The disordered and correlated insulator Bi2CrAl3O9

The disordered and correlated insulator Bi2CrAl3O9

The $3d$ transition metal insulator ${\mathrm{Bi}}_{2}{\mathrm{CrAl}}_{3}{\mathrm{O}}_{9}$ forms with a quasi-one-dimensional structure characterized by linear chains of edge-sharing, Cr-and Al-centered, distorted octahedra. The UV/Vis spectrum of high-quality single crystals is marked by broad absorption edges corresponding to direct transitions across a 1.36-eV insulating gap. Measurements of dc magnetic susceptibility $\ensuremath{\chi}$ reveal a fluctuating moment of $2.60\ifmmode\pm\else\textpm\fi{}0.01\phantom{\rule{4pt}{0ex}}{\ensuremath{\mu}}_{B}/\mathrm{Cr}$---reduced from the $3.87\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{B}/\mathrm{Cr}$ expected for ${\mathrm{Cr}}^{3+}$, while the Weiss temperature ${\mathrm{\ensuremath{\Theta}}}_{W}=\ensuremath{-}21\ifmmode\pm\else\textpm\fi{}1$ K implies that the prevailing local moment interactions are weakly antiferromagnetic in nature. Some 10% of the fluctuating moment is quenched, presumably due to the onset of an antiferromagnetic or spin glass phase at temperature ${T}^{★}=98\ifmmode\pm\else\textpm\fi{}3$ K, while measurements of magnetization versus field $H$ at $T\ensuremath{\le}10$ K scale as $H/{T}^{0.68(4)}$, suggesting the presence of quantum fluctuations associated with a disordered phase. Density functional theory calculations carried out within the generalized gradient approximation are in excellent agreement with experimental results, asserting that short-range magnetic interactions remnant above ${T}^{★}$ stabilize the insulating state.

三维过渡金属绝缘体\({\mathrm{Bi}}_{2}{\mathrm{CrAl}}_{3}{\mathrm{O}}_{9}\)具有准一维结构，其特征是由共棱且以\(Cr\)和\(Al\)为中心的扭曲八面体构成的线性链。高质量单晶的紫外/可见光谱具有宽吸收边，对应于跨越\(1.36 eV\)绝缘能隙的直接跃迁。直流磁化率\(\chi\)的测量结果显示出\(2.60\pm0.01\ {\mu}_{B}/\mathrm{Cr}\)的涨落磁矩——相较于\({\mathrm{Cr}}^{3 +}\)预期的\(3.87\ {\mu}_{B}/\mathrm{Cr}\)有所降低，而外斯温度\({\mathrm{\Theta}}_{W}=-21\pm1\ K\)表明主要的局域磁矩相互作用本质上是弱反铁磁性的。大约\(10\%\)的涨落磁矩被猝灭，可能是由于在温度\({T}^{★}=98\pm3\ K\)时出现反铁磁相或自旋玻璃相，而在\(T\leq10\ K\)时磁化强度随磁场\(H\)的变化关系为\(H/{T}^{0.68(4)}\)，这表明存在与无序相相关的量子涨落。在广义梯度近似下进行的密度泛函理论计算与实验结果高度吻合，断言在\({T}^{★}\)以上残留的短程磁相互作用稳定了绝缘态。