Crystal Structure and Physical Properties of Polymorphs of LnAlB₄ (Ln = Yb, Lu)

Single crystals of YbAlB4 were grown in excess Al flux. Plate- and needle-shaped crystals were found. The plates are found to be β-YbAlB4, which crystallizes with the ThMoB4 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 YbAlB4 which crystallizes with the YCrB4 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 Ising-like magnetic anisotropy, the electronic specific heat coefficients (γ) differ. The β-phase has a γ value near 300 mJ mol-1 K-2, more than twice that of the α-phase, γ = 130 mJ mol-1 K-2. A comparison of the structures and physical properties of both polymorphs is presented.通过过量铝助熔剂法生长了YbAlB4单晶体，所得晶体包含板状与针状两种形貌。经表征，板状晶体为β-YbAlB4，该物相以ThMoB4结构型结晶，空间群为Cmmm（序号65），晶胞内化学式单元数Z=4，晶格常数分别为a=7.3080(4)、b=9.3150(5)、c=3.4980(2) Å。针状晶体则为YbAlB4的α相，该物相以YCrB4结构型结晶，空间群为Pbam（序号55），晶胞内化学式单元数Z=4，晶格常数分别为a=5.9220(2)、b=11.4730(3)、c=3.5060(5) Å。尽管两种物相均为具有伊辛型磁各向异性（Ising-like magnetic anisotropy）的重费米子（heavy fermion）基态体系，但其电子比热系数（electronic specific heat coefficients，γ）存在差异。β相的电子比热系数γ约为300 mJ·mol⁻¹·K⁻²，是α相（γ=130 mJ·mol⁻¹·K⁻²）的两倍以上。本文对这两种多晶型物的晶体结构与物理性能进行了对比分析。