The effect of magnetic coupling on magneto-caloric behaviour in two 3D Gd(iii)–glycolate coordination polymers

Abstract

Two different 3D coordination polymers, [Gd(glc)(Hglc)(H₂O)]_n·nH₂O (1) and [Gd(Hglc)₃]_n (2) (H₂glc = glycolic acid), have been prepared based on the reaction of Gd(III) ions with H₂glc in different pH environments. Due to the deprotonation of the α-OH groups from half of the H₂glc ligands in 1, [Gd₂] units with a Gd–O–Gd bridging model are observed and suggest a stronger magnetic coupling than that in 2. Magnetic and heat capacity studies reveal the significant impact of the strength of magnetic coupling on the magneto-caloric effect (MCE) in systems. Although a theoretical calculation suggests the −ΔS_m (50.5 J kg⁻¹ K⁻¹) of 1 is larger than 2 (45.2 J kg⁻¹ K⁻¹), the stronger antiferromagnetic coupling in 1 decreases the number of low-lying excited spin states upon the lowering of temperature, thus giving a smaller value of −ΔS_m than 2 at various fields (e.g. −ΔS_m,max = 24.8 J kg⁻¹ K⁻¹ and 41.1 J kg⁻¹ K⁻¹ for 1 and 2 respectively at ΔH = 3 T). This case reveals that the effect of magnetic coupling on MCE plays a dominant role for designing low-temperature 3D Gd(III)-based magnetic coolants.