Characterization of advanced materialsW
High hydrostatic pressure in condensed matter physics
Introduction and scope:
High hydrostatic pressure, like temperature, electric and magnetic fields allows to alter the system and therefore is used to investigate the fundamental properties of the matter. The proposed Symposium is dedicated to the review of the latest studies on high pressure effects on solids and deals with research in the fields of structural, spectroscopic and electrical properties..
The symposium is dedicated to following specific areas.
- High hydrostatic pressure effects on band structure and energies of localized states related to transition metals and rare earth ions in luminescent materials: experiments and theory. This area covers the study of the influence of pressure on the energies of localized states of dopants and their location with respect of the conduction and valence bands of the host. Since sensitivity on pressure depends on several factors related to the symmetry and the localization of states, pressure can quench or switch on the luminescence processes. As a result, high pressure spectroscopy allows to determine the details related to deexcitation pathways in the material. The materials under consideration are phosphors, persistent phosphors and scintillators.
- Optical spectroscopy under high hydrostatic pressure. The most common experiment that involves high pressure applied in diamond anvil cells (DACs) is optical spectroscopy in the visible spectral region. The transparency of diamond allows to excite the system in the UV up to 250 nm and collect the luminescence in the visible and IR spectral region. As a result, high pressure is a very effective tool for the investigation of phosphors and luminescent materials. This technique is especially important for phosphors for white light emitting diodes (WLEDS).
- Ab initio calculations of structural, elastic and electronic properties of metals, semiconductors and dielectrics under high hydrostatic pressure. This area involves advanced ab initio modeling of physical properties of materials under pressure.
- X-ray spectroscopy and Raman under high hydrostatic pressure. This topic is related to investigation of crystal structure and phase transitions under high hydrostatic pressure.
- Conductivity and magnetic properties under pressure. This area is dedicated to investigation of electrical properties like conductivity, superconductivity and response to magnetic fields under pressure.
- Extremely high hydrostatic pressure technique. This area is dedicated to modern equipment for extremely high pressure generated in DACs as well as shock waves.
Hot topics to be covered by the symposium:
- actual location of the ground states of lanthanide ions with respect to the host band edges, determining the luminescence abilities and quantum efficiency of the system
- existence of partly delocalized states related to impurity trapped excitons,
- energies of compensating defects and existence of transition metal and rare earth ions in aliovalent states.
Tentative list of invited speakers:
- Agata Kamińska, Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warsaw, Poland
- Daniel Errandonea, Departamento de FísicaAplicada – ICMUV, Universitat de València, 46100 Burjassot, Valencia, Spain
- Mikhail Brik, Institute of Physics, University of Tartu, Riia 142, Tartu 51014, Estonia
- Christofilos Dimitrios, Physics Division, School of Technology, Aristotle University of Thessaloniki, 541 24 Thessaloniki, Greece
- Wenge Yang, Center for High Pressure Science and Technology Advanced Research (HPSTAR), Pudong, Shanghai 201203, China
- Luis Seijo, Dipartamento de Qimica , Universidad Utonoma de Madrid, 28049 Spain
- Andrzej Grzechnik, RheinWestfal TH Aachen, Inst Crystallog, D-52060 Aachen, Germany
- D. B. White, Department of Physics, University of California, San Diego, La Jolla, CA 92093, USA
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Authors : Ru-Shi Liu , Guogang Li , Agata Lazarowska , Sebastian Mahlik  and Marek Grinberg 
Affiliations :  Department of Chemistry, National Taiwan University, Taipei 106, Taiwan  Institute of Experimental Physics, University of Gdansk, WitaStwosza 57, 80-952 Gdansk, Poland
Resume : Tuning and optimizing luminescent properties of oxonitridosilicates phosphors are important for white light-emitting diode (WLED) applications. To improve the color rendering index, correlated color temperature and thermal stability of layer-structured MSi2O2N2:Eu (M = Sr, Ba) phosphors, cation substitutions have been used to adjust their luminescent properties. Spectroscopic properties of a series of (Sr0.98-xBaxEu0.02)Si2O2N2 (0 ? x ? 0.98) compoundshas beenstudied under high hydrostaticpressureappliedin a diamond anvil cellup to 200 kbar. At ambient pressure the crystal structures of (Sr0.98-xBaxEu0.02)Si2O2N2 (0 ? x ? 0.98) are related to the ratio of strontium to barium and three different phases exists: orthorhombic Pbcn(0.78 ? x? 0.98), triclinic P1 (0< x ?0.65) and triclinic P1 (0.65< x < 0.78). It wasfound that Eu2+ luminescence reveals abrupt changes under pressure (decay time, energy and shape) which indicate the variation of the local symmetry and crystal field strength in Eu2+ sites. These changes are attributed to the reversible pressure-induced structural phase transitions of triclinic (Sr0.98-xBaxEu0.02)Si2O2N2 into orthorhombic structure.Pressure in which phase transition occurs decreases linearly with increasing of Ba composition in (Sr0.98-xBaxEu0.02)Si2O2N2series. This study serves as a guide in developing oxynitride luminescent materials with controllable optical properties based on variations in local coordination environments through cation substitutions and pressure.
