Lithium-ion conduction in mixed-metal phosphates, LiMVM
III
(PO
4
)
3
[MV=Nb, Ta; M
III
=Al, Cr, Fe], possessing the rhombohedral (R
![[3 with combining macron]](https://www.rsc.org/images/entities/char_0033_0304.gif)
c) NASICON structure has been investigated. Among the phosphates investigated, LiTaAl(PO
4
)
3
exhibits the highest conductivity, σ≈1.0×10
–2
S cm
–1
at 350 °C (
E
a
=0.47 eV), comparable to the conductivity of LiTi
2
(PO
4
)
3
. Unlike LiTi
2
(PO
4
)
3
which contains lithium-reducible Ti
IV
, LiTaAl(PO
4
)
3
contains stable TaV and Al
III
oxidation states and hence deserves further attention towards tailoring new lithium-ion conductors for application as electrolytes in solid state lithium batteries.