Formation and interconversion of cobalt hydroxides have been investigated with XRD, FTIR, and compositional analysis. Effects of addition time, aging time, and preparative atmosphere on phase transformation and materials synthesis in ammoniacal solution have been systematically examined. A new precursor phase with hydrotalcite-like characteristics has been identified. While α-Co(OH)2 is prepared with short-aging in nitrogen atmosphere, this divalent metal (Co2+) hydrotalcite-like phase transforms into brucite-like β-Co(OH)2 under prolonged aging in nitrogen. The β-Co(OH)2 can be formed readily in air but later is converted into a new hydrotalcite-like phase which is stable and contains both Co2+ and Co3+ due to redox reactions. Interconversions among the four structural phases have been investigated and correlated.

Interconversion of brucite-like and hydrotalcite-like phases in cobalt hydroxide compounds

A new terephthalate-based cobalt hydroxide, Co2(OH)2(C8H4O4), was synthesized by the hydrothermal method. Its crystal structure has been determined by ab-initio XRPD methods (monoclinic, C2/m, a = 19.943(1), b = 3.2895(1), c = 6.2896(3) A, β = 95.746(3)°) and fully refined by the Rietveld technique down to Rp = 0.15 for 9301 observed data (178 independent reflections). The terephthalates are coordinated and pillared directly to the cobalt hydroxide layers and thus a three-dimensional framework is formed. Because of the bonds with the terephthalates, two crystallographically inequivalent cobalt sites are found inside the hydroxide layers, with different octahedral orientations. Magnetic studies show that the intralayer exchange interaction between Co(II) ions is ferromagnetic but the whole system orders antiferromagnetically at 48 K with a metamagnetic transition above a threshold field of 0.2 T. The existence of conjugated π electrons in terephthalates explains the antiferromagnetic interactions between t...

Ab-Initio XRPD Crystal Structure and Giant Hysteretic Effect (Hc = 5.9 T) of a New Hybrid Terephthalate-Based Cobalt(II) Magnet†

On the existence of hydrotalcite-like phases in the absence of trivalent cations

Through an atmosphere-controlled method, a new phase of hydrotalcite-like Co hydroxide with mixed valent states has been synthesised, along with preparations of known α and β phases. Structural and thermal behaviours of all the Co hydroxides have been compared. Three major stages of decomposition are found: (i) 149-164°C for dehydration of interlayer water, (ii) 185-197°C and (iii) 219-222°C for dehydroxylation of hydrotalcite- and brucite-like phases, respectively. Intercalated nitrate anions in hydrotalcite-like phases decompose largely during stage (ii). The oxide Co
 3
O
 4
 starts to form at temperatures as low as 165°C especially for hydrotalcite-like phases. An intermediate compound, HCoO
 2
, which is formed thermally, decomposes at 258-270°C. The Co
 3
O
 4
 oxide converts into CoO at 842-858 and 935-948°C respectively in nitrogen and air, which is much lower than the previously reported range of 1000-1200°C. Surface areas of calcined samples are found to be proportional to the intercalated anion content. The catalytic activity of the resultant Co
 3
O
 4
 oxides with nitrous oxide is 7.2-8.2 mmol N
 2
O g
 –1
 h
 –1
 at 375°C, which is comparable to some reported active catalyst systems.

Thermal evolution of cobalt hydroxides: a comparative study of their various structural phases

The reaction of manganese(II) and pyridine derivatives such as 3-methylpyridine (3-Mepy) and 3,4-dimethylpyridine (3,4-Dmepy) led to the new one-dimensional systems trans-[Mn(3-Mepy)2(N3)2]n (1) and trans-[Mn(3,4-Dmepy)2(N3)2]n (2). Compound 1 crystallizes in the monoclinic system, space group P2(1)/n, a = 11.201(3) A, b = 14.499(4) A, c = 14.308(4) A, Z = 6, and compound 2 crystallizes in the triclinic system, space group P1, a = 11.502(4) A, b = 14.246(5) A, c = 16.200(8) A, Z = 6. The two compounds show the same general one-dimensional arrangement of double azido bridges between neighboring manganese atoms with the unprecedented -Mn-(mu(1,3)-N3)2-Mn-(mu(1,3)-N3)2-Mn-(mu(1,1)-N3)2-Mn- sequence. Susceptibility and magnetization measurements reveal a ferrimagnetic-like behavior derived from the topology of the chain. A model of the Heisenberg chain, comprising classical spins coupled through alternating exchange interactions J1J1J2... is proposed to describe the magnetic behavior.

Topological Ferrimagnetic Behavior of Two New [Mn(L)2(N3)2]n Chains with the New AF/AF/F Alternating Sequence (L = 3-Methylpyridine or 3,4-Dimethylpyridine)

This paper deals with correlations between structural and magnetic properties of two layered parent compounds, Co(OH) 2 and Co 2 (NO 3 )(OH) 3 , which mainly differ by their interlayer distances. Measurements of magnetization, magnetic susceptibility, and specific heat show the occurrence of phase transitions at nearly the same temperatures, and this is explained by strong anisotropic in-plane interactions between cobalt(II) ions. Using HTS expansion for a triangular 2D lattice and numerical computations, the in-plane ferromagnetic interactions are shown to be J=9.2 K for Co(OH) 2 and 7.4 K for Co 2 (NO 3 )(OH) 3

P. Legoll

Papers

Ferromagnetism in triangular cobalt(II) layers: comparison of cobalt dihydroxide and cobalt nitrate hydroxide (Co2(NO3)(OH)3)

1D ferrimagnetism in copper(II) trimetric chains: Specific heat and magnetic behavior of A3Cu3(PO4)4 with A = Ca, Sr

Application du modèle Heisenberg à la chaîne ferrimagnétique d'ions Cu(II) dans Cu2OSO4