Perovskite-like Metal Formates with Weak Ferromagnetism and as Precursors to Amorphous Materials

Three isomorphous compounds M(CHOO)3[NH2(CH3)2] (M = Mn(1·Mn), Co(2·Co), Ni(3·Ni)) have been synthesized
in solvothermal conditions. Single-crystal X-ray diffraction shows that they are all crystallized in the trigonal space
group R 3̄c with small differences in the lattice parameters. Bridged by the three-atom single-bridge CHOO-, M
ions form a three-dimensional distorted perovskite-like structure with dimethylamine (DMA) cations located in the
cages of the network. Based on the magnetic data, these three 3D compounds are weak ferromagnets with the
critical temperature Tc = 8.5 K (1·Mn), 14.9 K (2·Co), and 35.6 K (3·Ni), and for 2·Co and 3·Ni, spin reorientation
might take place at 13.1 and 14.3 K, respectively. At 1.8 K, hysteresis loops can be observed for all three compounds
with the coercivity field ca. 90 Oe (1·Mn), 920 Oe (2·Co), and 320 Oe (3·Ni). The canting angles are estimated
to be 0.08°, 0.5°, and 0.6° for 1·Mn, 2·Co, and 3·Ni, respectively. The magnetic coupling between MnII ions in
1·Mn was estimated based on the model developed by Rushbrook and Wood for a Heisenberg antiferromagnet on
a simple cubic lattice and the best fit gives J = −0.23 cm-1. At the same time, according to molecular field theory
of antiferromagnetism, the J values for compounds 1·Mn, 2·Co, and 3·Ni were estimated to be −0.32 cm-1, −2.3
cm-1, and −4.85 cm-1, respectively. The spin cant in these compounds may originate from the noncentrosymmetric
character of the three-atom single-bridge CHOO-. Furthermore, amorphous materials 4·Mn238, 5·Mn450, 6·Co320,
and 7·Ni300 were prepared from precursors 1−3 under an argon atmosphere at different temperatures according
to the thermogravimetric analyses. As an interesting result, 5·Mn450 was confirmed to be an amorphous form of
Mn3O4 with a considerably large coercivity field HC = 4.1 kOe at 30 K compared to that value (250 Oe) for bulk
Mn3O4.