Bulletin of the American Physical Society
18th Biennial Intl. Conference of the APS Topical Group on Shock Compression of Condensed Matter held in conjunction with the 24th Biennial Intl. Conference of the Intl. Association for the Advancement of High Pressure Science and Technology (AIRAPT)
Volume 58, Number 7
Sunday–Friday, July 7–12, 2013; Seattle, Washington
Session T7: CM.2 Phase Transitions: Structural Transitions II |
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Chair: Stan Tozer, National High Magnetic Field Laboratory Room: Grand Crescent |
Thursday, July 11, 2013 9:15AM - 9:30AM |
T7.00001: Low-temperature high-pressure spectroscopy in Co$^{2+}$ -doped ZnO: effects of crystal structure and dimensionality Carlos Renero-Lecuna, Rafael Valiente, Jesus Gonzalez, Fernando Rodriguez, Rosa Martin-Rodriguez, Gloria Almonacid-Caballer, Alfredo Segura, Vicente Marin-Borras, Vicente Munoz-Sanjose Wide band-gap semiconductors doped with transition-metal ions have attracted a lot of attention in the past few years. Here we focus on the spectroscopic properties of Co$^{2+}$-doped ZnO in different structural conformations such as single crystal, thin film, nanorods and nanoparticles under different $P-T$ conditions. Optical measurements were obtained by means of Raman and time-resolved spectroscopy using high-pressure techniques. Our main goal is to investigate the pressure-induced Wurtzite-to-Rock-Salt phase transition and how the structural conformation affects the Raman spectra as well as the absorption and emission spectra associated with Co$^{2+}$ -doped ZnO. The effects of quantum confinement (dimensionality) are also investigated through the distinct spectral features observed in the samples as SC, TF, NW and NP helping us to clarify the still-unassigned absorption and emission spectra in terms of electron-phonon couplings and exchange interactions. [Preview Abstract] |
Thursday, July 11, 2013 9:30AM - 9:45AM |
T7.00002: Crystal-field dependence with Co$^{2+}$-F$^{-}$ distance in the perovskite KCoF$_{3}$ Fernando Rodriguez, Jose Barreda-Arg\"ueso, Fernando Aguado, Simon Redfern Perovskite crystals are attractive for structural studies at high pressure in a wide multidisciplinary science. Perovskite or distorted-perovskite oxides are relevant as solid state devices as many of them exhibit properties such as colossal magnetoresistance, exotic magnetism, or even high T$_{\mathrm{C}}$ superconductivity. In geoscience, the understanding of high-pressure posperovskite phase is noteworthy. In this sense, it must be noted that NaCoF$_{3}$ transforms to the postperovskite phase at a moderate pressure ($P =$ 15 GPa) in comparison to oxides [1]. However KCoF$_{3}$ seems to lack this transition as it remains in the perovskite structure up to higher pressure [2]. This work reports a structural study in KCoF$_{3}$ as a function of pressure in the 0-60 GPa range. Its large pressure perovskite stability is noteworthy. Therefore, this is a unique system to establish correlations between crystal and electronic structures in a high-symmetry phase, where Co$^{2+}$ ions are located in a perfect octahedral environment. Here we present an x-ray diffraction study in correlation with single-crystal optical absorption spectroscopy. The variation of the crystal-field strength and Racah parameters of Co$^{2+}$ with the crystal volume (and Co-F distance) up to 60 GPa are presented, discussed and compared with available data in other structures involving oxides and chlorides. [Preview Abstract] |
Thursday, July 11, 2013 9:45AM - 10:00AM |
