 BC is in good agreement with the expected value of 3.62 μ B for 4f 3 configuration. LMTO band structure calculations show that Nd 2 BC is metallic and the extra electrons occupy mainly bonding Nd-5d and antibonding π* states within the C—B—B—C units.