Numerical investigations on autogenous shrinkage of cement paste and mortar

Autogenous shrinkage of cement paste and concrete is defined as the macroscopic length change occurring with no moisture transferred to the exterior surrounding environment. It is a result of chemical shrinkage affiliated with the hydration of cement particles and the ongoing process of self-desiccation. The process of self-desiccation can be modeled starting from the formation of the capillary pore space during hydration in the cement paste. In this proposal a working model will be introduced explaining the difficulties to obtain the autogenous shrinkage strains directly from a simulated or measured microstructure of cement paste. In a second step the autogenous shrinkage of a hardening cement mortar was described on a mesoscopic level. It based on measurements on cement paste. The mortar simply consists of cement paste and a defined fraction of spherical aggregates with a known modulus of elasticity. Furthermore the influence of the interfacial transition zone (ITZ) is studied in numerical simulations. The results of these finite-element-calculations are introduced and compared with testing results of the autogenous shrinkage of hardening mortar samples. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Rydberg-Rydberg collisions: resonant enhancement of state mixing and penning ionization

In rubidium Rydberg states, the collision nD_(5/2)+nD_(5/2)-->(n-2)F_(7/2)+(n+2)P_(3/2) is nearly resonant in the vicinity of n=43. As a result, over a short range of n centered around n approximately 43 the Rydberg-Rydberg interaction potential is quite large and turns from repulsive to attractive [Phys. Rev. A 75, 032712 (2007)10.1103/PhysRevA.75.032712]. We use state-selective field ionization to investigate the effect of this resonance on instantaneous excitation of mixed two-particle states, state-mixing collisions, and Penning ionization. We find that these processes depend on the magnitude and sign of the two-particle interaction potential, and thus on n near the resonance. The large magnitude of the observed state mixing provides evidence for many-body effects.     

Time-dependent density-functional theory with a self-interaction correction

We discuss an implementation of the self-interaction correction for the local-density approximation to time-dependent density-functional theory. A variational formulation is given, taking care of the necessary constraints. A manageable and transparent propagation scheme using two sets of wave functions is proposed and applied to laser excitation with subsequent ionization of a dimer molecule.     

Novel Arachno‐type X56– Zintl Anions in Sr3Sn5, Ba3Sn5, and Ba3Pb5 and Charge Influence on Zintl Clusters

Two new binary Zintl phases, Sr₃Sn₅ and Ba₃Sn₅ were synthesized and structurally characterized. The revised structure of Ba₃Pb₅ is also reported. All three compounds are isotypic and crystallize with a modified Pu₃Pd₅ structure type. The anionic substructure is composed of X₅^6– square pyramidal clusters (X = Sn, Pb), which are described as arachno clusters according to the Wade‐Mingos electron counting rules. The electronic structure of the pyramidal Zintl anions and the influence of the number of skeletal electrons of these clusters are investigated using the electron localization function (ELF). The structural relationship between Ba₃Sn₅ and the Zintl phases Ba₃Si₄ and Ba₃Ge₄ are analyzed. Additionally, two new Zintl phases Ba₃Ge_2.82Sn_2.18 and Ba₃Ge_3.94Sn_0.06, have been synthezised and their structures are reported, which directly show that the exchange of tin against germanium leads to a change from the M₃X₅ to the M₃X₄ structure type. This effect is traced back to the maximal charge acquisition property of the Zintl anions of heavier and lighter tetralides.