Authors : Tadeusz Lesniewski, Justyna Barzowska, Sebastian Mahlik, Hyo Jin Seo, Marek Grinberg
Affiliations : Institute of Experimental Physics, Faculty of Mathematics Physics and Informatics, University of Gdansk Wita Stwosza 57 80-308 Gdansk; Institute of Experimental Physics, Faculty of Mathematics Physics and Informatics, University of Gdansk Wita Stwosza 57 80-308 Gdansk; Institute of Experimental Physics, Faculty of Mathematics Physics and Informatics, University of Gdansk Wita Stwosza 57 80-308 Gdansk; Department of Physics and Interdisciplinary Program of Biomedical, Mechanical & Electrical Engineering, Pukyong National University, Busan 608-737, Republic of Korea; Institute of Experimental Physics, Faculty of Mathematics Physics and Informatics, University of Gdansk Wita Stwosza 57 80-308 Gdansk
Resume : Luminescent pressure sensors are the easiest way to determine pressure inside a diamond anvil cell (DAC). The most common method is measurement of pressure shift of R1 and R2 spectral lines of ruby (Al2O3:Cr3+). Some alternatives have also been utilised, based on luminescence of Cr3+, Sm2+ as well as other rare-earth ions. Common feature to most of above mentioned materials is that their luminescence spectra lie in red and near infrared part of the spectrum. This poses a difficulty for optical measurements in that spectral region due to possible overlap of emission spectra of measured sample and pressure sensor, which not always are easy to separate. In the case of weakly emitting samples this may render the sensor inapplicable. As a solution we propose KMgF3:Eu2+ as a near UV luminescent pressure sensor. KMgF3:Eu2+ is one of few Eu2+ activated materials where electronic levels of 4f65d configuration lie sufficiently high above the excited states of 4f7 configuration, that material exhibits solely 4f-4f intraconfigurational, narrow-line emission, related to 6P7/2 → 8S7/2 transition. The emission spectrum consists of strong zero phonon line (ZPL) situated at 27862 cm-1 (358.9 nm, ambient pressure), accompanied by much weaker phonon sidebands. The suitability of KMgF3:Eu2+ for pressure sensing has been determined by measuring photoluminescence excitation (PLE) and emission (PL) spectra of KMgF3:Eu2+ at pressure range 1 bar - 300 kbar. The KMgF3:Eu2+ excitation spectra show several overlapping bands related to transition from 8S7/2 ground state to electronic levels of 4f65d excited configuration of Eu2+. The edge of the lowest excitation band lies at 325 nm and does not noticeably shift with increasing pressure. This indicates that KMgF3:Eu2+ luminescence can be efficiently excited with He-Cd UV laser (325 nm). The KMgF3:Eu2+ emission spectrum exhibits constant pressure shift towards longer wavelengths of magnitude -8.11 cm 1/kbar (ZPL) throughout the measurement range 1 bar – 300 kbar. The ZPL was clearly discernible from phonon sidebands at all pressures. Additionally, no interfering emission from 4f65d levels has been detected at pressure up to 300 kbar.
Authors : Anna Baran, Sebastian Mahlik, Marek Grinberg, Adam Watras, Robert P?zik, Przemys?aw Dere?
