Magnetic configuration of an epitaxial thin film of U2N3

UN is a prominent “advanced nuclear fuel”, due to its high uranium density and good thermal conductivity. However, its dissolution in water is worse than that of UO2 and is poorly understood. A strong possibility is that there are U6+ ions present, as these are known to be highly soluble, as opposed to the normal U4+ ions that dominate UO2 and are insoluble in water. These U6+ ions could come from U2N3, which is formed at the UN interface on oxidation and contains two U sites. U2N3 orders antiferromagnetically, and experiments on a thin epitaxial film at Diamond in Jan 2019 showed AF order at ~ 73 K with q = 1. No bulk single crystals exist and the phase is hard to stabilize. Since U6+ has no 5f electrons it cannot support magnetic order. We wish to measure the intensities of the first 6 AF reflections to determine the individual moments, and hence whether such a U6+ site exists.

氮化铀（UN）是一种重要的先进核燃料（advanced nuclear fuel），因其具备高铀密度与优异的热导率。然而，其在水中的溶解性劣于二氧化铀（UO₂），且相关机理尚不明晰。一种极具可能性的推测是体系中存在六价铀离子（U⁶+）：已知此类离子水溶性极强，而主导二氧化铀体系的四价铀离子（U⁴+）则难溶于水。此类六价铀离子可能源自二氮化三铀（U₂N₃），该物质在氮化铀（UN）界面氧化时生成，包含两种铀位点。二氮化三铀（U₂N₃）具有反铁磁有序（AF，antiferromagnetic order）特性；2019年1月在钻石光源（Diamond）开展的外延薄膜实验显示，其反铁磁有序温度约为73 K，波矢q=1。目前尚未获得该物相的体相单晶，且该物相难以稳定制备。由于六价铀离子（U⁶+）不含5f电子，无法支撑磁有序结构。本研究拟测量前6个反铁磁（AF）反射峰的强度，以确定各铀位点的磁矩，进而验证此类六价铀位点是否存在。