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The pressure-induced structural evolution of Ca2SnO4, Sr2SnO4, and Zn2SnO4 has been characterized by powder X-ray diffraction up to 20 GPa using the ALBA synchrotron radiation source and density functional theory calculations. No phase transition was observed in Ca2SnO4 and Zn2SnO4 in the investigated pressure range. The observation in Zn2SnO4 solves contradictions existing in the literature. In contrast, a phase transition was observed in Sr2SnO4 at a pressure of 9.09 GPa. The transition was characterized as from the ambient-condition tetragonal polymorph (space group I4/mmm) to the low-temperature tetragonal polymorph (space group P42/ncm). The linear compressibility of crystallographic axes and room-temperature pressure-volume equation of state are reported for the three compounds studied. Calculated elastic constants and moduli are also reported as well as a systematic discussion of the high-pressure behavior and bulk modulus of M2SnO4 stannates.
RESUMO
We report a joint experimental and theoretical study of the structural, vibrational, and electronic properties of layered monoclinic arsenic sulfide crystals (α-As2S3), aka mineral orpiment, under compression. X-ray diffraction and Raman scattering measurements performed on orpiment samples at high pressure and combined with ab initio calculations have allowed us to determine the equation of state and the tentative assignment of the symmetry of many Raman-active modes of orpiment. From our results, we conclude that no first-order phase transition occurs up to 25 GPa at room temperature; however, compression leads to an isostructural phase transition above 20 GPa. In fact, the As coordination increases from threefold at room pressure to more than fivefold above 20 GPa. This increase in coordination can be understood as the transformation from a solid with covalent bonding to a solid with metavalent bonding at high pressure, which results in a progressive decrease of the electronic and optical bandgap, an increase of the dielectric tensor components and Born effective charges, and a considerable softening of many high-frequency optical modes with increasing pressure. Moreover, we propose that the formation of metavalent bonding at high pressures may also explain the behavior of other group-15 sesquichalcogenides under compression. In fact, our results suggest that group-15 sesquichalcogenides either show metavalent bonding at room pressure or undergo a transition from p-type covalent bonding at room pressure towards metavalent bonding at high pressure, as a precursor towards metallic bonding at very high pressure.
RESUMO
SbPO4 is a complex monoclinic layered material characterized by a strong activity of the nonbonding lone electron pair (LEP) of Sb. The strong cation LEP leads to the formation of layers piled up along the a axis and linked by weak Sb-O electrostatic interactions. In fact, Sb has 4-fold coordination with O similarly to what occurs with the P-O coordination, despite the large difference in ionic radii and electronegativity between both elements. Here we report a joint experimental and theoretical study of the structural and vibrational properties of SbPO4 at high pressure. We show that SbPO4 is not only one of the most compressible phosphates but also one of the most compressible compounds of the ABO4 family. Moreover, it has a considerable anisotropic compression behavior, with the largest compression occurring along a direction close to the a axis and governed by the compression of the LEP and the weak interlayer Sb-O bonds. The strong compression along the a axis leads to a subtle modification of the monoclinic crystal structure above 3 GPa, leading from a 2D to a 3D material. Moreover, the onset of a reversible pressure-induced phase transition is observed above 9 GPa, which is completed above 20 GPa. We propose that the high-pressure phase is a triclinic distortion of the original monoclinic phase. The understanding of the compression mechanism of SbPO4 can aid to improve the ion intercalation and catalytic properties of this layered compound.
RESUMO
Lanthanum orthovanadate (LaVO4) is the only stable monazite-type rare-earth orthovanadate. In the present paper the equation of state of LaVO4 is studied using in situ high-pressure powder diffraction at room temperature, and ab initio calculations within the framework of the density functional theory. The parameters of a second-order Birch-Murnaghan equation of state, i.e. those fitted to the experimental and theoretical data, are found to be in perfect agreement - in particular, the bulk moduli are almost identical, with values of 106â (1) and 105.8â (5)â GPa, respectively. In agreement with recent reported experimental data, the compression is shown to be anisotropic. Its nature is comparable to that of some other monazite-type compounds. The softest compression direction is determined.
