FERROMAGNETISM IN THE ALPHA AND BETA POLYMORPHS OF ANHYDROUS COPPER(II) FORMATE - 2 MOLECULAR-BASED FERROMAGNETS WITH ORDERING TEMPERATURESOF 8.2-K AND 30.4-K

The first direct synthesis, the crystal structure, and the magnetic pr operties of one polymorph of anhydrous copper(II) formate, the beta- [ Cu(HCOO)2] phase, as well as the magnetic characterization of the alph a-[Cu(HCOO)2] phase, we reported in this work. Beta- [Cu(HCOO)2] cryst allizes in the monoclinic system, space group P2(1)/a (No. 14), with Z = 2 and cell parameters a = 8.187(2) angstrom, b = 7.922(2) angstrom, c = 3.616(6) angstrom, beta = 122.14(2)-degrees and V = 194.6(5) angs trom3. Least-squares refinement of 795 reflections (I > 3sigma(I)) and 35 parameters gave a final R = 0.044. The structure comprises infinit e bis(mu-oxo-ligand)-bridged chains of edge-sharing [CuO6] elongated o ctahedra, running parallel to the [001] axis. Each chain is connected to four adjacent ones through carboxylate groups leading to a three di mensional network. The beta phase is metastable and slowly transforms into the alpha one at room temperature. A mechanism for the beta --> a lpha transformation is proposed. Microwave absorption (MAMMA) experime nts show that both alpha and beta phases achieve 3d magnetic order nea r 10 and 30 K, respectively. These two polymorphs of anhydrous copper( II) formate present unusual magnetic properties, which have been chara cterized through ac and dc susceptibility as well as magnetization mea surements. Alpha-[Cu(HCOO)2] shows ferromagnetic order at T(c) = 8.2 K , and beta-[Cu(HCOO)2] behaves as a weak ferromagnet with T(c) = 30.4 K. The magnetic behavior of both phases can be analyzed in terms of a 2d square planar Heisenberg model. The intralayer interaction results to be J/k = 4.3 K and J/k = -41.4 K and the interlayer one has been es timated (molecular field) to be J(z)/k = 0.38 and J(z)/k = 0.64 K for the alpha and beta phases respectively.