High-pressure phase transition of a natural pigeonite
High-pressure phase transition of a natural pigeonite
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天然鸽石的高压相变
DOI：
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发表时间：
2010
期刊：
影响因子：
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通讯作者：
V. Tazzoli
中科院分区：
文献类型：
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作者：
M. Alvaro;F. Nestola;T. B. Ballaran;F. Cámara;M.;C. Domeneghetti;V. Tazzoli
用金刚石压腔对成分约为Wo10En43Fs47的天然鸽子石样品进行了高压和室温单晶X射线衍射研究。晶胞参数是在18个不同的压力下测定的，最高可达6 Gpa。发现P21/c-C2/c在3.5~3.6 Gpa之间发生一级相变，伴随着b型反射(h+k=奇数)的消失和晶胞体积的强烈不连续(约1.7%)。在相变时，观察到一个很小的滞后(~0.3 GPa)。用三阶Birch-Murnaghan状态方程(BM_3-EOS)对低P相的10个P-V数据进行拟合，得到V_0=431.93(2)ä3，K_0=96.8(8)Gpa和K‘=8.5(6)。用二阶Birch-Murnaghan状态方程拟合C2/c高P相的8个P-V数据，得到V0=423.6(1)ä3和Kt0=112.4(8)，表明高P C2/c相明显比低P相更硬。在对同一样品的晶体进行的分离实验中，收集了强度数据，并在13个压力下对晶体结构进行了优化，最高可达9.4 Gpa。低磷P21/c相的M1-O和M2-O平均键长分别减少了0.7%和2.1%。两个不等价的A和B四面体链随着压力的增加而更加扭曲，其角度分别减小了2.2%和5.1%。在转变时，A链改变了旋转感，两条链变得等价和更扭曲，角度进一步减少2.5%，最高可达9.4 Gpa。考虑到结构的几何参数，进行了应变计算，讨论了自发应变和序参数随压力的变化，并与其他组分的现有数据进行了比较。
Abstract High-pressure and room-temperature single-crystal X-ray diffraction (XRD) studies have been performed on crystals of a natural pigeonite sample with composition ca. Wo10En43Fs47 using diamondanvil cells. The unit-cell parameters were determined at 18 different pressures up to about 6 GPa. A first-order P21/c-C2/c phase transition was found between 3.5 and 3.6 GPa, associated with the disappearance of the b-type reflections (h + k = odd) and a strong discontinuity (about 1.7%) in the unit-cell volume. At the transition, a small hysteresis (~0.3 GPa) was observed. A third-order Birch-Murnaghan equation of state (BM3-EoS) fit to the 10 P-V data of the low-P phase yielded V0 = 431.93(2) Å3, KT0 = 96.8(8) GPa and K′ = 8.5(6). A second-order Birch-Murnaghan EoS fit to the 8 P-V data (between 3.6 and 6 GPa) of the C2/c high-P phase yielded V0 = 423.6(1) Å3 and KT0 = 112.4(8), indicating that the high-P C2/c phase is significantly stiffer than the low-P phase. In a separated experiment with crystals of the same sample, intensity data were collected and crystal structures were refined at 13 pressures up to 9.4 GPa. The M1-O and M2-O mean bond lengths of the low-P P21/c phase decrease by 0.7 and 2.1%, respectively. The two non-equivalent A and B tetrahedral chains become more kinked with pressure, with a reduction of their angle by 2.2 and 5.1%, respectively. At the transition the A-chain changes sense of rotation and both chains become equivalent and more kinked, with a further reduction of their angle by 2.5% up to 9.4 GPa. Strain calculations have been performed and the evolution of the spontaneous strain and the order parameter variation with pressure are discussed, considering geometrical parameters of the structure and comparing our results with the available data for other compositions.