High-pressure polymorphism ofPbF2to 75 GPa

Abstract

Lead fluoride, PbF 2 , was investigated experimentally in the laser-heated diamond anvil cell by x-ray diffraction to pressures of 75 GPa at room temperature and to 64.5 GPa and 2430 K, as well as through first-principles density functional theory calculations up to 70 GPa. During room temperature compression, no discontinuous changes in the x-ray diffraction pattern or volume were observed, but the lattice parameters display highly anomalous trends between 10-22 GPa with enhanced compressibility along the a direction and reduced or even negative compressibility along b and c. Theoretical calculations of valence electron densities at 22 GPa show that α-PbF 2 has undergone a pressure-induced isosymmetric phase transition to a postcotunnite Co 2 Si-type structure and also reveal the detailed atomic rearrangements associated with the development of an extra Pb-F bond in the high-pressure phase. Our x-ray results and theoretical calculations are consistent with an isosymmetric phase transition occurring over 10- Upon heating above 1200 K at pressures at or above 25.9 GPa, PbF 2 partially transforms to the hexagonal Ni 2 In-type phase but wholly or partially reverts back to Co 2 Si-type phase upon temperature quench. From 43-65 GPa, nearly complete transformation to the Ni 2 In-type PbF 2 is observed at high temperature, but the material partially transforms back to the orthorhombic phase upon temperature quench. Our results show that high-pressure behavior of PbF 2 is distinct from that of the alkaline earth fluorides with similar ionic radii. Our results also have relevance to understanding the behavior of lanthanide and actinide dioxides, which have been predicted theoretically to exhibit similar isosymmetric transitions at Mbar pressures.

High-Pressure Phase Transitions of Zinc Difluoride up to 55 GPa

“…All transition metal difluorides that adopt the CaF2 structure at ambient conditions transform to the cotunnite structure (PbCl2, Pnma, Z = 4) at relatively low pressures (< 10 GPa). 7,[13][14][15][16] This structure is characterized be an nine-fold coordination of M 2+ in the form of a tricapped trigonal prism with the three anions capping the rectangular faces. At higher pressures the PbCl2 polytype transforms to an Ni2In structure in which two additional anions enter the coordination sphere of M 2+ leading to a 11fold coordination (pentacapped trigonal prism), the highest observed for metal difluorides.…”

“…4,5 One reason for this is the large variation of the M 2+ cationic radius which increases from 0.45 Å for Be 2+ to 1.35 Å for Ba 2+ (Shannon's effective ionic radius for six-fold coordination, ). 6 Another important feature of MF2 systems lies in the range of electronic configurations adopted by M 2+ cations: for group 2 metals they exhibit a closed-shell, for group 12 and 14 a filled sub-shell (d for group 12, s and d for group 14), while transition metal cations have an open d shell.…”

“…Numerous experimental and theoretical investigations were conducted in order to establish the phase sequence for these compounds. [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] The motivation for these studies was the fact that the behaviour of difluorides at these conditions can serve as an analogue of the high-pressure transformations of dioxides, in particular SiO2. 7,8 Moreover high-pressure studies on MF2 systems revealed a surprisingly wide range of structures adopted by these compounds.…”

High Pressure–Temperature Study of MgF2, CaF2, and BaF2 by Raman Spectroscopy: Phase Transitions and Vibrational Properties of AF2 Difluorides

“…21 The phase transition sequence of difluorides at high pressures is strongly dependent on the cation radius. [3][4][5]7,10,12,22 At 300 K, rutile-type difluorides such as MgF 2 with a small cation radius of 0.72 Å undergo a second-order transformation to the orthorhombic CaCl 2 -type structure (Pnnm, Z = 2) at ∼9 GPa and then transforms to the cubic HP-PdF 2 -type structure (Pa3̅ , Z = 4, also misidentified as a "pyrite-type" structure in some studies) at ∼14 GPa. 10,23,24 However, an additional α-PbO 2 -type phase was predicted by theoretical calculation to be stable between CaCl 2 -and HP-PdF 2 -type phases.…”

X-Ray Diffraction under Extreme Conditions at the Advanced Light Source

“…A variation of this involves loose, unpacked sample placed within a gasket hole which is then filled with either liquid or solid pressure medium ( Figure A3b). This type of loading is useful for instance if a sample is highly anisotropic under differential strain [106], as this allows as many grains as possible to be surrounded by pressure media. In the case of a heating experiment, solid pressure media may be used to form a sandwich ( Figure A3c); if this case is too non-hydrostatic, an open-faced configuration may also be used where both solid and gaseous or liquid media are employed to ensure better hydrostaticity ( Figure A3d).…”