Evidence for a Bond-Percolation Transition.*

Metal-insulator (M-I) and magnetic T_c's in RNiO₃ vary with rare-earth-ion R size; only LaNiO₃ (largest R) remains metallic. For smaller R³⁺, as T is lowered, thermal straightening of bond angles diminishes. We model the T→0 M-I transition with bond percolation on the nearly simple-cubic lattice, assuming only that each small R ion breaks all 12 surrounding bonds; we predict a universal T→0 M-I transition for all La_1-xR_xNiO₃ alloys. To augment evidence from the known alloy with R = Y (M-I transition at x_c ≈ 0.25–0.30^[1]), we are synthesizing La_1-xNd_xNiO₃. The tolerance factor t for this alloy at x = 0.5 places it within the metallic region (t ≈ 0.943), while bond percolation predicts an insulator (x > 0.29). We are investigating bulk syntheses in high oxygen pressure, high static pressure, and precursor routes, as well as epitaxial film growth using pulsed laser deposition. The model provides an exact mapping of the small-R fraction x to the bond fraction p, so that experiments can quantify critical behavior near the percolation threshold p_c; in particular, we find x_c = 0.2937343(7) for the known p_c = 0.2488126(5).

[1] Luke G. Marshall, Ph. D. Thesis, University of Texas at Austin, pp. 80--104 (2003).