Thermal analysis and crystal growth of doped Nb$_2$O$_5$

TL;DR

This study investigates the thermal properties, phase relationships, and crystal growth of doped Nb$_2$O$_5$ systems, revealing solid solutions, intermediate compounds, and stable phase modifications through thermal analysis, X-ray diffraction, thermodynamic simulations, and crystal growth experiments.

Contribution

It provides new insights into phase stability, solid solution formation, and crystal growth techniques for Nb$_2$O$_5$-based systems, including detailed phase relationships and modifications.

Findings

Solid solutions are possible in Nb$_2$O$_5$-Ta$_2$O$_5$ and Nb$_2$O$_5$-V$_2$O$_5$ systems.

Both systems contain intermediate compounds VNb$_9$O$_{25}$ and Ta$_2$Nb$_4$O$_{15}$.

Multiple stable phase modifications of Nb$_2$O$_5$ and Ta$_2$O$_5$ were identified.

Abstract

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.