Series of phase transitions in cesium azide under high pressure studied by in situ x-ray diffraction

In situ x-ray diffraction measurements of cesium azide (CsN${}_3$) were performed at high pressures of up to 55.4 GPa at room temperature. Three phase transitions were revealed as follows: tetragonal (I4/mcm, Phase II) → monoclinic (C2/m, Phase III) → monoclinic (P2${}_1/m$ or P2${}_1$, Phase IV) → triclinic (P1 or P$\overline{1}$, Phase V), at 0.5, 4.4, and 15.4 GPa, respectively. During the II–III phase transition, CsN${}_3$ keeps its layered structure and the azide anions rotate obviously. The compressibility of Phase II is dominated by the repulsions between azide anions. The deformation of unit cell is isotropic in Phases II and IV and anisotropic in Phase III. With increasing pressures, the monoclinic angle increases in Phase III and then becomes stable in Phase IV. The bulk moduli of Phases II, III, IV, and V are determined to be 18 ± 4, 20 ± 1, 27 ± 1 and 34 ± 1 GPa, respectively. The ionic character of alkali azides is found to play a key role in their pressure-induced phase transitions.

Figure 1

The crystal structure of Phase II. The Cs atoms are represented by black spheres and the N atoms by gray spheres.

Figure 2

X-raydiffraction patterns of CsN${}_3$ at selected pressures. (a) Different phases, shown by the Roman numerals to the right of the patterns. The coexistence of two phases at 4.4 and 15.4 GPa indicates the onset of the phase transitions. The asterisks indicate the appearance of new Bragg peaks due to the phase transitions. The three patterns on the top of the figure (with a letter “d” in parentheses besides the pressure) demonstrate the decompression process. (b) The Miller indices of the high-pressure phases.

Figure 3

Rietveld refinement patterns for Phase III at 2.9 GPa. The observed diffraction intensities are represented by the dots, and the calculated pattern is represented by the solid line. The solid curve at the bottom represents the difference between the observed and the calculated intensities. Short vertical bars below the observed and the calculated patterns indicate the positions of allowed Bragg reflections.

Figure 4

The crystal structure of Phase III yielded from the refinement at 2.9 GPa. The Cs atoms are represented by black spheres and the N atoms by gray spheres.

Figure 5

The pressure dependence of the cell parameters a, b, and c of CsN${}_3$.

Figure 6

The pressure dependence of the monoclinic angle β of Phases III and IV. The solid lines demonstrate the linear fitting of the P-β data.

Figure 7

Volume per formula unit change of CsN${}_3$ with pressure. The solid lines demonstrate the fitting of the Phase V data to the second-order Birch-Murnaghan equation of state.