Layered hydride CaNiGeH with a ZrCuSiAs-type structure: Crystal structure, chemical bonding, and magnetism induced by Mn doping

Article abstract of DOI:10.1021/ja3026104

Stimulated by the discovery of the iron oxypnictide superconductor with ZrCuSiAs-type structure in 2008, extensive exploration of its isostructural and isoelectronic compounds has started. These compounds, including oxides, fluorides, and hydrides, can all be simply recognized as valence compounds for which the octet rule is valid. We report herein the first example of a ZrCuSiAs-type hydride, CaNiGeH, which violates the octet rule. This hydride was synthesized by hydrogenation of the CeFeSi-type compound CaNiGe under pressurized hydrogen. Powder diffraction and theoretical simulation confirm that H enters into the interstitial position of the Ca₄ tetrahedron, leading to notable anisotropic expansion of the unit cell along the c axis. Density functional theory calculations indicate the modification of the chemical bonding and formation of ionic Ca-H bond as a result of hydrogen insertion. Furthermore, CaNiGeH shows Pauli paramagnetism and metallic conduction similar to that of CaNiGe, but its carrier type changes to hole and the carrier density is drastically reduced as compared to CaNiGe. Mn-doping at the Ni site introduces magnetism to both the parent compound and the hydride. The measurement demonstrates that hydrogenation of CaNi_1-xMn _xGe reduces ferromagnetic ordering of Mn ions and induces huge magnetic hysteresis, whereas the spin glass state observed for the parent compound is preserved in the hydride. The hydrogenation-induced changes in the electric and magnetic properties are interpreted in terms of development of two-dimensionality in crystal structure as well as electronic state.

Full text of DOI:10.1021/ja3026104

Article
pubs.acs.org/JACS
Layered Hydride CaNiGeH with a ZrCuSiAs-type Structure: Crystal
Structure, Chemical Bonding, and Magnetism Induced by Mn Doping
Xiaofeng Liu,†,⊥ Satoru Matsuishi, Satoru Fujitsu, Toru Ishigaki, Takashi Kamiyama,
and Hideo Hosono*
‡
†
§
∥
,†,‡
†
‡
Frontier Research Center, and Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori,
Yokohama 226-8503, Japan
§
Frontier Research Center for Applied Sciences, Ibaraki University, 162-1, Shirakata, Tokai, Naka, Ibaraki 319-1106, Japan
∥High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki, 305-0801, Japan
*
S Supporting Information
ABSTRACT: Stimulated by the discovery of the iron oxypnictide superconductor
with ZrCuSiAs-type structure in 2008, extensive exploration of its isostructural and
isoelectronic compounds has started. These compounds, including oxides, fluorides,
and hydrides, can all be simply recognized as valence compounds for which the octet
rule is valid. We report herein the first example of a ZrCuSiAs-type hydride,
CaNiGeH, which violates the octet rule. This hydride was synthesized by
hydrogenation of the CeFeSi-type compound CaNiGe under pressurized hydrogen.
Powder diffraction and theoretical simulation confirm that H enters into the
interstitial position of the Ca tetrahedron, leading to notable anisotropic expansion
4
of the unit cell along the c axis. Density functional theory calculations indicate the
modification of the chemical bonding and formation of ionic Ca−H bond as a result of hydrogen insertion. Furthermore,
CaNiGeH shows Pauli paramagnetism and metallic conduction similar to that of CaNiGe, but its carrier type changes to hole and
the carrier density is drastically reduced as compared to CaNiGe. Mn-doping at the Ni site introduces magnetism to both the
parent compound and the hydride. The measurement demonstrates that hydrogenation of CaNi1−xMn Ge reduces ferromagnetic
x
ordering of Mn ions and induces huge magnetic hysteresis, whereas the spin glass state observed for the parent compound is
preserved in the hydride. The hydrogenation-induced changes in the electric and magnetic properties are interpreted in terms of
development of two-dimensionality in crystal structure as well as electronic state.
