Tunable K vacancies in K1? xCo2Se2 and their effects on structure and ferromagnetism

Article abstract of DOI:10.1016/j.jmmm.2019.165473

Deeply understanding the role of intermediate metal A on structure and related properties of ThCr₂Si₂-type transition metal compounds ATM₂X₂ is of great importance for designing novel layered functional materials. However, inducing A vacancies usually trends to destroy the original structure in reported systems so far, which hampers the further research. Here we report the controllable K vacancies in K_{1? x}Co₂Se₂ system (0 ≤ x ≤ 0.3), where both the ThCr₂Si₂-type structure and intact tetrahedral [CoSe] layers can be maintained with the varying occupancies of K. By inducing K vacancies in structure, tetragonality of the lattice for K_{1? x}Co₂Se₂ increases with the shortened a and elongated c. The (CoSe₄) tetrahedron is also compressed perpendicular to the c direction resulted from the K deficiency. X-ray absorption near-edge structure reveals that the valence state of Co is basically unaffected by K deficient with the absorption edge of Co K-edge unchanged. Concerning the physical properties, K vacancies increase the resistivity of metallic K_{1? x}Co₂Se₂ due to the decreased charge transfer from K⁺ to [CoSe] layers. More importantly, the ferromagnetic interaction of K_{1? x}Co₂Se₂ is unexpectedly weakened by raising K vacancies with the Curie temperature shifted from 80 to 52 K, despite the shortened Co-Co distance. First-principles calculation reveals that the spin polarization is weakened resulted from the K vacancies, mainly attributed to the reduced charge transfer from K⁺ to [CoSe] host. Our results clearly indicate the domination of transferred electrons from intermediate metal A on the magnetic interaction of ATM₂X₂, and also show the feasibility to regulate the structure and related properties of ATM₂X₂ by controlling the A content.

Full text of DOI:10.1016/j.jmmm.2019.165473

JournalofMagnetismandMagneticMaterials490(2019)165473
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Research articles
Tunable K vacancies in K1−xCo2Se2 and their effects on structure and
ferromagnetism
Zhongnan Guoa, Xiaoxiao Yana, Lirong Zhengb, Xue Hana, Dan Wua, Ning Liuc, Fan Suna,
Da Wangc, Wenxia Yuana,⁎
a Department of Chemistry, School of Chemistry and Biological Engineering, University of Science and Technology Beijing, Beijing 100083, China
b Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
c Research & Development Center for Functional Crystals, Beijing National Laboratory of Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences,
Beijing 100190, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
KCo2Se2
ThCr2Si2-type structure
K vacancies
Ferromagnetism
Deeply understanding the role of intermediate metal A on structure and related properties of ThCr2Si2-type
transition metal compounds ATM2X2 is of great importance for designing novel layered functional materials.
However, inducing A vacancies usually trends to destroy the original structure in reported systems so far, which
hampers the further research. Here we report the controllable K vacancies in K1−xCo2Se2 system (0 ≤ x ≤ 0.3),
where both the ThCr2Si2-type structure and intact tetrahedral [CoSe] layers can be maintained with the varying
occupancies of K. By inducing K vacancies in structure, tetragonality of the lattice for K1−xCo2Se2 increases with
the shortened a and elongated c. The (CoSe4) tetrahedron is also compressed perpendicular to the c direction
resulted from the K deficiency. X-ray absorption near-edge structure reveals that the valence state of Co is
basically unaffected by K deficient with the absorption edge of Co K-edge unchanged. Concerning the physical
properties, K vacancies increase the resistivity of metallic K1−xCo2Se2 due to the decreased charge transfer from
K+ to [CoSe] layers. More importantly, the ferromagnetic interaction of K1−xCo2Se2 is unexpectedly weakened
by raising K vacancies with the Curie temperature shifted from 80 to 52 K, despite the shortened Co-Co distance.
First-principles calculation reveals that the spin polarization is weakened resulted from the K vacancies, mainly
attributed to the reduced charge transfer from K+ to [CoSe] host. Our results clearly indicate the domination of
transferred electrons from intermediate metal A on the magnetic interaction of ATM2X2, and also show the
feasibility to regulate the structure and related properties of ATM2X2 by controlling the A content.
1. Introduction
considered as the guest species, also play an important role in reg-
ulating the properties of ATM2X2 since the electrons can easily transfer
Layered transition metal chalcogenides/arsenides ATM2X2, which
contain the intermediate metal (A = alkalis, alkali earths or lantha-
nides) between [TMX] host layers (TM = transition metals, and X = S,
Se, P or As), usually crystallize with the ThCr2Si2-type structure in a
body-centered tetragonal lattice (space group: I4/mmm) [1–3]. Due to
the d electrons from transition metal, ATM2X2 exhibits very rich elec-
tromagnetic properties, such as high-temperature superconductivity
[4,5], interlayer magnetic interaction [6], and heavy fermion behaviour
[7] as well. From the structure point of view, the covalent bonded
[TMX] layers adopt fluorite structure with edge-sharing (TMX4) tetra-
hedrons and act as the functional layers. It is believed that the prop-
erties of these materials are originated from [TMX], mainly determined
by the species of transition metals [3]. The intermediate metals A,
from intermediate metals to the host. It has been reported that the
superconducting critical temperature can be adjusted from 18 to 46 K in
Fe selenides by changing the intermediate metals [8–10], and the
magnetic interaction ordering can also be modulated (from ferromag-
netic to anti-ferromagnetic) by adopting different A in layered Co ar-
senides [11–14].