Affiliations : Institute of Experimental Physics, Faculty of Mathematics, Physics and Informatics, University of Gdansk, Wita Stwosza 57, 80-308 Gdansk, Poland; Institute of Experimental Physics, Faculty of Mathematics, Physics and Informatics, University of Gdansk, Wita Stwosza 57, 80-308 Gdansk, Poland; Institute of Experimental Physics, Faculty of Mathematics, Physics and Informatics, University of Gdansk, Wita Stwosza 57, 80-308 Gdansk, Poland; Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 2 Okólna Street, 50-422 Wroclaw, Poland; Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 2 Okólna Street, 50-422 Wroclaw, Poland; Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 2 Okólna Street, 50-422 Wroclaw, Poland;
Resume : In this work effects of pressure and temperature on the luminescence of Eu2+ and Eu3+-doped Ba2K(PO3)5 are presented. The luminescence spectra and luminescence decays were measured as a function of temperature and pressure. Depending on the excitation wavelength phosphor shows different luminescence spectra. The emission color was bluish green, when only Eu2+ was excited, reddish orange when only Eu3+ was excited or white over simultaneous excitation of both ions. At room temperature under excitation with near UV light, the luminescence spectrum consists of broad emission band peaking at 480 nm due to the 4f65d1?4f7 (8S7/2) transitions of Eu2+ and several sharp lines between 580 and 710 nm region, ascribed to the 5D0 ? 7FJ (J = 0, 1, 2, 3 and 4) transitions in Eu3+. At low temperatures, we observed three different bands related to the 4f65d1 ?4f7 transitions in different Eu sites (at 415 nm (A), 450 nm (B) and 505 nm (C)): two Eu sites substituting for Ba2+ and one Eu site substituting for K+. Under fixed excitation wavelength the effect of increasing of the intensity of Eu2+ emission with respect to Eu3+ emission was observed for temperature range 5 ? 100 K. The nonradiative intersystem crossing was responsible for decreasing of the relative intensity of the Eu2+ luminescence for temperature range 150 ? 500 K and causes decreasing of the Eu2+ to Eu3+ luminescence intensity ratio for temperature higher than 150 K. Luminescence decays were measured for selected temperatures and pressures. At 10 K the decays of Eu3+ luminescence were single?exponential, with time constant being 3.6 ms. When temperature increases all emissions decay faster and become multiexponential. Effective decay times slightly decreased with increasing pressure. In the range of 10 ? 400 K the decays of 4f65d?4f7 emission in the Eu2+ were single?exponential, with time constant being 0.65 ?s, 0.62 ?s and 0.35 ?s for A, B and C emission bands, respectively, and did not depend on temperature. At higher temperatures (from 400 K to 500 K) the luminescence decays become shorter and non?exponential, as the result of thermal quenching. When pressure increases all emissions decay faster.
Authors : Miros?aw Behrendt1, Sebastian Mahlik1, Marek Grinberg1, Dagmara Stefa?ska2, Przemys?aw J. Dere?2
Affiliations : 1Institute of Experimental Physics, University of Gdansk, Wita Stwosza 57, 80-952 Gda?sk, Poland 2Institute of Low Temperature and Structure Research, PAS, Okólna 2, 50-422 Wroc?aw, Poland
Resume : The contribution presents spectroscopic characterization of LaAlO3 doped with 0,5 mol %. Eu3+. We measured steady state luminescence, luminescence excitation spectra, as well as the time resolved spectra and luminescence kinetics. The experiments were performed at high hydrostatic pressure applied in diamond anvil cell (DAC) which was changed from ambient to 250 kbar. We found that for all pressures the emission from the 5D0 and 5D1 excited emitting state of Eu3+ was delayed in time after excitation pulse whilst emission from the 5D2 appear immediately after excitation. At pressure above 12 kbar the strong magnification of the luminescence lines related to the transitions from the 5D3 state which were very weak at ambient condition is observed. The emission decay of the 5D3 luminescence become slower when pressure is increased. All these effects are attributed to pressure-induced increase of the energy of the ground electronic configuration 4f6 of the Eu2+ with respect to the valence band edge which results in the charge transfer state, and 5D3 level crossing.
Authors : Andreas Tröster
Affiliations : Vienna University of Technology, Institute for Material Chemistry, Getreidemarkt 9 1060 Wien, Austria
Resume : The concept of broken symmetry is central to many areas in physics. In particular, Landau theory (LT) is an essential cornerstone of the theory of structural phase transitions. On the other hand, the last decades have seen a tremendous success of ab initio methods in condensed matter physics. Yet, the concepts of DFT and LT are to some extent antipodal. Indeed, condensed matter broken symmetry phases usually appear at low temperature and are thus accessible by ab-initio methods. However, as a rule the high symmetry reference phase, which is the pivotal reference frame of LT, only exists at elevated temperatures. DFT and LT thus appear as complimentary concepts, and the question of how to blend these two approaches in an efficient way has been an active area of research for the last two decades. In particular, DFT calculations are indispensable for understanding high pressure phase transitions. Unfortunately, while imposing high pressure usually does not pose serious additional difficulties in DFT, an attempt to similarly extend LT to include high pressure phase transitions that involve nonlinear elasticity as a central ingredient turns out to be a non-trivial enterprise. Yet, recently we have succeeded in constructing such an extension [PRX 4, 031010 (2014)] and have demonstrated both its practical applicability as well as the tremendous increase in numerical precision over a standard Landau description by applying it to the archetypal perovskite SrTiO3. Essential for the success of this approach is the ab initio calculation of pressure-dependent elastic constants. The ferroelectric cubic-tetragonal phase transition in the closely related perovskite PbTiO3, one of the most-studied ferroelectrics with considerable importance in technology, provides a striking example in which standard LT not only fails quantitatively but actually breaks down completely. This is obvious taking into account that high precision x-ray data obtained by Janolin et al [PRL 101, 237601 (2008)] at room temperature reveal a second order pressure-induced phase transition at Pc=13GPa, whereas the T-driven ferroelectric transition at Tc ≈ 763K appears to be first order. Moreover, the spontaneous strain components accompanying the high pressure transition at room temperature show a peculiar pressure-dependence with curvatures that are completely at odds with the predictions of standard LT. In the present talk, I will demonstrate how our new flavor of DFT-aided finite strain LT combined with a quasiharmonic approximation immediately resolves these issues in a straightforward way. Our theory also holds considerable predictive power, as it offers to determine e.g. the (P,T)-dependence of elastic constants in the low symmetry phase, the pressure dependence of the soft mode frequency and the Pc(T) phase boundary including the location of the tricritical point.
