W0144

The Reversible Phase Transition of Compound [Cu^II(15-crown-5)(H₂O)₂](NO₃)₂. Xiang Hao, Sean Parkin and Carolyn P. Brock, Dept. of Chemistry, Univ. of Kentucky, Lexington, KY 40506-0055, USA.

The temperature dependence of the unit-cell dimensions of [Cu^II(15-crown-5)(H₂O)₂](NO₃)₂ over the range 90-320 K indicates the occurrence of a reversible phase transition at 315-320 K, which is also confirmed by Differential Scanning Calorimetry (DSC) measurements. The low-temperature phase (LTP) crystallizes in the monoclinic space group P2₁/n with Z’ = 5. The LTP is a modulated superstructure. The modulations are five-fold along the c direction and two-fold along a. Separate Wilson plots show that reflections of h = 2n and/or l = 5n are systematically stronger than those of h ≠ 2n and/or l ≠ 5n. The high-temperature phase (HTP) appears to be triclinic with pseudo C-centered symmetry. The space group of the HTP is probably P with Z’ = 2. Similar packing in the two phases indicates there is only a small energy difference between them, which accounts for the retention of the crystalline form through the transition.

Crystal structures of compounds with the same basic formula but different metal ions (Zn²⁺, Mg²⁺, Mn²⁺ and Co²⁺) have also been investigated. Several errors in previous work have been corrected. The Zn²⁺ and Mg²⁺ structures are similar to the LTP, while the Mn²⁺ structure is similar to the HTP. The Co²⁺ structure is different from both of the LTP and HTP structures.