Origins of multi-sublattice magnetism and superexchange interactions in double-double perovskite CaMnCrSbO6

The multi-sublattice magnetism and electronic structure in double-double perovskite compound CaMnCrSbO6is explored using density functional theory. The bulk magnetization and neutron diffraction suggest a ferrimagnetic order (TC∼49 K) between between Mn2+and Cr3+spins. Due to the non-equivalent Mn atoms (labelled as Mn(1) and Mn(2) which have tetrahedral and planar oxygen coordinations, respectively) and the Cr atom in the centre of distorted oxygen octahedron in the unit cell, the exchange interactions are more complex than that expected from a two sublattice magnetic system. The separations between the on-site energies of thed-orbitals of Mn(1), Mn(2) and Cr obtained from Wannier function analysis are in agreement with their expected crystal field splitting. While the DOS obtained from non spin-polarized calculations show a metallic character, starting from HubbardU = 0 eV the spin-polarized electronic structure calculations yield a ferrimagnetic insulating ground state. The band gap increases withUeff(U - J), thereby showing a Mott-Hubbard nature of the system. The inclusion of anti-site disorder in the calculations show decrease in band-gap and also reduction in the total magnetic moment. Due to the ∼90∘superexchange, nearest neighbour exchange constants obtained from DFT are an order of magnitude smaller than those reported for various magnetic perovskite and double-perovskite compounds. The Mn(1)-O-Mn(2) (out of plane and in-plane), Mn(1)-O-Cr and Mn(2)-O-Cr superexchange interactions are found to be anti-ferromagnetic, while the Cr-O-O-Cr super-superexchange is found to be ferromagnetic. The Mn(2)-O-Cr superexchange is weaker than the Mn(1)-O-Cr super-exchange, thus effectively resulting in ferrimagnetism. From a simple 3-site Hubbard model, we derived expressions for the antiferromagnetic superexchange strengthJAFMand also for the weaker ferromagneticJFM. The relative strengths ofJAFMfor the various superexchange interactions are in agreement with those obtained from DFT. The expression for Cr-O-O-Cr super-superexchange strength (J~SS), which has been derived considering a 4-site Hubbard model, predicts a ferromagnetic exchange in agreement with DFT. Finally, our mean field calculations reveal that assuming a set of four magnetic sub-lattices for Mn2+spins and a single magnetic sublattice for Cr3+spins yields a much improvedTC, while a simple two magnetic sublattice model yields a much higherTC.

Spin Frustration in Double Perovskite Oxides and Oxynitrides: Enhanced Frustration in La2MnTaO5N with a Large Octahedral Rotation

The B-site sublattice in the double perovskite oxides A2BB'O6 (B: magnetic cation; B': nonmagnetic cation) causes spin frustration, but the relationship between the structure and spin frustration remains unclear although a number of compounds have been studied. The present study systematically investigated A2MnIIB'O6 (S = 5/2) and found that the frustration factor, defined by f = |θW|/TN (θW: Weiss temperature; TN: Néel temperature), scales linearly with the tolerance factor t, i.e., octahedral rotation. Unexpectedly, La2MnTaO5N (space group: P21/n) synthesized under high pressure is more frustrated (f = 6) than oxides with similar t values, despite the large octahedral rotation due to the small t value of 0.914. Structural analysis suggests that the enhanced frustration can be attributed to the site preference of nitride anions at the equatorial positions, which reduces the variance of neighboring Mn-Mn distances. Our findings provide a new guide to control and improve spin frustration in double perovskites with multiple anions.

Connection between Unusual Lattice Thermal Expansion and Cooperative Jahn-Teller Effect in Double Perovskites LaPbMSbO6 (M = Mn, Co, Ni)

Lattice thermal expansion (LTE) has been investigated in double perovskites LaPbMSbO₆ (M = Mn, Co, Ni). Ordinary LTE behavior with good thermal stability is identified for the Mn sample, whereas unusual LTE with a preferably expanded interplanar distance of (040) is revealed for Co and Ni samples. Temperature-dependent X-ray diffraction patterns ( T-XRD), Raman spectra ( T-Raman), and specific heat capacities ( T- C_p) consistently indicate that a rare isostructural displacive phase transition (IDPT) with a second-order phase transition nature is predominant near the critical temperature. Refinements of neutron powder diffraction (NPD) and in situ T-XRD data present temperature-sensitive bond parameters which are relevant to planar oxygen O1. X-ray photoelectron spectra (XPS) further confirm the Jahn-Teller (J-T) activated Co²⁺ (HS) or Ni³⁺ (HS/LS) cations at the B-site sublattice. This unusual LTE behavior could be understood by the cooperative J-T effect contributed by a Pb²⁺ ion and Co²⁺/Ni³⁺ ion from A- and B-site sublattices, respectively. The importance of 6s(Pb)-2p(O)-3d(Co/Ni) extended orbital hybridization on affecting thermal expansion behavior is highlighted on the basis of temperature-induced phonon mode softening. This study presents a microscopic description of connection between anisotropic thermal expansion and a cooperative J-T effect, which inspired exploration of thermal-mechanical coupled functional materials based on LaPbMSbO₆ double perovskites.

Theory of order–disorder phase transitions of B-cations in AB′1/2 B′′1/2O3 perovskites

Perovskite-like oxides AB′1/2 B′′1/2O3 with two different cations in the B-sublattice may experience cation order–disorder phase transitions. In many cases the degree of cation ordering can be varied by suitable synthesis conditions or subsequent sample treatment, which has a fundamental impact on the physical properties of such compounds. Therefore, understanding the mechanism of cation order–disorder phase transition and estimation of the phase transition temperature is of paramount importance for tuning of properties of such double perovskites. In this work, based on the earlier proposed cation–anion elastic bonds model, a theory of order–disorder phase transitions of B-cations in AB′1/2 B′′1/2O3 perovskites is presented, which allows reliable estimation of the phase transition temperatures and of the reduced lattice constants of such double perovskites.

Electronic structure and transport properties of antiferromagnetic double perovskite Y2AlCrO6

The antiferromagnetic G-type magnetic ordering in Y₂AlCrO₆ (YAC) has been investigated by the electronic band structure calculations and successive experiments.

Exotic magnetism on the quasi-fcc lattices of the d3 double Perovskites La2NaB'O6 (B'=Ru, Os)

We find evidence for long-range and short-range (ζ=70 Å at 4 K) incommensurate magnetic order on the quasi-face-centered-cubic (fcc) lattices of the monoclinic double perovskites La2NaRuO6 and La2NaOsO6, respectively. Incommensurate magnetic order on the fcc lattice has not been predicted by mean field theory, but may arise via a delicate balance of inequivalent nearest neighbor and next nearest neighbor exchange interactions. In the Ru system with long-range order, inelastic neutron scattering also reveals a spin gap Δ ∼ 2.75 meV. Magnetic anisotropy is generally minimized in the more familiar octahedrally coordinated 3d3 systems, so the large gap observed for La2NaRuO6 may result from the significantly enhanced value of spin-orbit coupling in this 4d(3) material.