LaI: An Unprecedented Binary Rare Earth Metal Monohalide with a NiAs-Type Structure

摘要

For more than 30 years, considerable effort has been devoted to the discovery and characterization of binary halides M_aX_b rare earth metals in low oxidation states. Dihalide phases with X = Cl, Br, and I are known for a majority of the rare earth metals. These exhibit saltlike characteristics except t'or the metallic diiodides of La. Ce. Pr, Gd, and Sc. Other reduced halides M_2X_3 and M_2X_3 seem to be more limited and elusive. Nevertheless, the quest for binary monohalides of the rare earth metals has witnessed a tumultuous history. Numerous reports of supposed binary rare earth monohalides, in which the heavy atom arrangement is the same as in the truly binary phases ZrCl and ZrBr, were later shown to be MXH, phases ( approx =0.67 <= n <= 1) stabilized by hydrogen ato-ns in the tetrahedral interstices bt-tween the double layers. As a result, it has been generally concluded that binary monohalide phases of these elements do not exisx But if a monohalide were to exist, chemical experience wouldpredict that the larger, more polarizable iodide should provide the most stable compound either as a conventional salt or, more likelv, as a metallic phase like Lal_2 .