Experimental demonstration of the physics of resonant cavities

We describe an undergraduate experiment that demonstrates the physics of cavity resonators. A mobile wall lets students alter the position of the nodes, thus changing the mode pattern. The nodal structure is made apparent by placing a metallic plate at different positions inside the cavity. A technique for dielectric characterization also is introduced, which helps students understand the boundary conditions in dielectrics, as well as highlighting the characteristics of fields in cavities

Kinetic modeling of ion conduction in KcsA potassium channel

KcsA constitutes a potassium channel of known structure that shows both high conduction rates and selectivity among monovalent cations. A kinetic model for ion conduction through this channel that assumes rapid ion transport within the filter has recently been presented by Nelson. In a recent, brief communication, we used the model to provide preliminary explanations to the experimental current-voltage J‐V and conductance-concentration g‐S curves obtained for a series of monovalent ions (K+,Tl+, and Rb+). We did not assume rapid ion transport in the calculations, since ion transport within the selectivity filter could be rate limiting for ions other than native K+. This previous work is now significantly extended to the following experimental problems. First, the outward rectification of the J‐V curves in K+ symmetrical solutions is analyzed using a generalized kinetic model. Second, the J‐V and g‐S curves for NH4+ are obtained and compared with those of other ions (the NH4+ J‐V curve is qualitatively different from those of Rb+ and Tl+). Third, the effects of Na+ block on K+ and Rb+ currents through single KcsA channels are studied and the different blocking behavior is related to the values of the translocation rate constants characteristic of ion transport within the filter. Finally, the significantly decreased K+ conductance caused by mutation of the wild-type channel is also explained in terms of this rate constant. In order to keep the number of model parameters to a minimum, we do not allow the electrical distance (an empirical parameter of kinetic models that controls the exponential voltage dependence of the dissociation rate) to vary with the ionic species. Without introducing the relatively high number of adjustable parameters of more comprehensive site-based models, we show that ion association to the filter is rate controlling at low concentrations, but ion dissociation from the filter and ion transport within the filter could limit conduction at high concentration. Although some experimental data from other authors were included to allow qualitative comparison with model calculations, the absolute values of the effective rate constants obtained are only tentative. However, the relative changes in these constants needed to explain qualitatively the experiments should be of [email protected]

Investigation of nitrogen-related acceptor centers in indium selenide by means of photoluminescence: Determination of the hole effective mass

In this work we report on steady-state and time-resolved photoluminescence (PL) measurements in nitrogen doped p-type indium selenide in the 33-210-K temperature range. In samples with low nitrogen concentration the photoluminescence spectrum consists of exciton-related peaks and a band-to-acceptor recombination peak (2.1-μs lifetime) with LO-phonon replica. An ionization energy of 65.5 meV is proposed for the nitrogenrelated acceptor. A long-lived (18 μs) component, which consists of an asymmetric broadband centered around the acceptor peak, has been also detected by means of time-resolved PL. Samples with a higher nitrogen concentration show a PL spectrum that mainly consists of the asymmetric long-lived broadband that can be associated to a complex center. The asymmetric shape of this band is quantitatively accounted for in the framework of the configuration coordinate model for complex centers. Under the assumption that the nitrogen related acceptor is shallow, the Gerlach-Pollman theory allows an estimate of the hole's effective masses

Phase stability of lanthanum orthovanadate at high-pressure

When monoclinic monazite-type LaVO4 (space group P21/n) is squeezed up to 12 GPa at room temperature, a phase transition to another monoclinic phase has been found. The structure of the high-pressure phase of LaVO4 is indexed with the same space group (P21/n), but with a larger unit-cell in which the number of atoms is doubled. The transition leads to an 8% increase in the density of LaVO4. The occurrence of such a transition has been determined by x-ray diffraction, Raman spectroscopy, and ab initio calculations. The combination of the three techniques allows us to also characterize accurately the pressure evolution of unit-cell parameters and the Raman (and IR)-active phonons of the low- and high-pressure phase. In particular, room-temperature equations of state have been determined. The changes driven by pressure in the crystal structure induce sharp modifications in the color of LaVO4 crystals, suggesting that behind the monoclinic-to-monoclinic transition there are important changes of the electronic properties of LaVO4.Comment: 39 pages, 6 tables, 7 figure

