磷氯铅矿和砷铅矿的原位高压单晶X射线衍射研究

In-situ high-pressure single crystal X-ray diffraction of natural pyromorphite and mimetite

磷氯铅矿和砷铅矿是具有特征磷灰石结构的类质同象磷酸盐矿物,六方晶系,空间群均为P63/m。采用同步辐射光源和金刚石对顶砧技术,对两种矿物进行了室温下的原位高压单晶X射线衍射研究,实验最高压力为11.51 GPa。在实验压力范围内,未观察到样品发生相变。随着压力的升高,晶胞体积逐渐被压缩,其中砷铅矿a轴和c轴的压缩率几乎相等,磷氯铅矿的轴压缩率a>c。但其晶胞参数未出现明显变化。利用三阶Birch-Murnaghan状态方程拟合得到,砷铅矿体积模量K_0=87.4(37)GPa,磷氯铅矿体积模量K_0=52.3(17)GPa。

[Background] Pyromorphite [Pb5(PO4)3Cl] and mimetite [Pb5(AsO4)3Cl] are phosphate minerals with a typical apatite structure [A5(BO4)3X], which have many substitutions of both metal cations and anionic complexes. Therefore, it is important to figure out the effect of substitutions of different elements on compressibility by studying their high pressure structures.[Purpose] This study aims to reveal the crystal structure and compressibility of the natural pyromorphite and mimetite under high pressure.[Methods] High pressure single crystal X-ray diffraction experiments on pyromorphite [Pb5(PO4)3Cl] and mimetite [Pb5(AsO4)3Cl] were performed with the pressure up to 11.51 GPa at ambient temperature. Both the in-situ high pressure synchrotron radiation X-ray diffraction and diamond anvil cell were applied to the experimental tests.[Results] No phase transition of pyromorphite and mimetite was observed within the experimental pressure range. The P-V data was fitted by the third-order Birch- Murnaghan Equations of State (BM-EoS), where K0=52.3(17) GPa, V0=0.638 4(5) nm3, K0'=8.9(6) for pyromorphite and K0=87.4(37) GPa, V0=0.673 3(7) nm3, K0'=1.2(6) for mimetite.[Conclusion] The substitutions of B in BO4 tetrahedron have little effects on its cell parameters and bulk modulus, but have great influences on the axial compressibility.