Direct determination of the shape of potential well for off-center Ge atom in GeTe-SnTe solid solution by EXAFS technique

Physics Department, Moscow State University, 119899, Moscow, Russia

The question whether the ferroelectric phase transition in GeTe and its solid solutions is of displacive or order-disorder type has been open for many years. Some experimental data (like heat capacity measurements [1]) indicated the displacive type of the phase transition while the EXAFS data [2] showed that Ge atoms are off-center in both GeTe-PbTe and GeTe-SnTe solid solutions.

In the present work we used the EXAFS technique to determine the shape of potential well for the Ge atom in a series of Sn_1-xGe_xTe solid solution (x=0.2--1). Previously we have already used EXAFS to study the microscopic mechanisms of the influence of impurities on the ferroelectric phase transition in GeTe [3].

EXAFS data were collected at the synchrotron radiation source of Daresbury Laboratory (UK) for four compositions of the solid solution in the temperature range 80--300 K. The potential energy V(r) was expanded in a power series of atomic displacement up to the 6th order. Supposing that the movement of the Ge atom is classical, the parameterized expression for V was put into the general formulae for EXAFS function \chi(k). As the resulting expression was too complex and it was hard to treat it within the cumulant method, we developed a method to solve the entire 3-dimensional problem. The expansion coefficients for V(r) were then obtained by minimizing the deviation between experimentally and calculated \chi(k) functions.

The section of the potential energy surface in the direction perpendicular to the [110] axis for Sn_0.3Ge_0.7Te sample is shown in Figure. The potential wells are not very deep; their depth varies from W=10 meV for x=0.2 to 30 meV for x=1. It is seen that for all samples W is about a half of kT_c. It is interesting that the potential well is nearly the same for compositions above and below the tricritical point (observed in Sn_1-xGe_xTe at x=0.28 [4]), so that the change of the order of the phase transition cannot be attributed to a change of the shape of potential well. In summary, the results of determination of the shape of potential well indicates that the type of the phase transition in Sn_1-xGe_xTe is intermediate between the order-disorder and displacive ones.

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[4] R. Clarke, Phys.Rev.B, 18, 4920 (1978).