Transition metal compounds with two-dimensional triangular lattice, such as LiVO2, are of particular interest, because they exhibit peculiar structural and magnetic behavior involving the frustration present in these oxides. An orbital ordering transition occurs near 500 K, which leads to a suppression of magnetic moment below the phase transition temperature Tt. We synthesized a series of Mg-doped LiVO2 single-phase samples and reported their structural and magnetic properties. The samples were characterized by x-ray diffraction, scanning electron microscope, differential scanning calorimetry, electrical resistivity, magnetic susceptibility, and specific heat measurement. For Mg-doped samples Li1−xMgxVO2
x=0, 0.05, 0.10, and 0.15, the structural analysis show that, with increasing x, the lattice constants change monotonously; in the a-b plane, the lattice expands; while in the c-axis direction, the lattice is compressed. Substitution of Li with Mg ions results in the degeneration of two-dimensional characteristics and the distortion of the VO6 block, which significantly influence magnetic properties. The magnetic phase transition temperature falls with increasing x. The Mg-dopants play an important role on breaking the original moment equilibrium and suppressing the magnetic frustration.

1. Mg-doping effect on structural and magnetic properties on
two-dimensional triangular lattice LiVO2
Yang Li,1,2,a͒
Weipeng Wang,1
Xiaoxiang Li,1
Lihua Liu,2
Aihua Wang,3
Ning Chen,4
Yang Liu,4
and Guohui Cao1
1
Department of Physics, University of Science and Technology Beijing, Beijing 100083, China
2
Department of Engineering Science and Materials, University of Puerto Rico at Mayaguez, Mayagues
00681-9044, USA
3
Department of Physics, Capital Normal University, Beijing 100037, China
4
School of Material Science and Engineering, University of Science and Technology Beijing, Beijing
100083, China
͑Presented 22 January 2010; received 20 October 2009; accepted 2 February 2010;
published online 21 April 2010͒
Transition metal compounds with two-dimensional triangular lattice, such as LiVO2, are of
particular interest, because they exhibit peculiar structural and magnetic behavior involving the
frustration present in these oxides. An orbital ordering transition occurs near 500 K, which leads to
a suppression of magnetic moment below the phase transition temperature Tt. We synthesized a
series of Mg-doped LiVO2 single-phase samples and reported their structural and magnetic
properties. The samples were characterized by x-ray diffraction, scanning electron microscope,
differential scanning calorimetry, electrical resistivity, magnetic susceptibility, and specific heat
measurement. For Mg-doped samples Li1−xMgxVO2 ͑x=0, 0.05, 0.10, and 0.15͒, the structural
analysis show that, with increasing x, the lattice constants change monotonously; in the a-b plane,
the lattice expands; while in the c-axis direction, the lattice is compressed. Substitution of Li with
Mg ions results in the degeneration of two-dimensional characteristics and the distortion of the VO6
block, which significantly influence magnetic properties. The magnetic phase transition temperature
falls with increasing x. The Mg-dopants play an important role on breaking the original moment
equilibrium and suppressing the magnetic frustration. © 2010 American Institute of Physics.
͓doi:10.1063/1.3364058͔
The layered oxide LiVO2 recently has received renewed
attention due to its interesting structural and magnetic behav-
iors involving the two-dimensional ͑2D͒ magnetic frustration
in these systems.1–3
Magnetic V3+
͑S=1͒ ions with partly
filled t2g orbitals in this compound occupy the sites of a
triangular lattice.4–6
The system forms an orbitally ordered
state at low temperatures.2,7,8
LiVO2 crystallizes into a rhom-
bohedral structure ͓space group R3¯m ͑No. 166͔͒ with hex-
agonal dimensions a=2.83 and c=14.87 Å, in which the
distribution of ions can be represented by ͑Li+
͓͒V3+
͔O2
2−
.
