Photon Factory Activity Report 2022 #40 (2023)

BL-10A/2021G528

Structural Study on Low Symmetry Cassiterite
1Kazuki KANAZAWA1, Takahiro KURIBAYASHI1,*, Toshiro NAGASE2
Department of Earth Science, Graduate School of Science, Tohoku University, Sendai 980-8578,
Japan
2
The Tohoku University Museum, Tohoku University, Sendai 980-8578, Japan

1 Introduction
Cassiterite, SnO2, is an important mineral as the material
source of Sn. The crystal structure of cassiterite belongs totetragonal symmetry with the space group of P42/mnm, butsome of natural cassiterite samples show a lower symmetry
than tetragonal symmetry under microscopic observation.
There are few descriptions of the low-symmetry cassiterite.
So, the crystal structure of such a low-symmetry cassiterite
is still unknown in detail, and the reason for having lower
symmetry is not clear. Recently, we found the cassiterite
showing low symmetry from some mines in Japan. In thisstudy, we have conducted single-crystal X-ray diffraction
experiments using synchrotron radiation to investigate alow symmetry cassiterite, and to clarify the mechanism of
symmetry reduction of cassiterite.
2 Experimental Procedure
The natural cassiterite used for this study was from theOhtani Mine, Kyoto, Japan. We confirmed symmetry
reduction and sector structures such as {231} and {111}sectors in this sample from microscopic observation of thethin section perpendicular to the c-axis. Before sample
preparation for the X-ray diffraction experiments,
chemical properties were checked from BSE-image using
an SEM-EDS apparatus (JEOL, FE-SEM JSM-7001F;
Oxford, EDS INCA), and we observed chemical zoning ineach sector. The samples for X-ray diffraction experiments
were carefully picked-up from the area showing nochemical zoning in each sector. The sample dimensions(µm) were as follows: 70 × 70 × 50 for a {231} sector and90 × 90 × 70 for a {111} sector. The chemicalcompositions of the sample used for the X-ray diffraction
study were measured using a WDS apparatus for the {231}and the {111} sectors (JEOL, JXA-8800M). The chemical
formulae of the samples were determined as Sn0.993Ti0.007O2for the {231} and Sn0.988Ti0.012O2 for {111} sector,respectively.
Single-crystal X-ray diffraction experiments wereperformed using an automated four-circle X-ray
diffractometer installed at the beam line BL-10A, PF, KEK.
The wavelength (l = 0.70137 Å for the {231} sector andl = 0.70116 Å for the {111} sector) of synchrotron
radiation in each run was calibrated by the unit cell volumeof the NIST standard ruby at ambient temperature. The Xray diffraction intensity data were collected up to 2qmax =96º for the {231} sector and 2qmax = 99º for the {111} sector,
respectively by using the w-scan method. Structuralrefinements for both samples were conducted using
SHELX97 with WINGX software. The cell parameters foreach sector were determined in our laboratory using anautomated four-circle diffractometer (Rigaku, AFC-7S)
with MoKa radiation (l = 0.71073 Å). The unit cellparameters of each samples were determined from the 54
centered reflections such as {310}, {220}, {301} and{231} of in the 2q range between 23º and 35º.
3 Results and Discussion
The lattice constants of each sample are summarized inTable 1. The obtained cell parameters showed almost
tetragonal symmetry. However, the systematic absencerule for the space group of P42/mnm (h + l = 2n for h0l, k+ l = 2n for 0kl, h = 2n for h00, k = 2n for 0k0 and l = 2nfor 00l) was broken for both sectors from the analysis of
the distribution of X-ray diffraction intensity data. We also
checked these peaks breaking the rule using the psi-scanmethod, and confirmed that these peaks were not theresults of multiple diffraction. The candidates for the spacegroup of the cassiterite structure showing symmetry_reduction can be considered as follows: P2/m, Pm, P2, P1
and P1. In this report, the results of the structure refinement
for the {231} sector was shown only. We conductedstructure refinements for these all structural models foreach sector. The results of structural refinements for thesemodels were summarized in Table 2. The space group forthe structure of {231} sector in the low symmetry
cassiterite could be considered as P2/m.

* t-kuri@tohoku.ac.jp