Structural, magnetic, and thermionic emission properties of multi-functional La1-xCaxB6 hexaboride

Article abstract of DOI:10.1016/j.jallcom.2017.10.065

Herein, we report the synthesis of nanocrystalline La_1-xCa_xB₆ (0 ≤ x ≤ 1) hexaboride powders by solid-state reaction and their subsequent consolidation via spark plasma sintering. The structural, magnetic and thermionic emission properties of La_1-xCa_xB₆ hexaboride are investigated. All of the synthesized nanocrystalline hexaboride powders are single phase with the CsCl-type structure and no ferromagnetic impurity phases have been detected from X-ray diffraction. Magnetic measurements show that weak ferromagnetism at room temperature is found in nanocrystalline La_1-xCa_xB₆ hexaboride powders, and the magnetism was attributed to the presence of the intrinsic defects, based on the data of the HRTEM. Thermionic emission measurements indicate that the maximum emission intensity for bulk La_0.4Ca_0.6B₆ at 1873 K reached 20.02 A/cm², which is more than three times higher as compared to bulk CaB₆ (～6.04 A/cm²). When the La doping was increased to 40 at%, the work function of CaB₆ decreased from 2.95 to 2.76 eV, indicating an improvement in the thermionic emission performance. Therefore, the quasibinary La_1-xCa_xB₆ hexaboride may have an application as a promising cathode.

Full text of DOI:10.1016/j.jallcom.2017.10.065

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Journal of Alloys and Compounds 731 (2018) 332e338
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Structural, magnetic, and thermionic emission properties of
multi-functional La1-xCaxB6 hexaboride
Lihong Bao*, Xiaoping Qi, Tana Bao, O. Tegus
Inner Mongolia Key Laboratory for Physics and Chemistry of Functional Materials, College of Physics and Electronic Information, Inner Mongolia Normal
University, Hohhot 010022, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Herein, we report the synthesis of nanocrystalline La1-xCaxB6 (0 ꢀ x ꢀ 1) hexaboride powders by solid-
state reaction and their subsequent consolidation via spark plasma sintering. The structural, magnetic
and thermionic emission properties of La1-xCaxB6 hexaboride are investigated. All of the synthesized
nanocrystalline hexaboride powders are single phase with the CsCl-type structure and no ferromagnetic
impurity phases have been detected from X-ray diffraction. Magnetic measurements show that weak
ferromagnetism at room temperature is found in nanocrystalline La1-xCaxB6 hexaboride powders, and
the magnetism was attributed to the presence of the intrinsic defects, based on the data of the HRTEM.
Thermionic emission measurements indicate that the maximum emission intensity for bulk La0.4Ca0.6B6
at 1873 K reached 20.02 A/cm2, which is more than three times higher as compared to bulk CaB6 (~6.04
A/cm2). When the La doping was increased to 40 at%, the work function of CaB6 decreased from 2.95 to
2.76 eV, indicating an improvement in the thermionic emission performance. Therefore, the quasibinary
La1-xCaxB6 hexaboride may have an application as a promising cathode.
Received 5 March 2017
Received in revised form
8 October 2017
Accepted 9 October 2017
Available online 10 October 2017
Keywords:
Nanocrystalline
Rare-earth hexaborides
Magnetic properties
Work function
© 2017 Published by Elsevier B.V.
