paper

Room-temperature structural phase transition in the quasi-2D spin-1/2 Heisenberg antiferromagnet Cu(pz) $_{2}$ (ClO $_{4}$ ) $_{2}$

arXiv:1904.12802 · doi:10.1103/PhysRevMaterials.3.053602

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 $T_{N}$ = 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 $T^{⋆}$ = 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.

10 pages, 12 figures

Room-temperature structural phase transition in the quasi-2D spin-1/2 Heisenberg antiferromagnet Cu(pz)2​(ClO4​)2​

Abstract

Room-temperature structural phase transition in the quasi-2D spin-1/2 Heisenberg antiferromagnet Cu(pz) $_{2}$ (ClO $_{4}$ ) $_{2}$