Spin-glass behavior in LaFe_xCo_{2- x}P₂ solid solutions: Interplay between magnetic properties and crystal and electronic structures

To explore the evolution of magnetic properties from ferromagnetic LaCo₂P₂ to paramagnetic LaFe₂P₂ (both of ThCr₂Si₂ structure type) a series of mixed composition LaFe_xCo_2-xP₂ (x ≥ 0.5) has been comprehensively investigated by means of single-crystal and powder X-ray and neutron diffraction, magnetization and heat capacity measurements, Mössbauer spectroscopy, and electronic band structure calculations. The Curie temperature decreases from 132 K in LaCo₂P₂ to 91 K in LaFe_0.05Co_1.95P₂. The ferromagnetic ordering is suppressed at higher Fe content. LaFe_0.1Co_1.9P ₂ and LaFe_0.2Co_1.8P₂ demonstrate spin-glass-like behavior, which was also confirmed by the absence of characteristic features of long-range magnetic ordering, namely, a λ-type anomaly in the heat capacity, a hyperfine splitting in the Mössbauer spectrum, and magnetic reflections in the neutron diffraction pattern. Finally, both LaFe_0.3Co_1.7P₂ and LaFe _0.5Co_1.5P₂ exhibit paramagnetic behavior down to 1.8 K. The unit cell parameters of the mixed compounds do not follow the Vegard behavior as the increase in the Fe content results in the decrease of average M-M distances (M = Fe, Co). Quantum-chemical calculations and crystal orbital Hamiltonian population analysis reveal that upon aliovalent (nonisoelectronic) substitution of Fe for Co the antibonding character of M-M interactions is reduced while the Fermi level is shifted below the DOS peak in the 3d metal subband. As the result, at higher Fe content the Stoner criterion is not satisfied and no magnetic ordering is observed.