1. Introduction
doping La into PrB6 leads to a reduction in the effective work
function [8]. Moreover, multiple substituted rare-earth hex-
Recently, nanocrystalline lanthanum hexaboride (LaB6) was
found to possess a number of attractive physical and chemical
properties as compared to coarse-grained bulk samples of the same
material [1e3]. One of the important findings is that nano-scale
LaB6 can be used as a low work function material in the charge
collective interlayer in organic photovoltaic solar cells [4]. As a
result, this material can enhance the built-in voltage, resulting in a
higher fill factor (FF), which in turn can significantly improve the
power conversion efficiency. In addition, nanosized LaB6 is an
efficient optical absorption material and has potential applications
for improving the efficiency of organic photovoltaic cells, as they
are highly transparent in the visible light spectrum [5e7]. There-
fore, there are great possibilities to apply nanocrystalline LaB6 in
organic photovoltaic-devices.
aborides exhibit
a
much-improved emission performance
compared to pristine LaB6. Zhou et al. [9] reported enhanced
thermionic emission from partially Ba-doped LaB6 in bulk, poly-
crystalline form. Recently, after the observation of high temper-
ature ferromagnetism in La-doped CaB6 single crystals by Young
et al. [10], such materials have attracted a great attention in the
study of magnetic interactions in the absence of 3d or 4f electrons
[11]. Subsequently, there have been many proposed explanations
for the presence of weak ferromagnetism in CaB6, both experi-
ment and theory. One suggestion is that the weak ferromagnetic
properties of CaB6 single crystals are induced by ferromagnetic
impurity phases, such as iron or FeB compounds [12,13]. In
contrast, other researchers have discovered a room temperature
weak ferromagnetism in CaB6 or BaB6 thin films and related it to
the crystal defects [14,15]. These opposing view-points make the
hexaborides a subject deserving further study. Nevertheless, the
magnetic properties of nanocrystalline La-doped CaB6 systems
have rarely been reported, in spite of their interesting magnetic
properties. Consequently, further studies of this topic are one of
our present research objectives. Furthermore, determining
whether doping La into CaB6 can reduce the work function of the
In general, in order to further improve the performance of LaB6
in terms of its electron emission capacity or optical absorption
intensity, several compositional and microstructural factors
should be taken into consideration. It has been reported that
* Corresponding author. Tel./fax: þ86 4714393246.
E-mail address: baolihong@imnu.edu.cn (L. Bao).
https://doi.org/10.1016/j.jallcom.2017.10.065
0925-8388/© 2017 Published by Elsevier B.V.

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L. Bao et al. / Journal of Alloys and Compounds 731 (2018) 332e338
333
parent compound is another purpose of the research presented
here.
In the following, we report the synthesis of nanocrystalline La-
doped CaB6 powders via solid-state reaction and of bulk material
with micron-sized grains via spark plasma sintering. The structural,
magnetic and thermionic emission properties of the materials thus
prepared have undergone systenatic investigation, the results of
which are reported here.
2. Experimental section
2.1. Synthesis of nanocrystalline powder and bulk materials
Lanthanum oxide (99.99% purity, Baotou rare-earth Institute),
calcium oxide (99.95% purity, Aladdin) and sodium borohydride
(99.0% purity, Sigma-Aldrich) powders were mixed in a stoichio-
metric molar ratio of x:1-x:6(x ¼ 0e1). Then the mixtures were
pressed into the thin pellets, and placed in a resistance furnace at a
reaction temperature of 1150e1200 ꢁC for 2 h. The interacting
mixture was kept under a vacuum of 2 ꢂ 10ꢃ2 bar. After the reac-
tion, the products were washed with hydrochloric acid and distilled
water several times to remove the impurity phases of (La,Ca)BO3.
The synthesized nanocrystalline powders were placed into a
graphite die with an inner diameter of 15 mm for solid-state SPS
processing using a Sumimoto SPS-3.20 MK-V sintering system. The
following conditions were applied for SPS sintering: the axial me-
chanical pressure was 50 MPa, the heating rate was 110 ꢁC/min, the
sintering temperatures was 1500 ꢁC, and the holding time was
5 min.
Fig. 1. XRD patterns of nanocrystalline La1-xCaxB6 powders prepared at 1150 ꢁC.
