DIFFRACTION STUDIES OF CRYSTALS AT LOW-TEMPERATURES - CRYSTALLOGRAPHY BELOW 77 K

Presently two well established cooling techniques prevail in crystallography: cold nitrogen-gas-stream cooling, which allows a minimum temperature close to the boiling point of nitrogen, 77 K, and closed-cycle helium refrigeration, which can reach temperatures even below 10 K. Some advantages and disadvantages of the cooling techniques are contrasted and recent technical developments are presented, in particular regarding data collection below 77 K. Three examples of charge-density studies are used to illustrate improvements in results obtained in small moiety crystallography by collecting data at very low temperatures. H8Si8O12 is a case showing how temperature influences radiation damage, KTiO(PO4) is a case of a non-centrosymmetric structure where anharmonic motion was reduced and the charge density was established more convincingly. (ND4)(2)Cu(SO4)(2).6D(2)O is a case of a combined X-ray and neutron (X/N) diffraction study of a Jahn-Teller distorted Cu-II compound in a minimum energy conformation.