Inorganic Chemistry
Article
field-cooled (FC) susceptibilities were measured in a field of 0.01 T
over a temperature range of 2−300 K. As M(H) is linear in this field
range, the small-field approximation to the susceptibility, χ ≃ M/H,
was assumed to be valid. The data for each compound were corrected
for diamagnetism of the sample using Pascal’s constants.51
2.6. Powder Neutron Diffraction (PND). Powder neutron
diffraction measurements were carried out at the ISIS Pulsed Neutron
and Muon Source using the WISH (Mn(NCS)2, Co(NCS)2,
Ni(NCS)2) and GEM (Fe(NCS)2) instruments.52 Samples of
Mn(NCS)2 (4.76 g), Fe(NCS)2 (2.26 g), Co(NCS)2 (2.44 g), and
Ni(NCS)2 (4.76 g) were loaded into thin-walled vanadium canisters.
The canister diameters were 11 mm for Mn(NCS)2 and Ni(NCS)2, 6
mm for Co(NCS)2, and 6 mm with an indium seal for Fe(NCS)2.
Each sample was loaded to a height of at least 40 mm, to ensure the
full beam illuminates the sample.
Each sample was first cooled to the base temperature (1.5 K for
Mn(NCS)2, Co(NCS)2, and Ni(NCS)2 and 10 K for Fe(NCS)2), and
diffraction patterns were then collected at a series of temperatures
through TN. A complete list of temperature steps and data collections
for absorption effects using the Mantid software package.53
For each compound, the nuclear structure was determined by
Rietveld refinement against powder neutron diffraction data collected
above TN, using a model derived from the previously reported single
crystal structure of Ni(NCS)2.27 All refinements were carried out
using TOPAS Academic 6.0.50
and becomes increasingly antiferromagnetic as we move to
earlier first-row TM cations.
From PND measurements, Mn(NCS)2, Fe(NCS)2, and
Co(NCS)2 are observed to adopt the same commensurate
stripe-ordered magnetic ground state with ordering vector k =
[100]*. In contrast, Ni(NCS)2 adopts a ground state magnetic
structure with ordering vector k = [0012 ]*, consistent with its
very different (and positive) Weiss constant.
2. MATERIALS AND METHODS
2.1. Synthesis. The synthetic procedures for each member of the
M(NCS)2 (M = Mn, Fe, Co, and Ni) used were broadly similar. We
therefore provide a general synthetic route for M(NCS)2 here;
complete synthetic routes for each compound are given in the
For M = Mn, Co, and Ni, TM sulfate salts were dissolved in the
minimum volume of deionized H2O and added to a saturated solution
of Ba(SCN)2·3H2O. For M = Fe, a solution of KSCN in dry
acetonitrile was added to Fe(BF4)2·6H2O. In all cases, a white
precipitate (colored by the strongly colored solution) formed
immediately, and the reaction mixture was stirred in the air (M =
Mn, Co, and Ni) or under a nitrogen atmosphere (M = Fe). The
solvent was then removed in vacuo to generate a microcrystalline
powder.
2.2. Powder X-ray Diffraction (PXRD). Phase purity was
assessed via powder diffraction measurements on a PANalytical
Empyrean Diffractometer using Cu Kα radiation (λ = 1.541 Å) in
Bragg−Brentano geometry. Diffraction patterns were recorded over
the range 2θ = 5−80° using a step size of 0.02° and a scan speed of
0.01° s−1. Due to their sensitivity to moisture and air, the diffraction
patterns of Mn(NCS)2 and Fe(NCS)2 were measured by encasing the
samples between polyimide (Kapton) films. All diffraction patterns
were analyzed via Pawley46 and Rietveld47,48 refinements using
TOPAS Academic 6 structure refinement software.49,50
2.3. Diffuse Reflectance Spectroscopy. Diffuse reflectance
spectra were recorded on an Agilent Technologies UV−vis
spectrometer, connected via optical fiber to a Cary 50 Diffuse
Reflectance Accessory, using a wavelength range λ = 200−1000 nm,
with a step size of 1.00 nm and scan rate of 10 nm s−1. In all cases,
samples were diluted with BaSO4 powdereither in a 1:10 mass ratio
(Fe(NCS)2, Co(NCS)2, and Ni(NCS)2) or a 1:1 mass ratio
(Mn(NCS)2)and the mixture was ground to produce a
homogeneous powder, which was then loaded between two quartz
discs. For Mn(NCS)2 and Fe(NCS)2, the homogeneous powders
were prepared inside an Ar-filled glovebox and the quartz discs sealed
with Parafilm; the spectra for these compounds were acquired within
half an hour of removing the samples and discs from the glovebox.
