uranium oxide hydroxide and at high pHs it becomes an anionic hydroxide complex. When carbonate is added, uranium is converted to a series of carbonate complexes if the pH is increased. One effect of these reactions is increased solubility of uranium in the pH range 6 to 8, a fact that has a direct bearing on the long term stability of spent uranium dioxide nuclear fuels. Hydrides, carbides and nitrides Uranium metal heated to reacts with hydrogen to form uranium hydride. Even higher temperatures will reversibly remove the hydrogen. This property makes uranium hydrides convenient starting materials to create reactive uranium powder along with various uranium carbide, nitride, and halide compounds. Two crystal modifications of uranium hydride exist: an α form that is obtained at low temperatures and a β form that is created when the formation temperature is above 250 °C. Uranium carbides and uranium nitrides are both relatively inert semimetallic compounds that are minimally soluble in acids, react with water, and can ignite in air to form . Carbides of uranium include uranium monocarbide (UC), uranium dicarbide (), and diuranium tricarbide (). Both UC and are formed by adding carbon to molten uranium or by exposing the metal to carbon monoxide at high temperatures. Stable below 1800 °C, is prepared by subjecting a heated mixture of UC and to mechanical stress. Uranium nitrides obtained by direct exposure of the metal to nitrogen include uranium mononitride (UN), uranium dinitride (), and diuranium trinitride (). Halides All uranium fluorides are created using uranium tetrafluoride (); itself is prepared by hydrofluorination of uranium dioxide. Reduction of with hydrogen at 1000 °C produces uranium trifluoride (). Under the right conditions of temperature and pressure, the reaction of solid with gaseous uranium hexafluoride () can form the intermediate fluorides of , , and . At room temperatures, has a high vapor pressure, making it useful in the gaseous diffusion process to separate the rare uranium-235 from the common uranium-238 isotope. This compound can be prepared from uranium dioxide and uranium hydride by the following process: + 4 HF → + 2 (500 °C, endothermic) + → (350 °C, endothermic) The resulting , a white solid, is highly reactive (by fluorination), easily sublimes (emitting a vapor that behaves as a nearly ideal gas), and is the most volatile compound of uranium known to exist. One method of preparing uranium tetrachloride () is to directly