Ionic diffusion mastering using crystal-chemistry parameters: τ-Cu1/2Ag1/2V2O5 structure determination and comparison with refined δ-AgxV2O5 and ε-CuxV2O5 ones

τ-Ag1/2Cu1/2V2O5 compound crystallises in the monoclinic system space group C2/m with cell parameters a=11.757(4) Å, b=3.6942(5) Å, c=9.463(2) Å, and β=114.62(2)°. The structure is build up with V4O10 D4 double layer. The silver and copper ions are located in two different oxygenated tunnels. Examination of electronic density maps shows that while the silver ions are located in defined crystallographic sites, the copper ones are fully delocalised over the whole tunnel. Comparison with δ-AgxV2O5 and ε-CuxV2O5 refined structure allows to define crystal chemistry parameters governing the ionic delocalisation and give clues to predict from structural consideration the expected electrical behaviour with the aim to make possible a structural design to enhance guest species reactivity.