Vibrational and elastic properties as a pointer to stishovite to CaCl2 ferroelastic phase transition in RuO2

The high pressure behavior of RuO2 has been investigated using the first principles calculations based on the density functional theory. Here, we report the calculated properties on structure, electronic, elastic, phonon and electron–phonon coupling of rutile and CaCl2 type RuO2. Our calculated lattice constants and bulk modulus agree reasonably well with the available theoretical and experimental data. Results of our calculations of zone center phonon modes at ambient and high pressures are in good agreement with Raman scattering measurements. Elastic (shear) modulus and phonon analysis confirm the ferroelastic phase transition from rutile to CaCl2 phase in RuO2. It is clearly illustrated that the first transition is associated with macroscopic shear instability which arises from the strong coupling between elastic constants and softening of Raman active B1g mode. The observed pressure of phase transition in experimental measurements was reproduced more accurately than in previous calculations and the difference between observed and calculated transition pressure is only of the order of few percentage. We have also analyzed the rotation angle for O-R-O corresponding to B1g mode and found that the rotation of bond about 80 is required for transition from rutile to CaCl2 phase in RuO2. In addition, the mode Grüneisen parameter is discussed. The electron–phonon interaction in both rutile and CaCl2 type RuO2 at high pressure is also discussed.