Advanced
TEM
of
BiCu1‐xOS
oxysulfide: copper deficiency and electronic properties
Authors: Lebedev Oleg , Berthebaud David , Guilmeau Emmanuel , Maignan Antoine 
Published: December 2016

The BiCuOX (X=S, Se) oxychalcogenides have attracted much attention because of their properties which are performing thermoelectric material ( copper deficient BiCu
1‐x
OSe phase) and along the BiCuOSe‐BiCuOS solid solution, one of candidate for possible transparent conductor (BiCuOS) and also superconductivity for BiCu
1‐x
OS structure.

An oxysulfide series of nominal compositions BiCu
1‐x
OS with x<0.20 has been prepared and its structural properties characterized by combining XPRD and TEM techniques. It is found from XPRD that this oxysulfide is crystallized in the
P4/nmm
SG with a3.87Å, c8.6Å unit cell parameters.

In order to investigate the crystal structure and chemical nature of defect structure, such as dislocations and possible intergrowth at atomic level, which mightbe responsible for particular properties, advanced TEM was carried out. The high resolution [100] HAADF‐STEM images of BiCuOS perfect crystal are given in Fig. 1a.The careful inspection of the high resolution HAADF‐STEM image reveals the presence of local variation in the intensity of the atomic columns in some of the weak brightness rows located in between two high brightness zig‐zag rows (Fig1b) It is clear from the nature of HAADF‐STEM contrast that the bright zig‐zag dots correspond to Bi atomic columns whereas the less bright dots correspond to the Cu ones. The corresponding intensity plot profiles made along the Cu atom rows shows clear drop of the peak intensity with respect to the adjacent peaks. This is observed every two atomic columns (black arrow heads). The peaks of lower intensity definitely correspond to lower amount of Cu atoms in the column. Accordingly, the presence of Cu vacancies should also create re‐arrangements of the S atoms in the structure. The larger sensitivity to the light elements of ABF‐STEM images evidences it (Fig. 1c, insert in bottom panel). In the latter, some of the S atoms are slightly displaced from original position. They also exhibit less contrast with respect to the other S atomic columns suggesting S‐vacancies. Therefore, the presence of Cu vacancies was confirmed by HAADF‐STEM studies and shows that BiCu
1−x
OS tends to adopt a constant amount of copper vacancy corresponding to x=0.05 and is general to all the BiCu
1−x
OS (x≤0.15) samples.

Moreover, for larger Cu deficiencies (x>0.05 in the nominal composition), other types of structural nanodefects are evidenced from HAADF‐STEM imaging such as oxysulfides of the “BiOS” ternary system which might explain the report of superconductivity for the BiCu
1−x
OS oxysulfide (Fig.2) Intergrowths between different structures can be evidenced at the nanoscale level, which are related to deficient Cu regions. Interestingly, the layers stacking in the “I” region shows a continuous phase transformation from tetragonal BiCuOS structure to the orthorhombic copper free BiOS oxysulfide and, finally, to the cubic Bi
2
O
3
oxide (see structural mode in the left bottom panel). The structures are epitaxial intergrowth, showing that, by adding sulfur to the Bi
2
O
3
oxide, and then Cu, the material can evolve from the oxide to BiCuOS oxysulfide in only few unit cells. The region noted II shows a nanoparticle (~15nm) without copper which can be identified as the Bi
2
O
2
S structure surrounded on both its left and right sides by Bi
2
O
3
layers.

This is in contrast with the TEM investigation of “nominal” BiCuOSe oxyselenide. TEM results evidence that layer stacking in the structure is very regular. Neither intergrowth nor dislocation can be observed. More importantly, neither Cu nor other vacancies, including oxygen or sulfur were found in the case of BiCuOSe .

The lack of copper explains why Bi
2
O
3
can play the role of buffer layer to epitaxially adapt BiCuOS to Bi
2
O
2
S. The presence of defects with Bi
2
O
2
S composition, very close to that of the Bi
3
O
2
S
3
superconductor with T
C
=4.5K, also explains why superconductivity could have been observed in Cu deficient BiCuOS oxysulfide reported previously.

Finally, our study shows that the BiCuOS structure tends to adapt a limited amount of Cu deficiency corresponding to “BiCu
0.95
” through the partial occupancy of the Cu sites in the (ab) plane. For nominal contents of copper smaller than 0.95, other kind of defects are created implying nano‐epitaxy between the “BiCuOS” phase and both “Bi
2
O
2
S” and “Bi
2
O
3
” phases. The origin of the BiCuOS lack of structural flexibility for Cu amounts deviating from ~5% could be related to a very rigid skeleton imposed by the epitaxial relation between the two types of structural layers in contrast to the isostructural selenides. Consequently, this limitation of doping via Cu vacancy in BiCuOS calls for other types of chemical control to inject carriers in the conduction [Cu
2
S
2
]
2−
layers.