Structural distortion and electronic states of Rb
doped WO
3
by X-ray absorption spectroscopy
Y. C. Wang,abC. H. Hsu,aY. Y. Hsu,cdC. C. Chang,aC. L. Dong,deT. S. Chan,d Krishna Kumar,fH. L. Liu,bC. L. Chen*adand M. K. Wua
Rubidium tungsten bronzes (RbxWO3) have recently attracted much attention due to their intriguing
phenomena, such as complex structural phase transitions, strong electron–phonon coupling, and superconducting properties. This study investigates the local atomic and electronic structures of RbxWO3
(0.17# x # 0.33). X-ray powder diffraction patterns showed a hexagonal tungsten bronze (HTB) phase. X-ray absorption spectra (XAS) at the W L3-edge and Rb K-edge of RbxWO3were carried out. The XAS
analysis indicated a local distorted WO6octahedron which leads to a splitting of egand t2genergy states
in the tungsten 5d orbital and this splitting of energy levels exhibited an asymmetrical behavior at x ¼ 0.23 and 0.27. Overall analysis revealed a distortion of local atomic structure of the WO6octahedra by
rubidium doping, leading to the modification of the electronic structures of eg and t2g states in the
tungsten 5d orbital, thereby accounting for the property changes in CDW formation and superconducting transition temperature of these materials.
Introduction
There have been tremendous advancements both experimen-tally and theoretically in superconductivity research in the past 30 years, since the discovery of high temperature super-conducting cuprates in 1986. One most notable outcome of these efforts is the discovery of many new superconducting materials with a wide range of crystal structures and chemical compositions. Among the non-cuprate new superconductors discovered, Fe-based superconductors have become the focus of scientic investigations since their discovery in 2008. On the other hand, the physical properties of transition metal oxides have also been studied for decades. Their electrical properties are strongly affected by the surrounding lattice. For instance, in the case of WO3, the anisotropic-shaped d-orbital electrons in
tungsten interact differently with p-orbital electrons in oxygen.1–4This effect induces many unconventional behaviors, such as metal–insulator transition, magnetic ordering, etc. in materials like tungsten bronze (AxWO3), where A refers to an
alkali or alkaline metal (Na, K, Rb, and Cs).5–12Rubidium doped tungsten bronze shows superconductivity below 6 K and the
superconducting transition temperature (Tc) depends on Rb
doping.10 The transport properties of these materials showed interesting features such as an asymmetrical behavior for Tc
dependence on Rb(x) doping. Stanley et al., observed a Tcof5
K at x¼ 0.17 Rb concentration and a subsequent decrease to 2 K at x ¼ 0.23. However, no clear trend could be observed although a slight increase in Tcfrom 2 K at 0.26 < x < 0.33 was
reported and suggested a concentration dependent phase transition near x¼ 0.25 where the material did not show any sign of superconductivity.10The origin of how Rb varies the Tc
and crystal structure is still unclear. The Tc suppression of
tungsten bronze by doping causes a suppression of supercon-ductivity and exhibits distinct physical properties.10–14Further, in Rb doped system, a metal–insulator transition at around 250 K along c axis was observed from resistivity measurements and has strong correlation with superconductivity.10–15The origin of superconductivity in RbxWO3wasrst explained by Sato et al.
