Twinning and defects in N-Nb2O5

N-Nb(2)O(5) [C2/m (No. 12), a = 28.51, b = 3.830 and c = 17.48 A, and beta = 124.8 degrees ] has been investigated by means of selected-area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM). N-Nb(2)O(5) is domain twinned, with the twin plane perpendicular to the c* axis. The domains are rather small and the domain twinning can sometimes be best explained as stacking faults. A second type of coherent twinning at an angle of 90 degrees to the other two domain directions was also found. These domains are linked together by areas containing blocks of different sizes, similar to the disordered block arrangement observed in M-Nb(2)O(5).