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Condens. Matter 2019, 4, 80 8 of 12 be considered as a first guess only—are about twice as large as those for LFP. Obviously, the FP GB energies may still change when the studied GB is further elaborated to be better suitable for the Li Condens. Matter 2019, 4 8 diffusion study envisaged. Some ideas in this direction are given in the next section. FFigiguurere33..Σ4Σ4(1(0110)1/[)0/[1001]0G]GBBfofrorFPF:P(:a()aC)Conofnifigugurartaitoionn1;1(;b(b))FFPPcoconnfifigguuraratitoionn22..TThheecocololorrscshchememeeisisththee sasammeeaassininFFigiguurere11, ,ininclculuddininggththeeFFPPuunnititcceelllaannddssuuppeerrcceellltrtraannsslalatitoionnvveecctotorrss.. Let us now discuss other properties of the studied GB in the FP material, even if they should be Let us now discuss other properties of the studied GB in the FP material, even if they should be considered as preliminary. Bader charge analysis shows that for the bulk cell, charges are as follows: considered as preliminary. Bader charge analysis shows that for the bulk cell, charges are as follows: q(Fe) = +1.76e, q(P) = +3.55e, q(O(1)) = −1.32e, q(O(2)) = −1.34e, and q(O(3)) = −1.32e. The sum of charges q(Fe) = +1.76e, q(P) = +3.55e, q(O(1)) = −1.32e, q(O(2)) = −1.34e, and q(O(3)) = −1.32e. The sum of charges in the formula unit, i.e., q(Fe) + q(P) + q(O(1)) + q(O(2)) + 2q(O(3)), results in +0.01e, which is again in the formula unit, i.e., q(Fe) + q(P) + q(O(1)) + q(O(2)) + 2q(O(3)), results in +0.01e, which is again practically zero, taking into account rounding errors. With the charge neutrality being fulfilled, one can practically zero, taking into account rounding errors. With the charge neutrality being fulfilled, one observe that the FP system becomes more covalent bonded compared to LFP (see also the discussion in can observe that the FP system becomes more covalent bonded compared to LFP (see also the Ref. [5]), since the magnitude of ionic charges diminishes (except for Fe, which is assumed to be an Fe3+ discussion in Ref. [5]), since the magnitude of ionic charges diminishes (except for Fe, which is ion);thephosphate3g+rouphasthecharge−1.76e.ThisindicatesthatinLFP,theLielectron—whichis assumed to be an Fe ion); the phosphate group has the charge −1.76e. This indicates that in LFP, the supposed to be transferred solely to the Fe ion—is partially transferred also to the phosphate group Li electron—which is supposed to be transferred solely to the Fe ion—is partially transferred also to (thereby oxygen anions). The more open FP structure with respect to LFP (see Figures 1 and 3) is also the phosphate group (thereby oxygen anions). The more open FP structure with respect to LFP (see an indication of more covalent bonds in FP. In the case of the C1 GB, average ionic charges change Figures 1 and 3) is also an indication of more covalent bonds in FP. In the case of the C1 GB, average slightly only (0.01–0.02e), but Fe ions close to the interface have a lower charge by about 0.04e (the ionic charges change slightly only (0.01–0.02e), but Fe ions close to the interface have a lower charge largest changes occur for Fe ions with fourfold coordination). The C2 GB behaves in about the same by about 0.04e (the largest changes occur for Fe ions with fourfold coordination). The C2 GB behaves way as the C1 GB concerning ionic charges, but for Fe ions with fourfold coordination, the charge in about the same way as the C1 GB concerning ionic charges, but for Fe ions with fourfold decreases even by 0.11e on average compared to the reference bulk system. Clusters of atoms were coordination, the charge decreases even by 0.11e on average compared to the reference bulk system. also investigated, but for the FP GB configurations, we chose a somewhat different approach because Clusters of atoms were also investigated, but for the FP GB configurations, we chose a somewhat of a less regular GB structure compared to LFP. Namely, 24 atom clusters (four formula units) were different approach because of a less regular GB structure compared to LFP. Namely, 24 atom clusters considered (instead of six atom clusters) forming just one unit cell of FePO . Such 24 atom clusters (four formula units) were considered (instead of six atom clusters) formin4g just one unit cell of are ‘parallel’ with the GB interface, and are located on either side of the interfaces, as well as in the FePO4. Such 24 atom clusters are ‘parallel’ with the GB interface, and are located on either side of the bulk-like regions. There are totally eight clusters in the supercell of each configuration. For both cases, interfaces, as well as in the bulk-like regions. There are totally eight clusters in the supercell of each there is a charge modulation going along the last supercell dimension. At the interfaces, one side is configuration. For both cases, there is a charge modulation going along the last supercell dimension. At the interfaces, one side is charged positively, and the other one is charged negatively. The magnitudes of the cluster charges peak at 0.10e for C1 and 0.03e for C2. The overall charge is, of course, zero, since the boxes have neutral charge. Again, the GB C2 interface with a better coherence exhibits lower cluster charges.PDF Image | First-Principles Grain Boundary Formation in the Cathode Material LiFePO4
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