Iron-doped lithium niobate crystals are illuminated with a sinusoidal light pattern (period length 374 nm) at a temperature of about 180°C. Electrons are redistributed and ions drift in the electronic space-charge pattern (‘thermal fixing’). At room temperature the ions are almost immobile. A density grating (space-charge field plus inverse-piezoelectric effect) and the ionic grating yield a refractive-index modulation for neutrons (‘photorefractive effect’). Neutrons (wavelength 1.39 nm) are diffracted from this grating with an efficiency up to $1.2\times10^{-3}$ . Usually hydrogen ions form the ionic grating. The ions responsible for charge compensation in dehydrated crystals have a much \smaller coherent neutron-scattering length and might be identified as lithium.