8
−10
1
. INTRODUCTION
Compounds with the two-dimensional (2D) ZrCuSiAs-type
1111) and related structures have become of particular interest
due to the discovery of the F-doped LaFeAsO superconductor
leading to ReTXH,
where H is negatively charged and thus
serves as the electron sink. For the case of Ae-based 111-type
compounds, such as AeTGe (Ae = Mg, Ca, Sr, and Ba; T = Mn,
(
1
1−13
Fe, Co, and Ni),
the octet rule is already obeyed as they
1
−3
are electronically balanced. In this respect, the exploration of
the hydride for these Ae-based compounds will provide new
insight into the understanding of hydrogenation behavior and
coordination for hydrogen in these systems.
in early 2008.
This 1111-type structure with the formula of
RTAB is built up from alternating layers of edge-sharing R4
tetrahedrons [R: rare earth (Re) or alkali earth (Ae) elements]
filled with B atoms (B: small anions such as O , F , or H )
2−
−
−
After hydrogenation, the initial crystal of structure of the
“111” is almost preserved in the “1111” type hydride, despite
the anisotropic expansion of the unit cell. These systems could
be therefore ideal platforms for studying the hydrogenation
effect on the crystallographic, magnetic, and transport proper-
ties. Besides geometrical change, hydrogenation in general
causes localization of conducting electrons at the negatively
charged H ions, and hence the electric resistance notably
increases as compared to their parent compounds. On the
contrary, we cannot generalize the hydrogenation effect on the
magnetic behavior. From a general perspective, hydrogen
and A tetrahedrons (A: group IV, V, or VI elements) filled
with T atoms (T: transition metal). The common feature
4
4
possessed by most of these quaternary compounds, including
oxide, fluoride, and hydride, is that they all can be regarded as
valence compounds because the octet rule is obeyed by simply
assigning integer oxidation state for each constituent atom (e.g.,
3+
2+ 3− 2−
La Fe As O ).
The octet rule is, however, not always valid for the ternary
CeFeSi-type (111) compounds (formulated by RTA), which
5
−7
can be viewed as an“unfilled” ZrCuSiAs-type structure.
Because of the absence of the B atoms in the R tetrahedrons,
4
insertion into the R tetrahedrons of the ReTX compounds
these compounds are always not electronically balanced. For
instance, the Re-based 111-type compounds are expressed in
4
3
+ 2+ 4− −
the form of Re T X ·e (X: Si or Ge). The excess electrons
Received: March 17, 2012
Published: June 29, 2012
can be consumed by insertion of H into the Re tetrahedron,
4
©
2012 American Chemical Society
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leads to the weakening of magnetic interaction for Re, which
bonds exclusively to H. However, the change of magnetism in
fact varies depending upon the nature of each compound and
the magnetic ions. Chevalier et al. reported that hydrogenation
triggered transition from antiferromagnetic (AFM) to spin
grains. For each batch, the amount of Ca was in excess by 5% as
compared to the stoichiometric ratio to compensate its evaporation
loss during melting. The as-melted metallic ingots were crushed into
fine particles, and then loaded into a tube furnace filled with 2.0 MPa
H , where the samples were allowed for hydrogenation at 573 K for
2
durations up to 40 h. After hydrogenation, the samples were pulverized
into fine powders due to the large volume expansion as a result of
hydrogenation. Analogously, CaNiGeD was synthesized under the
same temperature and pressure conditions in the atmosphere of D2.
Structural Characterization. The stoichiometry of H in the
1
4,15
fluctuation for CeCoSi(Ge),
from heavy fermion to AFM
1
6
for CeRuSi, and from AFM to ferromagnetic (FM) for
1
7
NdCoSi. Hydrogenation also affects the magnetism of
transition metal that does not bond directly to hydrogen. As
Mn is always magnetically ordered in these compounds,
NdMnSi was studied by the same author, and a weakening of
AFM interaction for Mn in NdMnSiH as compared to NdMnSi
compounds CaNi1−xMn GeH were determined by thermogravimetry/
x
mass spectroscopy (TG-MS) using a Bruker AXS TG-DTA/MS9610
system. The samples were heated from room temperature to 900 at 20
°C/min under a continuous argon flow, and the evolution of hydrogen
from the sample was monitored by mass spectrum. Powder X-ray
diffraction (XRD) pattern was collected with a Bruker D8 Advance
diffractometer equipped with Cu Kα radiation operating at 45 kV and
1
8
was unraveled. The origin of these changes in the magnetic
properties upon hydrogenation can be rationalized with the
19
synergistic influence of geometric and electronic factors.
