The crystal structure and superstructure of the K phase, Mn77Fe4Si19

Acta Crystallogr Sect B

1978

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Section: Ordering Of the Atomssupporting

confidence: 72%

Section: Introductionmentioning

confidence: 99%

Refinement of an R phase, Mn_85.5Si_14.5

Shoemaker¹,

Acta Crystallogr Sect B

1978

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“…(g) Shoemaker & Shoemaker (1963). (h) Shoemaker & Shoemaker (1977); data given are for the substructure, which differs from the superstructure only in site compositions. (i) Komura, Sly & Shoemaker (1960); data given are for the hexagonal cell.…”

Section: Discussionmentioning

confidence: 99%

“…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…”

Section: Ppqqrxx -~ ( Px2)( Q2r2x3)j( R3x)kmentioning

confidence: 96%

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

Shoemaker¹,

Acta Crystallogr B Struct Sci

1986

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102

Since diffuse intensity distributions calculated for the models with s = 1, where s is the number of layer units necessary to distinguish the stacking disorder, did not resemble the broad maxima observed in Fig. 1, the various models with s = 2 were calculated. The results for the model of Table 1 were in agreement with the observed diffuse maxima, as shown in Fig. 4.In the model of Table 1, the sequences such as Q1-P1-Q1, in which the second-neighbor layers are the same, do not occur, and Q1-P1-Q2 and Q1-P1-Q3 occur with the same probability and so on. The distance between the ordered Ti sites which belong respectively to the second-neighbor layers are shorterand so on. The calculated results excluded sequencesThis fact is reasonable with respect to the Coulomb interaction between Ti atoms and vacancies. The diffuse streaks have not yet been detected in X-ray photographs. The calculated intensities shown in Fig. 4 are the results of kinematical scattering theory. On the assumption that the geometry of the diffuse maxima is not affected by dynamical interactions, the model of Table 1 gives an explanation for the diffuse maxima observed. The designation Til.43S2-4H-Hx/~av/3a(SRO) is adopted for the structure discussed above, according to the notation system for polytypes and superstructures described previously (Onoda & Saeki, 1983 Abstract Tetrahedrally close packed (t.c.p.) metal or alloy crystal structure types are those in which the interstices are exclusively tetrahedral and the coordination types are restricted to a particular set of four, which are here called P, Q, R, and X, with fully triangulated coordination polyhedra and coordination numbers (CN) respectively 16, 15, 14, and 12. At least 20 of these are known.

“…Previously we found that these silicon-containing structures are still ideally tetrahedrally close-packed (t.c.p.) up to 19 at.% Si (K phase: Mn77Fe4Si19, Shoemaker & Shoemaker, 1977), whereas in the D phase with 29 at.% Si (MnsSi 2, Shoemaker & Shoemaker, 1976) some irregular coordinations occur. The study of the I phase was of interest because its Si content (up to 27 at.%) lies between that of the K and the D phases.…”

Section: Introductionmentioning

confidence: 99%

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

Shoemaker¹,

Acta Crystallogr Sect B

1981

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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.