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Structures of four new polytypes of lead iodide

1985; Wiley; Volume: 41; Issue: 10 Linguagem: Inglês

10.1107/s010827018500796x

1600-5759

B. Pałosz, S. Gierlotka, B. Wiktorowska, D. Dziąg,

X-ray Diffraction in Crystallography

Crystals of PbI 2 were grown from a gel at temperatures from 293 to 323 K. Structures of four new polytypes were identified: two hexagonal polytypes belonging to space group P3ml: 8H2:(f2fl)2/(211)2 and 30HI: tf2fl(t)3f2fl(t)sf2fl/211222211(22)3- 112; and two rhombohedral polytypes belonging to space group R3m: 24R1:f2of2f4/l(1 1)23 and 36R1: f2(o)3f2f4/1 (11)43 (t-o-f/Zhdanov notations). Struc- tures of four already known polytypes were recognized: 6H1, 12H1, 18RI and 18R2. Introduction. The polytypism of PbI 2 discovered by Mitchell (1959) has been investigated extensively in the last two decades: Hanoka & Vand (1968), Agrawal, Chadha & Trigunayat (1970), Prasad & Srivastava (1973), Minagawa (1975, 1979), Chand & Trigunayat (1975a,b, 1976), Chadha (1976), Chaudhary & Trigun- ayat (1983). Tens of multilayer polytypes occurring in the crystals obtained from a gel were reported and the structures of 17 were determined completely (these polytypes were listed by Patosz, 1983). It was estab- lished that three polytypes may be considered basic for PbI 2 (Mitchell, 1959): 2H, 4H and 12R with Zhdanov sequences 11, 22 and (13) 3, respectively (o, t and f4f5 in the t-o-f notation, Patosz, 1980). 2H is the low-temperature modification (2H dominates in the crystals grown at room temperature), and 12R is the high-temperature structure: only 12R is obtained from a vapour phase and this polytype forms from other polytypes, also 2H, at high temperature (Prasad & Srivastava, 1973; Minagawa, 1975). Based on the experimental data published on the growth and thermal behaviour of the crystals it was concluded by Patosz (1983) that polytypism of PbI 2 is a function of temperature: the greatest number of polytypes form in the temperature range 320-400 K and the composition of the polytypes changes with temperature. In the present work the effect of temperature on the formation of the polytypes grown from a gel at temperatures between 293 and 323 K is discussed. Experimental. Preparation of the crystals. PbI 2 crystals were grown in silica gel: Mitchell (1959), Henisch