text: Similar to yttrium, rare-earth metals from Gd to Lu can form REBSi-type boride. The first such compound was synthesized by solid-state reaction and its structure was deduced as YB. X-ray powder diffraction (XRD) and electron diffraction indicated that YB has an orthorhombic structure with lattice constants a = 1.66251(9), b = 1.76198 and c = 0.94797(3) nm. The space group was assigned as P222. Because of the close similarity in lattice constants and space group, one might expect that YB has the γ-AlB-type orthorhombic structure whose lattice constants and space group are a = 1.6573(4), b = 1.7510(3) and c = 1.0144(1) nm and P222. YB decomposes at ~1750 °C without melting that hinders growth of single crystals from the melt. Small addition of silicon made YB to melt without decomposition, and so enabled single-crystal growth from the melt  and single-crystal structure analysis.
  The structure analysis indicated that YBSi has not the γ-AlB-type lattice but a rare orthorhombic crystal structure (space group: Pbam, No. 55) with lattice constants of a = 1.674(1) nm, b = 1.7667(1) nm and c = 0.9511(7) nm. There are 58 independent atomic sites in the unit cell. Three of them are occupied by either B or Si atoms (mixed-occupancy sites), one is a Si bridge site and one is Y site. From the remaining 53 boron sites, 48 form icosahedra and 5 are bridging sites. Atomic coordinates and site occupancies are summarized in table III.
  The boron framework of YBSi consists of five B icosahedra (I1–I5) and a BSi polyhedron shown in figure 8a. An unusual linkage is depicted in figure 8b, where two B-I5 icosahedra connect via two B atoms of each icosahedron forming an imperfect square. The boron framework of YBSi can be described as a layered structure where two boron networks (figures 9a,b) stack along the z-axis. One boron network consists of 3 icosahedra I1, I2 and I3 and is located in the z = 0 plane; another network consists of the icosahedron I5 and the BSi polyhedron and lies at z = 0.5. The icosahedron I4 bridges these networks, and thus its height along the z-axis is 0.25.
  The I4 icosahedra link two networks along the c-axis and therefore form an infinite chain of icosahedra along this axis as shown in figure 10. The unusually short distances (0.4733 and 0.4788 nm) between the neighboring icosahedra in this direction result in the relatively small c-axis lattice constant of 0.95110(7) nm in this compound – other borides with a similar icosahedral chain have this value larger than 1.0 nm. However, the bonding distances between the apex B atoms (0.1619 and 0.1674 nm) of neighboring I4 icosahedra are usual for the considered metal borides.
  Another unusual feature of YBSi is the 100% occupancy of the Y site. In most icosahedron-based metal borides, metal sites have rather low site occupancy, for example, about 50% for YB and 60–70% for REAlB. When the Y site is replaced by rare-earth elements, REBSi can have an antiferromagnetic-like ordering because of this high site occupancy.