3
60 mA under ambient conditions. The XRD pattern can be
unambiguously indexed with the P4/nmm structure, and it was further
analyzed by Rietveld method using the EXPGUI-GSAS program. To
initialize the refinement, we adopted the structural model of the
ZrCuSiAs-type compound SrFeAsF,
H occupy the corresponding Sr(2c), Fe(2b), As(2c), and F(2b) sites
in SrFeAsF. Peak profile and the background were described with the
pseudo-Voigt function and the shifted Chebyschev polynomial,
respectively. The March−Dollase function was employed to describe
the observed strong preferred orientation along the [001]
23
2
4−26
where Ca, Ni(Mn), Ge, and
1
1,20,21
2
7,28
direction.
The refined crystal structures are visualized using
29
software VESTA. Neutron powder diffraction (NPD) data for
CaNiGeD was taken using a high throughput neutron diffractometer,
iMATERIA, at the Japanese Particle Accelerator Research Complex (J-
PARC). The measurement time for data collection in the 0.2MW
beam power was about 4 hours for 2.4 g powder sample contained in a
3
0
6
mm diameter cylindrical vanadium cell. Rietveld refinement for time-
of-flight neutron powder diffraction data was performed using the
program Z-Rietveld.
3
1,32
Physical Property Measurements. The powders of hydride were
pressed into high-density pellets, and then cut into rectangular bars for
resistivity measurement using a Quantum Design (QD) Physical
Property Measurement System (PPMS) by the conventional four-
probe method. Magnetism measurement was performed for particulate
samples using the QD Superconducting Quantum Interference Device
(
SQUID) equipped with a vibrating-sample magnetometer (VSM).
The Seebeck coefficient was determined by measuring thermoelectric
voltage between the two sides of the sample bar under temperature
gradient (ΔT < 5 °C) at room temperature (24 °C).
Figure 1. Crystal structure of (a) the parent compound CaNiGe with
the CeFeSi-type structure, (b) the hydride CaNiGeH with the
ZrCuSiAs-type structure, and (c) the magnetic hydride CaN-
Calculation Details. Density functional theory (DFT) calculations
were performed using the generalized gradient approximation (GGA)
with Perdew−Burke−Ernzerhof (PBE) functional and the projected
augmented planewave method implemented in the Vienna ab initio
i1−xMn GeH after spin injection by Mn doping at the Ni site. The
x
3
3−35
2
-fold and 4-fold Ge−Fe−Ge bonding angles are designated as α and
simulation program (VASP).
The lattice parameters and atomic
β, respectively.
positions were fully relaxed by a structural optimization procedure
minimizing the total energy and force. A conventional cell containing
two chemical formulas and 6 × 6 × 4 Monkhorst−Pack grids of k
points was used, and the plane-wave basis-set cutoff was set to 600 eV.
The projected density of states (PDOS) of each atom for both the
CaNiGe and the CaNiGeH were obtained by decomposing the charge
CaMnGe and MgTGe (T: Mn, Fe, and Co) did not lead to
single phase hydride. Here, in this Article, we present the
structure and chemical bonding for the nonvalence compound
CaNiGeH. Experiment combined with theoretical calculations
density over the atom-centered spherical harmonics with the same
/3
Wigner−Seitz radius r = (3V
/4πN)1 = 1.55 Å, where Vcell
point out that hydrogen atoms enter into the Ca tetrahedrons
cell(CaNiGeH)
4
and N are the unit-cell volume and the number of atoms in a unit cell,
of CaNiGe, leading to ionic Ca−H bonding and a notable
increase in electric resistivity. Furthermore, hydrogen insertion
in the Mn-doped CaNiGe removes FM ordering of Mn and
induces huge magnetic hysteresis. The observed results are
explained in terms of electron localization and the development
of 2D crystal and electronic structures.
respectively.
3
. RESULTS AND ANALYSIS
Hydrogenation Behavior. The interaction of CeFeSi-type
AeTX compounds with pressurized hydrogen (2−4 MPa) is
quite different from that of their Re-based counterparts ReTX,
most of which form monohydrides upon hydrogenation.
