Existence of Two Concomitant Magnetic Structures Below T_Néelfor the Natrochalcite, NaFe^II₂(H₃O₂)(MoO₄)₂

Abstract

A comparative study of the magnetic properties and magnetic structures of the natrochalcite, NaFe(2)(D(3)O(2))(MoO(4))(2) (FeD) to those of the isostructural NaCo(2)(D(3)O(2))(MoO(4))(2) (CoD) and NaNi(2)(D(3)O(2))(MoO(4))(2) (NiD) is presented. The structural change is a shrinking of the unit cell in the order of the ionic radii of the transition metal, FeD> CoD > NiD. While NiD and CoD are canted-antiferromagnets with T(N) = 28 and 21 K, respectively, FeD is an anisotropic 2D-Ising antiferromagnet (T(N) = 17 K) with a spin-flop field of 14 kOe at 2 K and the presence of a hysteresis loop reaching only (1)/(4) of the saturation magnetization in 70 kOe. The critical field decreases almost linearly on warming to T(N). The neutron diffraction patterns of FeD below T(N) display numerous magnetic Bragg peaks which cannot be assigned to any one magnetic structure but fits well to two superposed sets, one with a temperature independent line width and has a propagation vector k(1) = (0, 0, 0) while for the other there is a clear dependence and k(2) = (0, 0, ½). In the k(1) = (0, 0, 0) magnetic structure the moments are parallel to each other within one chain and lie along the a-axis but are antiparallel to those in neighboring chains. In contrast CoD and NiD, for which k = (0, 0, 0), have their moments aligned along the b-axis and ac-plane, respectively. The second magnetic structure, k(2) = (0, 0, ½), is characterized by four sublattices, two per layer, where the moments are in the ab-plane and canted with a resultant along the a-axis which is compensated by those of the adjacent layers. For the k(2) = (0, 0, ½) structure, the scattering coherent length decreases, and the moments tend progressively toward the a-axis upon increasing temperature. The coexistence of two concomitant magnetic structures is unprecedented for compounds containing transition metal moment carriers.

Crystal and Magnetic Structures and Magnetic Properties of Selenate Containing Natrochalcite, AIMII2(H3O2)(SeO4)2Where A = Na or K and M = Mn, Co, or Ni

“…The first of these sets are canted antiferromagnets, the second are ferrimagnets, and the last are ferromagnets. The last family of compounds that we are studying is that of natrochalcite, AM 2 (H 3 O 2 )(TO 4 ) 2 . , …”

“…The synthesis and crystal chemistry using X-ray, and in some cases neutron diffraction, of the mineral natrochalcite, AM 2 (H 3 O 2 )(TO 4 ) 2 , family are well documented, − but their magnetic properties and magnetic structures remained unknown until the reported antiferromagnetism of NaCu 2 (D 3 O 2 )(SO 4 ) 2 ( T N = 3.3 K) and NaM 2 (H 3 O 2 )(MoO 4 ) 2 (M = Ni ( T N = 21 K) and Co ( T N = 28 K)). , Previous works were concerned principally with the syntheses, characterization, and identification of the geometry and function of the H 3 O 2 anion. It has been demonstrated that a wide range of salts can be prepared with A of varying sizes (Na, K, Cs, Rb, Tl, NH 4 , and Ag), M of the divalent transition metals (Mn, Fe, Co, Ni, Cu, and Zn), and the central atom of the tetrahedral anion (T) being S, Se, or Mo. − Although the known salts contain principally divalent metals, one has been reported to contain trivalent iron. Such a variety of possible cations (A, M, and T) in the three crystallographic sites provide a handful of materials for systematically studying their influence on the magnetic properties and magnetic structures as a function of cationic sizes, spin, and magnetic anisotropy. , …”

“…It has been demonstrated that a wide range of salts can be prepared with A of varying sizes (Na, K, Cs, Rb, Tl, NH 4 , and Ag), M of the divalent transition metals (Mn, Fe, Co, Ni, Cu, and Zn), and the central atom of the tetrahedral anion (T) being S, Se, or Mo. − Although the known salts contain principally divalent metals, one has been reported to contain trivalent iron. Such a variety of possible cations (A, M, and T) in the three crystallographic sites provide a handful of materials for systematically studying their influence on the magnetic properties and magnetic structures as a function of cationic sizes, spin, and magnetic anisotropy. , …”

Simultaneous Presence of Two Different Magnetic Structures in a Single-Crystalline Solid? Hydrogen-Distribution-Dependent Magnetism

“…1,2 Thus, it is highly unlikely that a crystalline magnetic solid can exhibit two widely different magnetic structures simultaneously. Nevertheless, precisely such an observation has been reported for NaFe 2 (H 3 O 2 )(MoO 4 ) 2 , 3 and the cause for this phenomenon has not been understood yet. In this Communication, we explore the reason for this finding on the basis of density functional theory (DFT) calculations to find that the chemically meaningful structures of NaFe 2 (H 3 O 2 )(MoO 4 ) 2 require the partitioning of H 3 O 2 units into H 2 O and HO groups and that the spin exchanges of the resulting structures depend on the H-atom distributions, although the Fe/O framework remains unchanged.…”

“…It is remarkable that the intrachain Fe−O−Fe superexchange is AFM in the q 2 structure but is FM in the q 1 structure, despite the fact that the Fe/O framework is identical in both structures. Because ∠Fe−O−Fe of the edge-sharing FeO 4 chains is considerably greater than 90°(i.e., 98.8°), 3 one would have expected that the FeO 4 chains should be AFM chains on the basis of the Goodenough rule. 4 This is not true for the q 1 structure.…”