Lanthanum hexaboride crystal

An alternative version of the lanthanum hexaboride crystal structure has the boron octahedra occupying the body centered positions of the cubic array of lanthanum atoms (Figure 10.8). This version makes it clear that in order to plastically shear the structure, the boron octahedra must be sheared. Note that the octahedra are linked together both internally and externally by B-B bonds. 

Figure 10.7 illustrates the prototype hexaboride crystal structure, that of lanthanum hexaboride. It consists of a simple cubic array of boron octahedra surrounding a metal atom at the body center of each cube. The octahedra are linked by B-B bonds connecting their comers. This makes the overall structure relatively hard with approximately the hardness of boron itself since plastic shear must break B-B bonds. The open volumes surrounded by boron octahedra are occupied by the relatively large lanthanum atoms as the figure shows schematically. 

Figure 10.7 Crystal structure of Lanthanum Hexaboride (prototypre hexaboride). The black circles represent boron octahedra. They form a simple cubic arrangement surrounding the central metal atom.

Figure 10.8 Alternative drawing of the crystal structure of Lanthanum Hexaboride with the metal atoms occupying the cube corners.

M. M. Korsukova and V. N. Gurin, Single Crystals of Lanthanum Hexaboride Their Preparation, Properties and Prospective Uses, in Current Topics in Materials Science , ed. E. Kaldis, Elsevier Science, New York, 1984, Vol. 11, p. 389. 

Regardless of which approach is used, it is necessaiy, of course, to know the instrumental function. As we have previously recalled, one of the methods for determining the contribution from the instrament is to calculate it from the diffractometer s physical characteristics [CHE 04]. This aspect has been the subject of research for about 20 years and was described in Chapter 3. However, the most commonly used method consists of producing a diffraction pattern with a sample assumed to be comprised of perfect crystals. In practice, several materials are used. Only one of them, lanthanum hexaboride which is sold by the NIST [NIST] is a standard acknowledged by the international community. 

Loboda, P. I. (1999). Anisotropy of microhardness of directionally crystallized materials based on lanthanum hexaboride. Materials Science, 35(4), 552-560. doi 10.1007/BF02365754. 

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