実験報告書様式（一般利用課題・成果公開利用）（ ※本報告書は英語で記述してください。ただし、産業利用課題として採択されている方は日本語で記述していただいても結構です。 ）MLF Experimental Report
課題番号 Project No. 2012B0234

提出日 Date of Report
June 24, 2013
装置責任者 Name of responsible person
Kittiwit Matan

実験課題名 Title of experiment

装置名 Name of Instrument/（BL No.）

Nuclear and magnetic structures in the spin-1/2 kagome latticeBL-08

antiferromagnet Cs2Cu3SnF12

実施日 Date of Experiment

実験責任者名 Name of principal investigator

March 11-15, 2013

Kittiwit Matan
所属 Affiliation
Mahidol University
試料、実験方法、利用の結果得られた主なデータ、考察、結論等を、記述して下さい。（適宜、図表添付のこと）
Please report your samples, experimental method and results, discussion and conclusions. Please add figures andtables for better explanation.
1. 試料 Name of sample(s) and chemical formula, or compositions including physical form.
A power sample of Cs2Cu3SnF12 is filled in a vanadium can with the diameter of 1 cm.

The can is about 60%

full.

2. 実験方法及び結果 （実験がうまくいかなかった場合、その理由を記述してください。）
Experimental method and results.

If you failed to conduct experiment as planned, please describe reasons.

1Time-of-flight powder neutron diffraction was performed on the distorted kagome lattice antiferromagnet
Cs2Cu3SnF12 to study its nuclear and magnetic structure at low temperatures.

A powder sample is filled in

a 1cm-diameter vanadium can and cooled to base temperature of 10 K using a closed cycle He-4 cryostat.
The data are collected by three detector banks, which we will called LA (low-angle bank), QA (90-degreebank), and BS (back-scattering bank) with the highest resolution.

The measurements were done at fourtemperatures, 10 K (below TN = 20 K), 25 K, which is right above TN, 150 K (below Tf = 185 K, where thestructural phase transition occurs), and 200 K. The counting time at each temperature is about 30 hours.
After the data reduction process and normalization, the reduced scattering intensity of each detector bankis plotted as a function of either time-of-flight or d-spacing.
At the base temperature of 10 K. we observe peak magnetic Bragg peaks on top of the fundamental nuclear
Bragg peaks at Q = (2,0,2) and (2,2,0).

We note that the peaks are indexed using an approximate structure,

where the space group is R-3, and the lattice parameters are a = 14.21 and c = 20.18, since the true structure
at low temperature of this material is not yet known.

Figure 1 shows extra scattering intensity at 10 Kcompared to the data the 25 K at (2,0,2) and (2,2,0) for the LA, QA and BS detectors, indicative of themagnetic Bragg peaks.

Figure 1. Top panels show the magnetic Bragg peak at (2,0,2) in the LA (left) and QA (right) detectors. Bottom panels show
the magnetic Bragg peak at (2,2,0) in the QA (left) and BS (right) detectors.

22. 実験方法及び結果（つづき） Experimental method and results (continued)
At the structural phase transition, we observe the doubling of a unit cell, resulting in superlattice reflections. Figure 2
shows a group of superlattice peaks observed at 150 K but not at 200 K indexed by odd integers in H or/and K. Thesesuperlattice peaks are relatively small compared to the fundamental reflections; in the figure the fundamental reflections are(2,4,4) shown on the left and (0,0,9) shown on the right of the figure.

In addition, we observe the splitting of the (2,4,4) peaksuggesting that the true crystal structure is not exactly R-3. However, currently, the correct space group is unknown andfurther analysis is needed. We note that the splitting of some of the fundamental reflections can only observed using thehigh-resolution BS detector.

The resolution of the LA and QA detectors is not enough to resolve the splitting and hence

the data from those two sets of detectors can be refined using the spaces group R-3.

Figure 2. Superlattice reflections at odd integers in H or K between two fundamental reflections (2,4,4) at d = 2.112
angstroms, which splits into two peaks, and (0,0,9) at d = 2.112 angstroms, which does not split.

In summary, we performed high-resolution time-of-flight powder neutron diffraction on Cs2Cu3SnF12 to study its nuclear andmagnetic structure at low temperatures. We observe the magnetic Bragg peaks at 10 K, which disappears above TN = 20 K.
This magnetic peaks are very weak and lie on top of the structural peaks, making it difficult to detect.
only observe two of such magnetic peaks.
transition at Tf = 185 K.

As the result, we canIn addition, we observe the superlattice peaks, which are due to the structural

These superlattice peaks are small compared to the fundamental peaks indicating of small lattice

distortion. Furthermore, the splitting of some fundamental Bragg peaks with finite H and K is observed. The splitting is notobserved at (0,0,L).

This splitting of the fundamental peaks suggests that the symmetry of the crystal is not exactly R-3 as

we have expected.

We note that Rb2Cu3SnF12, which is a closely related compound to Cs2Cu3SnF12, crystallizes in R-3 atroom temperature.

This difference of the crystal structure could explain the observed difference in the magnetic groundstates of these two compounds; while the ground state of Rb2Cu3SnF12 is valence bond solid, S = ½ spins in Cs2Cu3SnF12order.

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