Kinetics of Phase Transitions in Superionic Pbsnf 4 Versus Temperature
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Mat. Res. Soc. Symp. Proc. Vol. 398 0 1996 Materials Research Society
pairs perpendicular to the ?' axis of the unit-cell, thereby making the structure of PbSnF 4 very highly anisotropic. Several phases of PbSnF 4 have been reported. By precipitation from aqueous solutions, we prepared orthorhombic o-PbSnF 4 and highly stressed tetragonal a-PbSnF 4 [2]. By direct reaction of the two fluorides at high temperature, unstressed tetragonal a-PbSnF 4 and tetragonal P3-PbSnF 4 are obtained [3]. All these phases are sufficiently stable, or metastable, at ambient temperature, to make it possible to study them in details. To the contrary, a non-quenchable phase, cubic ,-PbSnF 4, is observed above 3900C, and shows complete Pb/Sn disorder [2]. The preparation and sequence of phase transitions we had observed in PbSnF 4 prior to this work have already been published [1-3]. Two monoclinic phases have also been reported to be prepared from aqueous solutions or by cooling below 70K [4]. The purpose of this work was to investigate further the phase transformations of PbSnF4 versus temperature, particularly in samples heated for long period of times. EXPERIMENTAL Stressed a-PbSnF 4 and o-PbSnF 4 were prepared by adding dropwise a 1.7M aqueous solution of Pb(N0 3)2 to a 1.5M fresh aqueous solution of SnF 2 upon stirring. SnF 2 was dissolved inH2 0 for the preparation of a-PbSnF 4 and in aqueous HF for the preparation of o-PbSnF 4 , such that Vol.(40% HF)/[Vol.(H 20)+Vol.(40% HF)] = 0.05. Unstressed a-PbSnF 4 and 13-PbSnF 4 were prepared by direct reaction between stoichiometric quantities of SnF 2 and PbF 2 in sealed copper tubes, under dry nitrogen, according to literature procedures [3, 5]. For the study of the kinetics of phase transitions, the samples were heated at each temperature versus time. Thermal analysis was performed on an Omnitherm DTA/TGA instrument equipped with a STA 1500 module and ATII hardware and software. X-ray diffraction was carried out on a Philips X-ray powder diffractometer automated with the SIEl12 Progress of phase transformations were estimated system from Sietronics. qualitatively from the intensity of the Bragg peaks of each phase present however, because of the high degree of preferred orientation due to the layered structure, quantitative analysis is not possible. RESULTS AND DISCUSSION Thermal analysis The DTA curves for the four samples are shown on figure 2. o-PbSnF 4 has a small endothermic peak at ca. 700C, whereas the others do not. This is in agreement with our DSC results which show an endothermic peak at 730C for o-PbSnF 4 and a very diffuse less intense peak also centered at around 70-750C for a-PbSnF 4, the
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I -PFb
a-PbSnF 4
f3-PbF 2 0: F-
*
: Pb 2 รท
0 :Sn(lI) with lone pair
Figure 1: Projection of the structures of f3-PbF 2 and a-PbSnF 4 in the (tic plane (p-PbF 2 axes).
I
100
200
I
I
300
400
31
32 Deg. (20)
TEMPERATURE (oC) Figure 2: DTA curves of (a): o-PbSnF 4,
Figure 4: X-ray diffraction pattern of
(b): stressed ct-PbSnF 4, (c): unstressed ct-PbSnF 4 , a
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