Syntheses, Structural Analyses, and Unusual Magnetic Properties of Ba2CoSi2O7 and BaCo2Si2O7

The compounds Ba 2 CoSi 2 O 7, 1, and BaCo 2 Si 2 O 7, 2, were obtained by heating mixtures of BaCO 3, CoCO 3, and SiO 2 in the proper stoichiometries to 1300 °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 [Si 2 O 7 ] 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 [Si 2 O 7 ] 6 - ions in an extended two-dimensional arrangement. The barium ions are arranged in layers that run parallel to the Co[Si 2 O 7 ] layers. In 2 each cobalt ion is also surrounded by an arrangement of four oxygen atoms derived from the Si 2 O 7 groups in a distorted tetrahedral shape. These tetrahedra are linked into chains through the sharing of one of the oxygen atoms from the Si 2 O 7 groups. The CoO 4 chains are crosslinked via O−Si−O−Si−O bridges from the Si 2 O 7 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 c can be explained by spin canting and spontaneous domain formation. The canting angle is estimated to be about 5° 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 Ba 2 CoSi 2 O 7, a = 8.450(1) Å, b = 10.729(1) Å, c = 8.474(1) Å, β = 111.365(8)°, monoclinic, space group = C 2/ c, Z = 4; for 2, formula BaCo 2 Si 2 O 7, a = 7.2131(6) Å, b = 12.781(1) Å, c = 13.762(1) Å, β = 90.299(8)°, monoclinic, space group = C 2/ c, Z = 8.