Page 61 - ILLS Annual Report 2018

SCIENTIFIC HIGHLIGHTS
58-59
and organic ligands can produce a compound with an order/disorder phase transition involving the hydrogen bond network [4, 5] and resulting in electric order.
The compound studied is both antiferroelectric-like ordered and weak ferromagnetic ordered below 15 K. However, the change in electric behaviour occurs at much higher temperatures. The electric changes are correlated with changes in the crystal structure, and therefore the use of neutron diffraction is an essential tool for exploring these slight changes and relating them to the macroscopic properties of this material.
In the present study we investigate the structural, magnetic and dielectric properties of the [CH3NH3][Co(COOH)3] perovskite-like metal-organic compound, through variable-temperature neutron and X-ray single crystal and powder diffraction, magnetic susceptibility measurements and dielectric constant in the form of relative permittivity and pyroelectric current measurements. These analyses reveal an unreported structural phase transition at around 90 K, correlated with a change in electric behaviour. This phase transition consists of a change from Pnma space group at RT to P21/n space group (non-standard space group of
P21/c) at low temperature (see figure 1). Full datasets collected on the single-crystal neutron diffractometer D19 at RT, 135 K and 45 K revealed that this phase transition involves slight changes in the orientation of the methylammonium counterions, which are weakly anchored in the cavities, as well as in the CoII octahedral environments. These modifications in the structure are associated with the occurrence of an electric phase transition from paraelectric to antiferroelectric-like state (see figure 2).
Figure 2
Relative permittivity as a function of temperature (red curve on cooling, black curve on warming) together with a view of the hydrogen bonds between the hydrogens of the methylammonium counterion and the oxygen atoms of the formate ligands at 135 K and 45 K.
The powder X-ray studies carried out on BL04-MSPD beamline at ALBA synchrotron show a notable mixture of orthorhombic and monoclinic phases, even well below the nuclear phase transition. The different ratios between the phases in the different measurements suggest
a dependence on the cooling/warming process. Moreover, the broadening of some reflections after the structural transition denotes an important strain contribution (see figure 3) in the powder samples.
Together with the occurrence of weak electric transition, the [CH3NH3]
with a long-range magnetic order at about 15 K. To investigate the magnetic contribution, neutron powder diffraction experiments were performed on the high-intensity powder diffractometer D1B. This analysis concluded that a weak ferromagnetic component appears due to the occurrence of a non-collinear antiferromagnetic structure where magnetic moments are not strictly compensated. Therefore, below 15 K this material shows the coexistence of an antiferroelectric-like behaviour originating from a structural phase transition, non-significantly coupled with the weak ferromagnetism arising from the non-compensation of
the non-collinear antiferromagnetic structure of the metal–organic host framework. Even if weak ferromagnetism is commonly attributed to the Dzyaloshinskii–Moriya interaction, the ab initio calculations performed using the CASSCF/NEVPT2 method [6] indicate that the single-ion anisotropy of CoII ions is the dominant term in this case.
The magnetic reflections in the neutron measurements at 2 K are compatible with Pn’ma’ and P2’1/n’ magnetic models. The best refined models give rise to magnetic structures where the magnetic moments
are antiferromagnetically coupled with a small, non-compensated component of the magnetic moment, in agreement with the ab initio calculations (see figure 4). Moreover, the magnetic structures obtained at zero-field are not compatible with the magneto-electric behaviour previously observed with this compound under applied field. Therefore, further work is needed to understand the origin of the magneto-electric coupling under applied magnetic fields.
Figure 3
Experimental (open red circles) synchrotron X-ray powder diffraction data and calculated Rietveld refinement (black solid line) pattern. The upper row of vertical green marks represents the position of the Bragg reflections for the orthorhombic phase, while the lower row shows the position corresponding to the monoclinic phase, with refined cell parameters
of a = 8.26226(3), b = 11.6471(2) and c = 8.15991(3) Å,
α = β = γ = 90.0 ° and a = 8.16148(2), b = 8.26332(1) and
c = 11.65342(7) Å, β = 91.676(1) ° for the orthorhombic and monoclinic phases, respectively. The intensity corresponding to the orthorhombic and monoclinic phases (in the inset), is represented by pink and green solid lines, respectively. It should be noted that the orthorhombic (1 2 1) reflection, denoted by the † symbol, is split into (-1 1 2), while (1 1 2) reflections in the monoclinic phase are denoted by the ‡ symbol in the figure.
[Co(COOH) ] compound presents an overall antiferromagnetic coupling 3
Figure 4
left) Perspective view along the b-axis of the magnetic structure refined in the Pn’ma’ Shubnikov space group.
right) Detailed view along the a-axis. The crystal and magnetic unit cell [k = (0, 0, 0)] is shown in blue, the magnetic moments in light green. The nuclear structure uses the same colour coding as that in previous figures.
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