$\beta$-Technetium: An allotrope with a nonstandard volume-pressure relationship

We report the synthesis and structure of the second allotrope of technetium, $\beta \text{−}\mathrm{Tc}$. Transformative pathways are accessed at extreme conditions using the laser-heated diamond anvil cell and confirmed with in situ synchrotron x-ray diffraction and Raman spectroscopy. $\beta \text{−}\mathrm{Tc}$ is fully recoverable to ambient conditions, although counter to our DFT calculations predicting a face-centered-cubic lattice, we observe a tetragonal structure $(I4/mmm)$ that exhibits further tetragonal distortion with pressure. $\beta \text{−}\mathrm{Tc}$ has an expanded volume relative to the hcp ground state phase, that when doped with nitrogen has an unexpected volume lowering. Such anomalous behavior is possibly indicative of a rare electronic phase transition in a $4d$ element.

Figure 1

XRD patterns of a Tc at room temperature pre (hcp, bottom) and post ($\beta \text{−}\mathrm{Tc}$, top) laser heating. Black open circles are the experimental data, the Le Bail refinement is shown in red, and the residual is shown in blue. All peaks were attributed to Ar or Tc; tick marks for each phase shown below peaks. Inset: Raman spectrum of a room temperature Tc sample pre (hcp, black) and post ($\beta \text{−}\mathrm{Tc}$, blue), laser heating.

Figure 2

Diagram showing the phase stability domains of hcp and $\beta \text{−}\mathrm{Tc}$. The open markers represent a pressure-temperature condition where a single phase of hcp (squares) or bct (circles) could be confirmed by XRD. Stars indicate a point where the hcp-bct transformation was observed. Data at 15 GPa and 1800 K representing the transformation of $\beta \text{−}\mathrm{Tc}$ was uncharacteristically high due to a jump in temperature during laser heating, yet it is shown for completeness. Triangles indicate the P,T conditions where N-doped $\beta \text{−}\mathrm{Tc}$ is formed.

Figure 3

(a) The difference in compression behavior of $\beta \text{−}\mathrm{Tc}$ at room temperature when formed in different PTM compared to simulations. Tabulated data points in Supplemental Material [15]; hcp experimental data is from Ref. [3]. (b) Left: Peak splitting of the 200 and 112 reflections as a function of pressure in $\beta \text{−}\mathrm{Tc}$. Right: The absence of peak splitting in equivalent peaks of ${\mathrm{TcN}}_x$.

Figure 4

EBSD results of the recovered Tc sample. (a) processed EBSD pattern and (b) indexed pattern as Tc phase with MAD = 0.6. The results of EBSD point analysis identifies an fcc lattice, and is consistent with the N-doped $\beta \text{−}\mathrm{Tc}$ phase as confirmed by Rietveld analysis using XRD.

Figure 5

(a) Enthalpy variation of the DFT optimized $tI2$ structures along the Bain path at 0 GPa (black), 5 GPa (red), and 25 GPa (blue). (b) Enthalpy variation of the DFT optimized $hR1$ structures with $\alpha$ at 0 GPa (black), 5 GPa (red), and 25 GPa (blue). (c) Heat map of the per atom enthalpy variation of a 0 GPa modified $hR1$ structure where $a=b=c$ but $\alpha =\beta \ne \gamma$. All enthalpies are relative to the fcc Tc structure, i.e., $c/a=\sqrt{2}$ for $tI2$ or $\alpha =\beta =\gamma ={60}^{\circ }$ for $hR1$.