During our material exploration, we performed hydrogenation
for several AeTGe compounds (Ae = Mg, Ca; T = Mn, Fe, Co,
2. EXPERIMENTAL SECTION
Sample Synthesis. The parent compounds of CaNi1−xMn Ge
x
were synthesized by arc-melting from Ca, Ni, Mn, and Ge (purity 4 N)
1
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Article
Ni), and we found that only CaNiGe absorbs hydrogen to form
the ZrCuSiAs-type hydride CaNiGeH. In contrast, annealing of
CaMnGe in H2 resulted in the formation of CaH2 and
CaMn Ge . Other AeTX compounds, such as MgTGe (T: Mn,
2
2
Fe, Co), did not absorb hydrogen under the same condition.
This difference in the hydrogenation behavior may correlate
with the difference in the electron count and chemical bonding
of different AeTX compounds.
In our previous study, we obtained the single phase of
CaNi1−xMn Ge in both the Ni-rich (0 ≤ x ≤ 0.10) and the Mn-
x
rich (0.90 ≤ x ≤ 1.0) composition regions. Here, we find that
single phase hydride of the compounds in the Ni-rich side can
be synthesized through annealing in pressurized H , while
2
hydrogenation of the Mn-rich compounds leads to the
disproportionation reaction similar to the hydrogenation
behavior of CaMnGe. To determine the stoichiometry of the
obtained hydride, we recorded the TG-mass profile for the
hydride. Our assumption of the formation of monohydride
CaNiGeH is well supported by the total weight loss of 0.576%
(
theoretical value 0.58%) as observed from TG profile given in
Figure 2. Similarly, the compound CaNiGeD obtained under
Figure 3. Structural characterization for CaNi1−xMn GeH by XRD. (a)
x
XRD pattern together with Rietveld refinement for CaNiGeH. The
insert shows the XRD pattern from 20° to 90°. (b) Dependence of
lattice constants, cell volume, and the relative change of these
structural parameters (defined as x /x , x: a, c, and V) for the hydrides
H
P
with different Mn concentrations. The subscripts H and P represent
the hydrides and the parent compounds, respectively.
Figure 2. TG-DTA-mass profiles for CaNiGeH. An endothermic peak
centered at 408 °C is observed from TG-DTA curves, which is
associated with the evolution of gaseous hydrogen due to
decomposition of the CaNiGeH as can be seen from the mass
spectrum.
ray scattering length of H atoms. To confirm the position of
hydrogen, we performed NPD for the deuteride CaNiGeD.
Figure S3 (in Supporting Information) shows the Rietveld
refinement pattern of NPD for CaNiGeD. The result of NPD
the same conditions shows a weight loss of 1.12%, which again
confirms that the number of D (or H) atoms is the same as the
confirms that the D atoms enter into the Ca tetrahedrons
4
(Table S2). The structural model of hydrogen insertion is also
supported by structural relaxation based on DFT calculations.
The optimized lattice parameters a and c are 4.0010 and 7.7972
Å, respectively, which are very close to the experimental results.
The obtained structural parameters and atomic coordinates are
listed in Table 1. Similar to the structural evolution of CeFeSi
number of Ca tetrahedrons (and also number of Ca atoms) in
4
the deuteride (or hydride). The decomposition of CaNiGeH
begins from 370 °C and completes at 430 °C, as indicated by a
sharp endothermic peak in the DSC curve and rapid hydrogen
evolution in the mass spectrum. Further examination on
CaNiGeH reveals that the hydrogenation process for CaNiGe
is not reversible. The product after dehydrogenation contains
Ca and CaNi Ge (observed by XRD), and rehydrogenation of
36
upon hydrogenation, the compound CaNiGe undergoes an
overall cell volume expansion of 6.83%, together with a slight
shrinkage along the a axis by 4.5%, and a notable stretching by
17.2% along the c direction (Figure 3b). In addition, hydrogen
insertion also shifts the atomic coordinates along the c axis: ZCa
varies from 0.1863 (in CaNiGe) to 0.1507, and ZGe from
0.6769 (in CaNiGe) to 0.6605. Through a closer comparison of
2
2
the sample under the same annealing condition leads to
CaNi Ge , CaH , and a trace of NiGe alloy.
