Yttrium Borides - YB₂₅

YB₂₅

The structure of yttrium borides with B/Y ratio of 25 and above consists of a network of B₁₂ icosahedra. The boron framework of YB₂₅ is one of the simplest among icosahedron-based borides – it consists of only one kind of icosahedra and one bridging boron site. The bridging boron site is tetrahedrally coordinated by four boron atoms. Those atoms are another boron atom in the counter bridge site and three equatorial boron atoms of one of three B₁₂ icosahedra. The yttrium sites have partial occupancies of about 60–70%, and the YB₂₅ formula merely reflects the average atomic ratio / = 25. Both the Y atoms and B₁₂ icosahedra form zigzags along the x-axis. The bridging boron atoms connect three equatorial boron atoms of three icosahedra and those icosahedra make up a network parallel to the (101) crystal plane (x-z plane in the figure). The bonding distance between the bridging boron and the equatorial boron atoms is 0.1755 nm, which is typical for the strong covalent B-B bond (bond length 0.17–0.18 nm); thus, the bridging boron atoms strengthen the individual network planes. On the other hand, the large distance between the boron atoms within the bridge (0.2041 nm) reveals a weaker interaction, and thus the bridging sites contribute little to the bonding between the network planes.

YB₂₅ crystals can be grown by heating a compressed pellet of yttria (Y₂O₃) and boron powder to ~1700 °C. The YB₂₅ phase is stable up to 1850 °C. Above this temperature it decomposes into YB₁₂ and YB₆₆ without melting. This makes it difficult to grow a single crystal of YB₂₅ by the melt growth method.