The Predicted Structures of the New Pnictides HfMQ in Contrast to ZrMQ (M = Ti, V; Q = P, As)

Abstract

The new pnictides HfTiP, HfTiAs and HfVAs have been prepared through arc-melting. These arsenides form the predicted structures with (distorted) TiAs 4 and VAs 4 tetrahedra, while the structures of both Zr analogs (ZrTiAs and ZrVAs) are comprised of TiAs 4 and VAs 4 square planes, respectively. These differences stem from significant differences in the metal atom substructures. On the other hand, HfTiP and ZrTiP are isostructural. These variations were predicted by utilizing a structure map for metal-rich pnictides and chalco- IntroductionThe ternary equiatomic early transition-metal pnictides MMЈQ (M, MЈ: element of groups 3Ϫ5; Q: P, As, Sb) typically occur in one of two common crystal structures. These structures differ in the metalϪnonmetal coordination spheres (but with the same coordination numbers) as well as in the metalϪmetal bonding, while the nonmetal atoms are always surrounded by nine metal atoms in form of a tricapped trigonal prism, and the larger M atoms are situated in a square pyramid formed by five Q atoms. In the first variant, the Co 2 Si type (ordered variant: TiNiSi [1] ), the smaller MЈ atoms are tetrahedrally coordinated by four Q atoms, whereas in the second variant, the La 2 Sb type (ordered variant: CeScSi [2] ), the MЈQ 4 polyhedra are best described as square planes.These differences may be understood to stem from different metal atom substructures, which contain stronger MϪM and MϪMЈ bonds in case of the Co 2 Si structures. As shown earlier, [3] the major structure determining factors for such metal-rich pnictides and chalcogenides are the valence-electron concentration of M, the relative extension of the d orbitals of M (reflected in the principal quantum number), the radius ratio r M /r Q , and the electron deficit of the Q atoms. As for the metal atoms, higher valence-electron concentration, larger d orbitals and larger atomic radii all facilitate stronger MϪM bonding. On the other hand, larger atomic sizes and larger electron deficits of the Q atoms would lead to larger coordination numbers for the Q [a]

Planar Nets of Ti Atoms Comprising Squares and Rhombs in the New Binary Antimonide Ti2Sb

“…Thus far, the usability of this map was demonstrated in the subsequent uncoveries of new members of this M 2 Q family, namely Zr 1 -δ Ti 1+ δ As, Zr 1 -δ V 1+ δ As, and HfMQ with M = Ti, V; Q = P, As . Recently, the first example of a new type in this M 2 Q family was detected, namely Zr 2 S 0.33 Te 0.67 .…”

“…This map was previously utilized to correctly predict the hitherto uncovered arsenides ZrTiAs and ZrVAs that exhibit undistorted Ti and V square nets, respectively, , both falling into the La 2 Sb domain. Our latest examples were a series of isoelectronic Hf phosphides and arsenides occurring in the Co 2 Si region, that do not consist of square nets …”