Lattice relaxation in the highly-contrasted Zn1-xBexSe alloy: An extended x-ray absorption fine structure study

We have performed extended x-ray absorption fine structure (EXAFS) measurements at the Zn and Se K edges of (Zn,Be)Se alloy. This alloy exhibits an unusually large contrast in the physical properties (bond stiffness and bond lengths) of its constituting bonds, leading to a uniquely well-resolved 1-bond -> 2-mode behavior in the already reported Raman spectra of this material system. The nearest and next nearest neighbor distances of Zn and Se atoms, the pseudo Debye-Waller factors (sigma(2)) for these distances, as well as various bond angles, are determined as a function of the alloy composition after fitting the EXAFS equations. The lattice relaxation is discussed in terms of the distorted tetrahedral structure of the ternaries. We conclude that the accommodation of the local strain due to the difference in Zn-Se and Be-Se bond lengths is predominantly achieved by a displacement of the Se sublattice toward Be, the (Be,Zn)-mixed cation sublattice remaining quasirigid. Besides, we find that the 1-bond -> 2-mode behavior in the bond force constants (Raman data) is not paralleled by an obvious 1-bond -> 2-mode behavior in the bond lengths (EXAFS data). However, the dependence of sigma(2) for the nearest neighbor bond length as a function of the alloy composition is basically consistent with such a behavior. The difference in Be-Se bond lengths as predicted by earlier ab initio calculations, of 0.04 angstrom, is probably too small to be resolved by our present EXAFS measurements. Further, by combining the analysis of EXAFS data with available Raman data, we were able to provide a preliminary estimate of the BeSe TO mode Gruneisen parameter. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3483944]