The V-O coordination is octahedral VO6 site. As shown in
the inset of Fig. 1, Li, V, and O layers stack along c-axis
direction. Each of these layers forms a triangular 2D lattice.
LiVO2 exhibits a first-order magnetic phase transition9
at
around Tt=500 K. The interesting property is a change from
a high temperature paramagnetic phase with a large negative
Curie–Weiss temperature ⌰=−1800 K in ␹ϰ1/͑T+⌰͒, cor-
responding to a large antiferromagnetic coupling, to a low
temperature “nonmagnetic system” without any sign of long-
range order.10
The high temperature susceptibility is consis-
tent with a local moment of S=1, as expected for a d2
con-
figuration in the presence of a strong on-site Coulomb
interaction. Based on the model of d2
configuration in a 2D
triangular lattice, Pen et al.’s theoretical simulation suggests
that this phase transition is driven by a peculiar type of or-
bital ordering that removes the frustration inherent in the
triangular lattice, and LiVO2 undergoes a phase transition
into a spin-singlet phase at low temperature.2
Goodenough1,11
interpreted the phase transition in terms of
the formation of trimers below Tt. In this model, the non-
magnetic behavior is attributed to molecular orbital forma-
tion in the basal plane, which would quench the local spin
moment. The trimerization model is supported by Tian et al.’
electron diffraction observation on LiVO2 single crystals.3
The superlattice reflections disappear above the phase tran-
a͒
Author to whom correspondence should be addressed. Electronic mail:
ylibp@hotmail.com.
10 20 30 40 50 60 70
(113)
(110)
(108)
(107)
(009)
(105)
(101)
(104)
(006)
2θθ ((deg.))
Li0.85
Mg0.15
VO2
(003)
O layer
Li layer
V layer
FIG. 1. ͑Color online͒ X-ray refinement for Li1−xMgxVO2 ͑x=0.15͒. Upper
curve: data and fit, with difference plot below. Ticks show peaks indexed
according to rhombohedral structure. Insert: crystal structure of LiVO2.
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2. sition temperature Tt, and reappear below the Tt, which sug-
gests that Goodenough’s trimer forms below Tt and vanishes
above Tt.
It is significant that the magnetic frustration of LiVO2
heavily depends on microstructure, stoichiometry, and ionic
valence. The investigation on the substitution effect on V and
Li positions is important for understanding magnetic corre-
lation mechanism of V moments. The Cr- and Ti-doped
LiVO2 experiments show that Cr and Ti occupy the V site
and result in a decrease in phase transition temperature with
doping content increasing.1
It can be explained that Cr and Ti
moments suppress magnetic frustration in V planes. Until
recently, only a few studies have concentrated on the substi-
tution effect on electrical and magnetic character of LiVO2.
The orbital ordering mechanism of the magnetic phase tran-
sition in LiVO2 remains an open question. To the authors’
knowledge, there has not been a general study about the
Mg-doping effect in LiVO2. Unlike Cr and Ti ions on V site,
Mg ions preferentially occupy Li sites, which would indi-
rectly modify the magnetic frustration structure of V mo-
ment. The Mg doping will adjust the microstructure and
ionic valence of VO6 block. In particular, the quasi-2D char-
acteristic would be also modified. The investigation on Mg-
doped LiVO2 will be helpful to explain the nature of the
magnetic phase transition of this system.
A series of samples of Li1−xMgxVO2 ͑x=0, 0.01, 0.15,
and 0.2͒ were prepared by solid state reaction. Li2CO3, MgO,
and V2O3 were mixed in an agate and sintered at 750 °C for
24 h. This precursor of Li–Mg–V–O were ground and mixed
fully again, then pressed into a pellet, and annealed at
900 °C for 48 h followed by furnace cooling. The pellets
were under the atmosphere of inert gas flow during the entire
annealing process.
