Crystalline forms of silver iodide II: Determination of phase transformations

Abstract

In order to obtain appropriate forms of AgI (β- and γ-), a procedure was developed to synthesize AgI at room temperature (23°C), whereby samples of varying crystallographic purity and of varying crystallographic contents of the different forms were obtained. This paper presents the results of investigations of the influence of the manner of preparation of β-AgI and γ-AgI samples and the sample heating rate on the phase transformations and their temperatures. During the heating of non-ground, ground and pressed synthesized AgI samples, the phase transformations and the corresponding temperatures for one β-AgI and four γ-AgI samples with different β-AgI contents (representing a crystallographic impurity) were identified. The following phase transformations were observed for the non-ground AgI samples: β-AgI → α-AgI at 149.6°C (for the β-AgI sample) and γ-AgI → α-AgI at 148.7°C or 148.2°C for the γ-AgI samples with a minimum content of β-AgI (less than 7 %), as a crystallographic impurity. The phase transition γ-AgI → α-AgI was irreversible because β-AgI was obtained whenever the samples were heated up to 260°C. Manual sample grinding, as well as pressing at p1 = 650 MPa and p2 = 900 MPa resulted in the β-AgI → γ-AgI phase transition in all the investigated cases. .

Keywords

• silver iodide
• heat treatment
• mechanical treatment
• phase transformation
• dsc
• x-ray diffractometry