from neutron scattering experiments.12,13They proposed the low frequency vibration of rubidium ion in hexagonal channels (h-WO3) as Einstein type phonon modes, which has substantial
correlation to the electron–phonon coupling in superconduc-tivity in addition to the highly mobile Rb ions in the hexagonal cage along c direction. The Einstein type phonon mode was also
observed by Sagar et al. in KWO3 from Raman scattering
experiments.8 They suggested that the metal–insulator transi-tion in KWO3was caused by a competition between the lattice
structural disorder and charge ordering in the WO6octahedron,
which might have induced a weak charge density wave (CDW) ground state especially at low temperatures. Although some earlier reports prevail, a detailed investigation on the lattice aInstitute of Physics, Academia Sinica, Nankang, Taipei, Taiwan. E-mail: chen.cl@
nsrrc.org.tw
bDepartment of Physics, National Taiwan Normal University, Taipei, Taiwan cProgram for Science and Technology of Accelerator Light Source, National Chiao Tung
University, Hsinchu, Taiwan
dNational Synchrotron Radiation Research Center, Hsinchu, Taiwan eDepartment of Physics, Tamkang University, New Taipei city, Taiwan
fDepartment of General Studies, Physics Division, Jubail Industrial College (JIC), Jubail
Industrial City 31961, Kingdom of Saudi Arabia Cite this: RSC Adv., 2016, 6, 107871
Received 31st August 2016 Accepted 5th November 2016 DOI: 10.1039/c6ra21777j www.rsc.org/advances
PAPER
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structural disorder and charge ordering in the WO6octahedron
in hexagonal tungsten bronze (HTB) using advanced charac-terization tools is worthwhile to unravel the distinct supercon-ductivity behavior exhibited by these type of materials. This is
because in simple BCS theory one would expect Tc to be
a monotonic increasing function of x, assuming a constant electron–phonon interaction. Earlier, studies have been carried out on the electron–phonon interactions in hexagonal alkali tungsten bronze.8–15 The variation of T
c with Rb doping in
tungsten bronze is discussed on the basis of the CDW instability along the c-direction which further depends on type of lattice phonons produced as the crystalline structure is distorted from octahedral symmetry due to doping.10–15The CDW instability is similar to the rst order Jahn–Teller instability in molecules with incomplete degenerate levels and its formation reduces the carrier density at the Fermi level.11Further, the valence states of cations were suggested to have correlation with the exhibited physical properties. Consequently, the interaction between the W 5d-electrons and oxygen 2p-orbitals is of great
impor-tance as Rb ions are doped into the WO6 octahedron. We
envisage that the results from our paper would provide a rich ground for the detailed study into the mechanism responsible for these kind of non-cuprate superconductivity. Advanced technology arises from the understanding of fundamental science, and depends importantly on characterization tools for directly observing both physical and chemical properties. Without knowledge of the fundamental electronic and atomic structures of materials and the changes in their structures at various conditions, engineering these materials to widen their range of practical applications is tough. It is well known that X-ray absorption spectroscopy (XAS), including X-X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption ne structure (EXAFS) is a powerful tool to study the local electronic and atomic structure of materials. In this work, XAS at W L3-edge and Rb K-edge of RbxWO3 at x ¼ 0.17 to 0.33
revealed a striking correlation between the CDW formation and the extent of local structural distortion due to Rb doping. Based on analytic results, a schematic model is proposed for the structural distortion in the WO6 octahedron with Rb doping,
which is likely to dictate the Tcvalue of the system and hence
the superconductivity with Rb doping. Many new exciting results that are of both fundamental and technological interest have emerged in a relatively short amount of time. This report elucidates a relevant but rarely utilized means of elucidating the fundamental electronic/atomic structure and lays a foundation
for engineering better high Tc non-cuprate new
superconductors.
Experiments
High quality tungsten bronze were grown by an optical zone-melting method, which is similar to the previous study on the growth by sealed quartz tubes as described by Stanley et al. and R. Brusetti et al.10,14,15The concentration were measured by the X-rayuorescence spectrometer (ZSX Primus II, Rigaku, Japan) to describe the Rb content. The sample quality was character-ized with X-ray powder diffraction using Phillips diffractometer.
The magnetic properties were measured using a SQUID
Magnetometer (Quantum Design make) and resistivity
measurements were performed using a physical property measuring system (PPMS) of the same make; Tcwas conrmed
from both transport and magnetic measurements.10XAS at W L3-edge and Rb K-edge were recorded at beamline BL01C and
BL17C at National Synchrotron Radiation Research Center (NSRRC), Hsinchu, Taiwan. All spectra were normalized to unit step height in the absorption coefficient from well below to well above the edges. The standard metal foils and oxide powders of WO3were used for energy calibration.
Results and discussion
Fig. 1(a) presents the powder X-ray diffraction patterns of RbxWO3at various Rb doping concentrations (x from 0.17 to
0.33). The diffraction peaks are indexed according to the hexagonal structure with the space group P63/mcm (JCPDS
34-0394). No diffraction peak from pure Rb metal is noticed within the detection limit. The main phase of tungsten oxide have been identied as h-WO3. A small additional diffraction peak is
observed in the low Rb content, at around 23, when x < 0.21. It can be identied as intergrowth tungsten bronzes (ITB) phase, 3-ITB phase.16The h-WO3structure builds up by WO6
octahe-dral, sharing their corner oxygen atoms with each other to form
Fig. 1 X-ray diffraction pattern of RbxWO3. Inset shows the lattice
constant (a and c values marked in blue and red respectively) as a function of Rb doping(x).