Charge transfer from intermediate metals A is highly relevant to the
electronic structure of ATM2X2. Inducing the vacancies at A sites can
give rise to the adjustment of carrier concentration in [TMX] layers,
leading to the regulation on related electromagnetic properties.
Compared with the vacancies at TM sites which generally destroy the
related feature [15,16], tunable A vacancies in ATM2X2 are expected to
be more gentle and effective to modulate the related properties since
⁎ Corresponding author.
E-mail address: wxyuanwz@163.com (W. Yuan).
https://doi.org/10.1016/j.jmmm.2019.165473
Received 6 December 2018; Received in revised form 9 April 2019; Accepted 18 June 2019
Availableonline19June2019
0304-8853/©2019PublishedbyElsevierB.V.

Z. Guo, et al.
JournalofMagnetismandMagneticMaterials490(2019)165473
the [TMX] host would be maintained and intact. Consequently, a deep
understanding on this regulation will be of great help for the material
design in ATM2X2 system. However, tuning the A vacancies in ATM2X2
trends to be unattainable in arsenides due to the request of valence
balance from the negative [TMAs]- [17,18]. For TM chalcogenides with
electrically neutral [TMX], the deficient A was reported to reduce the
structural stability in Ni-based system, leading to the collapse of
ThCr2Si2-type structure [19]. These issues hinder us to thoroughly un-
derstand the regulation of intermediated metal A on the structure and
related properties of ATM2X2. In order to solve this problem, a system
where the content of intermediate metal A can be easily controlled
without inducing any TM vacancy or structural instability is required.
Layered cobalt selenide KCo2Se2 with ThCr2Si2-type structure was
reported to show A-type ferromagnetism [6]. Wide investigations have
been done on KCo2Se2 and its sister compounds ACo2Ch2 (A = Rb, Cs,
Tl and Ch = Se, S) by chemical doping on A, Co and Ch sites, leading to
a series of novel magnetic phase transitions [20–25]. However, the
effort to adjust the A vacancies in these layered Co selenides has not
been reported. In this work, we report the intrinsic feature of tunable K
vacancies in nominal K1−xCo2Se2 (0 ≤ x ≤ 0.3). It is indicated that
both the ThCr2Si2-type structure and intact [CoSe] layers can be well
maintained when the composition of intermediate metal is adjusted in
this system. With the K vacancies increased, the lattice parameter a is
shortened while c is elongated, resulting in an increased tetragonality
(c/a). Concerning physical properties, K vacancies significantly in-
creases electric resistivity of K1−xCo2Se2 by reducing the transferred
electrons. More interestingly, in spite of the shortened Co-Co distance in
[CoSe] functional layers, inducing the K vacancies leads to weaker
magnetic interaction with Curie temperature (TC) of K1−xCo2Se2 de-
creased from 80 to 52 K. This suggests that the ferromagnetic interac-
tion in K1−xCo2Se2 is mainly governed by the charge transfer, rather
than the structural parameters. First-principle calculation is also carried
out to explore the mechanism of these effects from K vacancies.
stoichiometric KCo2Se2 structure was used as the starting model to re-
fine the K1−xCo2Se2 samples with x = 0, 0.1, 0.2 and 0.3. The mor-
phology of samples was investigated via scanning electron microscopy
(SEM, Hitachi S-4800) and transmission electron microscopy (together
with the selected area electron diffraction (SAED)) (TEM, JEOL JEM-
2100F). The component analysis of Co, Se and K was made by energy
dispersive X-ray spectroscopy (EDS). The result for each sample was
obtained based on the average of 7–10 sets of data. The valence state of
Co was investigated by X-ray absorption fine structure (XAFS) which
was collected on the 1W1B beamline at Beijing Synchrotron Radiation
Facility (BSRF). The samples were recorded at the Co K-edge
(E0 = 7709 eV) at room temperature. The magnetization susceptibility
was measured by using a vibrating sample magnetometer (VSM,
Quantum Design). The electrical resistivity was measured with the cold-
pressed samples under a uniaxial stress of 600 kg·cm−2 via a typical
four-probe method on physical property measurement system (PPMS).
2.3. First-principles calculation
Electronic structure calculations were performed using the CASTEP
program with plane-wave pseudopotential method [27]. Generalized
gradient approximation (GGA) in the form of the Perdew-Burke-Ern-
zerhof was chosen to solve the exchange-correlation potentials. The
ultrasoft pseudopotential with a plane-wave energy cutoff of 330 eV
and a Monkhorst Pack k-point separation of 0.04 Å−1 in the reciprocal
space were used for all the calculations [28]. The self-consistent field
was set as 10−6 eV/atom. A 2 × 2 × 1 supercell with Co vacancy sites
was built to simulate the K0.75Co2Se2 with 25% K vacancies.
3. Results and discussion
The obtained K1−xCo2Se2 samples are dark microcrystalline solids,
which are unstable to air and moisture. So the samples were kept in the
glove box before they were mounted for characterization. Fig. 1 shows
the PXRD patterns for a series of K1−xCo2Se2 with nominal x = −0.2,
0, 0.1, 0.2, 0.3, 0.4 and 0.5 collected at room temperature. The main
phase of each pattern can be well indexed by the body-centered tetra-
gonal cell with space group I4/mmm (No. 139). For sample with excess
K (x = −0.2) an impurity marked by # is assigned to K2CoSe2 (ICSD
PDF: 79–2149). And for samples with x = 0.4 and 0.5, binary CoSe
(ICSD PDF: 70–2870) was observed, marked by asterisk. It can be seen
that the impurity CoSe also arises in sample with x = 0.3 but with a
very small amount. For samples with x = 0, 0.1 and 0.2, pure phase can
be obtained. It suggests that the K1−xCo2Se2 shows solid solution re-
spect to K content with the boundary around x = 0.3. This solid solu-
tion could be further confirmed by the inset of Fig. 1, where the (0 0 2)
and (2 0 0) peaks gradually shift to lower and higher 2θ angles re-
spectively with the nominal x increased from 0 to 0.3.
