The structure of the I phase, V41Ni36Si23, a pseudo superstructure

Abstract

The I phase, V41Ni36Si23, is monoclinic, a = 13.462 (6), b = 23.381 (9), c= 8.940 (4) A, fl= 100.34 (4) ° , 228 atoms per unit cell, space group Cc (C4). Almost all strong reflections have k= 3n, indicating a pseudo superstructure. Structure models were obtained with MULTAN in space group C2/c, which could not be refined. A Fourier synthesis in space group Cc, phased by a group of atoms common to these models, yielded with small changes a model which refined by full-matrix least squares to Rw(F)= 0.08 for all 4065 reflections.The structure may be visualized as a stacking in the b direction of 12 rumpled layers consisting of pentagons and triangles creating a pseudo repeat of ]b and many pseudo symmetry elements. The asymmetric unit comprises three of these layers, with their associated 'subsidiary' atoms. The structure is ideally 'tetrahedrally close-packed' (t.c.p.) with V in the 12 CN 16, the 6 CN 15 and the 6 CN 14 positions. Eighteen Ni and eleven Si are ordered in 29 of the 33 CN 12 positions and a mixture of Ni and Si occurs in the remaining four CN 12 positions. Si-Si contacts do not occur; however, a few contacts occur between positions occupied by Si and mixtures. Contacts of the latter type were not found in t.c.p, structures containing less Si (v, X, K, and R phases). The coordination distances cover a range of 2.12-3.02 A and depend in the same way on CN as found for other t.c.p, structures.

Metallic alloys at the edge of complexity: structural aspects, chemical bonding and physical properties*

“…The remaining complex FK alloys include Mn 77 Fe 4 Si 1 [63], Mn 45 Co 40 Si 15 [64,65], V 41 Ni 36 Si 23 [66] and V 2 (Co,Si) 3 [67] types and show major Si contents. Three formers are exclusive representatives of their own structure types while the last one contains six alloys.…”

Concerning the relative numbers of atomic coordination types in tetrahedrally close packed metal structures

“…(m) Shoemaker & Shoemaker (1967). (n) Shoemaker & Shoemaker (1981). (o) ; their results are reported in space group B2/m.…”

“…Y & K gave examples of how some complex structures can be built up of fragments of the three base structure types, but recognized that 'the most complex structure types are not obliged to be combinations of the corresponding fragments'. That the observation of Y & K has predictive power is illustrated by the fact that the four structure types known to us that have been determined since the (a) (c) publication of the Y & K paper also fit equation (1); these are the K phase Mn77Fe4Si19 (Shoemaker & Shoemaker, 1977), the I phase V41Ni368i23 (Shoemaker & Shoemaker, 1981), the H phase (Ye, Li & Kuo, 1984), and K7Cs 6 (Simon, Br/imer, Hillenkrtter & Kullmann, 1976 This formulation fits all known t.c.p, structure types because…”