Among a new materials for development of the mid-IR optoelectronic or the thermo-electric devices based on quantum dots the (Pb,Cd)Te solid solution attracted a lot of attention within the last few years. These crystals in the composition range corresponding to PbTe-based matrix containing a lot of CdTe precipitates may be considered as composites perfectly matching principal properties required for future thermoelectric applications. However, it is difficult to obtain a uniform material of such matrix. The reason is extremely low solubility limit of both materials [1] resulting from the difference in their crystal structure in spite of very similar values of lattice parameters. Both compounds crystallize in the fcc structure but of different type: PbTe in the rock salt (RS) type and CdTe in the zinc blende (ZB) one. So far, in the case of bulk crystals grown by the Bridgman technique or by rapid quenching and annealing method only polycrystalline samples were obtained. For such samples the phase diagram and the solubility limit have been determined long time ago [1-3].

The recent successful growth of metastable, big, single, (Pb_,Cd)Te crystals with the RS-type structure and the CdTe content up to about 12% at the Institute of Physics of the Polish Academy of Sciences in Warsaw opened new research opportunities. The crystals were grown by self-selecting vapour transport method (SSVT [4]), the details of the growth procedure can be found elsewhere [5, 6]. Characterization of numerous samples at ambient conditions allowed to determine their structure and the dependence of the lattice parameter value on the cadmium content in the solid solution. Due to their one-phase character and the perfect structure quality these samples were studied by several methods [5-8].

In this work we report the results of synchrotron X-ray diffraction studies in a wide temperature range from about 20 K to 1100 K, performed on a set of (Pb,Cd)Te solid solutions with different chemical compositions. These measurements were executed at the B2 beamline at Hasylab (DESY) using the polycrystalline diffractometerwith Debye–Scherrer geometry. The samples were prepared as a mixture of powdered (Pb_,Cd)Te crystals and fine diamond powder (applied as an internal standard), and placed in a thin-wall quartz or glass capillary, rotating inside a graphite heater during measurements. Figure 1 show the typical pattern set for 5.6% CdTe reach (Pb_,Cd)Te sample. The structural analysis of collected patterns sets was performed with the help of the Rietveld refinement and demonstrated non-monotonous evolution of the lattice parameter with temperature. The solubility limit calculated on the basis of present results does not agree with the previous data Ref. [1-3] and suggests the necessity of some correction of the relevant phase diagram in region of a low CdTe content. This correction will be shown, its possible influence on planned or expected applications of the solid solution under interest will also be discussed.

            The big-size, single crystal containing about 9% of CdTe has also been characterized by neutron scattering technique at Laboratoire Léon Brillouin. Fig. 2 shows the map of 200 Bragg peak intensity distribution in the reciprocal space determined by the elastic neutron scattering. The measurements were performed at RT with the use of 2T1 spectrometer installed on the thermal neutron beam (k_f = 2.66 Å^-1). The result confirms the single crystal character of investigated sample, which has been then applied for further studies of the phonon dispersion. The preliminary results of the last experiment will also be shown and discussed in comparison to the last exciting findings for PbTe [9].

Fig. 2. The 200 Brag peak intensity distribution for a whole (Pb,Cd)Te crystal containing 9% of CdTe taken with the use of neutron scattering.

This work has been partially supported by the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 226716 and by the European Union within the European Regional Development Fund, through grant Innovative Economy (POIG.01.01.02-00-108/09) and through the 'Research Infrastructures' action of the 'Capacities' Programme, NMI3-II Grant number 283883.

[1] A.J. Rosenberg, R. Gierson et al., Trans. Metall. Soc. A.I.M.E. 230, 342 (1964).
[2] P.M. Nikolic, Brit. J. Appl. Phys. 17, 341 (1966).
[3] V. Leute and R. Schmidt, Z. Phys. Chem. 172, 81 (1991).
[4] A. Szczerbakow and K. Durose, Prog. Cryst. Growth Character. Mater. 51, 81 (2005).
[5] M. Szot, A. Szczerbakow, K. Dybko et al., Acta Phys. Pol. A 116, 959 (2009).
[6] M. Szot, K. Dybko, P. Dziawa et al., Cryst. Growth Des. 11, 4794 (2011).
[7] R. Minikayev, E. Dynowska, P. Dziawa, et al., Synchr. Rad. Nat. Sci. 8, 83 (2009)
[8] R. Minikayev, E. Dynowska, E. Kaminska, et al., Acta Phys. Pol. A 119, 699 (2011).
[9] O. Delaire, J. Ma, K. Marty et al., Nature Materials 10, 614 (2011).

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