Unique Nanomechanical Properties of Diamond–Lonsdaleite Biphases: Combined Experimental and Theoretical Consideration of Popigai Impact Diamonds

For the first time, lonsdaleite-rich impact diamonds from one of the largest Popigai impact crater (Northern Siberia) with a high concentration of structural defects are investigated under hydrostatic compression up to 25 GPa. It is found that, depending on the nature of a sample, the bulk modulus for lonsdaleite experimentally obtained by X-ray diffraction in diamond-anvil cells is systematically lower and equal to 93.3–100.5% of the average values of the bulk moduli of a diamond matrix. Density functional theory calculations reveal possible coexistence of a number of diamond/lonsdaleite and twin diamond biphases. Among the different mutual configurations, separate inclusions of one lonsdaleite (001) plane per four diamond (111) demonstrate the lowest energy per carbon atom, suggesting a favorable formation of single-layer lonsdaleite (001) fragments inserted in the diamond matrix. Calculated formation energies and experimental diamond (311) and lonsdaleite (331) powder X-ray diffraction patterns indicate that all biphases could be formed under high-temperature, high-pressure conditions. Following the equation of states, the bulk modulus of the diamond (111)/lonsdaleite (001) biphase is the largest one among all bulk moduli, including pristine diamond and lonsdaleite.本研究首次针对西伯利亚北部波皮盖陨石坑（Popigai impact crater）这一巨型陨石坑中富含朗斯代尔石（lonsdaleite）且结构缺陷浓度较高的冲击钻石，开展了最高压力达25 GPa的静水压压缩实验研究。研究发现，根据样品特性的差异，通过金刚石压腔（diamond-anvil cells）内X射线衍射（X-ray diffraction）实验测得的朗斯代尔石体积模量（bulk modulus）系统性偏低，仅为钻石基体体积模量平均值的93.3%~100.5%。密度泛函理论（density functional theory）计算结果显示，多种钻石/朗斯代尔石双相结构及孪晶钻石双相结构均可能共存。在多种共存构型中，每4个钻石(111)晶面对应1个朗斯代尔石(001)晶面的独立包裹体结构的单位碳原子能量最低，这表明嵌入钻石基体的单层朗斯代尔石(001)片段更易于形成。计算得到的形成能与实验测得的钻石(311)、朗斯代尔石(331)粉末X射线衍射图谱表明，所有双相结构均可在高温高压条件下形成。根据状态方程（equation of states），钻石(111)/朗斯代尔石(001)双相结构的体积模量在所有测试体系的体积模量中最大，涵盖纯相钻石与纯相朗斯代尔石。