Negative Thermal Expansion in Anhydrous Salts of Acetylenedicarboxylic Acid

The vast majority of materials expand on heating they possess a positive thermal expansion coefficient. There are, however, certain compounds, whose volumes decrease on heating, so called NTEmaterials (negative thermal expansion). ZrW2O8, ZrP2O7 and Sc2(WO4)3 are prominent examples [1]. In many cases this unusual behaviour can be attributed to transverse vibrations of bridging ligand atoms in combination with specific structural requirements, i.e. ‘space’ for the transverse vibrations. Therefore, NTE behaviour has also been found in some zeolites [2]. One of our research projects is the synthesis and characterisation of coordination polymers based on salts of acetylenedicarboxylic acid (H2ADC). Beside many hydrates Sr[C2(COO)2] was synthesised as a first example of an anhydrous salt of acetylenedicarboxylic acid (I41/amd, Z 4, see Figure) [3]. Surprisingly, a small negative thermal expansion was found between 30 and 280 K and almost no thermal expansion was observed between room temperature and 573 K. This effect can be rationalised by ‘guitar string’ vibrations of bridging oxygen atoms accompanied by specific structural features. Therefore, we were aiming to synthesise compounds isostructural to Sr[C2(COO)2] to scrutinise our assumptions.