a hexagonal tunnel along c-axis and the Rb ions are located in the hexagonal tunnels of the lattice. The radius of the tunnel is about 2 ˚A and the Rb ionic radius is 1.47 ˚A.9,10Therefore, Rb ions are free to occupy randomly in the tunnel site. A compar-ison with the undoped WO3show a shi in the diffraction peak
position with Rb doping, denoting a change in lattice parame-ters. The lattice parameters a and c estimated at various Rb doping are plotted in the inset of the Fig. 1, which clearly shows a signicant decrease and increase in a and c-parameters respectively, at x > 0.23. These variations in lattice parameters are likely to cause a distortion of WO6octahedron to different
extents, though the crystallinity of the h-WO3 appears to be
same. Brusetti et al., speculated a remodeling of WO6octahedra
at low doping which make some associated phonons more active in the electron–phonon coupling.14,15
XANES provides information on the symmetry of the unoc-cupied electronic states. Fig. 2(a) presents W L3-edge XANES
spectra of RbxWO3, x is ranged from 0.17 to 0.33. The strong
resonance near the absorption edge in the energy region 10 190–10 225 eV is due to the excited electron transfer from W 2p3/2 to 5d unoccupied states with multiple excitations for
hybridized W 5d–O 2p conduction band states.17–20 XANES is sensitive to the local structural symmetry and to the inuence of the bonding effect with oxygen ligands. The W L3-edge spectra
exhibit several features and are consistent with the h-WO3
electronic structural calculations on crystal-eld 10 Dqresults of
the Ohsymmetries.18Thegure shows two prominent features
A1and A2, which are attributed to the splitting of W 5d orbital
into t2gand egdegenerate states, respectively, due to crystaleld
effect. To study the symmetry of W 5d electronic states, the decomposed A1(t2g) and A2(eg) features are obtained by
subtracting the arctangent (edge jump for step function) curve with best tted Gaussian curves (absorption white line) (as shown in the bottom of Fig. 2(a)).19,20The tted spectrum is consistent with the result from rst principle calculation, as shown in the bottom of Fig. 2(a) which depicts a W L3-edge
XANES spectra of h-WO3 and the corresponding density of
states (DOS) t2g and eg orbitals.18The band-structure
calcula-tions were carried out in the scheme of generalized gradient approximation GGA with the on-site Coulomb interaction U taken into account,18i.e., GGA+U calculations, are presented to interpret the XAS results. Fig. 2(a) shows that the line proles of the spectra are nearly identical for RbxWO3, irrespective of Rb
Fig. 2 (a) W L3-edge XANES spectra of RbxWO3. The DOS calculation and x ¼ 0.25 spectra with the estimated t2gand egstates is shown in the
bottom offigure. (b) The absorption edge position as a function of Rb doping, the edge of h-WO3is at 10 211.797 eV. Inset shows thefirst
derivative W L3-edge XANES spectra at various Rb doping. (c) The crystalfield splitting energy (D0, left black colored axis) and the measured
resistivity (r, right red colored axis) as a function of different Rb doping.
Fig. 3 The full width at half maximum (FWHM) values of egstates
(marked in blue) at various Rb doping. CDW onset temperature TBas
a function of x in RbxWO3is also shown (adapted from ref. 10).
