Commensurate-Glass Phase Transitions in Staged SbCl 5 -GIC
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PHASE TRANSITIONS IN STAGED SbCI 5 -GIC
L. SALAMANCA-RIBA,t G. TIMP,* L.W. HOBBS§ and M.S. DRESSELHAIJS* Center for Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
ABSTRACT An experimental study is reported of the commensurate (/7 x /7)R19.1* to glass phase transition in stages I < n < 4 graphite-SbCl 5 using electron and x-ray diffraction. The electron diffraction studies show an in-plane phase transition to a glass at low temperatures. The (00) x-ray diffraction scans show that the c-axis layered structure of the intercalate is unchanged by the phase transition. A driving mechanism for the commensurate to glass transition is suggested.
INTRODUCTION Although graphite intercalation compounds (GIC) show a variety of structural phase transitions, those occurring in the SbCI 5 system are of particular interest because they involve a commensurate to glass transition, and it is the low temperature phase which is the glassy phase [Il. In the present work, (hkW) electron diffraction is used to establish the temperature dependence of the in-plane intercalate structure and to monitor the phase transition from a (6/7 x /7)R19.1 0 structure to a glassy phase on cooling and heating the SbCl 5 GIC samples for stages 1 < n < 4. The present work elaborates on the previously reported transition for stage 2 compounds [1], and emphasizes the stage dependence of this phenomenon. The two dimensionality of the information provided by the transmission electron microscopy is supplemented with (00) x-ray diffraction to obtain structural information about the c-axis ordering. Particular attention is given to the temperature dependence of the interlayer distances, especially in the temperature ranges where the commensurate to glass phase transition occurs. All samples and all stages studied in this work exhibited this phase transition and showed a large hysteresis under temperature cycling in the range 10 < T < 300 K, consistent with transport properties [2]. In its pristine form, SbCl 5 retains its molecular bonding in both the liquid and solid phases. In the isolated molecule, the Sb atom is at the center of a trigonal bipyramid that has the five Cl atoms at its vertices, such that dSb_3Cl = 2.29 A, and dSb_2Cl = 2.34 A. Solid SbCl 5 forms a hexagonal lattice (space group P6 3 /mmc) with two molecules per unit cell, and having the following parameters: a = 7.49 A, and c = 8.01 A [3]. These lattice parameters for SbCI 5 suggest that SbCl 5 intercalated into graphite would form a (3 x 3)R30* commensurate superlattice. Solid SbCl 3 forms an orthorhombic crystal structure (space group Pbnm) with a tetramolecular cell of dimensions ao - 6.37 A, bo = 8.12 A, co - 9.47 A. The SbCl 3 molecules are trigonal pyramids having Sb atoms at the apices [3]. In contrast to the SbCl 5 lattice constants, the magnitude of a o - 6.37 A suggests that the SbCI 3 intercalated into graphite would form a (U7x /7)R19.1 0 commensurate superlattice. K6ssbauer results previously pubtDepartment of Physics. *Departme
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