Synthesis and structural investigations of tantalates phosphates in the flux system Na2O-P2O5-Ta2O5-MoO3

The complex sodium-tantalum phosphates (Na
2
Ta
6
P
4
O
26
, Na
3
Ta
7
P
4
O
29
, Na
4
Ta
8
P
4
O
32
) have been synthesized using the flux method in the quasi-quaternary system Na
2
O-P
2
O
5
-Ta
2
O
5
-MoO
3
. These compounds are isostructural with family of sodium-niobium phosphates Na
x
Nb
2m
P
4
O
6m+8
and tightly related to monophosphate tungsten bronzes (MPTB
p
) with pentagonal-shaped tunnels. The structure of these phosphates has been investigated by X-ray powder and single crystal diffraction. Their three-dimensional frameworks consist of slabs of corner-shared TaO
6
distorted octahedra connected via isolated PO
4
tetrahedra and delimit pentagonal tunnels running along c, where Na atoms are located. The solid state band gap measurements gave reasonable prospects for its photocatalytic activity.