Kyushu Synchrotron Light Research Center（ Form: No.5）

Experiment Report for Prefectural Beamline
Proposal no. : 1801144F

Beamline no.:Report date

BL15: 06/06/2018

L o c a l - a n d i n t e r me d i a t e - r a n g e s t r u c t u r e s o f As - S e g l a s s e s b e l o w
the stiffness transition region
J.R. Stellhorn and S. Hosokawa
Kumamoto University※１

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１ ． Summary（ Note: Please include conclusions）

We conducted a first experiment of anomalous x -ray scattering (AXS) at BL15.
The aim of our research project is the investigation of As-Se glasses in the floppy
glass-forming region. In this experiment, we obtained the differential structure
factor of the As K edge of As 25 Se 75 . Using this AXS data, element-specificcontributions can be determined in this materi al even though As and Se possessvery similar atomic numbers (Z = 33 and 34). These data will help to understandthe relation between the structure and the stiffness transition of As-Se glasses.

２．Purpose of experiment and backgroundMean-field constraint theory [1, 2] for network glasses provides a powerful tool to explain
numerous anomalies around the critical composition of rigidity percolation threshold at anaverage coordination number 〈r〉 = 2.4, where the number of constraints per atom is equal tothe degree of freedom. The character of the network glass undergoes a first-order-like
transition from floppy at 〈r〉 < 2.4 to rigid at 〈r〉 > 2.4. In case of glassy (g-)AsxSe1-x systems,
this corresponds to x = 0.40, i.e. the stoichiometric compound As2Se3, because thecoordination numbers of As and Se are believed to be 3 and 2, respectively. Temperature
modulated differential scanning calorimetry (MDSC) measurements on bulk AsxSe1-x glasses
provide evidence for a multiplicity of stiffness transitions from floppy to rigid glasses: anonset point at x = 0.29, and a completion point at x = 0.37. [3, 4] The intermediate phaserepresents an unstressed rigid glass phase.
We have previously carried out partial structure analyses for g-AsxSe1-x around the stiffnesstransition region by the AXS method. It was found that the prepeak at 1.2 Å-1 in thestructure factor S (Q) is mainly composed of the As-As correlation, but there is a contributionof the As-Se correlation as well.[5, 6] The x dependence of the position and width of theprepeak in SAsSe(Q) is inconsistent with the results of ab-initio MD simulations by Bauchy etal. [7], while that of the As-As wrong bond fractions is consistent with this theory. To confirm
these results, experimental data in the floppy region of x < 0.29 are necessary.

Therefore, we have carried out an AXS experiment on an As25Se75 sample. A major problems
to analyze the structure of these compounds with usual x-ray techniques is that theelements possess very similar atomic numbers (Z = 33 and 34), and also have similarbackscattering intensities in x-ray absorption spectroscopy. The proximity of the absorption
edges is another difficulty for an extended x-ray absorption fine structure investigation.
Anomalous x-ray scattering, however, can distinguish these element-specific contributions.
For example, the relative weighting of the As-As correlation is only 5% in the normal S (Q),but enhanced to about 17% in ΔAsS(Q).
３．Experimental（Note: Description of sample，method of experiment and analysis, etc.）

The AXS experiment was carried out using a standard diffractometer installed at BL15. Anenergy-dispersive SDD detector was used to discriminate the elastic scattering signal fromspurious contributions of fluorescence lines and inelastic contributions. The energy
resolution of the detector was about 220 eV near 10 keV. We also used a second SDD detector
to monitor the fluorescence signals independently during the entire experiment, which
greatly facilitates the data analysis.

a) top viewb) side viewFig. 1: Schematic view of the setup for the AXS experiment from the top (a) and from the side (b).
One SDD detector is used to collect the scattered intensity in a 2θ range, the other detector isfixed in a backscattering position and mainly monitors the fluorescence signals during the entire
measurement.

The experiment itself was conducted at two incident x-rays energies of about 20 eV (nearedge) and 200 eV (far edge) below the K absorption edge of As (11.686 keV). The scatteringintensity was measured by a SDD detector in an angular range of about 2θ = 2° ~ 140°, inconstant steps in Q space of about 0.05 Å-1.
The data were corrected for absorption effects and Compton scattering, and normalizedusing the Norman-Krogh-Moe method [8,9]. Further details on the theoretical andexperimental background of AXS can be found elsewhere [10-14].

