Sublattice-Resolved Magnetoelastic Coupling and Opposing Pressure Responses in the Frustrated Magnetocaloric GdMn₂O₅

Frustrated multiferroic manganites exhibit strong lattice-charge-magnetic coupling, enabling complex phase transitions and tunable magnetocaloric responses for cryogenic cooling. Here, we systematically investigate GdMn₂O₅ to elucidate its structure, magnetic ordering, magnetocaloric properties, and pressure response. Structural studies reveal an orthorhombic Pbam framework with no global phase transition from 4-300 K, but instead show anisotropic thermal expansion, including negative expansion along the b-axis, driven by magnetoelastic coupling. Microscopy and X-ray absorption spectroscopy confirm Mn³⁺/Mn⁴⁺ charge ordering between octahedral and pyramidal sites, with an effective coordination number of 5.5. The compliant Mn-Gd interface acts as the primary transducer of magnetoelasticity, exhibiting anomalous spin-phonon coupling. Magnetic measurements reveal successive transitions: T_N ≈ 39 K (Mn³⁺/Mn⁴⁺ antiferromagnetic ordering), T_SR ≈ 34 K (spin-reorientation), and T_Gd ≈ 3.5 K (Gd³⁺ ordering). Under 0-7 T, the material displays a conventional magnetocaloric effect with a maximum entropy change of 16.1 J/kg K at ∼8 K and a relative cooling power of 272 J/kg, surpassing prior reports. Hydrostatic pressure reveals opposing responses: T_N decreases, consistent with weakened Mn-O-Mn exchange, while T_Gd increases, suggesting enhanced Mn-Gd coupling under compression. These results highlight GdMn₂O₅ as a promising cryogenic magnetocaloric material and reveal a sublattice-dependent pressure-tuning mechanism relevant to frustrated multiferroics.