Synthesis, structural, and transport properties of Cr-doped BaTi2As2O

Article abstract of DOI:10.1021/ic502192h

The interplay between unconventional superconductivity and the ordering of charge/spin density wave is one of the most vital issues in both condensed matter physics and material science. The Ti-based compound BaTi₂As₂O, which can be seen as the parent phase of superconducting BaTi₂Sb₂O, has a layered structure with a space group P4/mmm, similar to that of cuprate and iron-based superconductors. This material exhibits a charge density wave (CDW) ordering transition revealed by an anomaly at around 200 K in transport measurements. Here, we report the synthesis and systematical study of the physical properties in Cr-doped BaTi_2-xCr_xAs₂O (x = 0-0.154) and demonstrate that the transition temperature of the CDW ordering is suppressed gradually by the doped Cr element. The magnetization measurements confirm the evolution of the CDW ordering transition. These observed behaviors are similar to that observed in iron-based superconductors, but no superconductivity emerges down to 2 K. In addition, the first-principles calculations are also carried out for well-understanding the nature of experimental observations.

Full text of DOI:10.1021/ic502192h

Article
pubs.acs.org/IC
Synthesis, Structural, and Transport Properties of Cr-Doped
BaTi2As2O
Qiucheng Ji,† Yonghui Ma,† Kangkang Hu,†,‡ Bo Gao,† Gang Mu,*,† Wei Li,†,§ Tao Hu,†
Ganghua Zhang,∥ Qingbiao Zhao,∥ Hui Zhang,∥ Fuqiang Huang,∥ and Xiaoming Xie†
†State Key Laboratory of Functional Materials for Informatics and Shanghai Center for Superconductivity, Shanghai Institute of
Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China
‡College of Sciences, Shanghai University, Shanghai 200444, China
§State Key Laboratory of Surface Physics and Department of Physics, Fudan University, Shanghai 200433, China
∥CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai
200050, China
S
* Supporting Information
ABSTRACT: The interplay between unconventional super-
conductivity and the ordering of charge/spin density wave is
one of the most vital issues in both condensed matter physics
and material science. The Ti-based compound BaTi2As2O,
which can be seen as the parent phase of superconducting
BaTi2Sb2O, has a layered structure with a space group P4/
mmm, similar to that of cuprate and iron-based super-
conductors. This material exhibits a charge density wave
(CDW) ordering transition revealed by an anomaly at around
200 K in transport measurements. Here, we report the
synthesis and systematical study of the physical properties in
Cr-doped BaTi2−xCrxAs2O (x = 0−0.154) and demonstrate
that the transition temperature of the CDW ordering is suppressed gradually by the doped Cr element. The magnetization
measurements confirm the evolution of the CDW ordering transition. These observed behaviors are similar to that observed in
iron-based superconductors, but no superconductivity emerges down to 2 K. In addition, the first-principles calculations are also
carried out for well-understanding the nature of experimental observations.
1. INTRODUCTION
Since the discovery of cuprate and iron-based superconduc-
tors,1,2 the search for high-temperature superconductivity and
novel superconducting mechanisms has become one of the
most challenging tasks of condensed matter physicists and
material scientists. From the viewpoint of theoretical and
experimental evidence,3−5 these superconductivities often
emerge in the proximity of the ordering of spin/charge density
wave instabilities, in contrast to that of conventional BCS
superconductivity.6 In recent years, the studies of the
BaTi2Pn2O (Pn = As, Sb, Bi) system associated with charge
density wave instabilities have attracted great research
Figure 1. (a) Structure of BaTi2As2O. (b) X-ray diffraction patterns of
attentions since the discovery of the superconductor
BaTi2−xCrxAs2O with different doping levels. Main peaks can be
indexed to the layered structure of BaTi2−xCrxAs2O. Small amounts of
Ba1−xNaxTi2Sb2O (Tconset = 5.5 K).7 The crystal structure of
BaTi2Pn2O combines the features of cuprate and iron-based
superconductors (see Figure 1a): It consists of edge-shared
[Ti2Pn2O] layers separated by the layers of Ba2+. The
conductive layer is the [Ti2Pn2O] layer, where the atoms
show a unique connective feature. It has a Ti2O square net with
an anti-CuO2-type structure of the cuprate superconductors,
where the Pn atoms are located above and below the Ti2O
impurities are marked by the asterisks.
squares. Four Ti atoms and two Pn atoms form the octahedral
sharing the corners with the neighboring one.
Received: September 17, 2014
Published: December 3, 2014
© 2014 American Chemical Society
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Interestingly, an anomalous phase transition has been
observed in both the Sb- and As-based compounds by the
susceptibility and resistivity measurement.8−12 Such an anomaly
was ascribed to the charge density wave order.13 The
substitution of Ba2+ by Na+ in BaTi2Sb2O as hole doping
leads to the suppression of charge density wave ordering phase
and the emergence of superconductivity with the highest
superconducting transition temperature Tc = 5.5 K when x ≈
0.15.7,14,15 On the other hand, Chen and co-workers
successfully synthesized BaTi2As2O in 2010 and increased the
transition temperature of charge density wave order to 200 K.16
It was reported that element doping is very difficult and only
Li+ has been doped into interstitial sites of BaTi2As2O, but no
superconductivity was observed. Therefore, more efforts are
needed in this system to explore the possible superconductivity
and to study the nature of the interplay between super-
conductivity and charge density wave (CDW) order.
In this work, chromium element was doped into BaTi2As2O,
and their physical properties were fully investigated. We show
