1D Heterometallic Oxalate Compounds as Precursors for Mixed Ca-Cr Oxides - Synthesis, Structures, and Magnetic Studies

摘要

The heterodimetallic compounds {[CaCr_2(L)_2(C_2O_4)_4]·xH_2O}_n [L = 2,2'-bipyridine (bpy), x = 0.83 (1); L = 1,10-phenanthroline (phen), x = 0.33 (2)] were synthesized and characterized by IR spectroscopy, X-ray diffraction and magnetic measurements. The crystal structure of 2 is reported: the chelation of a [Cr(phen)(C_2O_4)_2]~- units towards Ca cations results in 1D double zigzag chains. The ability of 1 and 2 to act as singlesource precursors for the formation of Ca-Cr oxides is explored by thermal analysis, powder X-ray diffraction and IR spectroscopy. The thermal processing of 1 and 2 at 1100 ℃ in air led to the appearance of the β-CaCr_2O_4 phase as the major crystalline oxide. By heating the samples up to 1100 ℃ in nitrogen, only the β-CaCr_2O_4 phase formed, whereas heating at 1400 ℃ caused the crystallization of two polymorphs, α- and β-CaCr_2O_4. The magnetic behaviour of β-CaCr_2O_4 pointed to the freezing of magnetic moments in coexistence with a weak ferromagnetic order.