Tatiana I. Chupakhina

arXiv2026

Ambiguity in B-Site cation ordering: A Case study of the double perovskite Ca$_2$CoNbO$_6$

The ordering of cations in the B sublattice remains a challenging issue in double perovskites. In this work, a combined experimental and theoretical approach was employed to investigate the Co/Nb distribution in Ca$_2$CoNbO$_6$ and its influence on magnetic and transport properties. The density functional theory, supported by magnetic susceptibility measurements, indicates that Co adopts a high-spin Co$^{3+}$ state. No long-range magnetic ordering was observed down to low temperatures; however, the presence of short-range correlations points to the partial disorder in the Co/Nb sublattice. This interpretation is further supported by electron paramagnetic resonance, which also reveals slight oxygen nonstoichiometry. Electrical transport follows a small-polaron hopping mechanism with an activation energy of 0.25 eV. The Seebeck coefficient reaches 0.4 mV/K at 600 K.

cond-mat.mtrl-sci↗

arXiv2024

Magnetic structure and electronic properties of mixed-metal Ruddlesden-Popper oxide LaSrCo$_{1/2}$Fe$_{1/2}$O$_4$

Atomic, electronic, and magnetic structure of LaSrCo$_{1/2}$Fe$_{1/2}$O$_4$ mixed-metal Ruddlesden-Popper oxide is investigated theoretically using self-consistent ACBN0 DFT + $U$ approach. We show that the electronic and magnetic properties strongly depend on the distribution of transition-metal and La/Sr ions in the lattice. Fe-Fe exchange is found to be antiferromagnetic, whereas Co-Co and Fe-Co exchange is ferromagnetic. We find that Co spin states depend on the distribution in both La/Sr and transition-metal ions. The most energetically favorable configuration is ferromagnetic, whereas the majority of metastable configurations are antiferromagnetic. LaSrCo$_{1/2}$Fe$_{1/2}$O$_4$ was synthesized using spray-pyrolysis method. Magnetization measurements revealed antiferromgnetic behavior, thus indicating presence of metastable configurations in the synthesized material. In addition, we show that in complex systems, such as mixed-metal Ruddlesden-Popper phases, Hubbard $U$ values have to be determined for each crystallographically non-equivalent site.

cond-mat.mtrl-sci↗