doping and closely resemble that of h-WO3. Thesendings are
consistent with the XRD data where no major change was noticed in the hexagonal structure irrespective of Rb doping. Further, it is noticed that the integrated area of W L3-edge for
the RbxWO3almost matches with the h-WO3 standard which
means that the total unoccupied state of W 5d hole does not change when Rb is doped into WO6 octahedra. This nding
imply that the valence of W in all the RbxWO3samples is not
altered and may therefore be conrmed as 6+. Fig. 2(b) presents the shi of W L3-edge as a function of Rb doping which are
obtained from rst derivative W L3-edge XANES spectra.19,20
Inset of Fig. 2(b) shows W L3-edge XANESrst derivative spectra
of Rb-doped WO3and reference h-WO3. A shi of 0.5 eV in the
absorption edge between the x¼ 0.17 and 0.23 are observed (region I). A shi in the W L3-edge XANES spectra of HxWO3
from that of pure W and h-WO3spectrum was reported earlier,
which was due to the displacement of the electron density of W atoms.17The absorption edge shi in W L3-edge XANES,
espe-cially at x# 0.23 may be attributed to a change in the energy gap, as suggested by Sato et al.12They suggested an alteration in the energy gap and Fermi level position in the electronic structure of tungsten, with a maximum around x¼ 0.25. Unlike the region I in which the energy of absorption edge position is monotonically decreased with an increase in Rb concentration, the absorption edge shis to high energy as x ¼ 0.23 increases to 0.27 in region II. Indeed to fully understand the structure of RbxWO3, its phase transition and the lattice distortions in WO6
octahedra with doping, it is necessary to have a knowledge on how the structural models were built. K. S. Lee et al.11performed
the electronic band structure calculations on RbxWO3 and
presented a detailed structural model to explain the metal– semiconductor–metal phase transition. Further, P. Kr¨uger et al.,18carried out a density functional theory study on pure and potassium doped tungsten trioxide which has a similar struc-ture to RbxWO3 and estimated the DOS. In Fig. 2(b), three
distinct regions (I, II, III) with different colors represent the phase transition. The green and yellow regions represent the metal and semiconductor phases of RbxWO3respectively. From
a band structure calculation,18the Fermi level (E
F) of WO3can
be tailored by introduction of Rb and subsequent lattice struc-tural distortion. Thus the lower part of conduction band which is composed of W 5d-t2gband and 5d-egband is critical. The W
5d-t2g band (unoccupied states above EF) exhibit mainly
elec-tron–orbital interaction in 5d characters. Above results suggest that presence of Rb can cause the energy shi of W 5d states and thus modify the DOS around the Fermi level. BCS theory suggests that superconducting state is strongly correlated to the electron–phonon interactions. Tcincreases with an increase in
the electron–phonon coupling. A reduced electron–phonon interaction and thus a decrease in Tcat x¼ 0.25 was reported
earlier by K. S. Lee et al.11As x decreases (x < 0.33) in Rb
xWO3,
cation vacant sites increases and are randomly distributed creating a random potential for conducting electrons. However, this is expected to suppress the superconductivity. Hence, disappearance of superconductivity at x¼ 0.25 is likely to be due to CDW formation associated with the 1D Fermi surface which removes lattice phonons that contribute to supercon-ductivity. Their calculations also showed the presence of 1D as well as 3D Fermi surfaces in the system.11 In general, more doping of Rb ions increase the stiffness of the WO3lattice which
reduces the chance of CDW formation, whereas the electronic instability favors. Briey, the CDW formation depends on the interplay of lattice stiffness and the electronic instability.11It is
Table 1 Estimatedfitting parameters for the first and second shell from RbxWO3and h-WO3 EXAFS spectra. N stands for the oxygen
coordination number, s2 is the Debye–Waller factor, and R is the
length of W–O bonding
Sample Shell N s2 R (˚A) h-WO3 1 4 0.005 1.7714 (0.003) 2 2 0.004 2.0456 (0.005) Rb0.17WO3 1 4.009 0.007 1.8303 (0.010) 2 1.982 0.005 2.0123 (0.001) Rb0.19WO3 1 4.009 0.006 1.8403 (0.002) 2 1.973 0.004 2.0172 (0.006) Rb0.21WO3 1 3.977 0.006 1.8405 (0.003) 2 1.984 0.006 2.0017 (0.009) Rb0.23WO3 1 3.782 0.004 1.8451 (0.010) 2 2.158 0.004 1.9854 (0.006) Rb0.25WO3 1 4.442 0.007 1.8559 (0.008) 2 1.479 0.007 2.0423 (0.005) Rb0.27WO3 1 4.543 0.009 1.8622 (0.002) 2 1.367 0.009 2.0700 (0.008) Rb0.30WO3 1 4.7 0.009 1.8779 (0.003) 2 1.2 0.009 2.1399 (0.007) Rb0.33WO3 1 4.824 0.008 1.8888 (0.006) 2 1.002 0.005 2.0159 (0.008)
Fig. 4 EXAFS results (solid line) from the W L3-edge. The FEFFfitting
(open symbols) presents the WO6octahedral coordination. The inset
shows the k space results.
worth to recall that an interesting behavior of RbxWO3 type
materials is their composition dependent resistivity anomaly that has strong correlation with CDW formation. The super-conductivity behavior of these materials is also correlated to the CDW onset temperature TBat which resistivity anomaly occurs
(an anomalous hump in the resistivity).8,10,11A few reports also indicated an electron–hole correlations in AxWO3due to W ions
at distorted lattice structure which may inuence the electronic transport behavior.21,22
Fig. 2(c) depicts theD0(D0¼ A2(eg) A1(t2g)¼ 10 Dq),
esti-mated by the Ohcrystaleld theory, as a function of Rb doping.