Rietveld refinements were carried out on PXRD patterns of samples
with x = 0–0.3 to solve the structure of these tetragonal phases. A ty-
pical refinement of sample x = 0.3 is plotted in Fig. 2a, showing a sa-
tisfactory level for fitting the experimental pattern. All the obtained
structural parameters are listed in Table 1. It needs to be mentioned
that only sample x = 0.3 contains 6.6 at.% CoSe, while the other three
show no diffraction peak from any impurity. The K-site occupations for
different samples show significant deviation from each other, varying
from 0.98(1) (x = 0) to 0.69(1) (x = 0.3). On the contrary, both Co and
Se sites are fully occupied. So it is indicated that the vacancies of K are
tunable in this ThCr2Si2-type compound with maximum content ~30%
(Fig. 2b). The observation of hexagonal CoSe impurity in sample
x = 0.3 could be mainly resulted from the loss of alkali metal during the
sample preparation, which leads to less K content in the reaction. EDS
measurements were used to confirm the variation of K content in these
samples and the measured chemical compositions of sample x = 0, 0.1,
2. Experimental
2.1. Synthesis
The polycrystalline samples K1−xCo2Se2 (with x = −0.2, 0, 0.1,
0.2, 0.3, 0.4 and 0.5) were synthesized by solid state reaction. Powders
of Co (99.8%) and Se (99.95%) obtained from Alfa Aesar were mixed
and ground in an agate mortar, followed by cold-pressing into disks
(1 mm in diameter) with a 200 kg/cm2 uniaxial stress. The disks were
put into the alumina crucibles together with the cut K ingot (97%, from
China National Accord Medicines Corporation) with predetermined
compositions and sealed into quartz tubes under vacuum (10−2 mbar).
The tubes were heated to 473 K with the heating rate of 100 K/h, held
for 12 h and then to 973 K for 48 h to obtain the precursors. After
grinding, the samples were re-heated at 923 K for 72 h to obtain the
homogeneous phase and then cooled down to room temperature by
turning off the furnace. All manipulations for sample preparation were
carried out inside an argon-filled glove box (O2 < 1 ppm) in order to
prevent the oxidation.
2.2. Characterization
Powder X-ray diffraction (PXRD) of the nominal samples
K
1−xCo2Se2 were collected at room temperature on a PANalytical dif-
fractometer (X’Pert PRO MRD) equipped with CuKα radiation
(λ = 1.5148 Å) operation at 40 kV and 40 mA and a diffracted-beam
graphite monochromator in a reflection mode (step = 0.017°2θ, scan
speed = 0.07 s/step for phase define and 0.47 s/step for structure
analysis). Temperature-dependent in situ PXRD was performed at 200
and 100 K by using a Rigaku SmartLab SE instrument (CuKα radiation)
equipped with an Anton Paar nonambient sample stage. Rietveld re-
finements were performed using the FULLPROF package [26]. The
0.2 and 0.3 are
K1.02Co1.96Se2, K0.87Co2Se2, K0.76Co1.98Se2 and
K
0.67Co2.01Se2 respectively, which are comparable to our refinements. A
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Z. Guo, et al.
JournalofMagnetismandMagneticMaterials490(2019)165473
Fig. 1. PXPD patterns for nominal K1−xCo2Se2
(x = −0.2, 0, 0.1, 0.2, 0.3, 0.4 and 0.5). The re-
flections marked by (*) and (#) are assigned to
hexagonal CoSe and K2CoSe2, respectively. Sample
with x = −0.2 represents the nominal composition
K1.2Co2Se2. Inset shows the enlarged (0 0 2) and
(2 0 0) peaks of K1−xCo2Se2, and the blue and red
area are eye guide for peak shifting. (For inter-
pretation of the references to colour in this figure
legend, the reader is referred to the web version of
this article.)
Fig. 2. (a) PXPD pattern for nominal K0.7Co2Se2 (the sample with x = 0.3) with the final Rietveld refinement. (b) Crystal structure for K0.7Co2Se2 with deficient K in
the lattice. (c) EDS spectrum for K0.7Co2Se2 with the electronic microscope photograph of a crystal particle inset. (d) SAED pattern for K0.7Co2Se2.
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Z. Guo, et al.
JournalofMagnetismandMagneticMaterials490(2019)165473
Table 1
and 0.2 were also collected at room temperature and plotted in Fig. 3a
together with that of Co foil as reference. It can be seen that the Co K-
edge X-ray absorption near-edge structure (XANES) spectra of KCo2Se2
and K0.8Co2Se2 are basically the same, but obviously different from that
of Co foil. A weak pre-edge feature around 7708 eV can be observed,
which is attributed to 1 s → 3d transition of transition metal [29].
Meanwhile, both two curves show a strong asymmetrical absorption
located at around 7725 eV, corroborating the high-spin Co2+ ions in
tetrahedral coordination in two samples (the absorption of octahedral
Co3+ ions is located around 7729 eV, as reported in literatures)
[30,31]. Comparing with the data from literature [32], K1−xCo2Se2
system shows very similar spectra with that of CoO, implying the oxide
state in our sample is close to +2. The first-derivatives of XANES
spectra inset show the absorption edges of Co K-edge for KCo2Se2 and
Refinement parameters for nominal K1−xCo2Se2 (x = 0, 0.1, 0.2 and 0.3).
