Structure of the intermediate phase of PbTe at high pressure

The evolution of PbTe with pressure has been reexamined using synchrotron x-ray diffraction. The phase transition at 6 GPa is not to the GeS (B16) or TlI (B33) type structures, as previously reported, but to an orthorhombic $Pnma$ structure, with cell parameters $a=8.157\left(1\right)$, $b=4.492\left(1\right)$, and $c=6.294\left(1\right)\mathrm{\AA }$ at 6.7 GPa. This structure corresponds to a distortion of the low-pressure NaCl structure with a coordination intermediate between the sixfold B1 (NaCl) and the eightfold B2 (CsCl) structure. We discuss the stability of this new structure with respect to other proposed phases using numerical methods. These results may modify the admitted paths of phase transitions between the B1 and B2 structures.

Figure 1

Rietveld refinement of the NaCl phase of PbTe, recorded at 0.4 GPa, at ESRF $(\lambda =0.3738\mathrm{\AA })$. Dots are data points, vertical bars are the Bragg positions. The continuous line is the calculated pattern. Inset: Structure of the NaCl phase of PbTe. Open circles are Pb atoms, filled circles are Te atoms. The NaCl cell is indicated. The dotted lines indicate the intermediate $Pnma$ cell of PbTe at high pressure (see Sec. 3B) .

Figure 2

(a) and (b) Calculated patterns for PbTe with the TlI and the GeS type structure. (c) Observed pattern of PbTe at 6.7 GPa at ESRF. (d) Pattern recorded at SRS, after annealing at 200 °C for 14 h, for which the pressure had dropped from 8.8 to 6.5 GPa.

Figure 3

Rietveld refinement of the intermediate phase of PbTe at high pressure at 6.5 GPa from SRS data after annealing 14 h at 200 °C $(\lambda =0.4654\mathrm{\AA })$. Inset: Rietveld refinement of the intermediate phase of PbTe at high pressure from ESRF data, at 6.7 GPa $(\lambda =0.3738\mathrm{\AA })$. Dots are data points, vertical bars are the Bragg positions. The continuous line is the calculated pattern. Note that there is a small amount of untransformed NaCl phase present in the ESRF data (lower tick marks).

Figure 4

Structure of the intermediate phase of PbTe at high pressure (left) and of the GeS type structure (right). Open circles are Pb atoms, filled circles are Te atoms. Three views are displayed: perpendicular to the ${\left[\overline{1}10\right]}_c$ (top), ${\left[110\right]}_c$ (middle), and ${\left[001\right]}_c$ (bottom) NaCl cubic axes. Note that the major atomic displacements are along ${\left[001\right]}_c$ for PbTe, whereas they are along ${\left[\overline{1}10\right]}_c$ for the GeS type. One can also notice the difference in coordination in the view perpendicular to the ${\left[110\right]}_c$ axis: it is 7 for PbTe and 6 for the GeS type structure.