标题： 压力诱导的反尖晶石型 Co$_2$TiO$_4$体积塌缩和金属化 显示英文标题

标题： Pressure-Induced Volume Collapse and Metallization in Inverse Spinel Co$_2$TiO$_4$

摘要： 反尖晶石$Co_2TiO_4$ (CTO-Sp) 在高压 (HP) 条件下的结构、振动、电子和磁性性质通过X射线衍射、拉曼光谱、原位光学显微镜和第一性原理密度泛函理论 (DFT) 计算进行了系统研究。 在常压条件下，CTO-Sp 呈现立方相，空间群为$Fd\bar{3}m$，并在高压下经历两次显著的结构相变。 第一次相变发生在约7.2 GPa，导致四四方形-$I4_1/amd$相，单位晶胞体积变化最小。 随后，在约17.3 GPa时观察到一个正交相，其空间群为$Fddd$和$Cmcm$。 第二次结构相变对应于一级相变，单位晶胞体积显著减少约17.5$\%$。 CTO-Sp 及其高压后尖晶石相的体压缩率几乎等于各相内多面体的平均压缩率。 超过23 GPa没有拉曼模式，这是金属化的有力证据。 DFT 计算显示高自旋到低自旋的转变，并伴随着 Cmcm 正交相中局部磁矩的坍塌，导致样品的压力诱导金属化。 显示英文摘要

摘要： The structural, vibrational, electronic, and magnetic properties of inverse spinel $Co_2TiO_4$ (CTO-Sp) under high pressure (HP) conditions are systematically investigated using X-ray diffraction, Raman spectroscopy, in situ optical microscopy, and first-principles density functional theory (DFT) calculations. At ambient conditions, CTO-Sp exhibits a cubic phase with a space group $Fd\bar{3}m$ and it undergoes two notable structural phase transitions at HP. The first transition, occurring at approximately 7.2 GPa, leads to the tetragonal-$I4_1/amd$ phase with minimal alteration in unit cell volume. Subsequently, a second transition to an orthorhombic phase with a mixed space group $Fddd$ and $Cmcm$ is observed around 17.3 GPa. This second structural transition corresponds to a first-order phase transition involving a significant reduction in unit cell volume of approximately 17.5$\%$. The bulk compressibility of CTO-Sp and its HP post-spinel phases is almost equal to the average polyhedral compressibility within each phase. The absence of Raman modes beyond 23 GPa is compelling evidence for metallization. DFT calculations reveal a high-spin to low-spin transition, accompanied by the collapse of local magnetic moments in the Cmcm orthorhombic phase, leading to the sample's pressure-induced metallization.