Nominal
x = 0
x = 0.1
x = 0.2
x = 0.3
Refined
a (Å)
c (Å)
αSe-Co-Se (°)
βSe-Co-Se (°)
Co-Se (Å)
Rwp (%)
χ2
K0.98(1)Co2Se2
3.8832(3)
13.5941(7)
109.18(8)
110.06 (9)
2.3694(16)
2.15
K0.85(1)Co2Se2
3.8520(4)
13.7520(8)
107.18(8)
110.63(8)
2.3932(14)
2.54
K0.78(1)Co2Se2
3.8233(3)
13.8601(6)
106.80(8)
110.82(8)
2.3812(15)
3.02
K0.69(1)Co2Se2
3.8117(4)
13.9145(8)
106.38(9)
111.04(9)
2.3804(17)
3.83
2.95
5.21
4.49
3.62
K
0.8Co2Se2 are located at 7713 eV. This suggests that the K vacancies in
structure hardly affect the Co valence state. Additionally, the Co K-edge
extended XAFS (EXAFS) k3χ(k) functions of KCo2Se2 and K0.8Co2Se2 are
basically the same (inset of Fig. 3b), and the Fourier transform (FT) of
EXAFS of two samples are also similar, both showing a single peak
(Fig. 3b). This clearly validates that the K vacancies do not break the
(CoSe4) tetrahedron in K1−xCo2Se2.
It is suggested that a certain amount of K is still requested to sta-
bilize the ThCr2Si2-type structure during high-temperature synthesis,
which is ~70% based on our work. Recently, Zhou et al reported the
synthesis of binary tetrahedral CoSe by de-intercalating K from KCo2Se2
precursor at room temperature, where the product is metastable in
thermodynamics [33]. This implies that the content of K vacancies
might be tuned in a wider range (> 30%) through low-temperature
synthesis. Although the regulating composition of K in K1−xCo2Se2 is
limited here, the high-temperature route used in our work can realize
much better crystalline and even large-size single crystals. On the other
hand, our attempt to introduce K vacancies in KCo2S2 by solid state
reaction was failed. We speculate that the suitable lattice parameters in
ATM2X2 system could also be a critical factor for the tunable vacancy of
intermediate metal A, since KCo2S2 is simply the isologous of KCo2Se2
with compressed lattice.
Based on the tunable K vacancies in K1−xCo2Se2, we could make a
thorough analysis of the influence of intermediate metal A on structure
parameters of this ThCr2Si2-type phase. The lattice constants and two
Se-Co-Se angles in K1−xCo2Se2 depended on the refined vacancy con-
tent x are schematized in Fig. 4. The obtained parameters of stoichio-
metric KCo2Se2 are comparable with the reported data [6]. By in-
creasing x to 0.3, a axis is significantly shortened from 3.8832(3) to
3.8117(4) Å, suggesting that the covalent bonding in [CoSe] host layers
is strengthened due to the induced K vacancies. On the contrary, c axis
lengthens from 13.5941(7) to 13.9145(8) Å with the reduced K com-
position. This can be explained by a smaller Coulomb force between
[CoSe] functional layers caused by less K+ cations [9,34]. Meanwhile,
the varying K content also leads to the distortion of (CoSe4) tetra-
hedron. In stoichiometric KCo2Se2, the tetrahedrons are slightly dis-
torted from the perfect (Se-Co-Se angle = 109.47°) with α = 109.18(8)°
and β = 110.06(9)°. With vacancies increased, the distortion is clearly
exacerbated with the difference between α and β getting larger (lower
part of Fig. 4). This suggests that the K deficiency also drives a com-
pression on CoSe4 tetrahedrons along the ab plane. It has been reported
that in tetragonal CoSe (which can be considered as the K1−xCo2Se2
with x = 1) this distortion is α = 105.8(2)° and β = 111.382(63)° [33],
which is in line with the tendency from our results [35].
Fig. 3. (a) The Co K-edge XANES spectra of Co foil, KCo2Se2 (x = 0) and
K0.8Co2Se2 (x = 0.2). Inset shows the first-derivatives of the XANES spectra. (b)
Fourier transform (FT) of the Co K-edge EXAFS k3χ(k) functions for KCo2Se2
and K0.8Co2Se2. The inset displays their normalized EXAFS k3χ(k) functions.
typical result of sample with x = 0.3 is shown in Fig. 2c. Both Rietveld
refinement and EDS exclude the existence of obvious Co deficiency in
the series of samples. Hence, we indicate that the [CoSe] layers are
intact in this system when K vacancies are induced in the structure. To
further reveal the microstructure features of K1−xCo2Se2, SAED in-
vestigation was also carried out on the sample with x = 0.3 and as
shown in Fig. 2d, the diffraction spots can be well indexed by I4/mmm
space group, which is in agreement with the PXRD pattern. It is worth
noting that no superstructure has been found, because no extra spot
besides the ones from tetragonal lattice was observed. It suggests that
the K vacancies in K1−xCo2Se2 system are totally disordered.
Temperature-dependent in situ PXRD was also performed at 200
and 100 K for sample x = 0 and x = 0.2, to further investigate the
structure in low temperature range. As shown in Fig. 5, the I4/mmm
space group is maintained at 200 and 100 K for both two samples, with
no extra diffraction peak observed besides the one from the tetragonal
lattice. The enlarged (1 0 3) diffraction peaks do not split off, suggesting
that there is no decreased symmetry occurred in this Co-based system.
