The surface crystallography and chemistry of a LaAlO
3
single crystal, a material mainly used
as a substrate to deposit technol
ogically important thin films (e.g. for superconducting and
magnetic devices), was analysed using surface X-ray diffraction and low energy ion
scattering spectroscopy. The surfa
ce was determined to be terminated by Al-O species, and
was significantly different from th
e idealised bulk structure. Termination reversal was not
observed at higher temperature (600°C) and chamber pressure of 10
-10
Torr, but rather an
increased Al-O occupancy occurred, which was accompanied by a larger outwards relaxation
of Al from the bulk positions. Changing the oxygen pressure to 10
-6
Torr enriched the Al site
occupancy fraction at the outermost surface from 0.245(10) to 0.325(9). In contrast the LaO,
which is located at the next sub-surface atomic
layer, showed no chemical enrichment and the
structural relaxation was lower than for the top AlO
2
layer. Knowledge of the surface
structure will aid the understanding of how and which type of interface will be formed when
LaAlO
3
is used as a substrate as a function of temperature and pressure, and so lead to
improved design of device structures.