Water, vacancies, and competing exchange interactions in Prussian blue analogues: a neutron diffraction study of field and dehydration-driven magnetic transitions

N. S. Dhami; C. V. Colin; V. Nassif; O. Fabelo; T. Nait; A. Bleuzen; A. Bordage; V. Balédent

paper

Water, vacancies, and competing exchange interactions in Prussian blue analogues: a neutron diffraction study of field and dehydration-driven magnetic transitions

arXiv:2609.08976

Abstract

We report a neutron diffraction study of the structural and magnetic properties of a family of ferro- and ferrimagnetic Prussian blue analogues (PBAs), $A_{4}$ [Fe(CN) $_{6}$ ] $_{2.7}$ ( $A$ = Co, Mn, Ni), Rb $_{2}$ Ni $_{4}$ [Fe(CN) $_{6}$ ] $_{3.3}$ , and Mn $_{4}$ [Cr(CN) $_{6}$ ] $_{2.7}$ , as a function of temperature (2--450~K) and applied magnetic field. All bimetallic compounds of the Fm $3$ m family exhibit a broad diffuse feature at low scattering angle, which we identify, through comparison with the cation-stabilized Rb $_{2}$ NiFe framework, as an intrinsic signature of correlated vacancies and their associated interstitial water. High-temperature diffraction reveals a continuous crossover from positive to negative thermal expansion in CoFe and MnFe upon dehydration, while NiFe remains structurally robust up to 450~K. At low temperature, all compounds order in a collinear ferrimagnetic state with propagation vector $k = (0, 0, 0)$ , except MnFe, which adopts a partially frustrated magnetic structure with $k = (1, 0, 0)$ . A moderate magnetic field of $B_{c} = 1.3$ ~T drives a spin reorientation in MnFe toward the collinear $k = (0, 0, 0)$ ferrimagnetic state common to the other compounds; the same transition is independently induced by dehydration. A minimal Heisenberg model shows that this transition results from a near-compensation between antiferromagnetic Mn--Fe coupling and a geometrically frustrated antiferromagnetic Mn--Mn interaction on the face-centered-cubic Mn sublattice, placing MnFe in the vicinity of a magnetic compensation point. These results resolve a longstanding ambiguity in the interpretation of the Fe $K$ -edge XMCD response of MnFe-based PBAs, and establish water content as a key parameter controlling both the structural and magnetic stability of this family of materials, with direct relevance to their use as battery electrodes.