T7.00003: X-ray diffraction study of amorphous lithium disilicate glass processed under high pressure Altair Soria Pereira, Silvio Buchner, Joao Cardoso De Lima, Naira Maria Balzaretti Lithium disilicate glass (Li$_{2}$O.2SiO$_{2})$ is a very interesting vitreous material, because, besides several important technological applications (e.g. biomaterial to produce prostheses and implants), shows a large difference between the Tg and Tc values. This is important for the investigation of the crystallization mechanism on vitreous phases, as it allows the independent study of the nucleation and growth stages. In this work, we have investigated the effect of high-pressure processing in the induction of structural changes in the amorphous phase. This can give place to polyamorphism and/or to the generation of possible seeds for a crystalline phase nucleation. Using toroidal type high-pressure chambers, glass samples were processed at 2.5 GPa, 4 GPa, 6 GPa and 7.7 GPa at room temperature. Synchrotron X-ray diffraction was used to obtain the radial distribution functions (RDF), in order to follow the structural changes at different ranges. Compared to a pristine sample, the main change observed for the samples processed up to 6 GPa was associated to the distortion of the SiO$_{4}$ tetrahedra, as already observed in the literature. However, for the sample processed at 7.7 GPa, we have identified a drastic change in the RDF which points for the production of a different amorphous phase with a local structure closer to that observed for the Li$_{2}$SiO$_{3}$ crystalline phase. [Preview Abstract] |
Thursday, July 11, 2013 10:00AM - 10:15AM |
T7.00004: Cs in high oxidation states and as a p-block element Maosheng Miao A long time doctrine rooted in the atomic shell model states that the atoms maintain a complete inner shell while interacting with other atoms. Therefore, group IA elements, for example, are invariably stable in the $+$1 charge state because the p electrons of their inner shells do not react with other chemical species. This general rule governs our understanding of the structures and reactions of matter and has never been challenged. In this work, I will show for the first time a striking result that while mixing with fluorine under pressure, Cs atoms will share their 5p electrons and become oxidized to a higher charge state. It forms stable CsF$_{\mathrm{n}}$ compounds, consisting of neutral or ionic CsF$_{\mathrm{n}}$ molecules. Their geometry and bonding resemble isoelectronic XeF$_{\mathrm{n}}$ molecules, showing that Cs chemically behaves like a p-block element. The stability of these remarkable compounds will change the understanding of the periodic table and chemistry of elements. The work was done by using an automatic crystal structure search based on particle swarm optimization algorithm and the first principles total energy calculations. [Preview Abstract] |
Thursday, July 11, 2013 10:15AM - 10:30AM |
T7.00005: The consequences of the pressure-induced \textit{spinel }-- \textit{post spinel} transition upon the electronic properties of MgFe$_{2}$O$_{4}$ Mark Nikolaevsky, Eran Grinberg, Weiming Xu, Gregory Kh. Rosenberg, Moshe P. Pasternak, Aviva Malchior Similar to magnetite the ferrimagnetic magnesioferrite (SG \textit{Fd3m} T$_{N} =$ 710 K) is a an \textit{inverse} spinel (Fe$^{3+}$)$_{A}$(Mg$^{2+}$Fe$^{3+}$)$_{B}$ in which the $A$ and $B$ are 4- and 6- coordinated moieties, respectively. At $\sim$ 25 GPa a 1$^{st}$ order structural transition takes place into a denser, the so called, post-spinel (\textit{ps}) structure. The few studies carried out so far concluded that the \textit{ps} structure is either of the CaMn$_{2}$O$_{4}$ or CaTi$_{2}$O$_{4}$ -type. Our preliminary HP M\"{o}ssbauer studies (MS) have unambiguously revealed two Fe sites contradicting the one Fe-site typical of the CaMn$_{2}$O$_{4}$ or the CaTi$_{2}$O$_{4}$ polymorphs. This presentation describes the evolution of the electronic/magnetic properties of the \textit{ps} phase of MgFe$_{2}$O$_{4}$ up to 80 GPa. Experiments at variable-T were carried out with MS and electrical resistance $R$ both at compression and decompression. At 47 GPa (\textgreater $P_{s\to ps})$ the M\"{o}ssbauer spectra reveals a paramagnetic state at RT composed of two -quadrupole-split sites which magnetically orders at \textit{T \textless T}$_{N}$ ($=$ 220 K). Two fully magnetic split components are present at T \textless 53 K. $R(P$, 300K) $ R(P)$ increases sharply reaching a peak at 20 GPa and drops continuously by 10$^{-6}$ at $P$ \textgreater 50 GPa. By decompression to $\sim$ 4 GPa the electronic/magnetic properties remain typical of the \textit{ps} phase. [Preview Abstract] |
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