LaNbO4 crystallizes in a tetragonal high-temperature phase with space group I41/a, which transforms to a monoclinic phase upon cooling below 500 °C. The low-temperature phase has space group I2/a (C2/c) with a monoclinic angle β=94.1°. This system serves as a useful model of a ferroelastic transition of the 4/mF2/m type using the notation of Aizu . This transition produces ferroelastic domains, the boundaries between which are parallel to the monoclinic b-axis. The orientation of these boundaries relative to the monoclinic a- and c-axes has been predicted by Sapriel  for all 94 ferroelastic species, and calculations specifically for the LaNbO4 system have been presented by Jian and Wayman .
We present an accurate determination of the boundary orientation in LaNbO4 using selected area electron diffraction. The boundary planes are parallel to the (2 0 –5.10)/(5.10 0 2) planes of the two domains, as opposed to the predictions of Jian and Wayman which indicate that the domain boundaries should be oriented parallel to the (2 0 -4.04)/(4.04 0 2) planes. Our experimental results are in good agreement with the results of a previous study .
Furthermore, we present a simple geometric model for calculating the boundary orientation based only on the unit cell parameters of the monoclinic phase. This model gives a boundary orientation in excellent agreement with our experimental determination.