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.