Pm-3m (PDF cards: 00-034-0427 and 00-031-0254). The diffraction
peaks are well indexed and assigned to the parallel crystal planes of
(100), (110), (111), (210), (211), (220), (310) and (311). We can see
from Fig. 1 that the (100) and (210) peak intensities of CaB6 are
lower than those of LaB6. The main reason for this is that the Ca
atoms have a lower scattering ability of X-Rays as compared to the
La atoms, which causes the lower diffraction intensity of the (100)
and (210) peaks. Furthermore, we do not find any extra impurity
phases such as La2O3, CaO, or (La,Ca)BO3, confirming the high purity
of the reaction products. Moreover, we do not observe a coexistence
of the two isostructural phases of LaB6 and CaB6, indicating that Ca
atoms randomly occupy the lattice sites of La atoms.
Fig. 2 shows typical FE-SEM images of nanocrystalline La1-
xCaxB6 hexaboride powders prepared at various reaction temper-
atures. It can be seen from Fig. 2(a)~(d) that when the reaction
temperature is 1150 ꢁC, all of the synthesized hexaborides are
primarily composed of nanocubes with mean sizes of 50 nm, in
addition to small number of larger cubic crystals. When the reac-
tion temperature is increased to 1200 ꢁC, it can be seen in Fig. 2 (e)
~(h) that the grain sizes obviously increase to 150 nm, and the grain
morphology showed a higher tendency towards a perfect cubic
shape. According to our previous investigations [16,17], non-cubic
nanoparticles are initially formed at a reaction temperature of
1000 ꢁC. When the reaction temperature is further increased to
1200 ꢁC, the nanoparticles aggregated together and crystallized
into perfect cubic crystals through the increased diffusion of ions or
atoms. At the same time, grain growth is observed, as shown in
Fig. 2 (e)~(h). However, we note that it is difficult to obtain ho-
mogenous cubic nanocrystalline powder by solid-state reaction,
which leads to the observation of non-cubic morphologies of hex-
aboride powders.
2.2. Characterization
The phase identification was performed using X-ray diffraction
(Cu Ka radiation, Philips PW1830). The 2q scans were taken be-
tween 20ꢁ and 80ꢁ with steps of 0.05ꢁ, with 2s counting time per
angular value. The nanocrystalline morphology was characterized
using a field-emission scanning electron microscope (FESEM:
Hitachi SU-8010), and the microstructure was characterized using a
transmission electron microscope (TEM: FEI-Tecnai F20 S-Twin
200 KV). The magnetic properties of nanocrystalline hexaborides
were measured using a SQUID magnetometer (Quantum Design
MPMS, 7 T). For the bulk samples, electron backscattered diffraction
(EBSD) measurements were carried out in an FEINANO 200 scan-
ning electron microscope incorporating an EDAX TSL OIM5.2 sys-
tem. Testing of the thermionic emission properties was carried out
using home-made set-up at the University of Electronic Science and
Technology of China. The emission area of the cathode was 1 mm2.
The emission current densities were investigated at cathode tem-
peratures of 1673 K, 1773 K and 1873 K under a vacuum of
7 ꢂ 10ꢃ4 Pa. The cathode temperatures were measured using an
optical micropyrometer.
3. Results and discussion
3.1. Crystal structure, morphology and microstructure of
nanocrystalline La1-xCaxB6
Elemental mapping is an effective method to distinguish the
mixture of phases present in a selected microscopic zone. To clarify
whether individual LaB6 or CaB6 crystals were formed during the
solid-state reaction, the nanocrystalline powders of La0.2Ca0.8B6 as
an example were used for analysis of the elemental distribution
Fig. 1 shows the XRD patterns of nanocrystalline La1-xCaxB6
hexaboride powders with x ¼ 0, 0.2, 0.4, 0.6, 0.8 and 1 prepared at
1150 ꢁC for 2 h. It can be seen that the crystal structure of the
synthesized hexaborides with various Ca doping values can be
indexed using the CsCl-type cubic structure with a space group of

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L. Bao et al. / Journal of Alloys and Compounds 731 (2018) 332e338
Fig. 2. (a)~(d) FESEM images of nanocrystalline La1-xCaxB6 (x ¼ 0, 0.2, 0.4, 0.8) powders prepared at 1150 ꢁC. Fig. 2 (e)~(h) La1-xCaxB6 (x ¼ 0, 0.2, 0.4, 0.8) powders prepared at
1200 ꢁC. The lower image corresponds to the energy mapping results of La0.2Ca0.8B6 prepared at 1200 ꢁC.
