Synthesis, crystal structures and ionic conductivities of Bi14P4O31 and Bi50V4O85. Two members of the series Bi18−4mM4mO27+4m (M=P, V) related to the fluorite-type structure

Abstract The two hitherto unknown compounds Bi 14 P 4 O 31 and Bi 50 V 4 O 85 were prepared by the direct solid-state reaction of Bi 2 O 3 and (NH 4 )H 2 PO 4 or V 2 O 5 , respectively. Bi 14 P 4 O 31 crystallizes in a C -centred monoclinic symmetry ( C 2/ c space group) with the unit-cell parameters: a = 19.2745 ( 2 ) A , b = 11.3698 ( 1 ) A , c = 52.4082 ( 2 ) A and β = 93.63 ( 1 ) ° ( Z = 16 ). The symmetry of Bi 50 V 4 O 85 is also monoclinic ( I 2/ m space group) with lattice parameters of a = 11.8123 ( 3 ) A , b = 11.7425 ( 2 ) A , c = 16.5396 ( 2 ) A and β = 90.14 ( 1 ) ° ( Z = 2 ). Both structures correspond to a fluorite-type superstructure where the Bi and P or V atoms are ordered in the framework. An idealized structural model is proposed where the structures result of the stacking of mixed atomic layers of composition [Bi 14 M 4 O 31 ] and [Bi 18 O 27 ] respectively. This new family can be formulated Bi 18−4 m M 4 m O 27+4 m with M =P, V and where the parameter m ( 0 ⩽ m ⩽ 1 ) represents the ratio of the number of [Bi 14 M 4 O 31 ] layers to the total number of layers in the sequence. Bi 14 P 4 O 31 corresponds to m = 1 when Bi 50 V 8 O 85 corresponds to m = 1 / 3 . In this last case, the structural sequence is simply one [Bi 14 V 4 O 31 ] layer to two [Bi 18 O 27 ] layers. As predicted by the proposed structural building principle, Bi 14 P 4 O 31 is not a good ionic conductor. The conductivity at 650 °C is 4 orders of magnitude lower from those found in Bi 46 M 8 O 89 ( M =P, V) ( m = 2 / 3 ) and Bi 50 V 4 O 85 ( m = 1 / 3 ).