Variation of the Cooperativity in Diluted Hofmann-Based Spin-Crossover Coordination Solids {Fe_1–xM_x(pz)[Pd(CN)₄]}

Abstract

This work is devoted to the investigation of the influence of dopants on the cooperativity of 3D Hofmann-based spin-crossover solids {Fe 1−x M x (pz)-[Pd(CN) 4 ]}, as reflected in the abruptness, position, shape, and completeness of the thermal spin transition and on the width of accompanying hysteresis loop. We present systematic experimental studies, including FTIR, powder X-ray diffraction, energy-dispersive X-ray analysis, and magnetic susceptibility measurements for pure and diluted compounds using Zn, Ni, Mn, and Co as dopants. The shift of the thermal transition temperature toward lower temperatures is, in most cases, correlated with the dopant sizes and the effective dilution. By using a 3D mechanoelastic model considering the different sizes of dopants and the change in intermolecular interactions induced by them, we have satisfactorily reproduced the observed macroscopic experimental behavior. In addition, the microscopic scale images obtained during the HS-LS thermal transition show how the presence of the dopants prevents the spreading of the clusters, reducing the cooperativity in the system.

The effect of inert dopant ions on spin-crossover materials is not simply controlled by chemical pressure

“…The FeZn materials 1a–1e show the expected trend, in that increasing zinc concentration leads to broadening of the transition, while significantly lowering T ½ . 32,40,43–49 Ruthenium doping in 2a–2d has a similar influence on the SCO cooperativity as the zinc dopant at each dopant concentration. However, the two dopant ions have opposite effects on the transition temperature, in that T ½ in 2a–2d increases at higher ruthenium dopant levels (Fig.…”

“…S1, ESI†). 32,33,48,49 In contrast, doping with Ru increases the transition temperature T ½ , stabilising the low-spin state. Such trends have previously been explained by chemical pressure arguments, where larger dopant ions progressively stabilise the larger high-spin form of the iron centres.…”

“…32–34 However, their effect on the transition temperature T ½ is more variable. While increased fractions of Mn 2+ , Co 2+ or Zn 2+ dopant ions consistently lower T ½ of an iron( ii ) SCO compound, 32,36–51 Ni 2+ dopants have a much smaller effect on T ½ of the same materials (Fig. S1, ESI†).…”

“…S1, ESI†). 36,46–54 That is usually explained by the chemical pressure exerted by each dopant ion. The ionic radii of Mn 2+ (83 pm), Co 2+ (74.5 pm) and Zn 2+ (74 pm) resemble high-spin Fe 2+ (78 pm), 55 so introducing those dopant ions into the lattice should favour the high-spin state of the SCO material.…”

Combined Experimental and Mechanoelastic Modeling Studies on the Low-Spin Stabilized Mixed Crystals of 3D Oxalate-Based Coordination Materials

“…The role of elastic interactions between the dopants and SCO molecules is explored in the Supporting Information (shown in Figure S13)in principle, larger interaction constants result in a larger LS residual fraction. In our previous work, when considering different size dopants in an Fe­(II) system, it was observed that larger dopants tend to favor the HS state . However, it is not a straightforward feature and surpasses the present study because the interaction constant between dopants and SCO molecules and the mobility of the dopants play crucial roles in stabilizing one of the spin states.…”