2
2
2
x
Crystal and Electronic Structure. The XRD pattern for
the hydride CaNiGeH is shown in Figure 3a. Similar to its
parent compounds, the hydride has the same 2D crystal
structure as given in Figure 1, and therefore a strong preferred
orientation along the [001] direction is observed. For the
Rietveld fitting, the position of H atoms was fixed at the center
the geometric change, it is clear that the Ca tetrahedrons
4
become smaller after hydrogen insertion, implying the
formation of Ca−H bonding. The formation of Ca−H bond
meanwhile weakens the bonds between Ca and Ge, and results
in an increment of the Ca−Ge distance for the hydride as
of Ca tetrahedron (1/4, 3/4, 0), and we found that the
4
removal of all H atoms from the structure only shows a
compared to the parent compound. Consequently, the NiGe
4
negligible influence on the result of fitting due to the small X-
tetrahedrons are stretched along the c axis, and therefore the 2-
1
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Table 1. Structure Parameters for CaNiGe and CaNiGeH
from XRD Rietveld Refinement, and Simulated Results for
CaNiGeH
1
1
a
formula
space group
Z
CaNiGeH
P4/nmm
2
CaNiGe
CaNiGeH
P4/nmm
2
P4/nmm
2
a (Å)
4.0038(1)
7.7650(1)
124.476
4.597
4.19341(3)
6.6264(1)
116.523(2)
4.885
4.00100
7.79720
124.817
4.584
c (Å)
3
cell volume (Å )
density (g/cm3)
dCa−Ca (Å)
3.67
3.780
dCa−Ge (Å)
3.19
3.101
dNi−Ni (Å)
2.83
2.965
dNi−Ge (Å)
2.36
2.433
b
αGe−Ni−Ge (deg)
116.19
106.22
y
119.03
104.92
z
c
βGe−Ni−Ge (deg)
atom
site
x
Uiso
Ca
Ni
Ge
H
2c
2b
2c
2a
1/4
3/4
1/4
3/4
1/4
0.1507(5)
1/2
0.6605(1)
0.042(3)
0.033(1)
0.039(1)
0.050(1)
1/4
1/4
1/4
0
a
b,c
Structure parameters obtained from simulation. α and β are the 2-
fold and the 4-fold Ge−Ni−Ge bonding angle, respectively (see Figure
).
1
fold Ge−Ni−Ge bond angle (α) decreases from 119.03° to
1
2
16.2°. These above results demonstrate the development of
D structure by hydrogenation, reflected in the formation of
the CaH and NiGe layers.
With an increment in the Mn concentration, the lattice
parameter c increases, whereas a decreases. This behavior is
consistent with the trend as observed in their parent
compounds. However, the cell volume reaches a maximum of
3
1
24.7 Å at a solubility limit of x = 0.1, beyond which the cell
volume decreases as the CaNi1−xMn Ge system probably
x
undergoes cluster formation or phase separation. The relative
Figure 4. Band structure (right panel), and total DOS and PDOS (left
panel) for (a) CaNiGe and (b) CaNiGeH. The Fermi level is set at 0
eV.
change of lattice constants, defined as a /a (or c /c ), remains
H
P
H
P
almost unaffected at different levels of Mn doping (Figure 3b).
As compared to hydride, the deuteride CaNiGeD shows a
slightly smaller unit cell (see Tables S1 and S2 in the
Supporting Information). This result seems quite abnormal
because the unit cells of deuteride are normally expanded as
compared to those of hydrides. The structural difference
between CaNiGeH and CaNiGeD is that the position of the Ca
atom has been shifted along the c axis, and thus the bond length
of Ca−H is shorter than that of Ca−D in CaNiGeD.
interpretation, while it is evident that a hole-like pocket is
present for CaNiGeH, while it is absent for CaNiGe,
reminiscent of a quasi 2D electronic/crystal structure.
