Study of phase transitions in some transition metal bis-pyridine halides, and application of a pseudospin-type model associated with the local ordering of the pyridine ring

Abstract

A nearly second-order phase transition around T _c ∼150±1 K has been confirmed in Co(NC₅H₅)₂Cl₂ single crystals from heat capacity (with a conduction-type calorimeter) and dielectric constant measurements. The transition at T _c was detected earlier by us from magnetic susceptibility and Mössbauer studies. A similar and new phase transition was also observed (first reported in this paper) in the corresponding diamagnetic Zn(py)₂Cl₂ crystals around T _c =284±1 K indicates that the phase transitions in the transition-metal bis-pyridine halides (including those previously observed in Cu(py)₂Cl₂ and Cu(py)₂Br₂ salts of this group) are not due to magnetic spin ordering (or exchange or superexchange type interaction). This is in contrast to some earlier observations. The phase transitions in these crystals have been attributed in the present paper to rotations/reorientations of the pyridine ring, similar to many other pyridine compounds. It has been suggested that a theoretical analysis of such phase transitions in these and many other similar crystals could be made in a unified way with the use of a pseudospin-type model.

Key Words:

• Co(py)₂Cl₂
• Zn(py)₂Cl₂
• specific heat
• dielectric constant
• pseudospin model