Abstract
Na 1− x K x MgF 3 (0≤ x≤1) solid-solutions were synthesized and the phase diagram of NaMgF 3–KMgF 3 system was determined by high temperature X-ray powder diffraction experiments and differential thermal analysis (DTA). This system is characterized by a complete series of solid solutions, with a minimum in the solidus at 30 mol% KMgF 3 and 1279 K. No immiscibility gap was found. The crystal system changes from orthorhombic to cubic at x=0.22 at room temperature. The volume change of the unit cell as a function of composition shows a large deviation (excess volume) from Vegard’s law for solid solution. The transition temperatures of NaMgF 3 and Na 0.9K 0.1MgF 3 from orthorhombic to cubic are 1043 and 723 K, respectively. The transition temperature decreases rapidly by the effect of replacing Na by K. Axial ratios of b/ a and c/√2 a in orthorhombic NaMgF 3 and Na 0.9K 0.1MgF 3 decrease linearly with temperature toward the transition and then discontinuously changes to cubic at the transition point.
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