Hence, it is indicated that there is not any structure distortion or phase
The X-ray absorption fine structure (XAFS) spectra of samples x = 0
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Z. Guo, et al.
JournalofMagnetismandMagneticMaterials490(2019)165473
to 0.65, 0.46 and 0.32 µB/Co for samples x = 0.1, 0.2 and 0.3, re-
spectively in Fig. 8b–d. These reduced magnetic moments are in
agreement with the subdued ferromagnetic interaction from the M−T
curves. Inset of Fig. 7 shows the inverse susceptibility (χ−1), whose
data in the higher temperature range (from 300 to 100 K) can be well
fitted by the modified Curie-Weiss law [23]:
C
χ = χ0
+
T − θ
where χ0 denotes the temperature-independent term, C is the Curie
constant and θ is the paramagnetic Curie temperature. From the
equation μeff
=
8C μB, the effective magnetic moments of Co were
calculated. The obtained µeff values are 2.48, 2.59, 2.64 and 2.95 µB/Co
for nominal x = 0, 0.1, 0.2 and 0.3, respectively. Compared with that of
Co2+ ion (3.87 µB), the effective magnetic moments of Co in all
K
1−xCo2Se2 samples are smaller, indicating the itinerant magnetism
nature in these phases.
Combining the lattice parameters obtained from the refinements, it
is interesting to find that the ferromagnetic interaction is weakened
with the K vacancies induced, despite the Co-Co distance in ab plane
(equal to 2 a) is obviously shortened from 2.746 to 2.695 Å with x
2
changed from 0 to 0.3. In general, the magnetic interaction in layered
Co-based materials should be highly related to the Co-Co distance in ab
plane, where the shorter Co-Co distance usually leads to stronger in-
teraction with higher critical temperature (Neel temperature TN or TC).
However, previous literatures reported the higher TC combined with
longer Co-Co distance in some Co-based phosphides (La1−xPrxCo2P2,
LaCo2−xFexP2, etc.) [37,38], which suggested that the magnetic inter-
action in these materials could be dominated by the electron structure
rather than the crystal structure parameters. K1−xCo2Se2 presented
here further confirms this feature that the magnetic interaction is
mainly determined by carrier concentration instead of the Co-Co dis-
tance in ab plane. Meanwhile the evidence in our work is even stronger
than the reported systems since no foreign element is involved in
Fig. 4. Lattice paramters a, c (upper) and two Se-Co-Se angles (lower) as a
function of refined K vacancy contents in K1−xCo2Se2.
K
1−xCo2Se2 but only the
K
deficiency induced, unlike the
La1−xPrxCo2P2 or LaCo2−xFexP2 systems with foreign species doped.
This novel phenomenon in Co-based 122-type materials could be at-
tributed to their itinerant magnetism nature, which is confirmed by the
calculations that we will show below.
transition in K1−xCo2Se2 system from room temperature down to 100 K.
By indexing the patterns, the lattice parameter a for K0.8Co2Se2 at 100 K
(3.8013(3) Å) is still smaller than that of the stoichiometric KCo2Se2
(3.8356(4) Å), showing the shorter Co-Co distance in ab plane in this K
deficient phase even in the low temperature range.
The temperature-dependent resistivity of K1−xCo2Se2 shown in
Fig. 6 exhibits the metallic behavior in measured temperature range
from 300 and 5 K with the resistivity of all samples below 3 mΩ·cm at
room temperature. It is noted that with the increased K vacancies, re-
sistivity increases with the data of sample x = 0.3 a fifth larger than
that of x = 0. This could be easily understood since the intermediate
metal K in this system plays the key role for charge transfer to [CoSe]
host layers. Increasing the K vacancies would certainly reduce the
carrier concentration in K1−xCo2Se2, hence enhance the electric re-
sistivity.
Fig. 7 shows the temperature dependence of magnetic susceptibility
χ of K1−xCo2Se2. All the curves clearly show the ferromagnetic or-
dering behavior below 100 K. It should be mentioned that the existence
of impurity phase in sample x = 0.3 does not interfere with the ferro-
magnetic transition, since the hexagonal CoSe is reported to be para-
magnetic [36]. With the K vacancies increased, the ferromagnetic in-
teraction is weakened with the Curie temperature TC decreased from
80 K for x = 0, to 67, 56 and 52 K for x = 0.1, 0.2 and 0.3 respectively
(TCs were obtained from the derivative susceptibility [15]). The hys-
teresis loops at 10 K shown in Fig. 8 confirm the ferromagnetism in
In order to understand the mechanism of the decline on ferromag-
netic ordering in K1−xCo2Se2 with the increased K vacancies, first-
principles calculation was carried out to calculate the electronic
structure of KCo2Se2 and nonstoichiometric K0.75Co2Se2 (it is con-
venient to induce 25% K deficiency in the structural model). As shown
in Fig. 9, both compounds exhibit metallic state with the density of
states (DOS) crossing the Fermi level, in accordance with the observed
metallic behavior in Fig. 6. Calculation on spin-polarized partial DOS
shows that the majority DOS near the Fermi level are contributed by Co
atoms (red curves in Fig. 9a and b). The magnetic moment for KCo2Se2
is calculated to be 2.29 µB/Co (consistent with the reported calculation
[39]), and this value is significantly reduced to 1.72µB/Co for K defi-
cient K0.75Co2Se2, implying the degenerative magnetic interaction. The
integral difference between the spin-up and spin-down DOS channels
crossing the Fermi level is 1.27 states/eV·Co for KCo2Se2 and this value
fells to 0.90 for K0.75Co2Se2 as shown in Fig. 9c, mainly attributed to the
reduced carrier concentration from the K vacancies. These results un-
ambiguously indicate that the ferromagnetic interaction in K-deficient
K
0.75Co2Se2 is weakened, compared with that of vacancy-free KCo2Se2.
