Phase Transitions in Lead(II) Fluoride Upon Milling
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ABSTRACT PbF 2 is known to exist under two different polymorphic structures. Orthorhombic oa-PbF 2 is stable at ambient temperature. It has the PbC12 structure. Cubic P3-PbF 2 is obtained by heating a-PbF 2. It does not transform back to a-PbF 2 on cooling, and it seems to be infinitely stable in the metastable state under ambient conditions. 13-PbF 2 crystallizes in the fluorite (CaF2) type. Owing to the large number of potential interstitial sites, many F- Frenkel defects can be formed, which make P3-PbF 2 the highest performance fluoride ion conductor among binary fluorides. In this work, both phases of PbF 2 have been ball milled. Milling ax-PbF 2 results in a partial transformation to microcrystalline 13-PbF 2 . The energy required for obtaining the high temperature phase is probably provided in the mechanical form. Milling f3-PbF 2 leads to partial amorphization and formation of a-PbF 2. In this case, milling transforms the high temperature polymorph to the low temperature form, by providing the energy required to overcome the activation energy that keeps PbF 2 trapped in the high temperature P-form after cooling INTRODUCTION Phase transformations in solids can be very sluggish, due to the difficulty for ions or atoms to move when they are trapped in a solid lattice. More particularly, the sudden decrease of temperature when samples are quenched from high temperature to ambient temperature can stabilize high temperature phases well below their temperature of thermodynamic stability for prolonged periods of time. This has been observed in SnF 2. The ambient temperature phase, monoclinic a-SnF2, when heated above 150 'C, gives tetragonal 7'-SnF 2. Upon quenching, ,-SnF2 remains metastable down to 66 'C, at which point it undergoes a fully reversible and nonquenchable second order phase transition to give orthorhombic P3-SnF 2 [1-3]. So, not only that a high temperature phase can be quenched below the minimum temperature where it becomes stable, but in addition, it can undergo its own phase transitions while being in the metastable state, thereby giving rise to phases that have no temperature of thermodynamic stability, which therefore can exist only in the metastable state. This is the case of 3-SnF2. The time length of metastability of P-SnF2 is highly unpredictable. We have observed the transformation back to stable a-SnF2 at ambient temperature to start within minutes of quenching and go rapidly to completion. In other occasions, we have also observed 1-SnF2 to remain metastable for several days, and once the transformation to a-SnF 2 started, it went very slowly, and was not complete yet after two months. Mechanical treatments, as mild as gentle manual grinding in a mortar, are always sufficient to trigger the transformation of P3-SnF 2 to stable a-SnF 2 and lead to rapid completion (usually in minutes). Ambient temperature a-PbF 2 is orthorhombic, space group Pbnm (standard space group: Pnma, No 62), PbC12 type. The unit-cell parameters are a = 7.63574 A, b = 6.42689 A, c = 3.89098 A at 18 TC. This structu
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