Pressure-induced phase transitions in rubidium azide: Studied by in-situ x-ray diffraction

We present the in-situ X-ray diffraction studies of RbN3 up to 42.0 GPa at room temperature to supplement the high pressure exploration of alkali azides. Two pressure-induced phase transitions of α-RbN3 → γ-RbN3 → δ-RbN3 were revealed at 6.5 and 16.0 GPa, respectively. During the phase transition of α-RbN3 → γ-RbN3, lattice symmetry decreases from a fourfold to a twofold axis accompanied by a rearrangement of azide anions. The γ-RbN3 was identified to be a monoclinic structure with C2/m space group. Upon further compression, an orthogonal arrangement of azide anions becomes energetically favorable for δ-RbN3. The compressibility of α-RbN3 is anisotropic due to the orientation of azide anions. The bulk modulus of α-RbN3 is 18.4 GPa, quite close to those of KN3 and CsN3. By comparing the phase transition pressures of alkali azides, their ionic character is found to play a key role in pressure-induced phase transitions.