Unified Study of Structural, Magnetic, Thermodynamic, Phonon Dynamics, Transport, Thermoelectric Properties, and Superconductivity (SC) of Rare-Earth 1111 Iron OxyPnictide (FeOPn) Compounds RO₁₋ₓ₋ᵧFₓFeAs under Doping and Pressure.

Comprehensive investigation of rare-earth FeOPn compounds RO₁₋ₓ₋ᵧFₓFeAs (R = La, Ce, Sm, Pr; 0 ≤ x ≤ 0.3, 0 ≤ y ≤ 0.5) integrates synthesis, structure, magnetic, thermodynamic, spectroscopic, transport, and high-pressure (HP) measurements. Polycrystalline and microcrystalline samples synthesized by controlled solid-state reactions and characterized by X-ray diffraction confirming the tetragonal P_4/nmm structure. Polarized Raman spectroscopy of RFeAsO (R = La, Sm, Pr) exhibits well-resolved A_₁g and B_₁g phonon modes with negligible electronic background, enabling quantitative analysis of lattice dynamics and electron–phonon coupling. Resistivity, magnetization, thermoelectric power S(T), and specific heat display correlated anomalies near 150 K, consistent with spin–density–wave driven lattice instability in undoped compounds. Fluorine substitution induces SC with T_c ≈ 55 K, while oxygen deficiency and quenching alter S(T), revealing role of lattice defects. HP studies show T_c is enhanced or suppressed depending on doping, with dT_c/dP trends replicating dT_c/dx correlations in cuprates. These results elucidate intricate coupling among lattice vibrations, carrier concentration, and spin fluctuations, establishing unified framework linking structural, magnetic, and electronic instabilities with SC in 1111 FeOPn.