Hysteresis in the β–α phase transition in silver iodide

Abstract

IntroductionSilver iodide can exist in three crystal polymorphs at atmospheric pressure [1]. Below 147°C the b-phase is stable; this has a wurtzite structure with the iodide ions arranged in a hexagonal close packed structure and with the silver ions occupying a sublattice of interstitial tetrahedral sites. Above 147°C silver iodide undergoes a crystalline transition to the high temperature a-phase, which has a body centred cubic arrangement of iodine ions with the silver ions distributed over a sublattice of interstitial sites whose number exceeds the occupancy of silver ions. This phase is stable up to the melting temperature of 555°C. Also present below 147°C is the metastable g-phase which differs from the b-phase in that it has a zinc blende structure with the iodide ions occupying a cubic close packed lattice. This can be viewed as a defect b-phase structure in which the iodide ions are layered ABABAB rather than ABCABC. Migration of Ag + though the disordered cationic lattice gives rise to exceptionally high ionic conductivity of the a-AgI phase [2]. The ordering of Ag + has been studied using computer simulations and it is the tendency of the cations to condense into a locally monoclinic arrangement which is unstable relative to the hexagonal (wurtzite) structure which is proposed as the driving force for the a-b phase transition [3].Previous investigators have reported the heat capacity of silver iodide using conventional and AC calorimetry [4,5]. These measurements have been carried out in heating only whereas Hanaya et al. have reported DSC studies on heating and cooling of the stabilisation of a-AgI by incorporating AgI inside porous silica [6]. In this work the nucleation of the low temperature phase is claimed to be inhibited by the reduction in AgI particle size. For bulk AgI claims have been made that the b-a phase transition can be made to occur around 100°C when the specimen is heated by microwave radiation rather than be conventional heating [7]. In this particular case, multi-phonon coupling between the electromagnetic field and vibrational motions within the AgI crystal are conjectured to stabilise the high temperature form in the same way that mechanical stress by reduction in particle size might also achieve the same effect. An alternative mechanism is the so-called 'ponderomotive effect' whereby microwave-excited ionic currents become locally rectified at interfaces (such as grain boundaries) so as to promote mass transport [8]. In the case of AgI there is no bulk diffusion of material during its structural transformation but it is not unreasonable to propose that this process might promote the formation of a-AgI.This work reports studies on the silver iodide a-b phase transition by conventional and modulated-temperature calorimetry. We also report dielectric property measurements under conventional and microwave heating. ExperimentalThe silver iodide powder (99.999%, Acros Organics) used for these studies was made into pellets with a density of 4.9±0.3 g cm -3 , i.e. 83% of theoretical d...

Giant barocaloric effects over a wide temperature range in superionic conductor AgI

“…The β polymorph has iodine anions in a hexagonal-close-packed structure 27 , 29 , 35 , the γ polymorph has iodine anions in a cubic-close-packed structure 27 , 36 , 37 . The interstitial silver cations in γ-AgI and β-AgI are relatively ordered, whereas in α-AgI they are disordered across a fraction of the tetrahedral interstices, such that the entropy difference |Δ S 0 | = 63 ± 4 J K −1 kg −1 for the α ↔ β + γ transition is large 38 – 40 .…”

Hybrid microwave∕conventionally heated calorimeter

“…28 Furthermore, because ␣-silver iodide couples very strongly with a the microwave field, it proved very difficult to cool specimens from this state using the compressed air supply in the presence of microwaves. However, studies on the structural transition in silver iodide by conventional thermal analysis techniques showed that the phase change exhibited hysteresis in that transformation is not perfectly reversible.…”

Switching behavior of bulk, fast ion conducting, vitreous AgI‐Ag2O‐MoO3 solids with inert electrode

“…Bulk (AgI) x ‐(Ag 2 O) 25 ‐(MoO 3 ) 75− x , for 60 ≤ x ≤ 40, vitreous solids have been prepared by microwave melting metal plate quenching method. The fundamental idea behind AgI‐based solid electrolytes with high ionic conductivity at room temperature is to hinder the α → β phase transition at lower temperatures, by introducing stabilizer ions into the lattice, that is, preserving the α‐AgI at a much lower temperature, in other words, a lower phase transition temperature . This can be efficiently achieved by microwave irradiation technique, the β—α phase transition occurs at around 100°C in this case .…”

“…The fundamental idea behind AgI‐based solid electrolytes with high ionic conductivity at room temperature is to hinder the α → β phase transition at lower temperatures, by introducing stabilizer ions into the lattice, that is, preserving the α‐AgI at a much lower temperature, in other words, a lower phase transition temperature . This can be efficiently achieved by microwave irradiation technique, the β—α phase transition occurs at around 100°C in this case . The microwave energy interacts with the crystal via phonon coupling and the low‐lying transverse optic (TO) modes dominate; Ag + mostly moves in these low energy modes that leads to Frenkel defects in the structure, allowing ionic conduction …”