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

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.

10 pages, 12 figures

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Cited by in corpus (1)

• The princess and the pea: on the outsized role of inter-layer ligands in copper-pyrazine antiferromagnets