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T. Onishi
[1], they have been focused on as solid electrolyte of lithium ion battery [2, 3]. In
solid oxide fuel cell (SOFC), proton and oxide ion conducting perovskite oxides
have been utilized for solid electrolyte, due to stable structure at high operation
temperature [4–7].
In proton and hydride ion conducting perovskites, hydrogen atom migrates
inside perovskites. Proton and hydride ion conductions can be distinguished from
formal charge of hydrogen atom before and after ion conduction: proton (+1) and
hydride ion (−1). However, during proton conduction, the real charge density is
different from the formal charge (+1). For example, the charge density is close
to zero, in proton conducting LaAlO 3 perovskite [8]. It is because OH and OHO
covalent bondings are formed during proton conduction. Norby et al. indicated the
possibility of hydride ion conduction in perovskite titanium oxides [9–11]. Recently,
in BaTiO 3 perovskite, Kageyama et al. showed that oxygen anion can be replaced
by hydride ion [12], and hydride ion acts as labile part [13]. From our previous
theoretical calculations, it was found that hydride ion migrates via oxygen vacancy,
and fluctuation of hydride ion occurs [14].
Here we focus on perovskite fluoride and hydride, which have a typical perovskite structure [15, 16], since they can be expected as fast ion conducting material.
In perovskite fluoride, hydride ion is incorporated by replacing fluorine anion at
lattice position [17, 18]. The hydride ion conducting mechanism is explained, from
the viewpoints of energetics and bonding.
2.2 Theoretical Approach: Molecular Orbital Calculation
and Chemical Bonding Rule
Molecular orbital (MO) calculations based on density functional theory (DFT) were
performed for perovskite fluoride and hydride. BHHLYP method, where functionals
of Hartree-Fock exchange, Becke exchange and LYP correlation are included,
was selected. 6-31G * basis set was used for magnesium, fluorine and hydrogen,
combined with MINI basis set for potassium. All calculations were performed by
using Firefly program package [19, 20]. The figures of MOs were depicted by
MOLEKEL [21]. Chemical bonding property was characterised by using chemical
bonding rule [22]. The rule is summarised as below.
Chemical Bonding Rule
For molecular orbitals including outer shell electrons, check whether orbital overlap
exists or not.
– With orbital overlap: Covalent.
– Without orbital overlap: Ionic.
T. Onishi
[1], they have been focused on as solid electrolyte of lithium ion battery [2, 3]. In
solid oxide fuel cell (SOFC), proton and oxide ion conducting perovskite oxides
have been utilized for solid electrolyte, due to stable structure at high operation
temperature [4–7].
In proton and hydride ion conducting perovskites, hydrogen atom migrates
inside perovskites. Proton and hydride ion conductions can be distinguished from
formal charge of hydrogen atom before and after ion conduction: proton (+1) and
hydride ion (−1). However, during proton conduction, the real charge density is
different from the formal charge (+1). For example, the charge density is close
to zero, in proton conducting LaAlO 3 perovskite [8]. It is because OH and OHO
covalent bondings are formed during proton conduction. Norby et al. indicated the
possibility of hydride ion conduction in perovskite titanium oxides [9–11]. Recently,
in BaTiO 3 perovskite, Kageyama et al. showed that oxygen anion can be replaced
by hydride ion [12], and hydride ion acts as labile part [13]. From our previous
theoretical calculations, it was found that hydride ion migrates via oxygen vacancy,
and fluctuation of hydride ion occurs [14].
Here we focus on perovskite fluoride and hydride, which have a typical perovskite structure [15, 16], since they can be expected as fast ion conducting material.
In perovskite fluoride, hydride ion is incorporated by replacing fluorine anion at
lattice position [17, 18]. The hydride ion conducting mechanism is explained, from
the viewpoints of energetics and bonding.
2.2 Theoretical Approach: Molecular Orbital Calculation
and Chemical Bonding Rule
Molecular orbital (MO) calculations based on density functional theory (DFT) were
performed for perovskite fluoride and hydride. BHHLYP method, where functionals
of Hartree-Fock exchange, Becke exchange and LYP correlation are included,
was selected. 6-31G * basis set was used for magnesium, fluorine and hydrogen,
combined with MINI basis set for potassium. All calculations were performed by
using Firefly program package [19, 20]. The figures of MOs were depicted by
MOLEKEL [21]. Chemical bonding property was characterised by using chemical
bonding rule [22]. The rule is summarised as below.
Chemical Bonding Rule
For molecular orbitals including outer shell electrons, check whether orbital overlap
exists or not.
– With orbital overlap: Covalent.
– Without orbital overlap: Ionic.