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Authors : Andrzej Grzechnik
Affiliations : Institute of Crystallography RWTH Aachen University Jägerstr. 17 - 19 52066 Aachen Germany
Resume : Vanadium occurs in more than one valence state in a solid state to form mixed-valence compounds. Its ability to change its valence state lies at the core of the functionalities important to materials sciences. Physical properties of vanadates are also closely linked to the redistribution and/or ordering of the charges. The aim of this presentation is to demonstrate how high-pressure techniques could be used to study phase transitions and chemical reactions in vanadate systems. The interplay between the effects of chemical substitutions and of external pressure on the stability of mixed-valence vanadates will be elucidated. As examples, the talk will present various pressure-induced phenomena in bronzes b-A0.33V2O5 (A = Li, Na, Ag) [1,2] and in vanadate fresnoites A2V3O8 (A = K, Rb, NH4, Cs) [3,4]. 1. A. Grzechnik, Y. Ueda, T. Yamauchi, M. Hanfland, P. Hering, V. Potapkin, K. Friese, Phys. Rev. B 91, 174113 (2015). 2. A. Grzechnik, Y. Ueda, T. Yamauchi, M. Hanfland, P. Hering, V. Potapkin, K. Friese, J. Phys.: Condens. Matter, 28, 035401 (2015). 3. A. Grzechnik, T.-Z. Ren, J.M. Posse, K. Friese, Dalton Trans. 40, 4572 (2011). 4. A. Grzechnik, J. Yeon, H.-C. zur Loye, K. Friese, J. Solid State Chem. 238, 252 (2016).
Authors : Moran Emuna (1) , Shlomi Matityahu (2), (3), Eyal Yahel (2), Guy Makov (1), and Yaron Greenberg (2)
Affiliations : (1) Department of Materials Engineering -1, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel (2) Department of Physics -2, NRCN, P.O. Box 9001, Beer-Sheva 84190, Israel (3) Department of Physics -3, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel
Resume : We present a novel experimental design for high sensitivity measurements of the electrical resistance of samples at high pressures (0-6GPa) and high temperatures (0-1000K) in a ?Paris-Edinburgh? type large volume press. Uniquely, the electrical measurements are carried out directly on a small sample, thus greatly increasing the sensitivity of the measurement. The sensitivity to even minor changes in electrical resistance can be used to clearly identify phase transitions in material samples. Electrical resistance measurements are relatively simple and rapid to execute and the efficacy of the present experimental design is demonstrated by measuring the electrical resistance of Pb, Sn and Bi across a wide domain of temperature-pressure phase space and employing it to identify the loci of phase transitions. Based on these results, the phase diagrams of these elements are reconstructed to high accuracy and found to be in excellent agreement with previous studies. In particular, by mapping the locations of several well-studied reference points in the phase diagram of Sn and Bi, it is demonstrated that a standard calibration exists for the temperature and pressure, thus eliminating the need for direct or indirect temperature and pressure measurements. The present technique will allow simple and accurate mapping of phase diagrams under extreme conditions and may be of particular importance in advancing studies of liquid state anomalies.
