  The systems Nb$_2$O$_5$-Ta$_2$O$_5$ and Nb$_2$O$_5$-V$_2$O$_5$ were
investigated using thermal analysis, X-ray powder diffraction and thermodynamic
simulations. Solid solution formation is possible for both systems;
furthermore, both contain one intermediate compound, VNb$_9$O$_{25}$ or
Ta$_2$Nb$_4$O$_{15}$, respectively. Phase relationships for pure
niobium(V)-oxide and tantalum(V)-oxide were studied under ambient pressure. It
was found that both compounds can occur in two stable solid modifications. For
niobium(V)-oxide this are the monoclinic high-temperature modification
(H-Nb$_2$O$_5$) and an orthorhombic low-temperature modification
(T-Nb$_2$O$_5$) and for tantalum(V)-oxide a tetragonal high-temperature form
($\alpha$-Ta$_2$O$_5$) and an orthorhombic low-temperature form
($\beta$-Ta$_2$O$_5$). Based on these results, crystal growth experiments with
various compositions from both systems were carried out using the optical
floating zone (OFZ) technique.
