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

The compounds Ba₂CoSi₂O₇, 1, and BaCo₂Si₂O₇, 2, were obtained by heating mixtures of BaCO₃, CoCO₃, and SiO₂ 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 [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_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 ⁴A₂ electronic ground state. Crystal data:  for 1, formula Ba₂CoSi₂O₇, a = 8.450(1) Å, b = 10.729(1) Å, c = 8.474(1) Å, β = 111.365(8)°, monoclinic, space group = C2/c, Z = 4; for 2, formula BaCo₂Si₂O₇, a = 7.2131(6) Å, b = 12.781(1) Å, c = 13.762(1) Å, β = 90.299(8)°, monoclinic, space group = C2/c, Z = 8.