In engineering, the most important neutron absorber is B, used as boron carbide in nuclear reactor control rods or as boric acid as a coolant water additive in pressurized water reactors. Other neutron absorbers used in nuclear reactors are xenon, cadmium, hafnium, gadolinium, cobalt, samarium, titanium, dysprosium, erbium, europium, molybdenum and ytterbium. All of these occur in nature as mixtures of various isotopes, some of which are excellent neutron absorbers. They may occur in compounds such as molybdenum boride, hafnium diboride, titanium diboride, dysprosium titanate and gadolinium titanate.