Reactions of LnCl(3)center dot 6H(2)O (Ln = La-Nd, Sm, Eu), concentrated (11 M) perchloric acid, and molten boric acid result in the formation of four different compounds. These compounds are Ln[B8O10(OH)(6)(H2O)-(ClO4)]center dot 0.5H(2)O (Ln = La-Nd, Sm), Pr[B8O11(OH)(4)(H2O)(ClO4)], Ln[B7O11(OH)(H2O)(2)(Cl((4))] (Ln = Pr, Nd, Sm, and Eu), and Ce[B8O11(OH)(4)(H2O)(ClO4)]. All Ln(III) cations are ten-coordinate with a capped triangular cupola geometry and contain an inner-sphere, monodentate perchlorate moiety. This geometry is obtained because of the coordination of the oxygen donors within the polyborate sheet which create triangular holes and provide residence for the lanthanide metal centers. Aside from Ln[B8O10(OH)(6)(H2O)-(ClO4)]center dot 0.5H(2)O (Ln = La-Nd, Sm), which are two-dimensional sheet structures, all other compounds are three-dimensional frameworks in which the layers are tethered together by BO3 units found roughly perpendicular to the sheets. Furthermore, a change in product is observed depending on the reaction duration while holding all other synthetic variables constant. This report also demonstrates that lanthanide borates can be prepared in extreme acidic conditions.