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Tantalum oxides (Ta$_2$O$_5$) are characterised by attractive physical andchemical properties, such as high dielectric constants and anti-reflectionbehaviour. Recently, Ta$_2$O$_5$ nanoparticles have also been proposed aspossible enhancers of the relative biological effectiveness in hadrontherapyfor cancer treatment. In principle, their electronic properties can beaccurately investigated from first-principles simulations. However, theexistence of several stable polymorphs of these oxides represents a majordifficulty in order to calculate and disentangle their respective spectralfeatures. To assess this problem, we use linear-response time-dependent densityfunctional to investigate the energy loss function $\mbox{Im}(-1/\bar{\epsilon})$, which is a unique fingerprint of the material, in theoptical limit for various polymorphs. We show that the experimental reflectionenergy loss signals can be rationalized and interpreted by assuming that the$\gamma$-phase of Ta$_2$O$_5$ represents the underlying structural model. Wenotice that both the inclusion of local field effects and spin-orbit couplingare crucial to compute the energy loss functions of this material. Finally, tofurther validate the $\gamma$-Ta$_2$O$_5$ polymorph as a model for experimentaltantalum oxide, we compute the reflection energy loss spectra using a MonteCarlo approach, finding an excellent agreement with the experimental data.

In search of the ground state crystal structure of Ta$_2$O$_5$ from ab-initio and Monte Carlo simulations