YCa3(CrO)3(BO3)4: A Cr(3+) Kagomé Lattice Compound Showing No Magnetic Order down to 2 K

We report a new gaudefroyite-type compound YCa3(CrO)3(BO3)4, in which Cr(3+) ions (3d(3), S = 3/2) form an undistorted kagomé lattice. Using a flux agent, the synthesis was significantly accelerated with the typical calcining time reduced from more than 2 weeks to 2 d. The structure of YCa3(CrO)3(BO3)4 was determined by combined Rietveld refinements against X-ray and neutron diffraction data. Symmetry distortion refinement starting from a disordered YCa3(MnO)3(BO3)4 model was applied to avoid overparameterization. There are two ordering models, namely, K2-1 and K2-2, with the space groups P63 (No. 173) and P3̅ (No. 147), respectively, that differ in the [BO3] ordering between different channels (in-phase or out-of-phase). Both models give similarly good fits to the diffraction data. YCa3(CrO)3(BO3)4 is an insulator with the major band gap at Eg = 1.65 eV and a second transition at 1.78 eV. Magnetically, YCa3(CrO)3(BO3)4 is dominated by anti-ferromagnetic exchange along edge-sharing CrO6 octahedral chains perpendicular to the kagomé planes, with Θ ≈ -120 K and μeff ≈ 3.92 μB. The compound shows no spin ordering or freezing down to at least 2 K.