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

## TL;DR

This study reveals a previously unreported second-order structural phase transition in Cu(pz)$_2$(ClO$_4$)$_2$ at 294 K, characterized by subtle lattice distortions without magnetic ordering, using multiple experimental techniques.

## Contribution

The paper uncovers a room-temperature structural phase transition in Cu(pz)$_2$(ClO$_4$)$_2$, combining magnetization, specific heat, NMR, and neutron diffraction data to characterize the transition.

## Key findings

- Evidence of a second-order phase transition at 294 K.
- Absence of magnetic ordering across the transition.
- Subtle angular distortions of molecular components above the transition.

## Abstract

Cu(pz)$_2$(ClO$_4$)$_2$ (with pz denoting pyrazine C$_4$H$_4$N$_2$) is a two-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 up to now. Here we report a study of the structural features of Cu(pz)$_2$(ClO$_4$)$_2$ in the paramagnetic phase, up to 330 K. By employing magnetization, specific heat, $^{35}$Cl nuclear magnetic resonance, and neutron diffraction measurements, we provide evidence of a second-order phase transition at $T^{\star}$ = 294 K, not reported before. The absence of a magnetic ordering across $T^{\star}$ 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.