Phase transition systematics in BiVO4 by means of high-pressure-high-temperature Raman experiments

"We report here high-pressure-high-temperature Raman experiments performed on BiVO4. We characterized the fergusonite and scheelite phases (powder and single crystal samples) and the zircon polymorph (nanopowder). The experimental results are supported by ab initio calculations, which, in addition, provide the vibrational patterns. The temperature and pressure behavior of the fergusonite lattice modes reflects the distortions associated with the ferroelastic instability. The linear coefficients of the zircon phase are in sharp contrast to the behavior observed in the fergusonite phase. The boundary of the fergusonite-to-scheelite second-order phase transition is given by TF-Sch (K) = -166(8)P(GPa) + 528(5). The zircon-to-scheelite, irreversible, first-order phase transition takes place at T-Z-(Sch )(K) = -107(8)P(GPa) + 690(10). We found evidence of additional structural changes around 15.7 GPa, which in the downstroke were found to be not reversible. We analyzed the anharmonic contribution to the wave-number shift in fergusonite using an order parameter. The introduction of a critical temperature depending both on temperature and pressure allows for a description of the results of all the experiments in a unified way.

LiCrO2 under pressure : In-situ structural and vibrational studies

The high-pressure behaviour of LiCrO, a compound isostructural to the battery compound LiCoO, has been investigated by synchrotron-based angle-dispersive X-ray powder diffraction, Raman spectroscopy, and resistance measurements up to 41, 30, and 10 Gpa, respectively. The stability of the layered structured compound on a triangular lattice with R-3m space group is confirmed in all three measurements up to the highest pressure reached. The dependence of lattice parameters and unit-cell volume with pressure has been determined from the structural refinements of X-ray diffraction patterns that are used to extract the axial compressibilities and bulk modulus by means of Birch-Murnaghan equation-of-state fits. The pressure coefficients for the two Raman-active modes, A and E, and their mode-Grüneisen parameters are reported. The electrical resistance measurements indicate that pressure has little influence in the resistivity up to 10 GPa. The obtained results for the vibrational and structural properties of LiCrO under pressure are in line with the published results of the similar studies on the related compounds. Research work reported in this article contributes significantly to enhance the understanding on the structural and mechanical properties of LiCrO and related lithium compounds

Phase Transitions of BiVO4 under High Pressure and High Temperature

We have studied the occurrence of phase transitions in two polymorphs of BiVO4 under high-pressure and high-temperature conditions by means of X-ray diffraction measurements. The fergusonite polymorph undergoes a phase transition at 1.5(1) GPa and room temperature into a tetragonal scheelite-type structure. The same transition takes place at 523(1) K and ambient pressure. A second phase transition takes place at room temperature under compression at 16(1) GPa. The transition is from the tetragonal scheelite structure to a monoclinic structure (space group P21/c). All observed phase transitions are reversible. The zircon polymorph counterpart also transforms under compression into the scheelite-type structure. In this case, the transitions take place at 4.3(1) GPa and room temperature and at 653(1) K and ambient pressure. The zircon–scheelite transition is nonreversible. The experiments support that the fergusonite–scheelite transformation is a second-order transition and that the zircon–scheelite transformation is a first-order transition. Finally, we have also determined the compressibility and the thermal expansion of the fergusonite, scheelite, and zircon phases

Pressure-Driven Isostructural Phase Transition in InNbO4: In Situ Experimental and Theoretical Investigations

