Nanostructured magnetic cuprate cluster:: Synthesis, structure, UV-vis spectroscopy, and magnetic properties of a new copper(II) arsenate NaCuAsO4 containing discrete [Cu4O16]24- clusters

Crystals of copper(II) arsenate NaCuAsO4 were grown by conventional high-temperature, solid-state methods in molten-salt media. The compounds were characterized by single crystal X-ray diffraction, UV-vis spectroscopy, and magnetic susceptibility measurements. NaCuAsO4 crystallizes in a monoclinic lattice with a 6,002 (1) Angstrom, b = 10.853 (2) Angstrom, c = 10.373 (2) Angstrom, = 91.50 (3)degrees, and V = 675.4(2) Angstrom(3); P2(1)/c (No. 14); Z = 8. The newly isolated sodium copper(II) arsenate reveals a pseudo-one-dimensional channel structure where the sodium cations reside. The extended framework contains nanostructured [Cu4O16](24-) magnetic clusters that are interlinked by closed-shell, nonmagnetic AsO43- oxy anions via sharing vertex oxygen atoms of the CuO5 and AsO4 polyhedral units. Each [Cu4O16](24-) cluster consists of four CuO5 square pyramidal units in a chair configuration centered by a center of inversion. The two crystallographically independent Cu2+ cations adopt the [4 + 1] CuO5 Jahn-Teller distortion giving rise to an intense d-d transition in UV-vis absorption spectra. The magnetic susceptibility measurements reveal that the title compound is antiferromagnetic. At high temperatures, the data follows a pure Curie law, suggesting noninteracting spins, but with a rapid suppression of the effective spin below T = 70 K. At low temperature, the susceptibility collapses, indicating spin gap formation as the magnetic-cluster material settles into the lowest energy magnetic singlet state. The current work in the exploratory synthesis of oxy compounds containing nanostructured transition-metal-oxide magnetic clusters leads to new materials for experimental and theoretical developments of magnetic models.