RESUMO
Dysprosium orthovanadate, DyVO4, belongs to a family of zircon-type orthovanadates showing a phase transition to scheelite-type structures at moderate pressures below 10 GPa. In the present study, the equations of state (EOSs) for both these phases were determined for the first time using high-pressure x-ray diffraction experiments and ab initio calculations based on the density functional theory. Structural parameters for scheelite-type DyVO4 were calculated from x-ray powder diffraction data as well. The high-pressure experiments were performed under pseudo-hydrostatic conditions at pressures up to 8.44 GPa and 5.5 GPa for the stable zircon-type and metastable (quenched) scheelite-type samples, respectively. Assuming as a compression model the Birch-Murnaghan EOS, we obtained the EOS parameters for both phases. The experimental bulk moduli (K0) for zircon-type and scheelite-type DyVO4 are 118(4) GPa and 153(6) GPa, respectively. Theoretical equations of state were determined by ab initio calculations using the PBE exchange-correlation energy functional of Perdew, Burke, and Ernzerhof. These calculations provide K0 values of 126.1 GPa and 142.9 GPa for zircon-type and scheelite-type DyVO4, respectively. The reliability of the present experimental and theoretical results is supported by (i) the consistency between the values yielded by the two methods (the discrepancy in K0 is as low as about 7% for each of the studied polymorphs) and (ii) their similarity to results obtained under similar compression conditions (hydrostatic or pseudo-hydrostatic) for other rare-earth orthovanadates, such as YVO4 and TbVO4.
RESUMO
El reciente progreso de la investigación científica ha aumentado notablemente el conocimiento básico, en biología molecular y genética, requerido en la formación de los médicos. Los profesores de Bioquímica en las Facultades de Medicina tienen un gran reto no sólo en transmitirles a los estudiantes una base sólida y actual de nueva información, sino motivarlos hacia la investigación. Una de las estrategias de educación que puede proporcionar este logro, es la adquisición de la información de una manera aprenderhaciendo. Para probar el efecto de una actividad práctica, la Cátedra de la Bioquímica de la Escuela de Medicina Luis Razetti, de la Universidad Central de Venezuela realizó en su curso regular 2005-2006 una actividad práctica piloto para introducir las técnicas básicas en biología molecular. En esta experiencia del laboratorio, los estudiantes pudieron aislar ADN de diversas muestras humanas, realizar una reacción de PCR y visualizar el producto de PCR mediante electroforesis en geles de agarosa. También incluimos información sobre tópicos éticos de la investigación en genómica como consentimientos informados y donación de muestras biológicas a un biobanco. Al final de la actividad, se les solicitó información usando una encuesta voluntaria y anónima, sobre el impacto de la actividad práctica en la adquisición del conocimiento propuesto. Las respuestas de los estudiantes demostraron que la actividad práctica les fue de gran utilidad, expresaron la importancia de esta clase de herramientas educacionales en su proceso de aprendizaje y recomendaron la inclusión de más actividades prácticas en el plan curricular de la asignatura Bioquímica
Recent progress of scientific research has notably increased the basic knowledge in genetic and molecular biology, required by medical doctors. Biochemistry educators in medical schools are challenged not only, to provide students with a strong and updated base with this new information, but also to engage them into the scientific process. One of the newest educational tools that could provide this achievement is the acquisition of information by learning-doing, like laboratory activities. In order to evaluate the impact of laboratory work over medical development a pilot experience was held at the Biochemistry Department of Luis Razetti Medical School, at Universidad Central de Venezuela from 2005 to 2006. Basic techniques in molecular biology were introduced to registered students and they were able to isolate DNA from different human samples, perform a PCR reaction and visualize the PCR product in agarose electrophoresis gel. We also included information about ethical issues regarding information for genetic and genomic research, such an informed consent and donation of biological samples to biobank, very actual and important information for medical doctor.At the end of the activity, the students were asked, by using a voluntary and anonymous survey, about the impact of the laboratory knowledge acquisition. The result of the survey reported, that the students considered these laboratory activities important and useful tools in their learning process. We received their suggestion to include more laboratory activities in the Biochemistry curricular program
