wangsheng chu beijing synchrotron radiation facility - … 2005 pdf/30 ix/chu ws... · buzea, c.,...

XAFS Study of the AlBXAFS Study of the AlB22--type type TransitionTransition--metal metal DiboridesDiborides

Wangsheng Chu

September 30, 2005

Beijing Synchrotron Radiation Facility ICXOM05 Frascati, September 25-30, 2005

Beijing Synchrotron Radiation Facility

OutlineOutline

Scientific backgroundScientific backgroundAim of the researchAim of the researchXANES study of the 3d TM XANES study of the 3d TM diboridesdiboridesLattice dynamics of 4d TM Lattice dynamics of 4d TM diboridesdiborides by by EXAFSEXAFSConclusions & PerspectivesConclusions & Perspectives


ScientificScientific backgroundbackgroundThe recent discovery of the MgB2superconductor, a system characterized by a remarkably high Tc (~39 K) stimulates the researches in related diboridessystems. Due to its simple structure, MgB2 is a precious system to investigate the superconductive mechanisms.

Nagamatsu, J., et al., Nature 410 (2001) 63-64


Many experimental and theoretical Many experimental and theoretical researches have been published on the researches have been published on the superconductive properties of the MgBsuperconductive properties of the MgB22; ; Most of the results support the idea that Most of the results support the idea that MgBMgB22 is a conventional superconductor is a conventional superconductor rather than a highrather than a high--TTc c system,system,Still MgBStill MgB2 2 is really a challenging systemis really a challenging system..

Scientific backgroundScientific background

Buzea, C., Yamashita, T., 2001. Review of superconducting properties of MgB2cond-mat/0108265, 1- 35.


Study of the influence of metal atoms Study of the influence of metal atoms on the superconductivity of on the superconductivity of diboridesdiborides. . Study of the electronic structure of Study of the electronic structure of diboridesdiborides by XANES spectroscopy.by XANES spectroscopy.Study of the lattice vibration of Study of the lattice vibration of diboridesdiboridesby temperatureby temperature--dependent EXAFS dependent EXAFS experiments.experiments.

Aim of researchAim of research


XANES study of the 3d TM XANES study of the 3d TM diboridesdiborides

-20 0 20 40 60

CrB2

VB2

TiB2E

DCB

A2

A1

N

orm

aliz

ed a

bsor

ptio

n

Energy (eV)

Normalized Ti, V and Cr K edges XANES spectra of 3d TM-diborides


-20 -10 0 10 20 30 40 50 60

Exp.

A2

ED

C

B

A1

Nor

mal

ized

Abs

orpt

ion

Energy (eV)

MS cal.

Comparison between MS calculations of the Ti K-edge XANES spectrum of TiB2 using a cluster of 179 atoms and experimental data.





-10 0 10 20 30 40 50 60 70

A2A1

EDC

B

7-shell

6-shell

4-shell

3-shell

Nor

mal

ized

Abs

orpt

ion

Energy (eV)

2-shell

MS calculations of the Ti K-edge XANES spectrum of the TiB2vs. cluster size.

the pre-edge feature A2 is due to transitions from the metal 1s core state to the central metal 4p state hybridized with 3d state of the higher-shell Ti atoms;the feature A1 at threshold is due to “long-term” MS contributions;the feature B arises by MS contributions of the photoelectron within the first B coordination shell; the features D and E are associated to SS processes between the photoabsorber and the first B shell and the second TM shell, respectively.

ResultsResults



3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

k2 χ(κ)

k (Å-1)

YB2

ZrB2

NbB2

Experimental k2χ(k) of the K edge of the TM in YB2, ZrB2 and NbB2


Lattice dynamics study of 4d TM Lattice dynamics study of 4d TM diborides diborides by EXAFSby EXAFS



0 1 2 3 4 5 6 7 8

|FT(

k2 χ(k

))|

R (Å)

ZrB2

Zr-B

Zr-Zr

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

k2 χ(k

)

k (Å-1)

0 1 2 3 4 5 6 7 8

Y-B

Y-YYB2

|FT(

k2 χ(k

))|

R (Å)

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

k2 χ(k

)

k (Å-1)

0 1 2 3 4 5 6 7 8

R (Å)

NbB2

Nb-B

Nb-Nb

|FT(

k2 χ(k

))|

3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5 1 6 1 7 1 8 1 9

k2 χ

(k)

k (Å -1 )

The k3-weighted FT (solid line) and fitted (dot) for the TM diborides, and the filtered and fitted k2χ(k) in the insets

0.0037(4)3.26(1)2Nb-Nb (out-plane)

0.0030(6)3.12(0)6Nb-Nb (in-plane)

0.0046(9)0.0046(9)2.43(8)2.43(8)1212NbNb -- BB1.371.37NbBNbB22

0.0029(8)3.54(2)2Zr-Zr (out-plane)

0.0026(5)3.18(0)6Zr-Zr (in-plane)

0.0042(6)0.0042(6)2.54(6)2.54(6)1212ZrZr--BB1.481.48ZrBZrB22

0.0069(3)3.84(9)2Y–Y (out-plane)

0.0040(2)3.31(5)6Y-Y (in-plane)

0.0054(5)0.0054(5)2.71(5)2.71(5)1212YY--BB1.621.62YBYB22

Sigma2RN (fixed)Distancerco (Å)Compounds

Structural parameters obtained by the EXAFS analysis for the first and second shell fittings.



