Phase Equilibria and Tin-Activated Luminescence in Strontium Orthophosphate Systems

Strontium orthophosphate undergoes a rapid, reversible transition at 1305°C. The high‐temperature β form of pure Sr 3 ( PO 4 ) 2 cannot be quenched to room temperature, even with extremely rapid cooling. Determination of phase relationships on the orthophosphate joins between Sr 3 ( PO 4 ) 2 and each of the three orthophosphates, Mg 3 ( PO 4 ) 2 , Ca 3 ( PO 4 ) 2 , and Zn 3 ( PO 4 ) 2 showed that they form β‐ Sr 3 ( PO 4 ) 2 solid solutions which could be cooled to room temperature to form the basis for the so‐called “modified strontium orthophosphate” phosphors. The ranges of composition and temperature over which the β‐ Sr 3 ( PO 4 ) 2 solid solutions exist were determined for each system, and the existence of two ternary compounds, SrMg 2 ( PO 4 ) 2 and SrZn 2 ( PO 4 ) 2 , was confirmed. The latter compound has a transition at 1035°C, but the high‐temperature form can be maintained at room temperature only by extremetly rapid quenching. Equilibrium diagrams of each of the systems are presented to show all stability relationships. The behavior of tin‐activated β‐ Sr 3 ( PO 4 ) 2 solid solution phosphors in each of the three systems was explored under 2537Aå excitation, and the relation of phosphor composition to quantum efficiency, brightness, and temperature stability in the lamp‐making range was determined. The tin‐activated luminescence of the two ternary compositions was investigated using 2537Aå, 3650Aå, and cathode ray excitation.