Single crystals of YbAlB 4 were grown in excess Al flux. Plate-and needle-shaped crystals were found. The plates are found to be -YbAlB 4 , which crystallizes with the ThMoB 4 structure type in space group Cmmm (No. 65), Z ) 4, with lattice parameters of a ) 7.3080(4), b ) 9.3150(5), and c ) 3.4980(2) Å. The needle-shaped crystals were identified as the first form of YbAlB 4 which crystallizes with the YCrB 4 structure type in space group Pbam (No. 55), Z ) 4, with lattice parameters of a ) 5.9220(2), b ) 11.4730(3), and c ) 3.5060(5) Å. While both compounds have heavy fermion ground states with Isinglike magnetic anisotropy, the electronic specific heat coefficients (γ) differ. The -phase has a γ value near 300 mJ mol, more than twice that of the R-phase, γ ) 130 mJ mol. A comparison of the structures and physical properties of both polymorphs is presented.

A polycrystalline sample of FeSe, which adopts the tetragonal PbO-type structure (P 4 /nmm) at room temperature, has been prepared using solid state reaction. We have investigated pressure-induced structural changes in tetragonal FeSe at varying hydrostatic pressures up to 0.6 GPa in the orthorhombic (T = 50 K) and tetragonal (T = 190 K) phases using high resolution neutron powder diffraction. We report that the structure is quite compressible with a bulk modulus ≈ 31 GPa to 33 GPa and that the pressure response is anisotropic with a larger compressibility along the c-axis. Key bond angles of the SeFe 4 pyramids and FeSe 4 tetrahedra are also determined as a function of pressure.