Reaction Pathways in the Formation of Intermetallic InPd₃ Polymorphs

Abstract

Abstract The atomic order in InPd 3 was determined by neutron powder diffraction on a series of samples synthesized under various conditions. While annealing at temperature T ≥ 800 K yields a fourfold superstructure of a cubic closest packing (ccp) of the ZrAl 3 type (space group I 4/ mmm (No. 139), a = 406.196(5) pm, c = 1520.31(2) pm, Z = 4), short reaction times at T ≤ 750 K produce InPd 3 in a twofold ccp superstructure of the TiAl 3 type (space group I 4/ mmm (No. 139), a = 409.96(2) pm, c = 747.59(5) pm, Z = 2). Crystallographic group‐subgroup relationships in terms of a Bärnighausen symmetry tree for a series of ccp superstructures reveal the simple formula n = 2 m ·3 p for the domain size n , indicating the number of like layers in the stacking sequence A n B n , where m and p are the number of e2 and e3 transitions within the space group types P 4/ mmm and I 4/mmm, respectively. From the factors governing the appearance and structural details of ccp superstructures atomic size seems to play a major role in determining the c / a ratio in MPd 3 compounds with M being a main group metal. Thermal analysis, X‐ray and neutron powder diffraction suggest that the formation of InPd 3 by an iodine catalyzed reaction starting from the elements procedes by the pathway: In + 3 Pd → 2 (In 0.5 Pd 0.5 )Pd [AuCu type] → InPd 3 [TiAl 3 type] → InPd 3 [ZrAl 3 type].