Abstract
The physical properties of La4Ni3O10 with a two-dimensional (2D)-like Ruddlesden–Popper-type crystal structure are extraordinarily dependent on temperature and chemical substitution. By introducing Al3+ atoms randomly at Ni sites, the average oxidation state for the two nonequivalent Ni cations is tuned and adopts values below the average of +2.67 in La4Ni3O10. La4Ni3–xAlxO10 is a solid solution for 0.00 ≤ x ≤ 1.00 and is prepared by the citric acid method, with attention paid to compositional control and homogeneity at low Al level (x) concentrations. The samples adopt a slightly distorted monoclinic structure [P21/a (Z = 4)], evidenced by peak broadening of the (117) reflection. We report a remarkable effect on the electronic properties induced by tiny amounts of homogeneously distributed Al cations, with clear correspondence between resistivity, magnetization, diffraction, and density functional theory (DFT) data. DFT shows that electronically, there is no significant difference between the nonequivalent Ni atoms and no tendency toward any Ni3+/Ni2+ charge ordering. The resistivity changes from metallic to semiconducting/insulating with increasing band gap at higher Al levels, consistent with results from DFT. The metal-to-metal (M-T-M) transition reported for La4Ni3O10, which is often interpreted as a charge density wave, is maintained until x = 0.15 Al level. However, the temperature characteristics of the resistivity change already at very low Al levels (x ≤ 0.03). A coupling of the M-T-M transition to the lattice is evidenced by an anomaly in the unit cell dimensions. Moreover, there is an excellent correlation between the resistivity and magnetization data shown by the metallic and Pauli paramagnetic regime for La4Ni3–xAlxO10 with x < 0.25. The introduction of the fixed +3 oxidation state of Al atoms randomly at the Ni sites reduces the overall oxidation state of Ni2.67+ by keeping the oxygen stoichiometry unchanged. In addition, the nonmagnetic Al3+ for Ni is likely to block Ni–O–Ni exchange pathways through −Ni–O–Al–O–Ni– fragments into the network of corner-shared octahedra with the emergence of possible short-range ferromagnetic ordering of the ferromagnetic domains/clusters that are formed due to Al substitutional disorder in a paramagnetic insulating matrix.
Highlights
We reported on crystal structure and phase transitions for La4Co3O10 and La4Ni3O10 and their solid solutions.[13,15−18] In high-resolution synchrotron powder diffraction patterns, the monoclinic distortion is manifested by peak broadening, and in some cases, peak splitting
The P21/a structure for La4Ni3O10 was recently evaluated by density functional theory (DFT) and found to be close in energy to variants with symmetries described in space groups Bmab, Pcab as well in a smaller volume P21/a structure (P21/aII),[2,19,20] likewise supported by single-crystal diffraction studies.[21]
We address several open questions: (i) will Al substitution trigger a metal-to-insulator (MIT) transition as a function of temperature and/or composition? (ii) will the charge density waves (CDW) transition of La4Ni3O10 disappear or change character? (iii) will the Pauli paramagnetism transform into Curie−Weiss-like paramagnetism with localized moments and long-range magnetic order emerging at low temperatures? (iv) will we observe positive or negative magnetoresistance and how does this correlate with Al content and electrical resistivity? (v) will a change in the formal Ni3+/Ni2+ ratio give rise to ferromagnetic-like interactions via a double-exchange mechanism?
Summary
The Ruddlesden−Popper (RP) series of nickelates Lan+1NinO3n+1 attract interest owing to their layer-like tchheairracptheyrs, icfeaal tpurroepserrteiseesm.1−b4linTghehRigPhn-Tccryssutpalersctrouncdtuurcetocros,nsainstds of n consecutive perovskite layers, (LaNiO3)n, alternating with rock-salt-like layers of LaO, i.e., (LaO)(LaNiO3)n.5 The presence of half a rock-salt-like layer influences the physical properties, and the number of the perovskite blocks (n) plays an important role due to their two-dimensional (2D)-like features.[6−8] With increasing n, the electrical behavior changes from insulating to metallic.[9,10] Their structures are closely related, though, with different symmetries, where La2NiO4 is tetragonal,[11] La3Ni2O7 is orthorhombic,[12] and La4Ni3O10 is slightly monoclinically distorted (pseudo-orthorhombic)[2,4,13] at room temperature. The presence of half a rock-salt-like layer influences the physical properties, and the number of the perovskite blocks (n) plays an important role due to their two-dimensional (2D)-like features.[6−8] With increasing n, the electrical behavior changes from insulating to metallic.[9,10] Their structures are closely related, though, with different symmetries, where La2NiO4 is tetragonal,[11] La3Ni2O7 is orthorhombic,[12] and La4Ni3O10 is slightly monoclinically distorted (pseudo-orthorhombic)[2,4,13] at room temperature. We note that the Bmab orthorhombic model well approximates the atomic arrange-
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