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The changes of Mn-Y (Y = S, Se, Te) bond lengths R with the Mn composition x in the diluted magnetic semiconductors Zn1-xMnxY are calculated from the optical spectra and their pressure shifts. The calculated results suggest that the Mn-Y bond lengths change little with the Mn composition x-much less than expected from lattice parameters as determined by X-ray diffraction. These results are also in good agreement with the observed values obtained from the EXAFS measurements. It appears that for ternary A(1-x)M(x)Y (M denotes the divalent transition-metal ion) alloys the changes of M-Y bond lengths R with the M composition x can be studied from their optical spectrum data. (C) 2000 Elsevier Science Ltd. All rights reserved.