４．Results and Discussions

In this experiment, we investigated the As25Se75 glass by AXS. The experiment is an
important step to demonstrate feasibility of the current AXS setup at BL15, to test thesimplified data analysis possible because of the special setup, and to prove the possibility togain element-specific information.
The total and differential structure factor of As are displayed in Fig. 2 along with thecorresponding pair correlation functions obtained by Fourier transform. The AsS(Q)function in general has a form similar to the total S(Q), but there are notable differences incertain peak intensities and forms, e.g. the first sharp diffraction peak (FSDP) around1.2 Å-1 becomes distinct and well separated from the ”main” peak around 2.2 Å-1. The peak at
2.2 Å-1 is smaller than in the total S(Q). These graphs confirm trends in the relative peakintensities also observed for larger As contents as discussed in refs. [5, 6], which shows thatthe main peak intensity is growing as a function of the As content. This is related to aminimum in the As-Se partial structure factor around 2 Å-1. A first interpretation of the data
can gained by a Fourier transform of the total and differential pair correlation functions, asshown in Fig. 5 b). Both functions exhibit similar features, but the Asg(r) shows a larger
signal in the first coordination shell around 2.4 Å. According to ref. [6], this is due to thelarger contribution of the As-Se correlation.

Fig. 2: Experimental scattering data of the As25Se75 glass collected at the As edge. (a) total (black)and differential (red) structure factors, and (b) total (black) and differential (red) pair correlation
functions.

５．Future issues

In subsequent experiment, we expect to derive partial structural data for several g-AsxSe1-xglasses, which has yet almost exclusively been the domain of the neutron scattering using
isotope enriched samples. From the Sij(Q) results across the rigidity percolation thresholdcomposition, the concentration variation of the short- and intermediate-range order in
g-AsxSe1-x can be discussed in detail, for example the conformation of the quasi-tetrahedral
units can be visualized, and their role for the floppy-rigid transition in this network glasses
can be clarified.
６．References[1] J. C. Phillips, J. Non-Cryst. Solids 34, 153 (1979).
[2] M. F. Thorpe, J. Non-Cryst. Solids 57, 355 (1983).
[3] D. G. Georgiev, P. Boolchand, and M. Micoulaut, Phys. Rev. B 62, R9228 (2000).
[4] P. Boolchand, X. Feng, and W. J. Bresser, J. Non-Cryst. Solids 293-295, 348 (2001).
[5] S. Hosokawa, A. Koura, J.-F. Brar, W.-C. Pilgrim, S. Kohara, and F. Shimojo,Europhys. Lett. 102, 66008 (2013).
[6] S. Hosokawa, W.-C. Pilgrim, J.-F. B´erar, and P. Boolchand, J. Non-Cryst. Solids 431,31 (2016).
[7] M. Bauchy, M. Micoulaut, M. Boero, and C. Massobrio, Phys. Rev. Lett. 110, 165501(2013).
[8] J. Krogh-Moe, Acta Cryst. 9, 951 (1956).
[9] N. Norman, Acta Cryst. 10, 370 (1957).
[10]Y.

Waseda,Anomalous

X-Ray Scattering for

Materials

Characterization,

Berlin-Heidelberg: Springer, 2002.
[11] S. Hosokawa, I. Oh, M. Sakurai, W.-C. Pilgrim, N. Boudet, J.-F. Bérar, and S. Kohara,

Phys. Rev. B 84, 014201 (2011).

[12] J. R. Stellhorn, S. Hosokawa, and W.-C. Pilgrim, Z. Phys. Chem. 228, 1005 (2014).
[13] S. Hosokawa, Y. Wang, J.-F. Bérar, J. Greif, W.-C. Pilgrim, and K. Murase, Z. Phys.

Chem. 216, 1219 (2002).
[14] S. Hosokawa, W.-C. Pilgrim, J.-F. Bérar, and S. Kohara, Eur. Phys. J. Special Topics
208, 291 (2012).

７．Publications, patents（Note: Typical deliverables related to this proposal．
）
８．Keywords（Note: 2-3 words about samples and experimental methods．）
As-Se, chalcogenide glass, anomalous x-ray scattering, stiffness transition
９．About the publication of research results
Please delete either item ① or item ② indicated below, according to a user’s choice of publications asdescribed in ※2. Also, please fill in an expected date of publication of a refereed journal article or anexpected date of submission of a SAGA-LS Research Report. For example, deadline of both publications
2017 is by the end of fiscal 2019
corresponding to the proposals performed in the fiscal 20132015 in both ① and②.
① Publication of a refereed journal article(the date of publication：January 2020）