Authors : Lars Ostheim, Peter J. Klar, Sven Liebich, Peter Ludewig, Kerstin Volz, Wolfgang Stolz
Affiliations : Lars Ostheim; Peter J. Klar Institute of Experimental Physics I, Justus-Liebig-University Giessen, Germany Sven Liebich; Peter Ludewig; Kerstin Volz; Wolfgang Stolz Material Sciences Center and Department of Physics, Phillips-University Marburg, Germany
Resume : Silicon photonics with the ultimate goal of integrating photonic and electronic devices is a very active research area. One possible approach towards silicon photonics is the monolithic integration of III-V semiconductors onto the silicon platform. Furthermore, this approach is also of interest for high-efficiency multi-junction solar cells on Si or Ge substrates. The monolithic integration of III-Vs with the larger lattice constants can be achieved either by alloying with smaller atoms like B on cation site or N on anion site. These alloys show unusual electronic effects which we investigate by pressure, magnetic field and temperature dependent transport experiments. N- and p-type (Bx,Ga1-x)(Asy,P1-y) samples are grown by MOVPE on a GaP substrate. While the incorporation of Te results in n-type doping of the samples, the incorporation of B into Ga(As,P) leads to the formation of localized electronic states resonant with the conduction band. In order to investigate the influence of these localized states on the transport properties, magnetotransport measurements were performed in a temperature range from 10 K to 300 K and at hydrostatic pressures up to 16 kbar. The results obtained indicate that a boron-related density of localized states exists in the vicinity of the conduction band edge of the alloy. These localized states act as electron traps as well as efficient scattering centers. By applying hydrostatic pressure the energetic positions of conduction band edge at the X-point and the localized boron states are shifted apart with respect to each other which has an impact on the electronic transport parameters of the alloy.
Authors : Agata Kaminska
Affiliations : Institute of Physics, Polish Academy of Sciences, 02-668 Warsaw, Poland; Cardinal Stefan Wyszynski University, College of Science, Department of Mathematics and Natural Sciences, Dewajtis 5, 01-815 Warsaw, Poland
Resume : Garnet crystals with various dopants are interesting for applications in the field of solid state laser materials. Especially important in this field are the garnets doped with rare earth ions, which can be easily grown using various methods. In this work the results of high pressure luminescence studies of Gd3Ga5O12 (GGG) crystals doped with Nd, Yb or Ce ions, and Y3Ga5O12:Ce (YGG:Ce) crystal will be presented. The origin of the removal of splitting of the 4F3/2 level of Nd3+ dopant in GGG by applying the pressure of about 10 GPa will be discussed. Afterwards the influence of hydrostatic pressure on the radiative intraconfigurational 4f → 4f transitions of Yb3+ ions will be shown. Finally, the pressure induced pronounced increase of the luminescence efficiency of Ce3+ doped GGG and YGG will be analysed. A model explaining this behaviour, based on the pressure-induced changes of the energy structure of the GGG:Ce and YGG:Ce systems, will be discussed.
Authors : J. P. Attfield
Affiliations : CSEC and School of Chemistry, University of Edinburgh, UK
Resume : High pressure methods are important for synthesising new materials, and exploring changes of structure and property in dense matter. This will be illustrated with reference to new transition metal oxide perovskite materials. High pressure often stabilises cations in unusual oxidation or coordination environments. Examples with unusual A cations are PbRuO3 which has a possible quantum critical point at 5.5 GPa where an orbital ordering transition is suppressed to zero temperature  and Mn-based perovskites such as MnVO3 . New magnetoresistive double perovskite materials such as CaCu3Fe2Re2O12 and Mn2FeReO6 have also been stabilised at high pressure [3,4]. Mn2FeReO6 has a high Curie temperature of 520 K and similar ferrimagnetic and spin-polarised conducting properties to the much-studied magnetoresistive double perovskite Sr2FeMoO6, but also shows a novel switch from negative to large positive magnetoresistances at low temperatures driven by Mn2+ spin ordering. In contrast, Mn2MnReO6 (Mn3ReO6) shows successive antiferromagnetic ordering transitions for Re and Mn spins at 99 and 109 K respectively.  Investigation of possible rare earth (R) analogues has led to discovery of a new ‘double double perovskite’ type MnRMnSbO6 (R = La, Pr, Nd, Sm) with simultaneous 1:1 cation order at both A and B sites . 1. A.F. Kusmartseva, A. Sinclair, J.A. Rodgers, S.A.J. Kimber and J.P. Attfield. Phys. Rev. B 87, 165130 (2013). 2. M. Markkula, A.M. Arevalo-Lopez, A. Kusmartseva, J.A. Rodgers, C. Ritter, H. Wu and J.P. Attfield. Phys. Rev. B 84, 094450 (2011). 3. Chen, WT; Mizumaki, M; Seki, H; Senn, MS; Saito, T; Kan, D; Attfield, JP; Shimakawa, Y. Nature Comm. 5, 3909 (2014). 4. Arévalo-López A.M., McNally G.M., Attfield J.P. Angew. Chem. 54, 12074 (2015). 5. A. M. Arévalo-López, F. Stegemann, J. P Attfield. Chem. Comm. 2016, 52, 5558. 6. E. Solana-Madruga et al, Angew. Chem. 2016, in press.