1.35 1.40 1.45 1.50 1.55 1.60 1.65

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

Bon

ding

Len

th (Å

)

Covalent Radii (Å)

RTM-B

Rin-planeTM-TM

Rout-planeTM-TM

NbB2ZrB2

YB2

1.35 1.40 1.45 1.50 1.55 1.60 1.650.002

0.003

0.004

0.005

0.006

0.007

0.008

Covalent Radii (Å)

σ2

TM-B

σ2 in-planeTM-TM

σ2 out-planeTM-TM

σ2(Å

2 )

NbB2 ZrB2

YB2

Bond distances of TM-B, TM-TM (in-plane) and TM-TM (out of plane) vs. the covalent radii of the TM atoms.

DW factors (σ2) of TM-B, TM-TM (in-plane) and TM-TM (out of plane) vs. the covalent radii of the different TM atoms.



Data address that the TM-TM bonds in the plane are stronger than those out of the plane.DW factors suggest that the chemical stability for these 4d TM diborides goes as ZrB2>NbB2>Yb2;A clear different vibrational behavior is observed for TM atoms in and out of plane. Data address the not negligible role of the metal atoms in the superconductivity of these diborides and an intimate relation between local atomic displacement and superconductive temperature.



ResultsResults

4 6 8 10 12 14 16 18 20

300K

228K

178K

128K

98K

78K

58K38K

k2 χ(κ)

k (Å-1)

28K

EXAFS oscillations (times k2) of the Zr K-edge absorption spectra of ZrB2 vs. T



0 1 2 3 4 5 6 7 80.00

0.01

0.02

0.03

0.04

0.05

0.06 28 K128 K178 K

FT A

mpl

itude

Distance (Å)

28 K 38 K 58 K 78 K 98 K 128 K 178 K 228 K 300 K

300 K228 K

FT of the EXAFS spectra vs. T



4 6 8 10 12 14 16 18

300K

228K

178K

128K

98K

78K

58K

38K

28K

k2 χ(κ)

k (Å-1)Experimental (solid lines) and fitted (dot lines) EXAFS oscillations forZr-B, Zr-Zr1 and Zr-Zr2 shells vs. T.



50 100 150 200 250 3001.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Zr-B shell Zr-Zr1 shell Zr-Zr2 shellσ2

(10-3

Å2 )

Temperature (K)

Comparison of DB factors between experimental data (points) and calculations (lines) for Zr-B and Zr-Zr shells in the ZrB2 compound.



251±20.0012±0.00013.54Zr-Zr (out-of-plane)271±20.0011±0.00013.18Zr-Zr (in-plane)499±110.0003±0.000032.55Zr-Bν (cm-1)σstat

2 (Å2)R (Å)Bonding

Static distortion and vibration frequency of Zr-B and Zr-Zr shells.

Zr-B shell: Einstein model (optical mode)

Zr-Zr shell: Debye model (acoustic mode)



The static disorder of Zr-B and Zr-Zr pairs in the ZrB2system is negligiblel, while the frequency of these pairs is larger than that of Zr-O and Zr-Zr pairs (e.g., strong Zr-B and Zr-Zr interaction).Although the crystal structure of this system is a layered structure, the EXAFS analysis do not show differences between the in-plane Zr-Zr vibration and the out-of-plane one (e.g., the acoustic phonon behavior does not show anisotropies).



ConclusionsConclusions

Future planFuture planThe EXAFS investigation of the local dynamical structure of other TM diborides vs. T is in progress. The influence of different TMs on the phonon behavior will be also analyzed.

The isotropy effect induced by B in Yttrium diboride will be also studied by temperature-dependent EXAFS.

Correlation between conductivity and phonon behavior in different TM diborides will be investigated and compared with the MgB2reference system.


AcknowledgmentsAcknowledgments

Prof. Wu Prof. Wu ZiyuZiyuProf. Liu Prof. Liu WenhanWenhanProf. A. Prof. A. BianconiBianconi, Prof. A. Marcelli and , Prof. A. Marcelli and Dr. N.L. Dr. N.L. SainiSainiProf. Prof. Hu Tiandu Hu Tiandu and Prof. and Prof. Xie YaningXie Yaning


Thanks for your attention!!

wangsheng chu beijing synchrotron radiation facility - … 2005 pdf/30 ix/chu ws... · buzea, c.,...

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