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Cu(pz)$_2$(ClO$_4$)$_2$ (with pz denoting pyrazine C$_4$H$_4$N$_2$) is atwo-dimensional spin-1/2 square-lattice antiferromagnet with $T_{\mathrm{N}}$ =4.24 K. Due to a persisting focus on the low-temperature magnetic properties,its room-temperature structural and physical properties caught no attention upto now. Here we report a study of the structural features ofCu(pz)$_2$(ClO$_4$)$_2$ in the paramagnetic phase, up to 330 K. By employingmagnetization, specific heat, $^{35}$Cl nuclear magnetic resonance, and neutrondiffraction measurements, we provide evidence of a second-order phasetransition at $T^{\star}$ = 294 K, not reported before. The absence of amagnetic ordering across $T^{\star}$ in the magnetization data, yet thepresence of a sizable anomaly in the specific heat, suggest a structuralorder-to-disorder type transition. NMR and neutron-diffraction data corroborateour conjecture, by revealing subtle angular distortions of the pyrazine ringsand of ClO$^-_4$ counteranion tetrahedra, shown to adopt a configuration ofhigher symmetry above the transition temperature.

Room-temperature structural phase transition in the quasi-2D spin-1/2 Heisenberg antiferromagnet Cu(pz)$_2$(ClO$_4$)$_2$