In consequence, we declare that in K1−xCo2Se2 system, the charge
transfer from intermediate metal K, which determines the charge
transfer to [CoSe] host layers, plays a more crucial role to regulate the
magnetism.
K
1−xCo2Se2 system and also indicate that the samples are soft ferro-
magnets according to the weak hysteresis (inset of Fig. 8). From Fig. 8a,
the saturated magnetic moment for stoichiometric KCo2Se2 is 0.73 µB/
Co, which is consistent with the value reported in literature (0.75 µB/
Co) [23]. As the K vacancies involved, the moments at 4 T are reduced
It is worth noting that an anomaly is also observed at 14 K in the
M−T curve of sample x = 0.3 (Fig. 7), which may be resulted from
another magnetic transition in lower temperature range. As reported in
literature, stoichiometric KCo2Se2 shows A-type ferromagnetic ordering
5

Z. Guo, et al.
JournalofMagnetismandMagneticMaterials490(2019)165473
Fig. 5. In situ PXRD patterns for sample x = 0 and 0.2 at 200 and 100 K. Inset shows the enlarged (1 0 3) diffraction peaks in the patterns.
Fig. 6. Temperature-dependent resistivity of nominal K1−xCo2Se2 with x = 0,
0.1, 0.2 and 0.3.
Fig. 7. Temperature-dependence of magnetic susceptibility of K1−xCo2Se2 with
x = 0, 0.1, 0.2 and 0.3 under the field of H = 1 T with zero-field cooling (ZFC).
Inset shows the inverse of susceptibility versus temperature with the modified
Curie-Weiss fitting (dash yellow lines). (For interpretation of the references to
colour in this figure legend, the reader is referred to the web version of this
article.)
with the magnetic moment of Co parallel to the ab plane. As the K
vacancies induced, a modulation on the original magnetic structure
may occur. Additionally, Zhou et al., reported the synthesis of tetra-
gonal CoSe by de-intercalating the potassium from KCo2Se2 and sug-
gested at first that CoSe is ferromagnetic [33]. Later, their colleagues
Wilfong et al., demonstrated the frustrated magnetic ordering instead
(spin-glass state) in tetragonal CoSe by more systematic magnetic re-
presentation [40]. In K0.7Co2Se2 with ~30% K deficiency, the ferro-
magnetic interaction may also be frustrated, resulting in the anomaly in
both M−T and M−H curves (Figs. 7 and 8d). Further investigation
should be done in future using more accurate characterization (such as
6

Z. Guo, et al.
JournalofMagnetismandMagneticMaterials490(2019)165473
Fig. 8. Hysteresis loops for K1−xCo2Se2 at 10 K with (a) x = 0, (b) x = 0.1, (c) x = 0.2, and (d) x = 0.3. Inset shows the enlarged curves near the coordinate zero
point, indicating the weak hysteresis in all four samples.
Fig. 9. Spin-polarized partial DOS for (a) KCo2Se2 and (b) K0.75Co2Se2. (c) Integral difference between the spin-up and spin-down DOS of KCo2Se2 and K0.75Co2Se2.
The dash line is the Fermi level.
7

Z. Guo, et al.
JournalofMagnetismandMagneticMaterials490(2019)165473
neutron powder diffraction) to determine the magnetic structure of
K0.7Co2Se2 sample at low temperature.
[12] C.M. Thompson, X. Tan, K. Kovnir, V.O. Garlea, A.A. Gippius, A.A. Yaroslavtsev,
A.P. Menushenkov, R.V. Chernikov, N. Büttgen, W. Kratschmer, Y.V. Zubavichus,
M. Shatruk, Synthesis, structures and magnetic properties of rare-earth cobalt ar-
senides, RCo2As2 (R = La, Ce, Pr, Nd), Chem. Mater. 26 (2014) 3825–3837.
[13] X. Tan, G. Fabbris, D. Haskel, A.A. Yaroslavtsev, H. Cao, C.M. Thompson, K. Kovnir,
A.P. Menushenkov, R.V. Chernikov, V.O. Garlea, M. Shatruk, A transition from
localized to strongly correlated electron behavior and mixed valence driven by
physical or chemical pressure in ACo2As2 (A = Eu and Ca), J. Am. Chem. Soc. 138
(2016) 2724–2731.
[14] X. Tan, A.A. Yaroslavtsev, H. Cao, A.Y. Geondzhian, A.P. Menushenkov,
R.V. Chernikov, L. Nataf, V.O. Garlea, M. Shatruk, Controlling magnetic ordering in
Ca1-xEuxCo2As2 by chemical compression, Chem. Mater. 28 (2016) 7459–7469.
[15] S. Shen, G. Wang, S. Jin, Q. Huang, T. Ying, D. Li, X. Lai, T. Zhou, H. Zhang, Z. Lin,
X. Wu, X. Chen, Tunable cobalt vacancies and related properties in LaCoxAs2, Chem.
Mater. 26 (2014) 6221–6225.
4. Conclusions
In conclusion, a series of K1−xCo2Se2 was successfully synthesized
by solid state reaction and the tunable K vacancies (up to ~30%) were
demonstrated in this system via analyzing the chemical composition
and structural parameters. We exclude the existence of Co deficiency
from refinements and EDS, suggesting the intact [CoSe] functional
layers can be maintained with the K vacancies regulated in this system.