Authors : Yuichi Shimakawa
Affiliations : Institute for Chemical Research, Kyoto University
Resume : We use high pressure in both material synthesis and property control. High pressure synthesis enables us to make novel materials, which cannot be obtained by synthesis at ambient conditions. We have been searching for such transition-metal oxide materials at high pressures up to about 15 GPa using multi-anvil type apparatuses. The compounds we found include new A-site ordered perovskites with unusually high valence Fe, like CaCu3Fe4+4O12 and LaCu3Fe3.75+4O12, which show very unusual physical properties of charge disproportionation and intersite charge transfer, respectively. The unusual electronic states of the transition-metal oxides with high-valence cations can also be modified under pressure. Both charge disproportionation and intersite charge transfer transitions are induced by applying pressure. Recent results of the high pressure research on a few transition-metal oxides are discussed.
Authors : Y. Qi, P. Naumov, O. Barkalov, W. Schnelle, C. Felser, S. Medvedev
Affiliations : Max Planck Institute for Chemical Physics of Solids, 01187 Dresden, Germany
Resume : Transition metal chalcogenides have attracted the research interest over the last few decades due to their interesting structural chemistry, unusual electronic properties, rich intercalation chemistry and wide spectrum of potential applications. One of the very interesting properties observed in TMC’s is the superconductivity appearing on the borderline with charge-density wave formation. Thus, TMC’s provide a platform for one of the most exciting and important areas of condensed matter research involving elucidation the competition between diverse and exotic phases in strongly correlated matter. The family of layered TXn materials is structurally well-defined: the structure is formed by stacks of the hexagonally packed planes in the sequence providing either the trigonal prismatic or octahedral coordination of metallic atoms. Here, we report on the electrical and structural properties of the layered MoTe2 and HfTe5 under high external pressures. The MoTe2 is recently predicted and experimentally confirmed to be a type-II Weil-semimetal whereas HfTe5 is predicted to locate close to the phase boundary between the weak and strong topological insulators and supply a platform to study topological quantum-phase transitions. Both studied compounds are found to be superconductors under high pressure with unusual pressure dependence of the Tc reflecting the fact that electronic band structure of topologically nontrivial materials around the Fermi level is extremely sensitive to changes in the lattice parameters.
Authors : T.R. Arslanov1, T. Chatterji2, and P.P. Khokhlachev1
Affiliations : 1Amirkhanov Institute of Physics, Daghestan Scientific Center, RAS, 367003 Makhachkala, Russia 2Institute Laue-Langevin, Boîte Postale 156, 38042 Grenoble Cedex 9, France
Resume : The chromium-based selenide CuCr2Se4 is well known for a long time as a material with the highest Curie temperature TC=430 K among chromium spinel chalcogenides . The metallic character together with high Curie temperature makes CuCr2Se4 promising candidate for spin-based electronic applications. The strong interaction between the electronic and magnetic subsystems results in drastic changes in the electronic transport and optical properties near TC. It has been predicted that with suitable doping CuCr2Se4 become half-metallic – being an excellent metal for one spin channel and excellent insulator for the other spin channel. Recent band structure calculations demonstrate that the density of states for spin-down electrons can be fully suppressed with cadmium doping to realize a perfect half-metallic regime . Remarkably, the Br-doped CuCr2Se4 shows a number of spin-dependent phenomena, such as: dissipationless anomalous Hall effect , anomalous Hall heat current and Nernst effect , colossal magnetoresistance  and anomalous thermoelectric transport caused by Berry phase . All these examples indicate that the