  A series of in-plane substituted compounds, including Cu-site
(SrZn$_x$Cu$_{2-x}$(BO$_3$)$_2$), and B-site
(SrCu$_2$(Si$_x$B$_{1-x}$O$_3$)$_2$) substitution, were synthesized by solid
state reaction. X-ray diffraction measurements reveal that these compounds are
single-phase materials and their in-plane lattice parameter depends
systematically on the substituting content $x$. The magnetic susceptibility in
different magnetic fields, the magnetization at different temperatures, and the
resistivity at room temperature were measured, respectively. It is found that
the spin gap deduced from the magnetic susceptibility measurements decreases
with increasing of $x$ in both Cu- and B-site substitution. No
superconductivity was found in these substituted compounds.
