The high-pressure phase transition sequence from the rutile-type through to the cotunnite-type structure in

Abstract

Four phase transitions were observed in under pressure up to 47 GPa using x-ray diffraction in a diamond anvil cell. At close to 4 GPa, rutile-structured underwent a second-order transition to an orthorhombic, -type phase. Above 7 GPa, this -type phase transformed to a cubic phase with a modified fluorite structure. A transition to an orthorhombic phase was observed at 11.4 GPa. The intensities of the diffraction lines in this orthorhombic phase indicate a displacement of the lead ions from the fcc positions occupied in the cubic phase. This orthorhombic phase has similar cell constants (a = 10.027(2) Å, b = 5.246(1) Å, and c = 5.116(1) Å at 26 GPa) to the orthorhombic I phase of and also that of , and could be isostructural. A further transition began above 29 GPa to a cotunnite-type phase, with space group Pnam, and Z = 4, and with a = 5.443(18) Å, b = 6.346(17) Å, and c = 3.368(8) Å at 47 GPa. The coordination number of the lead ion is 6 in the first two phases, 6 + 2 in phase III, most probably 7 in phase IV and 9 in phase V. The volume decreases observed in the three first-order transitions are 6.9, 1.4, and 7.5% at 7, 11.4, and 29 GPa, respectively. The two higher-pressure transitions were reversible, whereas the cubic phase transformed to upon decompression, and this was retained down to ambient pressure. This is the first time a cotunnite-type structure has been adopted by a group IVb dioxide, which has implications for the high-pressure behaviour of the homologous compounds with smaller cations: and .

Equation of state of the postperovskite phase synthesized from a natural (Mg,Fe)SiO 3 orthopyroxene

“…Despite being the high-pressure phase, it is possible that ppv has a lower bulk modulus than pv at deep mantle pressures. Reduction in bulk modulus across highpressure phase transitions has been observed in a number of oxide systems in which there are additional compression mechanisms available in the high-pressure phase (21). The enhanced compressibility along the b-axis may be responsible for this effect in the ppv phase.…”

Pressure-induced transformations of cristobalite

“…Another reason, why CaCl 2 -type phase did not appear in our experiments could be that in electrically heated DAC, we had practically stress-free conditions, while the stishovite ± CaCl 2 -type silica phase transition is very sensitive to stress conditions [17]. It is also possible that CaCl 2 -structured silica is a metastable phase similarly to the same polymorph of PbO 2 [22]. Further studies are required to resolve this issue.…”

Исследование Механических, Электронных И Колебательных Свойств Оксидов Свинца Из Первых Принципов

“…В зависимости от температуры [15][16][17][18], давле-ния [19][20][21][22][23][24] или условий синтеза [25][26][27][28] оксиды свинца обладают известным полиморфизмом [29][30][31][32]. Так, моно-оксид свинца существует в двух полиморфных формах: красный, тетрагональной модификации (глет, α-PbО), стабильный при комнатной температуре, и желтый, ромбической модификации (массикот, β-PbО), который стабилен при температуре выше 760 K и метастабилен при комнатной температуре.…”

“…Кристаллическая структура диоксида свинца изу-чалась в [23,39,40], отдельно α-PbO 2 в [41] и β-PbO 2 в [15,42]. Пространственная группа симмет-рии α-PbO 2 Pbcn, Z = 4, a = 4.9801 ¦ , b = 5.9588 ¦ , c = 5.4812 ¦ [41].…”

“…Четыре фазовых перехода под давлением до 47 GPa наблюдались в PbO 2 с использованием рентгеновской дифракции и алмазной ячейки в [23]. Ближе к 4 GPa структура рутила претерпевает фазовый переход второго рода в орторомбическую фазу PbO 2 -I со структурой Механические свойства оксидов свинца изучены недо-статочно подробно, несмотря на то что они активно используются при создании различных стекол и, как показано в [3], влияют на такие их характеристики как скорость звука или упругие модули.…”