The search for new thermoelectric (TE) oxide materials has been driven by the discovery of high figures of merit ZT in complex layered cobaltates such as Na(x)CoO2 or Ca3Co4O9. Relying on abundant and non-toxic transition metal elements, these materials can be an alternative to currently used TEs such as PbTe or Bi2Te3. Complex relaxor ferroelectric oxides crystallizing in the tetragonal tungsten bronze (TTB) structure are a new class of TEs, of which the prototypical example, Sr(x)Ba(1-x)Nb2O6 (SBN), shows promising ZT values. We wish to use E2 to study the crystal structure and the diffuse scattering in another TTB, Ca0.3Ba0.7Nb2O6 and its reduced variant Ca0.3Ba0.7Nb2O6-delta, around its para-to-ferroelectric transition temperature, as it was recently shown for SBN that the transition is driven by diffuse scattering related to polar nanoregions. This study complements a TAS study on V2.