neutron diffraction
Neutron diffraction is diffraction done with a beam of neutrons instead of X-rays, and the switch of probe changes what you can see in three deep ways. To do it you need a source of slow (thermal) neutrons — either a nuclear reactor or a spallation source where a particle accelerator knocks neutrons out of a heavy-metal target — because neutrons are not easy to make in bright beams. The neutrons are given a wavelength comparable to atomic spacings (around 1 to 2 angstrom) and scattered off a crystal exactly as X-rays are, obeying Bragg's law, producing a pattern whose peak positions give the unit cell and whose intensities give the atom positions.
The key difference is what the neutron scatters off. X-rays are scattered by the cloud of electrons around each atom, so heavy atoms (many electrons) scatter strongly and light atoms weakly. A neutron has no charge; it is scattered by the tiny NUCLEUS itself, through the nuclear force, and separately by any unpaired electron spins. This has three consequences that no X-ray experiment can match. First, light atoms are no longer overshadowed: hydrogen, lithium, carbon, oxygen scatter neutrons about as strongly as heavy metals, so you can pinpoint hydrogen atoms in a hydride or water molecule that X-rays barely register. Second, different isotopes of one element scatter differently, opening isotope-contrast experiments. Third, the neutron's own magnetic moment lets it diffract off ordered electron spins, so it can reveal magnetic structure.
There are further practical gifts. Because neutrons interact weakly and carry no charge, they penetrate deep into matter — centimetres of steel, not micrometres — so neutron diffraction can non-destructively measure residual stress deep inside a real engineering component, or study a sample inside a furnace or pressure cell. The honest cost is intensity: neutron sources are far weaker than X-ray tubes or synchrotrons, so you generally need larger samples and much longer counting times, and the work can only be done at a handful of large national facilities. Neutron diffraction is not a replacement for X-ray diffraction but a complementary probe you reach for when light atoms, isotopes, magnetism, or deep penetration are the point.
In studying ice, X-rays clearly show where the oxygen atoms sit but hydrogen (a single electron) is almost invisible. Neutron diffraction, for which hydrogen (or better, its isotope deuterium) scatters strongly, locates the hydrogen positions directly and settles how the water molecules are oriented and hydrogen-bonded in the lattice.
Neutrons scatter off nuclei, so light atoms like hydrogen show up strongly where X-rays see almost nothing.
Neutron scattering strength does not climb smoothly with atomic number the way X-ray scattering does — it jumps around erratically from element to element and isotope to isotope. That erratic table is exactly the source of the neutron's unique powers, not a defect.