Two modifications of layered cobaltous terephthalate: Crystal structures and magnetic properties

The synthesis, characterization, crystal structure, and magnetic properties of Co-2(OH)(2)BDC and Co(H2O)(2)BDC (BDC is benzene 1,4-dicarboxylate or t erephthalate, O2CC6H4CO2) are reported. Co-2(OH)(2)BDC (C2/m, a = 19.952, b = 3.286, c = 6.295 Angstrom, beta = 95.84, Z = 2, V = 410.6 Angstrom (3)) consists of two types of edge-sharing CoO6 chains that are connected to eac h other by OH (Co-O-Co bridges) to form layers that are further joined toge ther through terephthalate, It exhibits unusual magnetic properties: above 48 K it displays paramagnetism, between 44 and 48 K it behaves as a colline ar antiferromagnet, and below 44 K a weak spontaneous magnetization is obse rved in very low applied fields. At higher fields metamagnetic behavior is observed, Two types of hysteresis loops are observed, one centered about ze ro field and the second about the metamagnetic critical field, The critical field and the hysteresis width increase as the temperature is lowered; the latter exceeds 50 kOe at 2 K, The unusual large coercivity may be due to t he large single-ion anisotropy of Co(II) and its pronounced increase at low temperature, Co(H2O)(2)BDC (C2/c, a = 18.274, b = 6.543, c = 7.296 Angstro m, beta = 98.6 degrees, Z = 4, V = 862.5 Angstrom (3)) consists of layers o f separated octahedral CoO6 connected via Co-O-C-O-Co, It is paramagnetic a nd obeys the Curie-Weiss law (Theta = -27 K), (C) 2001 Academic Press.