The structure of the second phase ( (\beta)-) of (\mathrm{BaVSi}{2} \mathrm{O}{7}) has been solved by single crystal X-ray diffraction, and its magnetic properties have been examined in the temperature range (5-300 \mathrm{~K}). (\beta) (\mathrm{BaVSi}{2} \mathrm{O}{7}) crystallizes in the tetragonal system, space group (14 / m) (No. 87), with (a=7.0535(7) \AA, c=11.467(2), V=570.5(3) \AA^{3}), and (Z=4). This structure can be considered as consisting of (\mathrm{VO}{5}^{6-}) square pyramids and unbranched single rings of (\left[\mathrm{Si}{4} \mathrm{O}{12}\right]^{8-}), or of cage-like (\left[\mathrm{Si}{4} \mathrm{~V}{2} \mathrm{O}{18}\right]^{12-}) clusters formed by (\mathrm{SiO}{4}^{4-}) and (\mathrm{VO}{5}^{6-}). These building elements are cross-linked to form a pseudo-twodimensional (2D) network containing empty channels perpendicular to the (\boldsymbol{c})-axis. The 2D networks are held together by (\mathbf{B a}^{2+}) ions that occupy channels parallel to the (c)-axis. (\beta) - (\mathrm{BaVSi}{2} \mathrm{O}{7}) undergoes short range magnetic order below (\sim 20 \mathrm{~K}). Its behavior can best be described as an (S=\frac{1}{2}) dimer with (J / k=-19.0 \mathrm{~K}, \bar{g}=1.87). The fresnoite-type (\mathrm{Ba}{2} \mathrm{VSi}{2} \mathrm{O}_{8}) is found to be a Curie-Weiss paramagnet down to 4.5 K. 1994 Academic Press, Inc.