UNIDAD DE COMPETENCIA 3
3. Reconocimiento de las características de la piel y cuero
Although, as described in the beginning of this Chapter, synthesis of ‘pure’ perovskite compounds containing Sn2+ on the A-site remains difficult, there have been attempts to
infuse already grown perovskite materials with tin through doping [28, 195–199]. We have participated in an experimental collaboration that explored the effects of A- and B-site tin doping of epitaxial SrTiO3 films on SrTiO3 (001) substrates grown using a hybrid molecular
beam epitaxy (MBE) approach [28]. In addition to addressing the outstanding challenge of incorporating Sn2+ on the A-site of a perovskite structure, this investigation was focused
on understanding growth kinetics of Sn-alloyed perovskite oxides and its influence on the incorporation of Sn within the crystal lattice. Epitaxial Sn-doped SrTiO3 films were grown
on the SrTiO3 (001) substrates with MBE using hexamethylditin (HMDT) and titanium
tetraisopropoxide (TTIP) as the metal-organic chemical precursors for Sn and Ti, respec- tively. More specific details on the thin film synthesis and characterization are provided in
(a) (b) (c) (d)
Figure 4.6: Representative 2×2×2 supercells for (a) SrTiO3, (b) 1Sn[A] model with one Sn substituting Sr at an A-site, (c) 1Sn[A] + 1Sn[B] model with two Sn replacing one Sr at an A-site and one Ti at a B-site, (d) a variant of a 1Sn[A] + 2Sn[B] model with three Sn replacing one Sr at an A-site and two Ti at B-sites. Sr, Sn, Ti and O atoms are represented by green, black, light blue and red spheres, respectively. Note the structural models represented in (a) through (d) correspond to (Sr+Ti+Sn)Sn at. % ratio value of 0, 0.0625, 0.125 and 0.1875 respectively.
Ref. [28].
Structural models for Sn-alloyed SrTiO3 were created by taking a 40-atom supercell
consisting of a 2×2×2 perovskite unit-cell arrangement and then replacing some of the B- site Ti and/or A-site Sr with Sn. Examples of Sn arrangements on both A- and B-sites for some specific concentrations are shown in Fig. 4.6. DFT calculations were performed on these representative models to evaluate their lattice parameters and compare them with those obtained in experiments, making it possible to extract information about the relative fractions of Sn incorporation on A- and B-sites. Such highly-ordered models are available only for a small discrete set of stoichiometries and thus values of the (Sr+Ti+Sn)Sn at. % ratios, and cannot faithfully reflect the disordered nature of the alloyed compound. Nevertheless, they are easy to process with standard DFT techniques and can still provide useful information
on the basic geometrical parameters of the substituted system.
Determination of all the possible arrangements of Sn is especially straightforward in the case of the exclusively B-site doped SrTi1−xSnxO3 system. Only eight different stoichiomet-
rical variants are possible, with the (Sr+Ti+Sn)Sn at. % ratio values ranging from 0 to 12 in increments of 161 , where the limiting cases represent pure SrTiO3 and SrSnO3, for (Sr+Ti+Sn)Sn
at. % ratio of 0 and 12 respectively. For these compositions, ones corresponding to the ratios of 162 (12.50%) and 166 (37.50%) have five possible symmetrically distinct ordering arrange- ments of Ti and Sn on the B-sites, while those corresponding to the ratios of 163 (18.75%),
4
16 (25.00%) and 5
16 (31.25%) have six symmetrically distinct arrangements. The remaining
compositions of 161 (6.25%) and 166 (43.75%) are represented by one model each. None of the examined B-site Sn arrangements result in non-centrosymmetric space groups and therefore none of the corresponding structural models include any polar distortions. For each of the Sn concentrations, the value of the out-of-plane lattice parameter aop in the disordered sub-
stituted system was approximated by averaging of the aop values of all the possible ordered
structures.
Fig. 4.7 shows the aop of Sn-alloyed SrTiO3 as a function of the (Sr+Ti+Sn)Sn atomic % (at.
%) ratio measured using XPS (blue squares) [28]. The experimental data presented in the plot shows an unusual behavior of the aop with an increasing value of (Sr+Ti+Sn)Sn at. % ratio,
which initially increases and then decreases starting with the ratio of ∼ 0.1. The off-axis reciprocal space map (RSM) data showed a completely coherent film for all Sn concentration samples, suggesting that the origin of a reduced aop is not associated with strain relaxation.
Figure 4.7: The out-of-plane lattice parameter aop of the epitaxial SrTi1−xSnxO3 film as
a function of x/2. Dotted lines show calculated aop as a function of x/2 assuming Sn on
B-sites (red), and assuming Sn on both B- and A-sites (black) for completely strained films on SrTiO3 (001).
Dashed lines in Fig. 4.7 depict the calculated aop of the Sn-alloyed SrTiO3 films with B-
site doping (red dashed-dotted line) and mixed A- and B-site doping (black dashed line) as a function of the (Sr+Ti+Sn)Sn at. % ratio, assuming a completely strained film on the SrTiO3
(001) substrate. The computed results for the B-site doped structures follow a straight line in compliance with the Vegards law. All of these values are shifted up by 0.043 ˚A, which constitutes the difference between the experimental (3.905 ˚A) and theoretical (3.862 ˚
A) estimates of the lattice constant of cubic SrTiO3, to compensate for the well-known
“overbinding” behavior of the DFT-LDA techniques.
On the other hand, when Sn was allowed to occupy both B- and A-sites (black dashed line), for ratios above 0.18-0.20, the calculated aop behavior deviates strongly from the Veg-
ard’s law, remaining constant or even decreasing slightly with increasing Sn concentration. Although the DFT-based estimates for aop do not coincide precisely with those obtained
in experiments (potentially due to insufficient sampling of different Sn-dopant arrangements within the system), they do reproduce the same general trend, which suggests that the trans- fer of a certain number of Sn2+ ions onto the A-site, replacing Sr2+, results in a noticeable
decrease of the aop. This result also indicates that the films with measured (Sr+Ti+Sn)Sn at. %
ratio < 0.10 may also contain a small fraction of tin as Sn2+ and that the decrease in aop is
observed only when the fraction of Sn2+ at A-site increases. As a result of Sn ion occupying
A-site as Sn2+, it will off-center due to the lone-pair activity, thus breaking the inversion
symmetry of the system and making it polar. The polar character of the films was confirmed with the help of optical second harmonic generation (SHG) polarimetry measurements, thus providing strong experimental evidence for the presence of Sn2+ on the A-site of the doped
SrTiO3 thin film structure [28].