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.