"This document is the Accepted Manuscript version of a Published Work that appeared in final form in Inorganic Chemistry, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see http://pubs.acs.org/page/policy/articlesonrequest/index.html"[EN] The high-pressure behavior of technologically important visible-light photocatalytic semiconductor In.NbO4, adopting a monoclinic wolframite-type structure at ambient conditions, was investigated using synchrotron-based X-ray diffraction, Raman spectroscopic measurements, and first-principles calculations. The experimental results indicate the occurrence of a pressure-induced isostructural phase transition in the studied compound beyond 10.8 GPa. The large volume collapse associated with the phase transition and the coexistence of two phases observed over a wide range of pressure shows the nature of transition to be first-order. There is an increase in the oxygen anion coordination number around In and Nb cations from six to eight at the phase transition. The ambient-pressure phase has been recovered on pressure release. The experimental pressure volume data when fitted to a Birch-Murnaghan equation of states yields the value of ambient pressure bulk modulus as 179(2) and 231(4) GPa for the low and high-pressure phases, respectively. The pressure dependence of the Raman mode frequencies and Gruneisen parameters was determined for both phases by experimental and theoretical methods. The same information is obtained for the infrared modes from first-principles calculations. Results from theoretical calculations corroborate the experimental findings. They also provide information on the compressibility of interatomic bonds, which is correlated with the macroscopic properties of InNbO4.This research was supported by the Spanish Ministerio de Economia y Competitividad (MINECO), the Spanish Research Agency (AEI), and the European Fund for Regional Development (FEDER) under Grant Nos. MAT2013-46649-004-01/02/03-P, MAT2016-75586-C4-1/2/3-P, and MAT2015-71070-REDC (MALTA Consolider). J.A.S. acknowledges financial support through the Ramon y Cajal Fellowship.Garg, AB.; Errandonea, D.; Popescu, C.; Martínez-García, D.; Pellicer Porres, J.; Rodríguez-Hernández, P.; Muñoz, A.... (2017). Pressure-Driven Isostructural Phase Transition in InNbO4: In Situ Experimental and Theoretical Investigations. Inorganic Chemistry. 56(9):5420-5430. https://doi.org/10.1021/acs.inorgchem.7b00437S5420543056

Transition path to a dense efficient-packed post-delafossite phase. Crystal structure and evolution of the chemical bonding

[EN] A(I)B(III)O(2) delafossite-type oxides are important technological compounds characterized by the linear coordination of the monovalent A metal by oxygen atoms. Based on results of in situ synchrotron X-ray diffraction measurements and ab initio calculations, we herein report on the high-pressure behavior of AgGaO2, to the best of our knowledge the first compound showing step-wise transitions of Ag coordination from linear (2) to octahedral (6), through a leaning delafossite structure. These transformations take place at similar to 10.5 and similar to 16.5 GPa, respectively. Our structural analysis evidences that the initial rhombohedral delafossite structure first becomes dynamically unstable, and distorts continuously via a gliding motion of the [GaO2] octahedral layers within the ab plane, and subsequently transform into another rhombohedral phase 8% denser. This structural sequence is associated with a simultaneous decrease in the bond order of the Ag-O bonds and an increase in the ionicity of the crystal. These results may help to unveil the high-pressure phases of several delafossite compounds which were reported to undergo phase transitions under compression that could not be identified.We are thankful for the financial support received from the Spanish Ministerio de Ciencia e Innovacion and the Agencia Estatal de Investigacion under national projects PGC2018-094417-B-I00 (co-financed by EU FEDER funds), MAT2016-75586-C4-1-P/2-P, FIS2017-83295-P, PID2019-106383GB-C41/C42 and RED2018-102612-T (MALTA Consolider), and from Generalitat Valenciana under project PROMETEO/2018/123. D.S-P, A.O.R, and J.A.S acknowledge financial support of the Spanish MINECO for the RyC-2014-15643, RyC-2016-20301, and RyC-2015-17482 Ramon y Cajal Grants, respectively. Authors thank ALBA-CELLS synchrotron for providing beamtime (ALBA experiments 2012010170).Chuliá-Jordán, R.; Santamaria-Perez, D.; Pellicer-Porres, J.; Otero-De-La-Roza, A.; Martinez-Garcia, D.; García-Domene, B.; Gomis, O.... (2021). Transition path to a dense efficient-packed post-delafossite phase. Crystal structure and evolution of the chemical bonding. Journal of Alloys and Compounds. 867. https://doi.org/10.1016/j.jallcom.2021.15901215901286