2 Quantum Chemistry in Perovskite Fluoride and Hydride: Nanoscale. . .
29
2.3 Hydride Ion Conduction in Perovskite Magnesium
Fluoride: KMgF 3
The crystal structure of KMgF 3 perovskite was experimentally investigated from
very low temperature to 1200 K. KMgF 3 perovskite has a simple cubic structure
throughout the temperate range. The lattice parameter (Mg-F-Mg distance) is
3.973 Å [23, 24]. In relation to Jahn-Teller effect [25, 26], optical property and
magnetism, transition metals such as Co 2+ [27], Cu 2+ [28], Cr 3+ [29, 30], Ni 2+
[31, 32], etc. were doped at the lattice position of magnesium. No ion conductivity
was previously observed in KMgF 3 perovskite [33]. Here we investigate whether
hydride ion-doped KMgF 3 perovskite exhibits hydride ion conductivity or not, from
the viewpoints of energetics and bonding.
2.3.1 Calculation Models
Figure 2.1 depicts the calculation models for KMgF 3 perovskite. From our previous
MO calculations, it is known that conducting anion migrates along a diagonal line
connecting two lattice positions in a cubic perovskite (see arrows in Fig. 2.1). It is
because ion conduction via square centre (from F1 to F3) requires larger energy.
It is noted that fluorine anion vacancy, which can be introduced by counter cation
vacancy or different counter cation doping [3], is required to cause fluorine anion
conduction. To take the effect of charge compensation into account, potassium
vacancy is also considered.
2.3.2 Fluorine Anion Conduction in Perovskite Magnesium
Fluoride
Figure 2.2a, b show the potential energy curves of K 2 Mg 4 F 3 and KMg 4 F 3 models,
when displacing fluorine anion along the diagonal line. In K 2 Mg 4 F 3 model, local
minima are found between midpoint and lattice position (d = 0.4 and 2.4 Å), and
local maximum is found at the middle (d = 1.4 Å). Though the activation energy
for fluorine anion conduction is 2.23 eV, the partial activation energy crossing lattice
position is much smaller (0.49 eV). It is considered that fluorine anion is displaced
till local minimum at room temperature. It is because the partial activation energy is
as same as lithium conducting perovskite-type titanium oxides [1], where lithium
ion conduction occurs at room temperature. On the other hand, it is considered
that high temperature is required when crossing a midpoint. It is because the
activation energy is as same as proton conducting perovskite-type oxides [7, 8,
34], where proton conduction occurs at high temperature. The introduction of one
potassium vacancy causes one fluorine vacancy, due to charge compensation. In
order to investigate the effect, KMg 4 F 3 model was also constructed (see Fig. 2.1b).
29
2.3 Hydride Ion Conduction in Perovskite Magnesium
Fluoride: KMgF 3
The crystal structure of KMgF 3 perovskite was experimentally investigated from
very low temperature to 1200 K. KMgF 3 perovskite has a simple cubic structure
throughout the temperate range. The lattice parameter (Mg-F-Mg distance) is
3.973 Å [23, 24]. In relation to Jahn-Teller effect [25, 26], optical property and
magnetism, transition metals such as Co 2+ [27], Cu 2+ [28], Cr 3+ [29, 30], Ni 2+
[31, 32], etc. were doped at the lattice position of magnesium. No ion conductivity
was previously observed in KMgF 3 perovskite [33]. Here we investigate whether
hydride ion-doped KMgF 3 perovskite exhibits hydride ion conductivity or not, from
the viewpoints of energetics and bonding.
2.3.1 Calculation Models
Figure 2.1 depicts the calculation models for KMgF 3 perovskite. From our previous
MO calculations, it is known that conducting anion migrates along a diagonal line
connecting two lattice positions in a cubic perovskite (see arrows in Fig. 2.1). It is
because ion conduction via square centre (from F1 to F3) requires larger energy.
It is noted that fluorine anion vacancy, which can be introduced by counter cation
vacancy or different counter cation doping [3], is required to cause fluorine anion
conduction. To take the effect of charge compensation into account, potassium
vacancy is also considered.
2.3.2 Fluorine Anion Conduction in Perovskite Magnesium
Fluoride
Figure 2.2a, b show the potential energy curves of K 2 Mg 4 F 3 and KMg 4 F 3 models,
when displacing fluorine anion along the diagonal line. In K 2 Mg 4 F 3 model, local
minima are found between midpoint and lattice position (d = 0.4 and 2.4 Å), and
local maximum is found at the middle (d = 1.4 Å). Though the activation energy
for fluorine anion conduction is 2.23 eV, the partial activation energy crossing lattice
position is much smaller (0.49 eV). It is considered that fluorine anion is displaced
till local minimum at room temperature. It is because the partial activation energy is
as same as lithium conducting perovskite-type titanium oxides [1], where lithium
ion conduction occurs at room temperature. On the other hand, it is considered
that high temperature is required when crossing a midpoint. It is because the
activation energy is as same as proton conducting perovskite-type oxides [7, 8,
34], where proton conduction occurs at high temperature. The introduction of one
potassium vacancy causes one fluorine vacancy, due to charge compensation. In
order to investigate the effect, KMg 4 F 3 model was also constructed (see Fig. 2.1b).