Based on the controllable K vacancies in this ThCr2Si2-type compound,
the structure parameters and related properties can be effectively ad-
justed. By inducing the K vacancies, the lattice constant a is compressed
while c is enlarged, resulting in an increased c/a ratio. The distortion of
CoSe4 tetrahedron is also intensified resulted from the deficient K in the
structure. Although the oxide state of Co is basically unchanged, the K
vacancies raise the electric resistivity and most importantly, regulate
the strength of ferromagnetic interaction in this layered Co selenide
with Curie temperature TC varying from 80 to 52 K. First-principles
calculation shows that this regulation on magnetism is mainly attrib-
uted to the modulation of charge transfer from the intermediate metal K
to [CoSe] host layers. Our results not only offer a reliable route to
regulate the structure and electromagnetic properties of ATM2X2 ma-
terials, but also address the important interplay between the charge
transfer and magnetic interaction in ThCr2Si2-type materials.
[16] Z. Guo, F. Sun, B. Han, K. Lin, L. Zhou, W. Yuan, Iron vacancy in tetragonal Fe1-x
S
crystals and its effect on the structure and superconductivity, Phys. Chem. Chem.
Phys. 19 (2017) 9000–9006.
[17] C.M. Thompson, K. Kovnir, S. Eveland, M.J. Herring, M. Shatruk, Synthesis of
ThCr2Si2-type arsenides from Bi flux, Chem. Commun. 47 (2011) 5563–5565.
[18] P.L. Alireza, Y.T. Chris Ko, J. Gillett, C.M. Petrone, J.M. Cole, G.G. Lonzarich,
S.E. Sebastian, Superconductivity up to 29 K in SrFe2As2 and BaFe2As2 at high
pressures, J. Phys.: Condens. Matter 21 (2009) 012208.
[19] J.R. Neilson, T.M. McQueen, Bonding, ion mobility, and rate-limiting steps in
deintercalation reactions with ThCr2Si2-type KNi2Se2, J. Am. Chem. Soc 134 (2012)
7750–7757.
[20] G. Huan, M. Greenblatt, Antiferromagnetic-to- ferromagnetic transition in metallic
Tl1-xKxCo2Se2 (0≤ x ≤1.0) with ThCr2Si2-type structure, J. Less-Common Met. 156
(1989) 247–257.
[21] A. Broddefalk, P. Nordblad, R. Berger, Structural and magnetic properties of TlCo2-
xCuxSe2, 0≤ x ≤1, Physica B 284–288 (2000) 1317–1318.
[22] S. Ronneteg, R. Berger, R.Z. Levitin, E.A. Popova, R.M. Scherbov, A.N. Vasiliev,
Longitudinal spin fluctuations in thermal and magnetic properties of TlCo2Se2-xSx,
J. Magn. Magn. Mater. 281 (2004) 388–393.
[23] J. Yang, B. Chen, H. Wang, Q. Mao, M. Imai, K. Yoshimura, M. Fang, Magnetic
properties in layered ACo2Se2 (A = K, Rb, Cs) with the ThCr2Si2-type structure,
Phys. Rev. B 88 (2013) 064406.
[24] Z. Guo, H. Zhang, D. Wang, B. Han, S. Jin, W. Yuan, Ferromagnetic interlayer in-
teraction in KCo2Se2-xSx (0≤ x ≤ 2) and its chemical origin, Dalton Trans. 45
(2016) 8248–8252.
Acknowledgements
[25] J. Yang, W. Yang, Q. Zhu, B. Chen, H. Wang, Q. Mao, J. Du, Z. Lou, M. Fang, Spin-
flop transition and magnetic phase diagram in CsCo2Se2 revealed by torque and
resistivity measurements, J. Magn. Magn. Mater. 474 (2019) 70–75.
[26] J. Rodriguez-Carvajal, Recent advances in magnetic structure determination by
neutron powder diffraction, Phys. B 192 (1993) 55–69.
[27] J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made
simple, Phys. Rev. Lett. 77 (1996) 3865–3868.
We would like to thank Chen X.L. and Jin S.F. from Institute of
Physics, Chinese Academy of Sciences for the useful discussion. The
financial supports by the National Natural Science Foundation of China
(No. 51532010, 51872028 and 51832010) and the Fundamental
Research Funds for the Central Universities (FRF-BR-18-009B) are
gratefully acknowledged.
[28] H.J. Monkhorst, J.D. Pack, Special points for Brillouin-zone integrations, Phys. Rev.
B 13 (1976) 5188–5192.
[29] J.W. Kim, S.M. Park, In situ XANES studies of electrodeposited nickel oxide films
with metal additives for the electro-oxidation of ethanol, J. Electrochem. Soc. 150
(2003) E560–E566.
Conflicts of interest
[30] C. Roux, D.M. Adams, J.P. Itié, A. Polian, D.N. Hendrickson, M. Verdaguer,
Pressure-induced valence tautomerism in cobalt o-quinone complexes: an X-ray
absorption study of the low-spin [CoIII(3,5-DTBSQ)(3,5-DTBCat)(phen)] to high-
The authors declare no competing financial interest.
References
Ⅱ
spin [Co (3,5-DTBSQ)2(phen)] interconversion, Inorg. Chem. 35 (1996)
2846–2852.
[31] M.O. Figueiredo, T.P. Silva, J.P. Veiga, A XANES study of cobalt speciation state in
blue-and-white glazes from 16th to 17th century Chinese porcelains, J. Electron.
Spectrosc (Related Phenomena) 185 (2012) 97–102.
