$B1$-to-$B2$ phase transition of transition-metal monoxide $\mathrm{CdO}$ under strong compression

This paper reports the experimental research on the compressibility and phase transition of $\mathrm{CdO}$ up to $176\mathrm{GPa}$ at room temperature using high-resolution angular-dispersive x-ray diffraction from synchrotron source combined with the diamond anvil cell technique. The phase transition from $\mathrm{NaCl}$ $\left(B1\right)$- to $\mathrm{CsCl}$ $\left(B2\right)$-type structure for $\mathrm{CdO}$ was observed at pressure about $90.6\mathrm{GPa}$, which is uniquely observed in the transition-metal oxide family for the pressure-induced direct $B1\text{−}B2$ transition and in good agreement with the theoretical prediction. The bulk modulus of the $B1$ and $B2$ phases of $\mathrm{CdO}$ at zero pressure was estimated as 147 and $169\mathrm{GPa}$, respectively. The $3d$ and $4d$ transition-metal monoxides $\mathrm{ZnO}$ and $\mathrm{CdO}$ were both predicted to undergo a $B1$-to-$B2$-type structural transformation under high pressure based on first-principles total energy calculations; the difference in the bulk moduli between the results of this experiment and simulations, and the possibility of a similar $B1$-to-$B2$ phase transition in $\mathrm{ZnO}$ are discussed.

Figure 1

(Color online) The ADXRD patterns of $\mathrm{CdO}$ during the $B1$-to-$B2$ phase transition.

Figure 2

(Color online) A typical Rietveld refinement result($R_{wp}=1.66\%$, $R_p=1.23\%$) for the XRD pattern measured under $170\mathrm{GPa}$.

Figure 3

(Color online) Molar volume of $\mathrm{CdO}$ as a function of pressure (the error bars are omitted since they are smaller than the symbol size).