Samples were analyzed by x-ray powder diffraction, and
the x-ray diffraction ͑XRD͒ data refinement is performed us-
ing the general structure analysis software package.12,13
The
energy dispersive spectroscopy ͑EDS͒ compositional analy-
sis was carried out. Magnetic measurements were performed
on a superconducting quantum interference device magneto-
meter. The resistivity and specific heat were measured in the
physical property measurement system ͑PPMS͒.
Structural analysis for Li1−xMgxVO2 ͑x=0, 0.05 0.01,
and 0.15͒ by powder XRD showed a characteristic rhombo-
hedral pattern. All samples exhibit single-phase XRD pat-
terns. The Mg-doped LiVO2 crystallized in a rhombohedral
structure ͓space group R3¯m ͑No. 166͔͒. As shown in Fig. 1,
the XRD pattern and refinement of x=0.15 sample exhibited
no additional phases. The lattice parameters of the x=0.15
sample is a=b=2.869 Å and c=14.735 Å. The XRD refine-
ment shows that Mg preferentially occupies Li ion sites, and
the refined compositions of samples are close to the nominal
compositions, which are also supported by the experiments
of EDS analysis.
Figure 2 shows the variation with x of the refined lattice
parameters, c/a ratio, and the cell volume at room tempera-
ture. The substitution of smaller Mg2+
ion ͑rMg=0.66 Å͒ for
Li+
ion ͑rLi=0.68 Å͒ causes a nearly linear variation in lat-
tice parameters. Within the crystallographic a-b plane, the
lattice parameter a increase with x despite the smaller size of
a Mg2+
ion, which implies the localized-electron configura-
tions of the Mg2+
ions would not share in this basal-plane
Li–Li bonding. A similar anomalous increase in lattice pa-
rameter was observed in Cr-doped LiVO2 in which a smaller
Cr3+
ion substituted for a larger V3+
ion.1
However, along the
c-axis direction, a decrease in lattice parameter c and c/a
ratio with increasing x suggests an enhancement in bonding
among Li, O, and V ion layers. The Mg dopants result in a
stronger Mg–O hybridization in c-axis direction, which
weakens the 2D characteristic in LiVO2.
As shown in Fig. 3, all the compounds show semicon-
ducting behavior; the resistivity increases with decreasing
temperature. At room temperature the resistivity is
285 ⍀ cm for the x=0.15 sample. As temperature goes be-
low 150 K, the resistances are very large and out of the
available measurement range of the PPMS equipment. With
Mg-doping content, the resistivities increase, so we can an-
ticipate an increase in the gap between the valence and con-
duction band. Based on a single activation energy model, the
resistivity data are not fitted very well in the wide tempera-
ture region. On the other hand, taking into account of the
Li1−xMgxVO2 compounds with quasi-2D structure, we tried
to fit the resistivity in term of 2D-variable-range-hopping
͑VRH͒ model ͑␳ϳexp T−1/3
͒. However, unlike the doped-
FIG. 2. Variations in lattice parameters and cell volume with x for
Li1−xMgxVO2.
FIG. 3. ͑Color online͒ Temperature dependence of electrical resistivity ␳ of
Li1−xMgxVO2. Inset: ln ␳ vs T−1/4
based on a VRH model, where the dashed
lines indicate the calculated results with parameters provided in the text.
09E108-2 Li et al. J. Appl. Phys. 107, 09E108 ͑2010͒
Author complimentary copy. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

3. free single crystals,3
this 2D-VRH model cannot express the
temperature dependence of resistivity for our Mg-doped mul-
ticrystal compounds, in particular for samples with heavy
doping content. We noticed in the structural analysis that Mg
doping gives rise to a decrease in c/a ratio which implies the
degeneration of 2D characteristic. The three-dimensional
͑3D͒-VRH ͑VRH͒ model, ␳=␳o exp͑To/T−1/4
͒.14,15
appears
to hold for the Mg-doped compounds, as shown in the inset
of Fig. 3. The To increases with increasing Mg-doping con-
tent x, suggesting the wave function of carriers is localized as
x increases.16
In the Mg-doped LiVO2 structure the Mg–
O–Li interactions are still not sufficiently strong to lead to
itinerant electrons, the layer of Li͑Mg͒–O cannot provide
carriers. In addition, the heat capacity measurement of
Li0.90Mg0.10VO2 in low temperature ͑not shown͒, a phenom-
enon related to the frustration effect was observed.
