Structural and calorimetric investigations of a highly disordered crystal [Co(NH₃)₆](BF₄)₃

Abstract

Five crystalline phases of [Co(NH₃)₆](BF₄)₃ were identified by adiabatic calorimetry. Two phase transitions at T _C1(II → I) = 271.7 K and T _C2(III → II) = 152.2 K are accompanied by large entropy changes (8.4 and 6.5 J K¹ mol¹, respectively), whereas two additional transitions at T _C3(IV → III) = 67.4 K and T _C4(V → IV) = 48.9 K are accompanied by small entropy changes (0.31 and 0.15 J K⁻¹ mol⁻¹, respectively). X-ray single-crystal diffraction at 293 K demonstrates that phase I is cubic (Z = 4) and that two types of anions exist with different types of orientational disorder. In phase II, at 200 K, which is also a cubic system (Z = 32), five anions (out of 12) become ordered, and three cations (out of four) are deformed to give a lower symmetry. This is attributed to the orientational ordering of the anions triggered by NHF hydrogen-bonding interactions. Further ordering of the anions and the symmetry reduction of the crystal occurred at T _C2, but the structure of phase III still remained cubic. The energies required to reorient an ordered anion in the crystal lattice of phases II and III were estimated from the excess heat capacity, demonstrating that the NHF hydrogen-bonding interactions in this compound are considerably weaker than the NHO interactions in an isostructural compound, [Co(NH₃)₆](ClO₄)₃.

Keywords:

• X-ray diffraction
• Calorimetry
• Crystal structure
• Phase transition
• Disordered system

Acknowledgment

This study is Contribution No. 27 from the Research Center for Structural Thermodynamics.