Synthesis, characterization and magnetic property studies of copper-based phosphate and silicate compounds containing nanosize transition metal oxide frameworks

Abstract

The search for novel compounds with nanosize transition metal oxide frameworks is important for the study and understanding of materials with interesting and important magnetic and electronic properties. These low-dimensional structural features have become associated with unusual physical phenomena such as charge density waves, magnetic ordering, and superconductivity. Exploratory synthesis of copper(II) and copper(I/II) phosphates containing oligomers, chains and layers has been investigated through the use of low-melting eutectic fluxes. Additionally, magnetic property studies of many of these compounds have been undertaken and interesting ordering has been observed. Ultimately, these new compounds should lead to structural models for experimental and theoretical studies of electrons in a confined lattice. Several interesting and significant low-dimensional copper phosphates and silicates have been synthesized by employing low-melting eutectic fluxes for crystal growth. Characterization was carried out by single and powder X-ray diffraction, thermal analysis (DTA and TGA), infrared spectroscopy and magnetic measurements. Three compounds exhibiting low-dimensional features dependent on the versatility of Cu-O-P-O-Cu bonding have been found: $\rm Na\sb2CuP\sb2O\sb7,$ $\rm Ba\sb2Cu(PO\sb4)\sb2$ and $\rm BaCuPO\sb4Cl.$ Long-range antiferromagnetic ordering has been found despite the long Cu--Cu distances and lack of Cu-O-Cu interactions. $\rm Li\sb2Cu\sb3Si\sb4O\sb{12}$ and LiCu$\sb2$PO$\sb5$ have copper oxide trimers and layers with Cu-O-Cu bonds, respectively. The magnetic properties of the latter and $\beta$-NaCuPO$\sb4$ (a structurally known compound containing one dimensional (Cu$\sb2\rm O\sb8\!\sb\infty\sp{12{-}}$ chains) have been studied and show strong coupling. Finally, investigations into the mixed-valence copper(I/II) system has produced two welcome additions to a small family of electronically distinct copper compounds, Cu$\sb2$PO$\sb4$ and $\rm Na\sb{2.2}Cu\sb{3.8}P\sb2O\sb9Cl.$ These compounds both have fascinating low-dimensional features: an array of $\rm Cu\sp{I}O\sb2$ cross-linked by PO$\sb4$ tetrahedra and a chain of condensed copper oxide polyhedra, respectively. Our structure/property correlation studies should offer insight into the conducting mechanism of technologically important materials and in the development of new materials with desired properties.