[32] G. Jacobs, P.M. Patterson, T.K. Das, M. Luo, B.H. Davis, Fischer-Tropsch synthesis:
effect of water on Co/Al2O3 catalysts and XAFS characterization of reoxidation
phenomena, Appl. Catal. A-Gen. 270 (2004) 65–76.
[33] X. Zhou, B. Wilfong, H. Vivanco, J. Paglione, C.M. Brown, E.E. Rodriguez,
Metastable layered cobalt chalcogenides from topochemical deintercalation, J. Am.
Chem. Soc. 138 (2016) 16432–16442.
[34] Z. Guo, L. Zhou, S. Jin, B. Han, F. Sun, W. Yuan, Kx(C2H8N2)yFe2-zS2: synthesis,
phase structure and correlation between K+ intercalation and Fe depletion, RSC
adv. 7 (2017) 17539–17544.
[35] After de-intercalaltion of K in KCo2Se2, the (CoSe4) tetrahedrons could be re-
mained, but the binary CoSe shows a n-glide along ab plane from ThCr2Si2-type,
forming the anti-PbO structure with P4/nmm space group. So the lattice parameters
of binary CoSe reported by Zhou et al., (Ref. 33) are not plotted in Fig. 4 in this
work for comparison.
[1] P. Villars, International, A Pearson’s Handbook of Crystallographic Data for
Intermetallic Phases, 2nd ed., ASM International, Novelty, OH, 1991.
[2] X.Q. Zhou, E.E. Rodriguez, Tetrahedral transition metal chalcogenides as functional
inorganic materials, Chem. Mater. 29 (2017) 5737–5752.
[3] Z. Guo, F. Sun, W. Yuan, Chemical intercalations in layered transition metal chal-
cogenides: syntheses, structures, and related properties, Cryst. Growth Des. 17
(2017) 2238–2253.
[4] J. Paglione, R.L. Greene, High-temperature superconductivity in iron-based mate-
rials, Nature Phys. 6 (2010) 645–658.
[5] J. Guo, S. Jin, G. Wang, S. Wang, K. Zhu, T. Zhou, M. He, X. Chen,
Superconductivity in the iron selenide KxFe2Se2 (0≤ x ≤ 1.0), Phys. Rev. B 82
(2010) 180520.
[6] G. Huan, M. Greenblatt, M. Croft, New ternary transition metal chalcogenides
AM2X2 (A = K, Rb, Cs; M = Co; A = K, M = Ni; X = S, Se): magnetically ordered
metals with the ThCr2Si2-type structure, Eur. J. Solid State Inorg. Chem. 26 (1989)
193–220.
[7] J.R. Neilson, A. Llobet, A.V. Stier, L. Wu, J. Wen, J. Tao, Y. Zhu, Z.B. Tesanovic,
N.P. Armitage, T.M. McQueen, Mixed-valence-driven heavy-fermion behavior and
superconductivity in KNi2Se2, Phys. Rev. B 86 (2012) 054512.
[8] T.P. Ying, X.L. Chen, G. Wang, S.F. Jin, T.T. Zhou, X.F. Lai, H. Zhang, W.Y. Wang,
Observation of superconductivity at 30–46 K in AxFe2Se2 (A = Li, Na, Ba, Sr, Ca,
Yb, and Eu), Sci. Rep. 2 (2012) 426.
[9] T. Ying, X. Chen, G. Wang, S. Jin, X. Lai, T. Zhou, H. Zhang, S. Shen, W. Wang,
Superconducting phases in potassium-intercalated iron selenides, J. Am. Chem. Soc.
135 (2013) 2951–2954.
[10] S. Jin, X. Fan, X. Wu, R. Sun, H. Wu, Q. Huang, C. Shi, X. Xi, Z. Li, X. Chen, High-Tc
superconducting phases in organic molecular intercalated iron selenides: synthesis
and crystal structures, Chem. Commun. 53 (2017) 9729–9732.
[11] K. Kovnir, W.M. Reiff, A.P. Menushenkov, A.A. Yaroslavtsev, R.V. Chernikov,
M. Shatruk, “Chemical Metamagnetism”: from antiferromagnetic PrCo2P2 to fer-
romagnetic Pr0.8Eu0.2Co2P2 via chemical compression, Chem. Mater. 23 (2011)
3021–3024.
[36] A. Sobhani, M. Salavati-Niasari, Cobalt selenide nanostructures: hydrothermal
synthesis, considering the magnetic property and effect of the different synthesis
conditions, J. Mol. Liq. 219 (2016) 1089–1094.
[37] K. Kovnir, C.M. Thompson, H.D. Zhou, C.R. Wiebe, M. Shatruk, Tuning ferro- and
metamagnetic transitions in rare-earth cobalt phosphides La1-xPrxCo2P2, Chem.
Mater. 22 (2010) 1704–1713.
[38] K. Kovnir, V.O. Garlea, C.M. Thompson, H.D. Zhou, W.M. Reiff, A. Ozarowski,
M. Shatruk, Spin-glass behavior in LaFexCo2-xP2 solid solutions: interplay between
magnetic properties and crystal and electronic structures, Inorg. Chem. 50 (2011)
10274–10283.
[39] V.V. Bannikov, I.R. Shein, A.L. Ivanovskii, Structural, electronic, magnetic and
elastic properties of tetragonal layered diselenide KCo2Se2 from first principles
calculations, Physica B 407 (2012) 271–275.
[40] B. Wilfong, X. Zhou, H. Vivanco, D.J. Campbell, K. Wang, D. Graf, J. Paglione,
E. Rodriguez, Frustrated magnetism in the tetragonal CoSe analog of super-
conducting FeSe, Phys. Rev. B 97 (2018) 104408.
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