Identification of occupation site of Al doped in
Y2Ti2O7
based by ab initio calculation and statistical high‐angular resolution electron channeling X‐ray spectroscopy
Authors: Obata Yoshihiro , Oda Kenji , Ohtsuka Masahiro , Muto Shunsuke , Tanaka Makoto , Kitaoka Satoshi 
Published: December 2016

SiC
f
/SiC
m
is expected to be a turbine material of next generation aircraft engine, though the environment barrier coating (EBC) is indispensable to shield oxygen and reflect the radiant heat from the outer environment. It was reported that the periodic layers of Al
2
O
3
and Y
2
Ti
2
O
7
(YT) that have different refraction indices worked very effectively as EBC.
*1
However, a problem was found that Al effusion from Al
2
O
3
to Y
2
Ti
2
O
7
gave rise to collapse of the layer structure and the property of EBC was significantly deteriorated. In order to avoid this problem, it was also found that doping a small amount of Al into YT stabilized the layer structure and improved the oxygen permeation property. It is thus necessary to identify which atomic site and how much fraction Al occupies in Al doped Y
2
Ti
2
O
7
(AYT) to clarify the mechanism of this property improvement. In this study, we applied a statistical ALCHEMI technique, electron energy loss spectroscopy (EELS) and first principles theoretical calculation for the above purpose by comparing with the obtained macroscopic analysis.

ALCHEMI is a technique for quantitatively identifying the occupancies of substitutional impurities in a crystalline material. The method takes advantage of the electron channeling effect, which is a phenomenon that electron probability densities change with the incident electron beam direction with respect to the crystalline orientation. However, there is a possibility that we may obtain a wrong result using this method because of delocalization effects of electron that provides systematic errors from difference in the electron orbital spread of different atoms. The statistical ALCHEMI technique acquires a large amount of data points as an incoherent channeling pattern (ICP), followed by the statistical analysis, so that the site occupancies are very precisely determined, insusceptible to the delocalization effects.
*2

The samples used for the ICP measurement are three kinds of AYT (nominal Al concentration was approximately 1 cation%) annealed under different oxygen partial pressures (P
O2
=10
‐10
, 10
‐1
, 10
5
Pa) at 1300 °C for 50 hours after vacuum sintering at 1500 °C for 5 hours. We assumed that Al can only substitute for the Y and Ti sites in the crystals.

Fig.1 shows the measured ICP of these samples. The statistical ALCHEMI analysis showed that Al occupied both the Y and Ti sites at a ratio of approximately 1:1, and at the lower oxygen partial pressure the Al occupancy was slightly biased to the Ti site. The concentration of Al was estimated to be 1.6 cat%. It is considered that the concentration of oxygen vacancies in AYT should increase under the lower oxygen partial pressures, where AYT tends to be positive charged, which can be compensated for by preferential substation of Al
3+
for Ti
4+
to maintain the electrical neutrality.

We also measured the Al‐K and L
2,3
ELNES and compared them with those obtained by
ab initio
calculation to confirm if the Al substitution sites in AYT were consistent with the statistical ALCHEMI results. For the theoretical ELNES simulation we created the AYT conventional cell (a, b, c = 10.45 +, α, β, γ = 90°) by substituting an Al atom for a Y or Ti atom in the YT conventional cell and carried out the geometry optimization (Fig.2). The concentration of Al atoms is expected to be very small and the calculating cell must be large enough to ignore the interaction between two Al atoms in the neighboring cells. We used CASTEP code for the geometry optimization and the FEFF code based on a multiple scattering method for the ELNES calculation.

Fig.3 shows the theoretical (upper) and experimental (lower) Al‐K ELNES. ‘Al
Y
’ and ‘Al
Ti
‘ in the upper figure are calculated spectra which respectively substitute Al for the Y or Ti site. ‘Sum’ is the summed spectra of both with the Gaussian function convolved, to compare with the experimental spectrum with 1:1 occupation deduced by the ALCHEMI method above. The first peak of ‘Sum’ was aligned to match the peak position of the experimental spectrum. The ELNES result seems to be consistent with the ALCHEMI result.

In summary, it is considered that Al atoms substitute the Y and Ti atoms at a rate of 1:1. The more detailed discussion is presented in the poster.