DFT calculations and structural characterization have
clarified that H enters into the Ca tetrahedrons, leading to
4
Ca−H bonding. By integrating the PDOS for the energy region
from −12 eV to the Fermi level (0 eV), we may derive
quantitatively the contribution of each atomic state for CaNiGe
(Ca, 0.74; Ni, 10.28; and Ge, 3.58) and CaNiGeH (Ca, 0.69;
Ni, 10.37; Ge, 3.62; and H, 1.75). The relative large decrease in
the Ca state in the hydride indicates the formation of Ca−H
bond. This is also reflected by a large contribution of the H 1s
state, which suggests the localization of conduction electrons
on H and the formation of hydrogen anion. As compared to Ca,
Ni and Ge are almost not affected by hydrogenation due to the
large distances between H and Ni (or Ge). A similar change in
the electronic structure has also been observed after hydro-
genation of the CeFeSi and CeScSi-type compounds, despite
that the Re−H bond is in fact more covalent than the Ca−H
Figure 4 shows the band dispersion and the PDOS for
CaNiGe and CaNiGeH calculated based on DFT. The DOS for
CaNiGe is very similar to the results obtained by linear muffin-
12
tin orbital (LMTO) method. The observed metallic nature of
the parent compound CaNiGe and its hydride is consistent
with the presence of several bands that cross Fermi level and
the absence of band gap in DOS. The DOS at E [N(E )] is
f
f
slightly lowered after hydrogenation, which is in agreement
with the results from transport measurements. Moreover, in
CaNiGe, the atomic state of Ca contributes to the formation of
conduction band, while in CaNiGeH the participation of the
states of Ca and H is negligibly small, implying the reduction of
metallic bonding to Ca. We have also calculated the Fermi
surfaces (FS) for CaNiGe and CaNiGeH, as shown in Figure
S6 in the Supporting Information. The FS is notably simplified
after hydrogen insertion, and it is rather difficult to give a clear
1
5,18,37,38
bond.
Transport and Magnetic Properties. The electric
resistance was measured with rectangular bars of compacted
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2
2
powders. Because of the presence of considerable porosity,
precise determination of the resistivity was not possible;
however, the measurement clearly revealed an increase of
resistivity for the hydrides by at least 2 orders of magnitude as
compared to their parent compounds. This change can be
understood in terms of electron localization on H due to the
formation of ionic Ca−H bond. The increase in the resistivity,
or even transformation from metallic to semiconducting, has
been observed for the hydrides of many CeScSi-type and Zintl
ature. Figure 6a shows the temperature dependence of
magnetization for the hydrides with different Mn concen-
trations. A small amount of paramagnetic impurity (probably
due to NiGe alloys that also present in the parent compounds)
x
contributes to the slight upturn of magnetization at temper-
ature approaching zero, and this does not affect the following
discussions. From the zero-field cooled (ZFC) and field-cooled
(FC) curves, we can clearly observe that strong magnetic
irreversibility occurs after the smooth increase of magnetization
at around 20 and 80 K for x = 0.05 and 0.10, respectively. In
contrast, magnetic irreversibility develops from around 200 K
for x = 0.02. The same feature was also observed for its parent
compounds (CaNi0.98Mn0.02Ge), and this can be attributed to a
weak AFM ordering. For all of the studied Mn-doped hydrides,
the weak anomaly located at approximately 70 K can be
ascribed to AFM transition of Mn sublattice within the ab
plane. Unlike the parent compounds, no observable FM
transition is present for each of the hydrides examined,
implying weakening of magnetic interaction after hydro-
genation. However, the observation of bifurcation in the ZFC
and FC curves can be still interpreted in terms of SG behavior
due to competing AFM and FM interactions in this system.
Figure 6b shows the M−T curves for CaNi Mn Ge
3
7,39
compounds.
Figure 5 shows the normalized resistivity as a
0
.9
0.1
measured at magnetic fields from 200 to 10 000 Oe. It
becomes fairly clear that the magnetic glassy behavior is more
pronounced at higher fields. This is not a feature observed for
typical SG systems, in which the unparalleled spins are often
reoriented and the SG behavior is destroyed by the applied high
field. This result suggests a noncanonical SG behavior, and the
high magnetic hysteresis for this system implies strong FM
interaction.
Figure 5. Normalized resistivity as a function of temperature for
CaNi1−xMn GeH. The inset shows the normalized resistivity as a
x
function of ln T.
