The exploration of Li+A+Mo+P+O systems has allowed several Mo(V) phosphates, Li 4؊x A x (MoO) 3 (PO 4 ) 3 P 2 O 7 (A ؍ Na, Ag, Cs, K) and Li 3 Ba 0.5 (MoO) 3 (PO 4 ) 3 P 2 O 7 , to be synthesized. Two types of closely related structures are evidenced depending on the nature and content of the A cation. The type I structure has been re5ned from single-crystal X-ray re5nements for Li 3 Ba 0.5 (MoO)
ribbons forming six-sided Z-shaped tunnels. This structure is also observed for other phosphates, Li 4؊x A x (MoO) 3 (PO 4 ) 3 P 2 O 7 with A ؍ Na, K, Cs, Ag, but it is associated with a certain order+disorder phenomena, evidenced by the existence of streaks in ED patterns. The type II structure, observed for Li 2.5 K 1.5 (MoO) 3 (PO 4 ) 3 P 2 O 7 , exhibits the same [Mo 2 P 2 O 15 ] slices as type I, and di4ers only from the latter by the positions of the [Mo 2 P 6 O 25 ] ribbons which interconnect the [Mo 2 P 2 O 15 ]
slices. For this compound, streaks are also observed in ED patterns. Taking into consideration the close relationship between the two structures, a model based on the coexistence of the two structures in the same matrix (99intergrowth:: defects) is proposed to explain the disorder phenomena. Magnetic measurements con5rm the only presence of Mo(V). Ionic conductivity, due to lithium mobility, is evidenced for several phosphates with values ranging from 1.3؋10 ؊ 5 to 4.1؋10 ؊ 5 ( /cm) ؊ 1 at 3003C, and E a ranging from 0.73 to 0.89 eV.