Yttrium Borides - YB₆₆

YB₆₆

YB₆₆ was discovered in 1960 and its structure was solved in 1969. The structure is face-centered cubic, with space group Fm3c (No. 226), Pearson symbol cF1936 and lattice constant a = 2.3440(6) nm. There are 13 boron sites B1–B13 and one yttrium site. The B1 sites form one B₁₂ icosahedron and the B2–B9 sites make up another icosahedron. These icosahedra arrange in a thirteen-icosahedron unit (B₁₂)₁₂B₁₂ which is called supericosahedron. The icosahedron formed by the B1 site atoms is located at the center of the supericosahedron. The supericosahedron is one of the basic units of the boron framework of YB₆₆. There are two types of supericosahedra: one occupies the cubic face centers and another, which is rotated by 90°, is located at the center of the cell and at the cell edges. Thus, there are eight supericosahedra (1248 boron atoms) in the unit cell.

Another structure unit of YB₆₆ is B₈₀ cluster of 80 boron sites formed by the B10 to B13 sites. All those 80 sites are partially occupied and in total contain only about 42 boron atoms. The B₈₀ cluster is located at the body center of the octant of the unit cell, i.e., at the 8a position (1/4, 1/4, 1/4); thus, there are eight such clusters (336 boron atoms) per unit cell. Two independent structure analyses came to the same conclusion that the total number of boron atoms in the unit cell is 1584. The boron framework structure of YB₆₆ is shown in the figure to the right. A schematic drawing under it indicates relative orientations of the supericosahedra, and the B₈₀ clusters are depicted by light green and dark green spheres, respectively; at the top surface of the unit cell, the relative orientations of the supericosahedra are indicated by arrows. There are 48 yttrium sites ((0.0563, 1/4, 1/4) for YB₆₂) in the unit cell. Fixing the occupancy of the Y site to 0.5 results in 24 Y atoms in the unit cell and the chemical composition of YB₆₆; this occupancy of 0.5 implies that the yttrium pair has always one Y atom with one empty site.

YB₆₆ has density 2.52 g/cm3, low thermal conductivity of 0.02 W/(cm·K), elastic constants c₁₁ = 3.8×109 and c₄₄ = 1.6×109 Newton/m2 and Debye temperature of 1300 K. As all yttrium borides, YB₆₆ is a hard material and exhibits Knoop hardness of 26 GPa. High-quality YB₆₆ crystals of few centimeters in size can be grown by the multiple-pass floating zone technique and be used as X-ray monochromators.

The large unit cell of YB₆₆ results in large lattice constant of 2.344 nm. This property, together with high thermal and mechanical stability resulted in application of YB₆₆ as dispersive elements of X-ray monochromators for low energy radiation (1–2 keV).