High hydrostatic pressure in condensed matter physics

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).

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.

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.

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.

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 [1] and Mn-based perovskites such as MnVO3 [2]. 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. [5] 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 [6]. 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.

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 LaCu₃Fe^3.75+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.

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.

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 [1]. 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 [2]. Remarkably, the Br-doped CuCr2Se4 shows a number of spin-dependent phenomena, such as: dissipationless anomalous Hall effect [3], anomalous Hall heat current and Nernst effect [4], colossal magnetoresistance [5] and anomalous thermoelectric transport caused by Berry phase [6]. 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 [7], 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). [1] I. Nacatani, H. Nose, and K. Masumoto, J. Phys. Chem. Solids 39, 743 (1978). [2] Y. Wang, A. Gupta, M. Chshiev, and W. Butler, Appl. Phys. Lett. 92, 062507 (2008). [3] Wei-Li Lee, S. Watauchi, V. L. Miller, R. J. Cava, N. P. Ong, Science 303, 1647 (2004). [4] Wei-Li Lee, S.Watauchi, V. L. Miller, R. J. Cava, and N. P. Ong, Phys. Rev. Lett. 93, 226601 (2004). [5] K. Borisov, J. Alaria, J. M. D. Coey, and P. Stamenov, Journal of Applied Physics 115, 17C717 (2014). [6] Di Xiao, Yugui Yao, Zhong Fang, and Qian Niu, Phys. Rev. Lett. 97, 026603 (2006). [7] L. G. Khvostantsev, V. N. Slesarev, and V. V. Brazhkin, High Pressure Res. 24, 371 (2004).

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: [1] A.M. Srivastava, M.G. Brik, J. Lumin. 132 (2012) 579. [2] M.G. Brik, A.M. Srivastava, J. Lumin. 133 (2013) 69. [3] M.G. Brik, A.M. Srivastava, Opt. Mater. 35 (2013) 1251. [4] M.G. Brik, A.M. Srivastava, ECS J. Solid State Sci. & Technol. 2 (2013) R148. [5] M.G. Brik, S.J. Camardello, A.M. Srivastava, ECS J. Solid State Sci. & Technol. 4 (2015) R39. [6] M.G. Brik, S.J. Camardello, A.M. Srivastava, N.M. Avram, A. Suchocki, ECS J. Solid State Sci. & Technol. 5 (2016) R3067. [7] M.G. Brik, A.M. Srivastava, Opt. Mater. 54 (2016) 245.

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 [1]. 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 2H11/2,4S3/24I15/2 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 [1] Thomas Tröster, Optical Studies of Non-metallic Compounds Under Pressure, in Handbook on The Physics and Chemistry of rare Earths, 33, 515, (2003). [2] V. Lavín, I.R. Martín, C. K. Jayasankar and Th. Tröster, Phys. Rev. B, 66, 064207, (2002). [3] 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).

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.

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.