Tantalum pentoxide (${\mathrm{Ta}}_2{\mathrm{O}}_5$) is a component of technologies ranging from photographic lenses to medical implants. Despite more than six decades of interest, however, there is no consensus on the crystal structure of ${\mathrm{Ta}}_2{\mathrm{O}}_5$. Here, we report the results of neutron diffraction experiments on ${\mathrm{Ta}}_2{\mathrm{O}}_5$ powders at temperatures up to $1000{}^{\circ }\mathrm{C}$. Because neutron diffraction is sensitive to the positions of O atoms in the presence of heavy elements, our results provide strong evidence that the orthorhombic ${\mathrm{L}}_{\mathrm{SR}}$ structure can account for experimental observations better than any other proposal in the literature. We also determine the anisotropic crystallographic thermal expansion parameters of ${\mathrm{Ta}}_2{\mathrm{O}}_5$, which are important for modeling the performance of ${\mathrm{Ta}}_2{\mathrm{O}}_5$-bearing technologies.

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• Crystal structure
• Thermal expansion
• Neutron diffraction