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.