(see the lowest part of Fig. 2). The La, Ca, and B elements are
distributed homogeneously in the selected zone indicating the
formation of a single phase. As observed from the elemental anal-
ysis, no ferromagnetic impurity element could be found. As for the
presence of other impurity elements, it is possible that oxygen and
chlorine are introduced at the sample surface during the hydro-
chloric acid washing, silicon maybe introduced by diffusion from
the quartz tube at high reaction temperatures used, and the
aluminum signal comes primarily from the conductive adhesive.
Limitations in the resolution of the SEM meant an EDS analysis of a
single nanocrystal was not possible using this technique. Therefore,
the microstructure and detailed compositional information for
single nanocrystals of La0.4Ca0.6B6 was investigated using a trans-
mission electron microscope. It can be seen from Fig. 3 (a) that this
crystal has a cubic-shaped morphology with an average grain size
of 50 nm. Fig. 3 (b)-(e) displays the corresponding HAADFeSTEM
images and elemental mapping results. The La, Ca and B elements
are uniformly distributed in this single crystal, which indicates that
the Ca element has been successfully doped into the lattice of LaB6.
The EDS analysis of shown in Fig. 3(f), does not show the presence
of ferromagnetic impurity phases in the selected single crystal,
which is consistent with the SEM results.
3.2. Crystallographic orientation of bulk samples
Bulk La1-xCaxB6 samples with grain sizes on the micron scale
were prepared by the spark plasma sintering (SPS) method in an
oxygen-free system. By comparison with hot-pressing sintering,
this technique is a promising method to not only densify the
ceramic powders rapidly, but also prepare grain-orientated bulk
samples because of the special sintering mechanism [18]. Fig. 4
shows the EBSD pole figures and inverse pole figures for polished
bulk samples of CaB6 and La0.6Ca0.4B6, where the color depth in-
dicates the pole area density. It can be seen from Fig. 4 (a) and (b)
that both samples are characterized by the preferred orientation of
the {111}<-6,-34,31>, {111}<-12,-33,14>, and {111}<-2,34,37> di-
rections, which are marked by numbers and boxes. From the in-
verse pole figures of Fig. 4(c) and (d), both bulk samples displayed a
strong (111) texture. According to the earlier investigations [19], the
work functions (Ф) of hexaboride materials depend on the surface
orientation, whereby the (111) crystal surface should have a lower
work function than the others. Thus, we predict that the bulk La1-
xCaxB6 samples with (111) texture should exhibit excellent emis-
sion properties.

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L. Bao et al. / Journal of Alloys and Compounds 731 (2018) 332e338
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Fig. 4. (a) and (b) pole figures, (c) and (d) ND inverse pole figures for CaB6 and
La0.6Ca0.4B6 bulks.