The measurement of the magnetic susceptibility was per-
formed in an applied field of 1 kOe. DC magnetic suscepti-
bility in cooling and heating process for various Mg-doped
samples are shown in Fig. 4. For LiVO2 compound, the first-
order magnetic phase transition occurs at Ttϳ500 K ͑Ref.
3͒. So within the measurement temperature region ͑T
Ͻ400 K͒, we can anticipate the x=0 sample is in a “non-
magnetic” phase.10
As shown in Fig. 4͑a͒, the susceptibility
exhibits a small value and no thermal hysteresis in heating
and cooling process due to phase transition temperature Tt
higher than the measurement temperature. With increasing
Mg-doping content, for the x=0.05 sample, despite there is
still no thermal hysteresis the temperature dependence of
susceptibility exhibits a U-shaped upturn, which is attributed
to the trail of magnetic phase transition, as shown in Fig.
4͑b͒. The Mg doping clearly results in the temperature of
phase transition to shift to a lower temperature, which was
supported by the differential scanning calorimetry measure-
ment ͑not shown͒. With increasing Mg-doping content fur-
ther, the x=0.10 and 0.15 samples manifest a significant ther-
mal hysteresis in susceptibility, and the phase transition
temperature Tt on cooling process decreases to 377 and 364
K for x=0.10 and 0.15, respectively, as shown in Figs. 4͑c͒
and 4͑d͒. The decrease in Tt originates from the degeneration
of magnetic frustration induced by Mg doping. The Mg dop-
ants enhance the bonding between Li͑Mg͒ and O layer so as
to weaken the 2D characteristic. On the other hand, taking
into consideration the requirement of overall electroneutral-
ity, the substitution of Mg2+
ions for Li+
ions would result in
V2+
ions occurring. The valences of ion can be represented
by ͑Li+
1−xMgx
2+
͓͒V1−x
3+
Vx
2+
͔O2
2−
. Accordingly, the modifi-
cation of microstructure and ionic valence of VO6 block will
break the original moment equilibrium of V ions on the 2D
triangle lattice. Such changes in microstructure and valence
will further suppress the magnetic 2D magnetic frustration,
which in favor of formation of vanadium trimers1
and the
orbital ordering of 3d electron.2
Thereby, Mg doping drives
the magnetic phase transition occurring at lower temperature
in Mg-doped LiVO2 compounds.
In summary, the XRD refinement shows that Mg dopants
preferentially substitute Li ion in Mg-doped LiVO2 com-
pounds. With increasing Mg-doping content, the 2D struc-
tural characteristic decrease, which is supported by electrical
transport measurements as well. The resistivity of samples
obeys the Mott T−1/4
law, which indicates a 3D “variable
doping conduction” mechanism. Based on the magnetic mea-
surements, the Mg doping will undermine the balance of
spin-orbital moment within the VO6 system, which leads to a
decrease in phase transition temperature.
This work was supported by the National Natural Sci-
ence Foundation of Beijing ͑Grant No. 1072007͒, the Na-
tional Science Foundation under Grant No. DMR-0821284,
NASA PRSGC IDEAS-ER and INDUNIV Research Consor-
tium ͑Grant No. G-09-03͒.
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FIG. 4. ͑Color online͒ Temperature dependence of the dc magnetic suscep-
tibility of Li1−xMgxVO2, ͑a͒ x=0; ͑b͒ x=0.05; ͑c͒ x=0.10; and ͑d͒ x=0.15.
09E108-3 Li et al. J. Appl. Phys. 107, 09E108 ͑2010͒
Author complimentary copy. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp