**2.4 X-Ray crystallography**

A colorless platelet crystal of [(*n*-C4H9)4N]4[α-PW11{Al(OH2)}O39] (0.16 0.16 0.01 mm3) was mounted on a MicroMount. All measurements were made on a Rigaku VariMax with a Saturn diffractometer using multi-layer mirror monochromated Mo Kα radiation (λ= 0.71075 Å) at 93 K. Data were collected and processed using CrystalClear for Windows, and structural analysis was performed using the CrystalStructure for Windows. The structure was solved by SHELXS-97 (direct methods) and refined by SHELXL-97 (Sheldrick, 2008). Since one aluminum atom was disordering over twelve tungsten sites in [α-PW11{Al(OH2)}O39]4-, the occupancies for the aluminum and tungsten sites were fixed at 1/12 and 11/12 throughtout the refinement. Four tetra-*n*-butylammonium ions could not be modelled with disordered atoms. Accordingly, the residual electron density was removed using the SQUEEZE routine in PLATON (Spek, 2009).
