Angel R J, Bismayer U, Marshall W G
Crystallography Laboratory, Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24060, USA.
Acta Crystallogr B. 2004 Feb;60(Pt 1):1-9. doi: 10.1107/S0108768103026582. Epub 2004 Jan 21.
Pure lead phosphate, Pb(3)(PO(4))(2), undergoes a phase transition from C2/c to R(-)3m symmetry at a pressure of approximately 1.8 GPa and room temperature. Single-crystal X-ray diffraction measurements of the unit-cell parameters of a sample doped with 1.6% Ba(2+) for the Pb(2+) indicates that the doping reduces the transition pressure by approximately 0.1 GPa. The structural evolution of both samples through the phase transition has been determined by Rietveld refinement of neutron powder diffraction data collected to pressures of 6.3 and 3.3 GPa, respectively. There is no evidence for any significant change in the local structure at the phase transition at high pressures; the structure of the R(-)3m phase at pressures just above the phase transition includes disordered positions for several atoms. The observation of diffuse scattering from the R(-)3m phase at high pressure by single-crystal X-ray diffraction suggests that the disorder is static and arises from the presence of several orientations of the ordered microdomains of the monoclinic local structure. The macroscopic transition from monoclinic to trigonal symmetry therefore appears to correspond to the pressure at which the coherency strains between the locally monoclinic microdomains are sufficient to create a dimensionally trigonal lattice within which local displacements of atoms are still significant. A further pressure increase then decreases the magnitude of these displacements until at 3.5 GPa or higher they are not detectable by our current experimental probes, and the structure appears to have true local and global trigonal symmetry.
纯磷酸铅Pb₃(PO₄)₂在室温及约1.8吉帕的压力下会发生从C2/c到R-3m对称性的相变。对掺杂1.6% Ba²⁺替代Pb²⁺的样品进行单晶X射线衍射测量其晶胞参数，结果表明掺杂使转变压力降低了约0.1吉帕。通过对分别收集至6.3吉帕和3.3吉帕压力下的中子粉末衍射数据进行Rietveld精修，确定了两个样品在相变过程中的结构演变。没有证据表明在高压相变时局部结构有任何显著变化；刚好高于相变压力时R-3m相的结构包括几个原子的无序位置。通过单晶X射线衍射观察到高压下R-3m相的漫散射，这表明无序是静态的，且源于单斜局部结构有序微区的几种取向的存在。因此，从单斜到三角对称性的宏观转变似乎对应于局部单斜微区之间的相干应变足以形成一个三维三角晶格的压力，在该晶格中原子的局部位移仍然很显著。进一步增加压力则会减小这些位移的幅度，直到在3.5吉帕或更高压力下，我们目前的实验探针无法检测到这些位移，此时结构似乎具有真正的局部和全局三角对称性。
