The Role of Dislocations in the Thermal Dehydration of Lithium Formate Monohydrate

Abstract

The kinetics of Lithium Formate Monohydrate (LFM) dehydration (single crystal samples) has been investigated at constant temperature (317 < T < 353) under dynamic vacuum both optically and thermogravimetrically.The process takes place with an induction period followed by a constant rate period. The first one is characterized by a surface nucleation which occurs mostly in connection with dislocations emerging on the (021), (110) and (010) faces and at the vertex-intersections of these planes; the second one corresponds to the advancement of a reaction interface (formed on the planes with / 0) along the c-axis. This topochemical model has been interpreted on the basis of the different features of the actual dislocations and of the reticular geometry of the LFM crystal.This work, aiming at investigating the role of surface defects and reticular geometry on solid state reactions, reports the results concerning the dehydration of Lithium Formate Monohydrate (LFM), a crystal which finds application for non-linear optical devices in lasers [1].The structure of this compound has been recently determined [2], The lithium ion is tetrahedrally coordinated with three oxygen atoms of the formate group and one water oxygen. The water molecules, bound together by hydrogen bonds, form infinite zigzag chains parallel to the c-axis; they are also linked to the formate group through the following links: 0(H 2 0)Li and H(H,0) O (formate group).Furthermore, Klapper has investigated the structural defects of this compound by X-ray topography [3] Please order a reprint rather than making your own copy.Moreover, the heat of dehydration, the value of which was not available in the literature, has been determined. ExperimentalLFM single crystals were prepared by slowly evaporating at 303 K a solution obtained by neutralization of aqueous HCOOH (C. Erba RPE) with Li 2 C0 3 (Fluka puriss.) : the single crystals obtained had grown preferably along the c-axis and averaged 1 x 2.5 x 4 mm. X-ray analysis and polarizing microscopy allowed us to state that the most developed faces were the following: (110), (010) and (021).The etching tests were carried out by dipping these crystals for a few seconds into water (or a mixture water-propanol 1 : 1 by vol). Dislocation etch pits have been observed on all these faces. The etch pits on the (110) face had a rectangular basis with the longer side parallel to the [001] direction. Their centres were shifted toward the c-axis direction, thus showing [5] that the corresponding dislocation lines were not perpendicular to the plane considered (Figure 1 a, b).On the (010) face the etch pits, again with rectangular basis and with the longer side parallel to the [001] direction, were more symmetrical and lined-up along the [100] and [001] directions (Figure 2).The etch pits observed on the (021) face were highly asymmetrical (Figure 3).To gain direct information on the nucleation process, a small-sized vacuum-proof reactor, adjustable on the sample holder slide of the microUnauthenticated Download Date | 5/12/1...