The structure of Laves phase deuteride YFe2D4.2 has been investigated by synchrotron and neutron (ToP) powder diffraction experiments between 60 K and 370 K. YFe2D4.2 crystallizes below 323K in fully ordered monoclinic structure (s.g. Pc, Z 8, a = 5.50663(4), b = 11.4823(1), c = 9.42919(6) A, ~ 122.3314(5t, V = 503.765(3) A 3 at 290K) containing 4 yttrium, 8 iron and 18 deuterium atoms. Most ofD-D distances are withil1 the precision of the diffraction experiment longer than 2.1 A, the shortest ones are of 1.96 A. Seven iron atoms from eight are coordinated by deuterium in a trigonal bipyramid, similar to that in TiFeD1.95_2. The eights iron atom is coordinated by deuterium in a tetrahedral configuration. The iron coordination by deuterium, and iron-deuterium distances points to the importance of the directional bonding between iron and deuterium atoms. The lowering of crystal symmetry due to deuterium ordering occurs at much higher temperature than magnetic order, and is therefore one of the parameters which are at the origin of magnetic transition at lower temperatures.

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