  We use femtosecond electron diffraction to study ultrafast lattice dynamics
in the highly correlated antiferromagnetic (AF) semiconductor NiO. Using the
scattering vector (Q) dependence of Bragg diffraction, we introduce a
Q-resolved effective lattice temperature, and identify a nonthermal lattice
state with preferential displacement of O compared to Ni ions, which occurs
within ~0.3 ps and persists for 25 ps. We associate this with transient changes
to the AF exchange striction-induced lattice distortion, supported by the
observation of a transient Q-asymmetry of Friedel pairs. Our observation
highlights the role of spin-lattice coupling in routes towards ultrafast
control of spin order.