For all materials, the spectra were averaged over multiple
measurements; spikes in the average due to erroneous spikes in the
raw datai.e., spikes in one spectrum which do not repeat in the
other spectra, likely due to specular reflection from the powderwere
removed from the average and the “spiked” data point replaced with
the average intensity either side of the spike.
Rietveld refinements using the candidate magnetic irreducible
representations (irreps) were carried out for each compound
separately, which showed that in each case only one of the two
single irrep structures was consistent with the experimental data.
Including the second irrep did not significantly improve the fit to the
data. On this basis, we refined the magnetic structures using only the
+
+
mY2 irrep for Mn(NCS)2, Co(NCS)2, and Fe(NCS)2 and the mA1
irrep for Ni(NCS)2. All refinements were carried out by
simultaneously refining against data collected on multiple banks of
detectors: on WISH, for Mn(NCS)2, banks 2−5; for Co(NCS)2,
banks 1−5; and for Ni(NCS)2, banks 2−5; on GEM, for Fe(NCS)2,
banks 2−5.
In all cases, the background was fit with a 12-term Chebyshev
polynomial. For the final refinements of the data collected for
Mn(NCS)2 using the WISH diffractometer and for the final
refinement of the data collected for Fe(NCS)2 on the GEM
diffractometer, it proved necessary to refine the Voigt peak-shape
parameters separately for high Q and low Q data, due to their unusual
Q dependence.
For all refinements, the lattice parameters, atomic positions, and
magnitudes and directions of the magnetic moments were allowed to
refine freely, aside from restraints on the C−N (ca. 1.15 Å) and C−S
(ca. 1.65 Å) bond lengths. For Mn(NCS)2, Co(NCS)2, and
Ni(NCS)2, the same set of freely refining anisotropic atomic
displacement parameters was used for each atom, while the same
isotropic atomic displacement parameter was refined for each atom in
Fe(NCS)2 [Table S6]. The bond lengths and angles were consistent
with those expected from previous studies [Table S1].20,27
2.4. Thermogravimetric Analysis (TGA). Thermogravimetric
data for each compound were recorded with a Mettler-Toledo
Thermogravimetric Analysis/Simultaneous Differential Thermal
Analysis (TGA/SDTA) 851 Thermobalance. Each powder sample
(20−50 mg) was loaded into an alumina crucible and heated from 50
to 600 °C at a heating rate of 10 °C min−1 under a nitrogen
atmosphere. The data collected were measured relative to a
background blank TGA curve, recorded using the same alumina
crucible, temperature range, and heating rate, under a nitrogen
atmosphere.
2.5. Magnetic Susceptibility Measurements. The magnetic
susceptibility measurements were carried out on powder samples
(10−20 mg) using a Quantum Design Magnetic Property Measure-
ment System 3 (MPMS) superconducting quantum interference
device (SQUID) magnetometer. The zero-field cooled (ZFC) and
3. RESULTS
3.1. Bulk Characterization. The M(NCS)2 family
members (M = Mn2+, Fe2+, Co2+, Ni2+) were synthesized via
salt metathesis reactions, driven by precipitation of an
insoluble side-product (BaSO4 for M = Mn, Co, Ni; KBF4
for M = Fe). Apart from Ni(NCS)2, all compounds crystallized
as solvates; the cocrystallized solvent was removed by heating
either in vacuo (M = Fe, Mn) or in the air (M = Co). While
Co(NCS)2 and Ni(NCS)2 were stable in the air, Mn(NCS)2
and Fe(NCS)2 were moisture- and air-sensitive, respectively.
The phase purity of all materials was checked initially using
PXRD [Figure S1], revealing the presence of trace quantities
(<1 wt %) of impurities such as unreacted starting materials or
C
Inorg. Chem. XXXX, XXX, XXX−XXX