Polymorphism and Melting of Ammonium, Thallous, and Silver Nitrates to 45 kbar

Abstract

The phase diagrams of NH₄NO₃ and TlNO₃ were determined by means of differential thermal analysis to 40 kbar. The NH₄NO₃ I/VI, IV/VI, and I/IV transition lines meet at a triple point near 19.5 kbar, 256°C. The TlNO₃ I/II transition line can be represented by t(degrees Centigrade) = 144.6+7.91P−0.062P², and the melting curve of TlNO₃ I is given by P (kilobars) = 5.00 [(T/479)^3.531−1]. The phase diagram of AgNO₃ was determined by means of DTA and volumetric methods to 45 kbar. In addition to the four phases previously known, metastable AgNO₃ V can be obtained by suitable manipulation above 21 kbar at temperatures below 50°C. The metastable AgNO₃ I/V transition is easily reversible. AgNO₃ V reverts to stable AgNO₃ II if left under pressure overnight. The melting curve of AgNO₃ II is given by P (kilobars) = 9.17[(T/483)^4.438−1].