Synchrotron radiation powder diffraction patterns were taken on synthetic Co(3)Al(2)Si(3)O(12) garnet at BM8-GILDA (ESRF), at T = 298, 423, 573, 723 and 873 K. The cell parameters and atomic positions were refined by Rietveld analysis; the volume thermal expansion coefficient is alpha(V) = 28.5(9) x 10(-6) K(-1). The results on synthetic Co(3)Al(2)Si(3)O(12) garnet were compared with the high temperature structure data of X(3)Al(2)Si(3)O(12) garnets with X = Mg, Ca, Mn and Fe taken from literature. The thermal expansion of the unit cell is very similar in X(3)Al(2)Si(3)O(12) garnets, whereas the thermal expansion of the longest bonds in the distorted cubic cage surrounding the X cation decreases with increasing cation size. Contrarily to the compression behaviour, the thermal expansion in X(3)Al(2)Si(3)O(12) garnets does not change significantly as a function of composition.
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