Roomerature structural phase transition in the quasi-2D spin- 12 Heisenberg antiferromagnet Cu(pz)₂(ClO₄)₂

Cu(pz)₂(ClO₄)₂ (with pz denoting pyrazine, C4H4N2) is a two-dimensional spin-1/2 square-lattice antiferromagnet with TN=4.24 K. Due to a persisting focus on the lowerature magnetic properties, its roomerature structural and physical properties caught no attention up to now. Here we report a study of the structural features of Cu(pz)2(ClO4)2 in the paramagnetic phase, up to 330 K. By employing magnetization, specific heat, Cl35 nuclear magnetic resonance, and neutron diffraction measurements, we provide evidence of a second-order phase transition at Tâ=294 K, not reported before. The absence of a magnetic ordering across Tâ in the magnetization data, yet the presence of a sizable anomaly in the specific heat, suggests a structural order-to-disorder-type transition. NMR and neutron-diffraction data corroborate our conjecture by revealing subtle angular distortions of the pyrazine rings and of ClO4- counteranion tetrahedra, shown to adopt a configuration of higher symmetry above the transition temperature.