Deviation from Debye-Waller behavior in single crystalline freestanding <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:mrow><mml:mi>Ni</mml:mi><mml:mi mathvariant="normal">O</mml:mi></mml:mrow></mml:m

We realize single-crystal freestanding $\mathrm{Ni}\mathrm{O}$ films on $\mathrm{Si}\mathrm{N}$ membranes, enabling thin-film scattering experiments with little or no substrate background signal. These membrane samples provide a critical platform for performing ultrafast electron diffraction (UED) to probe the response of the lattice to laser excitation. We show that epitaxial $\mathrm{Ni}\mathrm{O}$ films on a water-soluble ${\mathrm{Sr}}_{2}{\mathrm{Ca}\mathrm{Al}}_{2}{\mathrm{O}}_{6}$ buffer layer on ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_{3}$ substrates can be transferred as freestanding films to the $\mathrm{Si}\mathrm{N}$ membranes while maintaining the epitaxial character of the films. An above-gap photoexcitation ($\ensuremath{\lambda}=266\phantom{\rule{0.2em}{0ex}}\mathrm{nm}$) incident on the sample stimulates an initial lattice response of the order of 1.1 ps, followed by the persistence of this state for more than 20 ps. We observe typical Debye-Waller behavior in peaks with $q$ above the (220) family, but a distinct deviation from this behavior is seen in low-$q$ peaks. We relate this behavior to a dynamic decrease in sample mosaicity in response to the optical pump of the order of 3% that endures for approximately $4\phantom{\rule{0.2em}{0ex}}\mathrm{ps}$. We suspect that this dynamic mosaicity is due to laser-induced strain propagation. This work demonstrates the viability of the thin-film transfer method for UED and paves the way for the study of the dynamic response of antiferromagnetic thin-film oxides.