The t2g–eg splitting energy levels in the 5d with Rb doping
suggests an instability and distortion of Oh symmetry. The
d orbital splitting directly reects the energy gap.20The result demonstrates thatD0is minimum at x¼ 0.23 and maximum at
x¼ 0.27. Interestingly, the room temperature resistivity (r) of RbxWO3at various doping levels shows a similar behavior toD0
(Fig. 2(c)). The resistivity gradually decreases from x¼ 0.17 to 0.23, exactly in the same trend as Tcdecreases with x and recall
that RbxWO3exhibit metallic properties in this region. Thus, it
can be deduced that both the effect of lattice structure modu-lation and electronic-orbital overlapping variations with doping lead to the state of semiconductor behavior along with no sign of superconductivity in the range 0.23 < x < 0.27. At the metal– semiconductor–metal phase transition region in RbxWO3, the
instability in the local structure as discussed above can make changes in the W 5d band structure and the position of EF. CDW
can cause an electronic instability, and it is strongly correlated to the variation in the crystal structure. The W 5d orbital symmetry is strongly correlated to the crystal lattice and corner-sharing atoms in WO6 octahedron. Thus, the values of full
width at half maximum (FWHM) of egstates (marked in blue)
are estimated and plotted as a function of Rb doping in Fig. 3. A maximum at x¼ 0.23 is observed in Fig. 3. As the eg(dz2and dx2y2) orbitals point directly to the corners of WO6octahedron, the variation in FWHM egunoccupied states with Rb doping is
likely to cause a distortion in the octahedron. The CDW onset temperature TBas a function of x in RbxWO3(adapted from ref.
10) is also included in Fig. 3 for comparison. Interestingly, the variation in TBshow a maximum near x ¼ 0.25. The similar
trend in the variation of FWHM egand TBagain suggest that the
Rb doping changes the structural ordering and modies the local structure symmetry of WO3.11,12Therefore, it is essential to
investigate the local atomic structure of RbxWO3using EXAFS.
Fig. 4 shows Fourier transform (FT) of EXAFS k3c data at the W L3-edge from k¼ 3 to 11 ˚A1(inset) for RbxWO3at various
Rb doping and the reference h-WO3. For all samples, radial
distribution of FT spectra are similar to each other. The FT proles in real (R)-space provide direct evidence on the changes in the W–O bond length, coordination number, and Debye– Waller factor (s2).23Therst two main peaks near 1.38 and 1.85 ˚A correspond to the rst (W–O1), nearest neighbor, and second (W–O2), next nearest neighbor, shells/bond length in the WO6
octahedron, respectively.19,20,24The O1 and O2 corresponds to the oxygen atom in the equatorial and axial positions in the octahedron, respectively. The EXAFS curves aretted by FEFF analysis using ARTEMIS program23(open symbols in Fig. 4) and the results are presented in Table 1. The spectra shows a best tting within the limits of 0.5 ˚A and 2.3 ˚A. Fig. 5(a) shows the variation of W–O bond length in the rst and second shell in WO6octahedron (from Fig. 4) as a function of Rb doping. The
gure demonstrates that the W–O1 bond length (blue) increases from 1.83 ˚A at x¼ 0.17 to 1.888 ˚A at x ¼ 0.33, the error in bond length is less than0.01. In contrast, the W–O2 bond length (red) slightly decreases with Rb doping initially (i.e. x¼ 0.17 to 0.23) and then increases. The W–O bond length variation (Fig. 5(a)), especially asymmetrical shi of the second shell near x¼ 0.23, imply that the structural symmetry of WO6(Oh)
octa-hedron has distorted keeping the hexagonal structure, which is consistent with the XRD analysis. The FWHM of W 5d-egorbital
exhibits a maximum value at x¼ 0.23 (Fig. 3). The increase in W–O bond length with Rb doping is attributed to the increased occupation of comparatively large atomic radius Rb compared to W. This probably create an increased disorder in the WO6
octahedron, with a local lattice distortion that may lead to changes in the lattice phonon behavior. Fig. 5(b) compares the Fig. 5 (a) The W–O bond length in first (blue) and second (red) shell,
(b) is the oxygen coordination number infirst and second shell.