Based on the above mentioned analysis, it can be inferred that
the weak ferromagnetism of nanocrystalline La1-xCaxB6 powders
should originate from the intrinsic defects. Cao et al. [24] have
calculated the magnetic moments of CaB6 with the local lattice
distortion induced by moving a boron cage. As a result, the B atoms
in this distorted system have a negative charge and provide 1/3 of
an extra electron, resulting in ferromagnetism. For non-magnetic
system such as ZnO or CuO [25,26], the atomic vacancies or crys-
tal defects are also related to the ferromagnetism. Therefore, to
better identify the crystal defects, a high resolution transmission
electron microscopy (HRTEM) was used to assess the crystal defects
in nanocrystalline La0.4Ca0.6B6 hexaboride. It can be seen from
Fig. 6 that the lattice fringes of d ¼ 0.42 nm agree well with the
(100) crystal plane, and some fuzzy regions marked by yellow
boxes indicate the evidence of crystal defects. To further clarify the
intrinsic defects at various crystal surfaces, Fig. 7 (b) and (d) show
the inverse FFT patterns along the (100) crystal plane, which is
transformed from the FFT patterns of Fig. 7(a) and (c) and corre-
spond to the marked zones of Fig. 6 (a) and (b). It can be clearly seen
from Fig. 7 (b) and (d) that there are many edge dislocations and
lattice distortions. Meanwhile, we can directly see the obvious edge
dislocation from the locally enlarged image of Fig. 7 (e), which is
denoted by the symbol “T”. This kind of dislocation is often related
to the lattice distortion proceeding in a thin layer containing 1e2
atomic planes. Therefore, we excluded the possibility of a ferro-
magnetic impurity phase contributing to the room-temperature
weak ferromagnetism and assume that it originates from the
intrinsic defects. As for the formation of crystal defects, it maybe
due to the different values of ionic radii of Ca2þ(~1.0 Å) and
La3þ(~1.18 Å), which can lead to the crystal defects. Another pos-
sibility is that the evaporation of Ca element also leads to edge
dislocation during the high temperature synthesis procedure. The
higher magnetization of CaB6 shown in Fig. 5(a), maybe associated
with the crystal's intrinsic defects. During the sample preparation
procedure, we find that the CaO starting material evaporates more
readily than La2O3 at the same reaction temperature, which in-
dicates that using the same chemical ratio of oxide to sodium
borohydride (1:6) it will not be possible to prepare a phase-pure
Fig. 3. (a) TEM analysis of the nanocrystalline La0.4Ca0.6B6 prepared at 1150 ꢁC, (b) the
HAADFeSTEM analysis, (c)~(e) La, Ca and B elements mapping, (f) elemental spectra.
3.3. Magnetic properties
Fig. 5(a) shows the room-temperature magnetization charac-
teristics of nanocrystalline La1-xCaxB6 (x ¼ 0.2, 0.4, 0.6, 1) hex-
aboride powders with a maximum magnetic field of 5 T after
subtracting the diamagnetic background of the tetrafluoroethylene
capsule, where a 10-mg sample is used for every magnetic mea-
surement. It is interesting to note that at the initial low magnetic
field, the magnetic moments of all hexaboride samples show an
abrupt increase and reach saturation magnetizations (Ms) of
2.12 ꢂ 10ꢃ8 Am2, 1.28 ꢂ 10ꢃ8 Am2, 1.8 ꢂ 10ꢃ8 Am2 and 3.24 ꢂ 10ꢃ8
Am2, respectively, exhibiting a room-temperature weak ferro-
magnetism. According to Sakuraba et al. [20], Ackland et al. [21,22]
and Liu et al. [23], room-temperature weak ferromagnetism was
also found in La-doped CaB6 or BaB6 thin film materials, which is
very similar to the present work. Thus, to clarify the mechanism of
the room-temperature weak ferromagnetism of nanocrystalline
La1-xCaxB6, we initially studied whether the ferromagnetic impu-
rity phase contributes to the magnetism. However, we did not find
any ferromagnetic impurity phase from the XRD analysis or EDS
analysis with SEM and TEM. Subsequently, we attempted to
calculate the magnetic moments of La1-xCaxB6 hexaboride theo-
retically using the DFT method. The nominal composition of a
La0.625Ca0.375B6 crystal with 2 ꢂ 2 ꢂ 2 supercells was selected, and
calculation results are shown in Fig. 5(b), where the green dotted
line is taken from Ref. [17] for comparison. It can be seen from the
calculation results that the high symmetry of spin up and spin
down total density of states indicate the zero local magnetic mo-
ments for La0.625Ca0.375B6.