Unlike their parent compounds, before entering into the SG
state, the hydrides do not exhibit characteristic paramagnetic
behavior, and thus simple Curie−Weiss (C−W) law does not
function of temperature recorded from 300 to 1.8 K. At high
temperatures, the temperature dependence of resistivity for all
of the examined samples exhibits good linearity, suggesting
typical metallic conduction. With a decrease of temperature, ρ
reaches minimum at around 20 K and then increases with a
further decrease of temperature for the Mn-doped samples. The
presence of a resistivity minimum at low temperatures and the
linear-like dependence of resistance on ln T might possibly be
attributed to Kondo-effect, which stems from the scattering of
the conducting electrons by the magnetic dopant of Mn.
The thermopower was measured to determine the type of
charge carrier. It is revealed that the carrier type for the parent
compound CaNiGe is electron as indicated by the negative
thermopower (Seebeck coefficient α) of −8.41 μV/K. After
hydrogen insertion, hole is the charge carrier, and the value
increases to 20.04 μV/K. The changing sign and increase in the
thermal power strongly support the DFT calculation result that
indicates the decrease in carrier density and the development of
a hole-like pocket for CaNiGeH. The result further suggests the
trapping of conduction electrons by H, leading to the negatively
charged hydrogen anion.
Hydrogenation also exerts a huge influence on the magnetic
behavior. Before presenting the magnetic measurement for the
hydride, we need to point out here that both the parent
compound and the hydride without Mn-doping show temper-
ature-independent susceptibility, PPM. Moreover, to under-
stand the magnetism of the hydride, we also need to recall the
results as revealed by our previous study that the doping of Mn
into the CaNiGe induces strong magnetic interaction into the
system, resulting in the onset of FM ordering at around 120 K,
followed by a magnetic glass transition at a lower temper-
apply for all of the studied hydrides in the range from T (glass
g
transition temperature) to 300 K. The deviation from C−W
behavior at temperatures higher than T is often observed for
and it is in general associated with the
g
4
1,42
SG systems,
presence of low-dimensional magnetic ordering, which could be
triggered by hydrogenation for the present system. It has to be
40
noted here that the system of CaNi1−xMn GeH is situated near
x
structural instability, and tends to compositional fluctuation or
cluster formation that might be of AFM or FM character. These
magnetic interactions present in the systems cause the non-C−
W behavior at high temperatures and finally lead to SG at lower
temperatures as we have observed by magnetization measure-
ments.
Isothermal magnetization curves were recorded at 5, 25, 150,
and 300 K for CaNi1−xMn GeH (x = 0.02, 0.05, and 0.1), as
x
shown in Figure 6c. At room temperature, all of the hydrides
with different compositions are in the paramagnetic state, while
after entering into the glassy state the M−H curves become S-
shaped, similar to their SG parent compounds. It seems quite
interesting that magnetic hysteresis at 5 K increases rapidly with
an increment of Mn-concentration, and a maxima coercive field
of H = 13.5 kOe is observed for the sample x = 0.1 from the
c
hysteresis loop given in Figure 6c, suggesting strong FM
interaction. In addition, the maxima magnetic moments at 5 T
are very close to results of the parent compounds. This result
implies the presence of strong FM interaction for samples with
high Mn concentration and points out that hydrogenation only
leads to weakening of FM interaction, but does not change the
character of SG behavior and the number of magnetic moment.
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Figure 6. Magnetic properties of CaNi1−xMn GeH. (a) Temperature dependence of magnetization for the CaNi Mn GeH measured at 500 Oe at
x
1−x
x
ZFC and FC mode. The inset gives the M−T curves for CaNi Mn Ge as a reference. (b) Temperature dependence of magnetization for
0.9
0.1
CaNi1 Mn GeH measured at magnetic fields from 200 to 10 000 Oe. (c) Isothermal magnetization curves recorded at temperatures of 5, 25, 150,
−x
x
and 300 K.
4
. DISCUSSION
Regarding magnetism, both CaNiGeH and CaNiGe are PPM
due to electron itinerancy, and hence Ni in the hydride
CaNiGeH does not carry a local magnetic moment. Similar to
the parent compound, the observed magnetic behavior can be
solely attributed to the magnetic dopant Mn. For Mn-based
CeFeSi and ThCr Si -type compounds, it has been generally
From crystal chemistry consideration, hydrogenation leads to
the development of two-dimensionality in both crystal and
electronic structure, resulting in the localization of conducting
electrons on H . Similar to iron-based superconductors, the
parent compound CaNiGe can be simply regarded as a valence
compound in the form of Ca Ni Ge , for which the octet
rule is valid. It is, however, not appropriate from chemists’
viewpoint because metallic bonding is present between Ca and
NiGe layer and strong covalent and metallic bonding is
involved for the NiGe layer as indicated by theoretical
calculation. To reflect the ionic bonding character between
Ca and NiGe layer, the compound CaNiGe should be better
−
3
2
2
accepted that the magnetic interaction between Mn ions varies
2
+
2+
4−
from AFM, FM, to nonmagnetic (NM) depending upon the in-
19,43
plane Mn−Mn distance.
