Przemyslaw Dera

Alumina (α-Al2O3) has been widely used as a pressure calibrant in static high-pressure experiments and as a window material in dynamic shock-wave experiments; it is also a model material in ceramic science. So understanding its high-pressure stability and physical properties is crucial for interpreting such experimental data, and for testing theoretical calculations. Here we report an in situ X-ray diffraction study of alumina (doped with Cr3+) up to 136 GPa and 2,350 K. We observe a phase transformation that occurs above 96 GPa and at high temperatures. Rietveld full-profile refinements show that the high-pressure phase has the Rh2O3 (II) (Pbcn) structure, consistent with theoretical predictions. This phase is structurally related to corundum, but the AlO6 polyhedra are highly distorted, with the interatomic bond lengths ranging from 1.690 to 1.847 Å at 113 GPa. Ruby luminescence spectra from Cr3+ impurities within the quenched samples under ambient conditions show significant red shifts and broadening, consistent with the different local environments of chromium atoms in the high-pressure structure inferred from diffraction. Our results suggest that the ruby pressure scale needs to be re-examined in the high-pressure phase, and that shock-wave experiments using sapphire windows need to be re-evaluated.

ABSTRACT Double-sided laser heating combined with synchrotron x-ray radiation for in-situ studies in the DAC using diffraction, emission and inelastic scattering methods has been the most productive and widely used high temperature-high pressure technique in past two decades. Equation of state, phase transformations, element partitioning, electronic and optical properties of various materials have been successfully studied at conditions relevant to the Earth's interior with help of lasers. High temperature data collected in the DAC are mostly consistent, however, there are some discrepancies in reported results among high pressure research groups performing experiments at different facilities, particularly for determinations of melting temperatures, Clapeyron slopes, elastic constants, and thermal expansion coefficients. Although differences in the samples themselves cannot be ruled out, an important contributor to inconsistent results is related to temperature non-uniformity in the analyzed volume. Here we report a new development in on-line, double-sided, laser heating systems based on diode pumped fiber lasers coupled with beam-shaping optics that allows control of the shape of the focused laser beam spot on the sample surface in the DAC with variable diameter from 8 to 40 mum. Varying the settings of the laser heating system, we were able to shape the beam to almost any desired intensity profile and size on the surface of the sample in the DAC including tight focus, flat top, trident and doughnut types. The significant advantages and excellent performance of the flat top laser heating (FTLH) technique will be demonstrated in melting experiments on germanium and iron compounds. During FT laser heating the molten sample doesn't escape from the homogeneously heated area as is usually observed for Gaussian or doughnut type laser spots in the DAC. The capability to maintain the molten sample in the DAC for a relatively long time (at least 60 s) allowed us to collect high quality x-ray scattering data suitable for structure analysis even from low-Z molten materials such as Si, SiO2, Fe, Fe:C etc. The FT-LH method opens a new era in high temperature high pressure studies using diamond anvil cell with combination of advanced synchrotron as well as lab techniques, and will lead to superior quality high temperature measurements including equation of state, melting curve, phase transformation, element portioning, elastic, electronic and optical properties.

Alumina (α-Al2O3) has been widely used as a pressure calibrant in static high-pressure experiments and as a window material in dynamic shock-wave experiments; it is also a model material in ceramic science. So understanding its high-pressure stability and physical properties is crucial for interpreting such experimental data, and for testing theoretical calculations. Here we report an in situ X-ray diffraction study of alumina (doped with Cr3+) up to 136 GPa and 2,350 K. We observe a phase transformation that occurs above 96 GPa and at high temperatures. Rietveld full-profile refinements show that the high-pressure phase has the Rh2O3 (II) (Pbcn) structure, consistent with theoretical predictions. This phase is structurally related to corundum, but the AlO6 polyhedra are highly distorted, with the interatomic bond lengths ranging from 1.690 to 1.847 Å at 113 GPa. Ruby luminescence spectra from Cr3+ impurities within the quenched samples under ambient conditions show significant red shifts and broadening, consistent with the different local environments of chromium atoms in the high-pressure structure inferred from diffraction. Our results suggest that the ruby pressure scale needs to be re-examined in the high-pressure phase, and that shock-wave experiments using sapphire windows need to be re-evaluated.

ABSTRACT Double-sided laser heating combined with synchrotron x-ray radiation for in-situ studies in the DAC using diffraction, emission and inelastic scattering methods has been the most productive and widely used high temperature-high pressure technique in past two decades. Equation of state, phase transformations, element partitioning, electronic and optical properties of various materials have been successfully studied at conditions relevant to the Earth's interior with help of lasers. High temperature data collected in the DAC are mostly consistent, however, there are some discrepancies in reported results among high pressure research groups performing experiments at different facilities, particularly for determinations of melting temperatures, Clapeyron slopes, elastic constants, and thermal expansion coefficients. Although differences in the samples themselves cannot be ruled out, an important contributor to inconsistent results is related to temperature non-uniformity in the analyzed volume. Here we report a new development in on-line, double-sided, laser heating systems based on diode pumped fiber lasers coupled with beam-shaping optics that allows control of the shape of the focused laser beam spot on the sample surface in the DAC with variable diameter from 8 to 40 mum. Varying the settings of the laser heating system, we were able to shape the beam to almost any desired intensity profile and size on the surface of the sample in the DAC including tight focus, flat top, trident and doughnut types. The significant advantages and excellent performance of the flat top laser heating (FTLH) technique will be demonstrated in melting experiments on germanium and iron compounds. During FT laser heating the molten sample doesn't escape from the homogeneously heated area as is usually observed for Gaussian or doughnut type laser spots in the DAC. The capability to maintain the molten sample in the DAC for a relatively long time (at least 60 s) allowed us to collect high quality x-ray scattering data suitable for structure analysis even from low-Z molten materials such as Si, SiO2, Fe, Fe:C etc. The FT-LH method opens a new era in high temperature high pressure studies using diamond anvil cell with combination of advanced synchrotron as well as lab techniques, and will lead to superior quality high temperature measurements including equation of state, melting curve, phase transformation, element portioning, elastic, electronic and optical properties.