the evolution of the CDW ordering transition in Ba-
Ti2−xCrxAs2O as a function of the substitution concentration.
Evidences are displayed to demonstrate that, with the increase
of chromium content, the CDW ordering transition temper-
ature (TCDW) is continuously lowered, but without the
emergence of superconductivity down to 2 K. In addition, we
carried out the first-principles calculations for both parent and
doped compounds for well-understanding the nature of our
experimental observations.
Figure 2. Temperature dependence of the resistance for Ba-
Ti2−xCrxAs2O samples, normalized to the data at 300 K. Inset is the
comparison of R−T and M−T curves for one typical sample with x =
0.034 to confirm the CDW transition at TCDW. The applied field of the
M−T curve is 1 T.
determined in the M−T curve is rather consistent with that in
the R−T curve, as shown in the inset of Figure 2. As more and
more chromium is doped into the sample, the CDW transition
temperature declines gradually, but no superconductivity
emerges down to 2 K.
2. RESULTS AND DISCUSSION
The actual doping levels of the samples were analyzed and
determined by the energy dispersive X-ray spectroscopy (EDS)
measurements. The results can be seen in the Supporting
Information, Figure S1. The values (x) determined from EDS
measurements will be used in the present paper. We also
examined the structure and purity of our samples by the
powder X-ray diffraction (XRD) measurements at room
temperature. In Figure 1b, we show the XRD data of the
BaTi2−xCrxAs2O samples with 0 ≤ x ≤ 0.154. The main
diffraction peaks can be indexed to the layered structure with
the space group P4/mmm as shown in Figure 1a. Tiny peaks
from small amounts of impurities can be detected as the doping
level increases. Moreover, we can see that the peaks move to
the right slightly with the increase of doping, indicating the
doping induced shrinkage of crystal lattice. To investigate the
influences on the crystal lattice by the Cr substitution
quantitatively, we obtained the lattice parameters by fitting
the XRD data. The a- and c-axis lattice parameters and volume
decrease gradually. The detailed descriptions and discussions
about the response of crystal lattice to Cr doping are presented
in the Supporting Information, Figure S2. The clear evolution
of the crystal lattice suggests the successful substitution of the
Cr element.
Resistance and magnetism measurements were performed to
study the transport and possible superconducting properties.
Figure 2 indicates the temperature dependence of normalized
resistance of our samples with doping 0 ≤ x ≤ 0.154. A clear
anomaly is observed around 200 K for the sample with x = 0,
which is consistent with the previous report.16 This anomaly
has been identified as a nematic CDW ordering transition.13
For the samples in low doping region, such a transition was also
detected by the magnetism measurements. Taking the sample
with x = 0.034 as an example, the transition temperature TCDW
In Figure 3, we show the doping dependence of the CDW
transition temperature TCDW. The dashed line is a guide for the
Figure 3. Doping dependence of CDW transition temperature, TCDW
.
The dashed line is the guide for the eyes. The slope curve of the R−T
curve for the sample with x = 0.078 is plotted in the left inset. Doping
dependence of S1 − S2 is shown in right inset.
eyes. TCDW goes down gradually and shows a roughly linear
tendency with the increase of doping concentration. The lowest
TCDW obtained from our experiment is 139 K. Further
suppression of TCDW is hampered due to the solubility limit
of Cr and the indistinctive resistive features from the CDW
transition in the high doping region. Here, we also attempted to
describe such resistive features of the CDW transition
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quantitatively. As shown in the left inset of Figure 3, the
temperature derivative of resistance, slope = d(R/R300K)/dT,
for the sample with x = 0.078 is plotted versus temperature. In
this figure, the onset point of the transition S1 and the
minimum point S2 are determined. The difference of slopes
between the two points S1 − S2 is used to describe the feature
due to the CDW transition. In the right inset of Figure 3, we
show the doping dependence of S1 − S2. The value of S1 − S2
also decreases linearly with the doping level x and becomes
nearly zero with x around 0.15, which suggests that the features
due to the CDW ordering are almost unobservable in the
resistance data in the high doping region.
In order to well-understand the nature of experimental
observations, we carry out the first-principles calculations17−20
to discuss the evolution of electronic structure as Cr doped in
BaTi2As2O. The calculated total and projected density of states
(DOS) for BaTi2As2O and Cr doped BaTi1−xCrxAs2O are
shown in Figure 4a,b, respectively. Here, we take x = 0.25 to
electron-type carriers by the Cr doping are evidenced by the
first-principles calculations.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details, details of the first-principles calculations,
EDS results, and lattice parameters. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Fax: +86-21-62127493. E-mail: mugang@mail.sim.ac.cn.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge the help of EDS experiments from Prof. W.
Peng and Dr. H. Jin. This work is supported by the Knowledge
Innovation Project of Chinese Academy of Sciences (No.
KJCX2-EW-W11), the Natural Science Foundation of China
(Nos. 11204338 and 11404359), and the “Strategic Priority
Program (B)” of the Chinese Academy of Sciences (Nos.
XDB04040300 and XDB04030000). W.L. also gratefully
acknowledges support from the Shanghai Yang-Fan Program
(No. 14YF1407100).
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3. CONCLUSION
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