Therefore, the observed SG-like
magnetic behavior can be interpreted in terms of competing
AFM and FM interactions involving Mn ions distributed
randomly in the 2D lattice. Because the system is metallic, the
SG behavior probably stems from Ruderman−Kittel−Kasuya−
Yosida (RKKY) interaction that is mediated by conduction
2+
2−
described as Ca (NiGe) . After insertion of H, the octet rule
is apparently not obeyed. Ca become more ionic, and each CaH
pair is positively charged in the form of [CaH] ; therefore, the
44−46
electrons.
After hydrogenation, the number of conduction
electrons available for magnetic coupling through RKKY
interaction is reduced as discussed previously, leading to the
weakening of magnetic interaction. Besides the electronic
factor, it has been observed that hydrogenation leads to the
shrinkage of the unit cell within the ab plane, and hence the
distance between two nearby Mn ions is reduced, which is also
expected to affect magnetic interaction. However, the interplay
of electronic and geometric factors on the magnetic behavior
still needs to be elucidated in a more precise manner by further
experiment.
+
+
−
hydride CaNiGeH can be viewed as [CaH] [NiGe] . It has to
be pointed out here that the integer charge numbers assigned
for individual atom (or atom pairs) do not represent real
charges. These formulations, however, reflect the bonding
character for both CaNiGe and CaNiGeH in a simplified
manner, despite that the real charges carried by each pair could
be far smaller because mixed bonding nature of these
compounds.
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The final remark concerns the magnetic hysteresis. For the
AUTHOR INFORMATION
Corresponding Author
hosono@lucid.msl.titech.ac.jp
■
system of CaNi1−xMn Ge, the origin of FM ordering is
x
explained to correlate with FM cluster precipitated as a result of
compositional fluctuation or phase separation as this system
22
Present Address
reaches solubility limit at x = 0.1. After hydrogenation, it is
reasonable to consider that this kind of structural inhomoge-
neity is preserved. From a general viewpoint, the coercive force
observed from the hysteresis loop for the hydride and the
parent compounds could relate inherently with the crystal
anisotropy, which impedes magnetization rotation with
⊥
Department of Colloid Chemistry, Max-Planck-Institute of
Colloids and Interfaces, Research Campus Golm, 14424
Potsdam, Germany.
Notes
The authors declare no competing financial interest.
47,48
magnetic anisotropy.
For the case of CaNi Mn GeH,
0.9 0.1
ACKNOWLEDGMENTS
the large magnetic hysteresis can be ascribed to the impeding of
magnetization reversal or pinning of domain walls by structural
■
This research was granted by the Japan Society for the
Promotion of Science (JSPS) through the “Funding Program
for World-Leading Innovative R&D on Science and Technol-
ogy (FIRST Program)”. We thank Dr. H. Mizoguchi for
helping with the Seebeck coefficient measurement. X.L. thanks
Dr. Yu Kang for stimulating discussions.
49
defects or secondary phase precipitates, which is induced by
the tendency to structure instability for this composition. On
the other hand, the sample of CaNi Mn GeH behaves
0
.9
0.1
somewhat like a metamagnet as can be inferred from the
5
0
double jumps in the M−H curves in Figure 6. This
explanation is in line with the interpretation of the SG behavior
originating from competing AFM and FM interaction. A high
magnetic field induces spin flips in the AFM domains and
hence leads to FM ordering of the spins through out the
sample. The presence of such a field-induced magnetic
transition inevitably results in the large magnetic hysteresis.
Although the metamagnet is a convincing candidate to explain
the large magnetic hysteresis, it is yet to be clarified by
systematical investigation on the magnetization of this system.
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