下地幔占地球体积一半以上。在高温高压下对下地幔真实组分所开展的矿物学和岩石学实验是了解深部地幔演化过程的必要途径。激光加热的金刚石对顶压砧（laser-heated diamond anvil cell, LHDAC）是开展这类高温高压实验最常用的工具，实验产物通常包括从上百纳米到数微米尺寸不一的大量晶粒所组成的多相集合体。这些下地幔相的晶体结构往往不能在卸压后保存下来，因此必须对它们进行原位表征。相对于同步辐射光源设备中可用的聚焦X射线光斑尺寸（3~5 μm），晶粒尺寸要小一个量级，所得到的X射线衍射（X-ray diffraction, XRD）图谱通常显示为衍射斑点和衍射环的混合。由于多相衍射峰重叠严重，粉晶X射线衍射法无法对新相和弱相进行鉴定，因此采用传统XRD技术测定高温高压下多相混合物非常困难。我们最近在高压研究中所发展的同步辐射多晶X射线衍射法可以通过测定多相组合中成千上百个晶粒各自的晶面取向，使多相体系中单晶的指标化成为可能。一旦完成指标化，多晶混合物中的每一个晶粒都可以看作是单晶。因此，我们可以同时确定多相体系中新相和弱相的晶体结构。随着这一新方法的发展，我们开辟了深下地幔高温高压条件下的结晶学新领域。本文阐述了多相体系研究中的关键技术挑战，并通过高压多相X射线衍射法的成功应用实例，论证了该方法的独特能力。

The lower mantle makes up more than a half of our planet's volume. Mineralogical and petrological experiments on realistic bulk compositions under high pressure–temperature (P–T) conditions are essential for understanding deep mantle processes. Such high P–T experiments are commonly conducted in a laser-heated diamond anvil cell, producing a multiphase assemblage consisting of 100 nm to submicron crystallite grains. The structures of these lower mantle phases often cannot be preserved upon pressure quenching; thus, in situ characterization is needed. The X-ray diffraction (XRD) pattern of such a multiphase assemblage usually displays a mixture of diffraction spots and rings as a result of the coarse grain size relative to the small X-ray beam size (3–5 μm) available at the synchrotron facilities. Severe peak overlapping from multiple phases renders the powder XRD method inadequate for indexing new phases and minor phases. Consequently, structure determination of new phases in a high P–T multiphase assemblage has been extremely difficult using conventional XRD techniques. Our recent development of multigrain XRD in high-pressure research has enabled the indexation of hundreds of individual crystallite grains simultaneously through the determination of crystallographic orientations for these individual grains. Once indexation is achieved, each grain can be treated as a single crystal. The combined crystallographic information from individual grains can be used to determine the crystal structures of new phases and minor phases simultaneously in a multiphase system. With this new development, we have opened up a new area of crystallography under the high P–T conditions of the deep lower mantle. This paper explains key challenges in studying multiphase systems and demonstrates the unique capabilities of high-pressure multigrain XRD through successful examples of its applications.