The Disordered and Correlated Insulator Bi2CrAl3O9

The 3d transition metal insulator Bi2CrAl3O9 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 chi reveal a fluctuating moment of 2.60 +/- 0.01 mu(B)/Cr-reduced from the 3.87 mu(B)/Cr expected for Cr3+, while the Weiss temperature Theta(W) = -21 +/- 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-star = 98 +/- 3 K, while measurements of magnetization versus field H at T <= 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-star stabilize the insulating state.