Fig. 6 The model of distortion in octahedral.
coordination number (CN) ofrst and second shells as a func-tion of Rb doping. The result indicates a discontinued change of CN at x¼ 0.23 for both the O1 (blue) and O2 (red) cases. The CN in therst shell indicates a substantial decrease (increase) at x # 0.23 (x $ 0.23). In contrast, the second shell demonstrate an opposite trend. These changes in the CN suggest an off center effect in the octahedron as predicted by earlier electronic structure calculation.6,25In order to get an insight into the local structural modication caused by Rb doping and the nature of distortion in WO6octahedral symmetry, a schematic model of
distortion duly taking account of W–O1 and W–O2 variation is presented in Fig. 6. The W–O bond length and CN estimated from EXAFS analysis are also considered. Analytic results from Fig. 5 and 6 infer that at low Rb doping, though the WO6
octahedron does not exhibit a major distortion, the W–O bond length along z-axis increases. As the Rb doping gradually increases to 0.23, it appears that the position of central W ions is modulated and shied in the xy-plane (equatorial) of octa-hedron. In this region, CN slightly increases in second shell (W– O2) and a corresponding decrease is observed in therst shell (W–O1). Further increase in Rb doping (x > 0.23) results the structure (octahedron) extend along z-axis at one side that lead to an increase in CN of W–O1 and a corresponding decrease in W–O2. To sum up, Rb doping caused a local structure distortion and the tungsten ions off center in the WO6octahedron. The
electron–phonon coupling mechanism in quasi-1-D lattice structure is contributed by vibration of Einstein-like phonon modes.10–15Hence, the change of local electronic structure and distortion of local atomic structure of the WO6 octahedron,
revealed by XANES and EXAFS, modify the electron–phonon coupling in RbxWO3 and generate an Einstein-like phonon
mode vibration. This alteration in the phonon mode vibration
accounts for the superconducting transition temperature and CDW formation in RbxWO3. Recall that the W L3-edges XANES
indicate a variation in the W5d-orbital electronic states due to the t2g–egsplitting of energy levels by the ligandelds of the
surrounding oxygen atoms and an associated distortion in the local structure symmetry by Rb doping. Further, it is worth to point out that our Rb K-edge XANES of RbxWO3, as shown in
Fig. 7, does not show any change in the Rb valence with Rb doping which is consistent with other studies on hexagonal alkali tungsten bronzes AxWO3 (A ¼ K, Rb and Cs) that A+
cations are occupied at the tunnel of hexagonal lattices. Finally, this study indicate an off center shi for tungsten ions in WO6
octahedral symmetry and an associated change in phonon mode of vibration. Analytic result from XAS evidences the changes in W 5d electronic states and the local structural distortion cause the suppression of superconductivity and the phase transition in RbxWO3.
Conclusion
We investigated the effects of Rb doping in WO3 using XAS
techniques. The local electronic and atomic structures, as well as their correlation to the superconducting properties are dis-cussed. XAS results corroborate with the CDW formation in RbxWO3likely due to changes in electro–phonon interaction in
the octahedral symmetry of WO6 octahedron. Spectroscopic
result indicates a shi in the Fermi level with Rb doping and a change in coordination number, and subsequent variation in electron–phonon coupling. An increase in the lattice phonon interaction is originated from the local structure distortion as a result of Rb doping. Combined XANES and EXAFS analyses reveals the WO3in RbxWO3has regular hexagonal phase with
a distortion in the Ohcoordination structure and an off center at
x# 0.23. These ndings suggest a change in phonon scattering which may be correlated with the distinct superconductivity behavior in RbxWO3. Our study suggests the possibility of
tuning the physical properties of these type of materials by correlating their electronic/atomic structures and CDW forma-tion. Further, an enhanced knowledge concerning the behavior and better understanding the mechanism of superconductivity in RbxWO3 and related compounds is very important for
potential tond new high Tcnon-cuprate superconductors.
Acknowledgements
This work is supported by the Ministry of Science and Tech-nology (MOST) (formerly the National Science Council (NSC)) of Taiwan, under contracts no. MOST 102-2112-M-001-004-MY3. The authors are grateful to Prof. H. C. Hsueh for band struc-ture calculation and NSRRC for providing beamtime and beamline support.
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