Crystal Chemistry and Phase Transitions of Perovskite inP–T–XSpace: Data for (KxNa1−x)MgF3Perovskites

Abstract

We report a high-resolution synchrotron X-ray powder diffraction study on (KxNa1−x)MgF3perovskites at high pressures and temperatures. The crystal structure of the perovskite transforms reversibly from orthorhombic (Pbnm) to tetragonal (P4/mbm) to cubic (Pm3m) with change of pressure and temperature and also with K+concentration. The tilt angle of the MgF6octahedral framework decreases continually and the octahedral Mg–F bond length increases slightly with increasing temperature. Correspondingly, at high pressures, the tilting angles increase slightly and the shortening of the Mg–F bond length is the dominant compression mechanism. Both octahedral tilting and bond length decrease with increasing K+participation. We have monitored superlattice diffraction associated with in-phase and antiphase octahedral tiltingsin situand in real time inP–T–Xspace. The intensities of the superlattice diffraction peaks decrease quickly toward zero with increasing temperature and/or K+concentration. Meanwhile, the doublet and triplet diffraction peaks, which directly reflect the dimensional differences of thea,b, andcaxes, converge continuously and finally coalesce into single peaks as the temperature reachesTcand/or the K+concentration reachesXc. The transition temperatures are plotted as a function of pressure (Tc–P), and as a function of composition (Tc–X). The structural phase transitions in (K,Na)MgF3perovskites are driven by two order parameters, which are represented by the octahedral tiltingsθ=(φ−x,φ−y, 0) andϕ=(0, 0,φ+z). The strong coupling between these two order parameters drives the end-member NaMgF3perovskite to transform directly from the orthorhombic to cubic phase at temperatureTc=1038 K. Such direct orthorhombic–cubic phase transition remains unbroken even at pressures up to 10 GPa. However, with increasing incorporation of larger K+cations into the crystal structure, the interaction between the octahedral tiltings was successively decoupled and the purely intermediate tetragonal phase thus appeared aroundX∼35%. The crystal structure of the (KxNa1−x)MgF3perovskites eventually becomes cubic atX∼40%.