physical properties of CuCr2Se4 can be qualitatively improved by chemical doping. In contrast to doping, the application of high pressure provides significant perturbation in electronic structure due to changes in the structural parameters of lattice volume, as well as the direct impact on the band gap. However, such studies on the bulk CuCr2Se4 are lack to date. Here we report the transport investigation under hydrostatic pressure up to 8 GPa performed on single crystal of CuCr2Se4. High pressure measurements were conducted in the Toroid type high-pressure cell , using six-probe method on the samples cutted along two crystallographic directions. The pressure transmitting medium used is a mixture of ethanol-methanol 4:1, which remains liquid up to 10 GPa at RT and ensures highly hydrostatic pressure generation. From the room temperature pressure dependence of resistivity and Hall coefficient (Hall concentration) we observed a dramatic changes around P=3 GPa on the background of weak dynamic of transport properties. In addition, we found that application of pressure leads to enhancement of negative magnetoresistance which is change the sign at P=3 GPa and becomes positive. We ascribe this behavior to the pressure-induced phase transition, which is most likely electronic nature. Our results suggested that the pressure-induced metallization is a route toward half-metallic state in CuCr2Se4 via pressure application. This work was supported by the RFBR (Grant No. 16-32-00661 mol_a).  I. Nacatani, H. Nose, and K. Masumoto, J. Phys. Chem. Solids 39, 743 (1978).  Y. Wang, A. Gupta, M. Chshiev, and W. Butler, Appl. Phys. Lett. 92, 062507 (2008).  Wei-Li Lee, S. Watauchi, V. L. Miller, R. J. Cava, N. P. Ong, Science 303, 1647 (2004).  Wei-Li Lee, S.Watauchi, V. L. Miller, R. J. Cava, and N. P. Ong, Phys. Rev. Lett. 93, 226601 (2004).  K. Borisov, J. Alaria, J. M. D. Coey, and P. Stamenov, Journal of Applied Physics 115, 17C717 (2014).  Di Xiao, Yugui Yao, Zhong Fang, and Qian Niu, Phys. Rev. Lett. 97, 026603 (2006).  L. G. Khvostantsev, V. N. Slesarev, and V. V. Brazhkin, High Pressure Res. 24, 371 (2004).
Authors : M.G. Brika, A.M. Srivastava
Affiliations : College of Sciences, Chongqing University of Posts and Telecommunications, Chongqing 400065, People’s Republic of China, Institute of Physics, University of Tartu, W. Ostwald Str. 1, Tartu 50411, Estonia, Institute of Physics, Jan Dlugosz University, PL-42200 Czestochowa, Poland, GE Global Research, One Research Circle, Niskayuna, New York 12309, USA
Resume : Spectroscopic properties of crystalline materials doped with the Mn4+ ions are intensively studied, both experimentally and theoretically, because of their applications for solid state lighting, holographic recording, optical data storage, dosimetry etc. These systems have been an object of our thorough investigations over several recent years [1-7 and references therein]. In the present work a particular emphasis is placed on calculations of the Mn4+ energy levels in solids by using crystal field and ab initio methods. An analysis of spectroscopic properties of Mn4+ ions allowed to establish correlation between the energy of the 2Eg→4A2g red emission transition (that is of growing importance in white LED technology) and degree of covalency of the “Mn4+ - ligand” chemical bonds, which is determined by the degree of reduction of the Racah parameters of Mn4+ ions in solids in comparison with those for a free state. Several practical recommendations on how to tune the Mn4+ red emission are suggested. References:  A.M. Srivastava, M.G. Brik, J. Lumin. 132 (2012) 579.  M.G. Brik, A.M. Srivastava, J. Lumin. 133 (2013) 69.  M.G. Brik, A.M. Srivastava, Opt. Mater. 35 (2013) 1251.  M.G. Brik, A.M. Srivastava, ECS J. Solid State Sci. & Technol. 2 (2013) R148.  M.G. Brik, S.J. Camardello, A.M. Srivastava, ECS J. Solid State Sci. & Technol. 4 (2015) R39.  M.G. Brik, S.J. Camardello, A.M. Srivastava, N.M. Avram, A. Suchocki, ECS J. Solid State Sci. & Technol. 5 (2016) R3067.  M.G. Brik, A.M. Srivastava, Opt. Mater. 54 (2016) 245.
