黑柱石的高压相变及其压缩性研究

Phase Transitions and Compressibility of Ilvaite at High Pressures

利用金刚石压腔(DAC)和原位同步辐射X射线衍射(XRD)技术,研究天然黑柱石单晶室温下的高压结构稳定性和压缩性,实验最高压力为39.5 GPa。实验中观察到黑柱石在19.2 GPa时从斜方晶系α相(空间群Pnam)转变为单斜晶系β相(空间群P2_(1)/a),转变过程中晶胞体积没有明显的不连续变化。利用Birch-Murnaghan状态方程,对黑柱石单胞体积随压力的变化关系(P-V)进行拟合,得到α相黑柱石状态方程参数为K_(0)=118(4)GPa,V0=671.9(5)Å^(3),K_(0)′=6.6(7);β相黑柱石初始相变压力P_(r)=19.2 GPa,状态方程参数为K_(r)=157(3)GPa,V_(r)=599.3(6)Å^(3),K_(r)′=4(固定)。此外,确定了α相和β相黑柱石的轴压缩系数,并从化学组成和晶体结构压缩性各向异性的角度讨论相变机理。

The high-pressure phase stability and compressibility of a natural orthorhombic symmetry ilvaite are investigated up to 39.5 GPa at room temperature using diamond anvil cell(DAC)combined with in-situ synchrotron radiation single-crystal X-ray diffraction.Experimental results show that ilvaite undergoes a structural transition from an orthorhombicα-phase(Pnam)to a monoclinicβ-phase(P2_(1)/a)at 19.2 GPa.This structural transformation fromαtoβphase is displaced without any obvious discontinuity in volume across the transition.The pressure-volume data of ilvaite are described by the third-order Birch-Murnaghan equation of state,yielding K_(0)=118(4)GPa,V_(0)=671.9(5)Å^(3),K_(0)′=6.6(7)forα-ilvaite and K_(r)=157(3)GPa,V_(r)=599.3(6)Å^(3),K_(0)′=4(fixed)with a phase transition pressure P_(r)=19.2 GPa forβ-ilvaite.In addition,the axial compression coefficients for bothα-andβ-ilvaites are determined,and the mechanism of the pressure-induced Pnam to P2_(1)/a phase transition is discussed in the views of crystal structure and the anisotropy of compressibility.