Twinning structure of ferroelastic LaNbO4 and NdNbO4 crystals

Refinement of the crystal structure is made for the room temperature phases of LaNbO4 and NdNbO4. The final R index is 0.0457 for LaNbO4 and 0.0342 for NdNbO4. From precise atomic parameters, the vectors of the shuffles for all atoms out of “the plane of shear” (010) are determined. Since the twins in the crystals are connected with the parents by a new kind of relation, in that they are folded only by a rotation about the b-axis normal to the plane of shear, they are called “twins of the third kind”.