Room-temperature structural phase transition in the quasi-2D spin-1/2 Heisenberg antiferromagnet Cu(pz)2(ClO4)2
Abstract
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 low-temperature magnetic properties, its room-temperature 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, 35Cl nuclear magnetic resonance, and neutron diffraction measurements, we provide evidence of a second-order phase transition at T = 294 K, not reported before. The absence of a magnetic ordering across T in the magnetization data, yet the presence of a sizable anomaly in the specific heat, suggest 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 ClO-4 counteranion tetrahedra, shown to adopt a configuration of higher symmetry above the transition temperature.
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