Band 216, Heft 9

Various approaches for the representation of equation of state data for solids under strong compression are discussed. The theoretical background for reasonable extrapolations to higher pressures and higher as well as lower temperatures is described. The distinction between ideal, regular, and anomalous behaviour allows to gain deeper insight into the electronic changes occurring in various solids under strong compression. The discussion of experimental data for various regular solids leads finally to an estimate of the accuracy obtained in the present realisation of a practical pressure scale based on equation of state measurements.

The symmetry relationships among zeolite-type ABW compounds are described in terms of the reductions of symmetry from the highest possible topological symmetry in space group Imam to eight possible subgroups. This family of compounds, also known as Icmm -type compounds, covers a wide range of chemical compositions. The distortion modes inducing the lowering of symmetry are described based on rotation and ordering processes. Atomic site relationships with their evolution from the aristotype are given for the recently described monoclinic P 112 1 / n structures with an ABW-type framework.

The high pressure behaviour of cubic silica sodalite has been investigated by ab initio calculations based on density functional theory. The experimentally observed phase transition from the fully-expanded ideal sodalite framework (space group Im 3̅ m ) to a collapsed framework with space group I 4̅3 m could be successfully reproduced to appear at 0.12 GPa. This transition is discontinuous. The mechanism of the transition can be understood in terms of a tilt of the framework and of deformations of the SiO 4 -tetrahedra. Discontinuities of the enthalpy, cell volume, Si-O bond lengths and angles at 1.15 and 3.01 Gpa point at further possible pressure-induced iso-symmetric phase transitions in the space group I 4̅3 m .

Single crystals of Al 1- x Ga x PO 4 of various mole fractions x were grown from phosphoric acid solutions under hydrothermal conditions using the slow-heating method. In order to relate the parameters of this low-quartz structure analogue and the observed phase transitions to the respective mole fraction, the crystals were investigated by means of electron probe microanalysis, X-ray powder diffraction and differential thermal analysis. The results are discussed in terms of the segregation effects occuring during growth. The limiting geometrical parameters for the existence of the high-quartz phase are compared to the values given in the literature.

The phase transitions of the solid solutions (Ca 1- x Sr x ) 2 MgSi 2 O 7 (0 ≤ x ≤ 0.32) of modulated melilites are studied by X-ray diffraction. The variation of the intensity and the FWHM of satellite reflections reveal different transition temperatures. Two ranges of different ordering are found in the range of incommensurate modulation. The mechanism of the phase transition into the incommensurately modulated structure is discussed by including the diffuse scattering above the transition.

New sulfamates KLi(NH 2 SO 3 ) 2 , RbLi ⋅ (NH 2 SO 3 ) 2 and CsLi(NH 2 SO 3 ) 2 were prepared from non-stoichiometric (ratio Li: Me = 3:1) aqueous solutions of LiNH 2 SO 3 and MeNH 2 SO 3 (Me = K, Rb, Cs) at room temperature. Large single crystals of KLi(NH 2 SO 3 ) 2 were grown from these solutions by controlled lowering of the temperature between 293 K and 288 K. The crystal structures were investigated using single-crystal X-ray diffraction methods. KLi(NH 2 SO 3 ) 2 : space group P 2 1 , Z = 2, a = 5.196(1) Å, b = 8.475(1) Å, c = 8.868(1) Å, β = 105.05(1)°, T meas . = 293 K, R 1 = 0.026; RbLi(NH 2 SO 3 ) 2 : space group P 1̅, Z = 4, a = 7.844(1) Å, b = 9.804(1) Å, c = 10.825(1) Å, α = 108.77(1)°, β = 93.29(1)°, γ = 98.64(1)°, T meas. = 293 K, R 1 = 0.033; CsLi(NH 2 SO 3 ) 2 : space group P 1̅, Z = 2, a = 5.535(1) Å, b = 7.877(1) Å, c = 9.857(1) Å, α = 74.07(1)°, β = 76.33(1)°, γ = 87.07(1)°, T meas. = 200 K; R 1 = 0.023. The common feature of the three crystal structures are alternating layers of Me + and lithium cations together with anionic [NH 2 SO 3 ] – tetrahedra. Li + coordination polyhedra and [NH 2 SO 3 ] – anions, linked via common corners, form one type of these sheets. Additionally, these layers are stabilized by hydrogen bonds. Me + coordination polyhedra are linked to each other and to the above mentioned layers by common corners and edges. RbLi(NH 2 SO 3 ) 2 and CsLi(NH 2 SO 3 ) 2 possess similar structures. Generally, the observed interatomic distances and angles agree with crystal chemical expectations. Differential scanning calorimetry measurements showed that KLi(NH 2 SO 3 ) 2 undergoes a first-order structural phase transition at 401 K and melts at 423 K. Between 153 K and 423 K no indications of further phase transitions were observed. In addition, the refractive indices and the tensor of thermal expansion were measured. Further, the elastic and thermoelastic constants of KLi(NH 2 SO 3 ) 2 were determined from ultrasonic resonant frequencies of thick plane-parallel plates and their shift upon variation of temperature, respectively. The elastic behaviour of KLi(NH 2 SO 3 ) 2 is similar to the average elastic properties of LiNH 2 SO 3 and KNH 2 SO 3 .