Crystal structure of hafnium palladium gallium, HfPdGa

Abstract

GaHfPd, hexagonal, P62c (no. 190), a = 7.1572(2) A, c = 6.8945(4) A, V= 305.9 A, Ζ = 6, R&(F) = 0.019, η>RKf(F) = 0.031, 7 =293 K. Source of material A 1:1:1 mixture of the elemental components (hafnium pieces, 99.9 %, ChemPur; palladium wire, 99.9 %, ChemPur; gallium pieces, 99.999 %, ChemPur) was arc-melted in a protective atmosphere of purified argon. The homogenization was performed in an evacuated quartz tube at 800 °C for two weeks followed by subsequent quenching in cold water. Single crystals which were suitable for structure refinements were obtained by annealing of pre-reacted reguli at temperatures slightly below the melting point (controlled visually) using a high frequency furnace [1]. Experimental details Sample characterization was performed by X-ray powder diffraction (Huber Image Plate Guinier camera G670, CuK^i radiation, λ = 1.54056 A, 5° < 2Θ < 100°) and optical metallography as * Correspondence author (e-mail: schwaiz@cpfs.mpg.de) well as energy dispersive X-ray spectroscopy (EDXS). The compound was found to be a single phase. EDXS confirmed the equiatomic composition. Lattice parameters were obtained by a leastsquares refinement of 36 reflections determined from a powder diffraction pattern (LaBe as internal standard, a = 4.15692 A). The refined lattice parameters were used in the last runs of the structure refinement and for calculations of interatomic distances. Discussion The new ternary equiatomic compound HfPdGa is isostructural with HfRhSn [2] which represents a distorted and ordered variety of the well-known Fe2P structure type [3] and its ternary derivative ZrNiAl [4,5]. Α klassengleiche symmetry reduction of index 2 (fe) from space group P62m (ZrNiAl) to P62c (HfPdGa) is accompanied by a doubling of the lattice parameter c which is caused by a slight shift of the Hf atoms within the (001) plane and a movement of the Pdl along [001]. Alternatively, HfPdGa can be described as a variant of the Mg2Ga type [6] with an ordering of hafnium and gallium atoms on the magnesium positions (6A and 6g, respectively), while palladium atoms occupy the gallium sites (2/and 4b). The number of phases reported with a HfRhSntype crystal structure is relatively small: ZrlrSn, HfCoSn [2], ZrRhSn, ZrPtGa [5], ScPdSn, ScPtSn [η. Palladium and gallium atoms in the crystal structure of HfPdGa build a 3D framework (figure, top) which is exclusively linked by heteronuclear bonds. Hafnium atoms are embedded in voids of the network and form zig-zag chains within large pseudohexagonal [PdGa] tubes oriented parallel [001]. The Hf—Hf distance of 3.4567(4) A within the chain is significantly longer than the shortest Hf—Hf contacts of 3.126 A in hep hafnium metal [8]. Each crystallographically distinct atom in the [PdGa] network is differently coordinated (figure, bottom). Pdl atoms are located on the three-fold axis and are trigonally non-planar coordinated with d(¥dl—Ga) = 2.6581(6) A (figure, bottom left). The displacement of Pdl from the plane which is formed by Ga atoms amounts to ±0.21 A along [001] and results in a short (3.028(2) A) and a long (3.867(2) A) distance to adjacent Pdl atoms. Both values are significantly larger than the closest contacts of 2.751 A observed in bcc palladium [8], Along [001], an alternation of expansion (d(Hf—Hf) = 4.0032(6) A) and contraction (d(Hf—Hf) = 3.5860(7) A) in the Hf3 triangles is observed. The difference between the two types of triangles correlates with the movement of the Pd species towards one of the triangular planes of the trigonal prisms Hfe. The short and long Pdl—Hf interactions correspond to 2.7630(8) A and 2.8329(9) A, respectively. The coordination sphere of Pd2 is an undistorted trigonal prism formed by Ga atoms (figure, bottom middle). Three additional Hf atoms above the rectangular prism faces are situated in distances of 2.8541(3) A from the central atom. It is remarkable that the Pd2—Ga distances of2.5789(3) A for six-fold coordinated Pd2 is even slightly shorter than the Pdl—Ga distances of 2.6581(6) A for three-fold coordinated Pdl. Ga atoms are tetrahedrally coor-