Polymorphic phase transitions in mixed alkali magnesium fluoride solid solutions

Phase transitions in the miscible solid solution Na1-xKxMgF3 were examined over a wide range of compositions by computer molecular dynamics, X-ray dif fraction, and thermal analysis in order to characterize the polymorphic pha se transitions as a function of alkali-metal content. The pure NaMgF3 compo sition has a single orthorhombic-to-cubic transition at 1038 K, but compute r modeled compositions with K+ partially substituted for Na+ ions have at l east two polymorphic transitions. The models also indicate that the transit ion temperatures decrease with increasing potassium content, Results from t hermal analyses and from literature give similar results, Computer simulati ons, experimental data, and literature values all show a room-temperature t ransition for the composition around Na0.65K0.35MgF3. (C) 2000 Elsevier Sci ence Ltd. Ail rights reserved.