Chair: Ladislav Bohaty (Germany), Co-chair: Vaclav Janovec (Czech Republic)

Karlheinz Schwarz (Vienna) began the session on Phase Transitions with an excellent review lecture about theoretical methods and calcula- tions of solids. He presented the Density Functional Calculations as a new tool to study phase transitions and demonstrated how effectiv and sophisticated the corresponding computer programs are. Periodic Nodal Surfaces (PNS) are a nice tool for describing crystal structures. Stefano Leoni (Zurich) explained that PNS can be usefully employed in the analysis of structural pathways relating different modifications of polymorphic compounds. Markus Braden (Saclay, Karlsruhe) showed in his lecture how powerful crystallographic methods are for the study of Spin-Peierls phase transi- tions. He discussed in detail structural aspects of inorganic Spin-Peierls compounds CuGeO³ and NaV2O5 and presented the latest results, which were elaborated in cooperation between four French and German groups. Stanislav Filatov (Saint Petersburg) presented unusual first-order phase transitions without change of space group symmetry and demonstrated the structural mechanism of such isosymmetrical phase transitions in alkali borates with framework structures. In the last 20 minutes of the microsymposium Theo Hahn (Aachen), Vaclav Janovec (Prague) and Helmut Klapper (Bonn) - who are colla- borating on a review article about twins and domains - gave some short insights into the current state of this topic. In the accompanying poster session more than 30 contributions were presented.