A Copper-Formate Framework Showing a Simple to Helical Antiferroelectric Transition with Prominent Dielectric Anomalies and Anisotropic Thermal Expansion, and Antiferromagnetism

• Published in: Chemistry : a European journal 2014-11, Vol.20 (48), p.15872-15883
• Main Authors: Shang, Ran, Chen, Sa, Wang, Zhe-Ming, Gao, Song
• Format: Article
• Language: English
• Subjects: anisotropic thermal expansion; Anisotropy; antiferroelectricity; Antiferromagnetism; Channels; Chemistry; dielectric anomaly; Dielectrics; Ferroelectricity; Helical; metal formate; Order disorder; phase transition; Thermal expansion
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201403466

We present here the compound [NH4][Cu(HCOO)3], a new member of the [NH4][M(HCOO)3] family. The Jahn–Teller Cu2+ ion leads to a distorted 49⋅66 chiral Cu–formate framework. In the low‐temperature (LT) orthorhombic phase, the Cu2+ is in an elongated octahedron, and the ${{\rm NH}{{+\hfill \atop 4\hfill}}}$ ions in the framework channel are off the channel axis. From 94 to 350 K the ${{\rm NH}{{+\hfill \atop 4\hfill}}}$ ion gradually approaches the channel axis and the related modulation of the framework and the hydrogen‐bond system occurs. The LT phase is simple antiferroelectric (AFE). The material becomes hexagonal above 355 K. In the high‐temperature (HT) phase, the Cu2+ octahedron is compressed, and the ${{\rm NH}{{+\hfill \atop 4\hfill}}}$ ions are arranged helically along the channel axis. Therefore, the phase transition is one from LT simple AFE to HT helical AFE. The temperature‐dependent structure evolution is accompanied by significant thermal and dielectric anomalies and anisotropic thermal expansion, due to the different status of the ${{\rm NH}{{+\hfill \atop 4\hfill}}}$ ions and the framework modulations, and the structure–property relationship was established based on the extensive variable‐temperature single‐crystal structures. The material showed long range ordering of antiferromagnetism (AFM), with low dimensional character and a Néel temperature of 2.9 K. Therefore, within the material AFE and AFM orderings coexist in the low‐temperature region. Unlike the others: [NH4][Cu(HCOO)3], a new member of the [NH4][M(HCOO)3] family, possessing a distorted 49⋅66 chiral Cu–formate framework due to the Jahn–Teller Cu2+ ion, displays a low‐temperature simple to high‐temperature helical antiferroelectric phase transition at 355 K (see figure), and relevant dielectric anomalies and anisotropic thermal expansion, as well antiferromagnetic ordering at 2.9 K.