Syntheses, structural analyses, and unusual magnetic properties of Ba2CoSi2O7 and BaCo2Si2O7

The compounds Ba(2)CoSi(2)O7, 1, and BaCo2Si2O7, 2, were obtained by heating mixtures of BaCO3, CoCO3, and SiO2 in the proper stoichiometries to 1300 degrees C for 48 h. Both materials were characterized by single-crystal X-ray diffraction and magnetization measurements on powder and partially oriented single crystals. Each cobalt ion in 1 is surrounded by four oxygen atoms derived from the [Si2O7](6-) groups in the form of a distorted tetrahedron. The cobalt ions are bridged by O-Si-O and O-Si-O-Si-O groups from the [Si2O7](6-) ions in an extended two-dimensional arrangement. The barium ions are arranged in layers that run parallel to the Co[Si2O7] layers. In 2 each cobalt ion is also surrounded by an arrangement of four oxygen atoms derived from the Si2O7 groups in a distorted tetrahedral shape. These tetrahedra are linked into chains through the sharing of one of the oxygen atoms from the Si2O7 groups. The CoO4 chains are crosslinked via O-Si-O-Si-O bridges from the Si2O7 groups. The barium ions occupy channels that run parallel to the crystallographic c-axis. Curie-Weiss behaviors and temperature independent paramagnetisms (TIP) coexist in the ranges 40-300 K and 100-300 K for 1 and 2, respectively. For both compounds the values of the Curie constant and TIP are consistent with tetrahedral Co(II) ions having three unpaired electrons. While compound 1 remains paramagnetic down to 5 K, 2 undergoes a transition into a state having a field-induced weak ferromagnetic response at T-c = 21 K. The weak ferromagnetic properties below T,can be explained by spin canting and spontaneous domain formation. The canting angle is estimated to be about 5 degrees at 5 K. Interestingly, 2 could represent the first example among the tetrahedral Co(II) compounds with the transition temperature (T-c = 21 K) being higher than the zero-field splitting of the (4)A(2) electronic ground state. Crystal data: for 1, formula Ba2COSi2O7, a = 8.450(1) Angstrom, b = 10.729(1) Angstrom, c = 8.474(1) Angstrom, beta = 111.365(8)degrees, monoclinic, space group = C2/c, Z = 4; for 2, formula BaCo2Si2O7, a 7.2131(6) Angstrom, b = 12.781(1) Angstrom, c 13.762(1) Angstrom, beta = 90.299(8)degrees, monoclinic, space group = C2/c, Z = 8.