Authors : Víctor Lavín, U. R. Rodríguez-Mendoza, and I. R. Martín
Affiliations : Departamento de Física, MALTA Consolider Team, IMN and IUdEA. Universidad de La Laguna. Apdo. 456. E-38200 San Cristóbal de La Laguna, Santa cruz de Tenerife, Spain
Resume : The luminescence properties of different rare earth ions (RE3+: Sm3+, Tb3+ and Er3+) in lithium fluoroborate glasses and oxyfluoride nanocrystalline glass-ceramics have been analyzed as a function of the pressure. The roles of the pressure-induced energy transfer between optically active ions, their host local structures and the energy trap centers in the quenching of the rare earth luminescence are discussed . The concentration and pressure dependent luminescence properties of the Tb3+ and Sm3+ ions in lithium fluoroborate glasses have been studied by analyzing the de-excitation processes of the 4G5/2(Sm3+) and the 5D4(Tb3+) levels at ambient conditions as well as a function of pressure up to 21 and 35 GPa, respectively, at room temperature [2,3]. Continuous red shifts as well as a progressive increase in the magnitude of the crystal-field splitting are observed in the luminescence spectra. The luminescence decay curves have been measured and analyzed in order to understand the dynamics of the de-excitation of these rare earth ions in these glasses. With increasing pressure these decays show a more pronounced non-exponential behavior, even for the low concentrated sample, accompanied by a fast decrease of the lifetimes. For the first time, the lifetime and the energy transfer parameter have been obtained and analyzed independently as a function of pressure. The luminescence properties observed with releasing pressure are slightly different to those obtained while increasing pressure suggesting a local structural hysteresis in the lithium fluoroborate glass matrix giving rise to the generation of a new distribution of environments for the RE3+ ions. Finally, the effect of the pressure is studied in the infrared-to-visible energy upconverted luminescence of the Er3+ ions in a nanocrystalline oxyfluoride glass-ceramic from ambient pressure up to 17 GPa. After exciting the sample at 800 nm with a Ti:sapphire laser, upconverted 2H11/2,4S3/24I15/2 green and 4F9/2→4I15/2 red emission are observed by the naked eyes, even inside the DAC. The time-resolved luminescence measurements as a function of pressure indicate that the dominant mechanism of the upconversion processes is energy transfer (ETU). References  Thomas Tröster, Optical Studies of Non-metallic Compounds Under Pressure, in Handbook on The Physics and Chemistry of rare Earths, 33, 515, (2003).  V. Lavín, I.R. Martín, C. K. Jayasankar and Th. Tröster, Phys. Rev. B, 66, 064207, (2002).  V. Venkatramu, P. babu, I.R. Martín, V. Lavín, J. E. Muñoz-Santiuste, Th. Tröster, W. Sievers, G. Wortmann and C.K. Jayasankar, J. Chem. Phys., 132, 114505, (2010).
Authors : Daniel Errandonea
Affiliations : Departamento de Física Aplicada, ICMUV, MALTA Consolider Team, Edificio de Investigación, Universidad de Valencia,C. Dr. Moliner 50, 46100 Burjassot, Spain
Resume : The relationships between the crystal structure and the electronic band gap and other physical properties of PbCrO4 will be discussed. A review of recent high-pressure studies carried out in lead chromate will be presented. This will include synchrotron X-ray diffraction, Raman, optical-absorption, Hall effect, and resistivity measurements carried out up to 50 GPa using different pressure media. The discovery of several phase transitions will be discussed and the crystal structure of the high-pressure phases reported. Three high-pressure phases with structures different than the ambient-pressure monazite-type were identified in the experiments and their crystal structures determined. The changes induced by pressure in the crystal structure at the successive transitions will be correlated with changes in the Raman spectrum, resistivity, and electronic band gap. In particular, we found that the first phase transition (at 3.5 GPa) involves a band-gap collapse and a large resistivity drop associated to an increase of the carrier concentration. In the pressure range covered by the experiments, compression transforms PbCrO4 from a wide band-gap (2.3 eV) semiconductor into a narrow band gap semiconductor (0.8 eV). The reported findings provide insights into the effects of pressure on the physical properties of PbCrO4. The results will be discussed in comparison with related compounds. The distinctive role played by Pb states in the band structure of PbCrO4 and it influence in the high-pressure behavior of the band gap will be examined.
Authors : Ru- Shi Liu
Affiliations : Department of Chemistry, National Taiwan University, Taipei 106, Taiwan
Resume : A SrLiAl3N4:Eu2+ (SLA) red phosphor prepared through a high-pressure solid state reaction was coated with organosilica layers in 400 ~ 600 nm thickness to improve its waterproof property. These 4f65d→4f7 transition bands are considered to result from the existence of Eu2+ occupying two different Sr2+ sites. Luminescence spectra measured at 10 K revealed two zero phonon lines at 15780 cm-1 for Eu(Sr1) and at 15377 cm‒1 for Eu(Sr2). The phosphors exhibited stable red emission under high pressure conditions of up to 312 kbar. The configurational coordinate diagram gave a theoretical explanation for Eu2+/3+ result. The coated samples showed excellent moisture resistance while retaining an external quantum efficiency (EQE) of 70% of its initial EQE after being aged for 5 days in harsh conditions. White light-emitting diodes of SLA red-phosphors and commercial Y3Al5O12:Ce3+ yellow-phosphor on a blue-InGaN chip were shown high color rendition (CRI = 89, R9= 69) and low correlated color temperature of 2406 K.
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