Lattice dynamics of Sb2Te3 at high pressures

We report an experimental and theoretical lattice dynamics study of antimony telluride (Sb 2Te 3) up to 26 GPa together with a theoretical study of its structural stability under pressure. Raman-active modes of the low-pressure rhombohedral (R-3m) phase were observed up to 7.7 GPa. Changes of the frequencies and linewidths were observed around 3.5 GPa where an electronic topological transition was previously found. Raman-mode changes evidence phase transitions at 7.7, 14.5, and 25GPa. The frequencies and pressure coefficients of the new phases above 7.7 and 14.5 GPa agree with those calculated for the monoclinic C2/m and C2/c structures recently observed at high pressures in Bi 2Te 3 and also for the C2/m phase in the case of Bi 2Se 3 and Sb 2Te 3. Above 25 GPa no Raman-active modes are observed in Sb 2Te 3, similarly to the case of Bi 2Te 3 and Bi 2Se 3. Therefore, it is possible that the structure of Sb 2Te 3 above 25 GPa is the same disordered bcc phase already found in Bi 2Te 3 by x-ray diffraction studies. Upon pressure release, Sb 2Te 3 reverts back to the original rhombohedral phase after considerable hysteresis. Raman- and IR-mode symmetries, frequencies, and pressure coefficients in the different phases are reported and discussed. © 2011 American Physical Society.This work has been done under financial support from Spanish MICINN under Project Nos. MAT2010-21270-C04-03/04 and CSD-2007-00045 and supported by the Ministry of Education, Youth and Sports of the Czech Republic (MSM 0021627501). E. P.-G. acknowledges the financial support of the Spanish MEC under a FPI fellowship. Supercomputer time has been provided by the Red Espanola de Supercomputacion (RES) and the MALTA cluster.Gomis Hilario, O.; Vilaplana Cerda, RI.; Manjón Herrera, FJ.; Rodríguez-Hernández, P.; Pérez-González, E.; Muñoz, A.; Kucek, V.... (2011). 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Structural and vibrational study of Bi2Se3 under high pressure

The structural and vibrational properties of bismuth selenide (Bi2Se3) have been studied by means of x-ray diffraction and Raman scattering measurements up to 20 and 30 GPa, respectively. The measurements have been complemented with ab initio total-energy and lattice dynamics calculations. Our experimental results evidence a phase transition from the low-pressure rhombohedral (R-3m) phase (B-Bi2Se3) with sixfold coordination for Bi to a monoclinic C2/m structure (B-Bi2Se3) with sevenfold coordination for Bi above 10 GPa. The equation of state and the pressure dependence of the lattice parameters and volume of a and B phases of Bi2Se3 are reported. Furthermore, the presence of a pressure-induced electronic topological phase transition in B-Bi2Se3 is discussed. Raman measurements evidence that Bi2Se3 undergoes two additional phase transitions around 20 and 28 GPa, likely toward a monoclinic C2/c and a disordered body-centered cubic structure with 8-fold and 9- or 10-fold coordination, respectively. These two high-pressure structures are the same as those recently found at high pressures in Bi2Te3 and Sb2Te3. On pressure release, Bi2Se3 reverts to the original rhombohedral phase after considerable hysteresis. Symmetries, frequencies, and pressure coefficients of the Raman and infrared modes in the different phases are reported and discussed.This work was done under financial support from Spanish Ministry of Science and Innovation under Projects No. MAT2007-66129, No. MAT2010-21270-C04-03/04, and No. CSD-2007-00045 and from the Valencian government under Project No. Prometeo/2011-035. It is also supported by the Ministry of Education, Youth and Sports of the Czech Republic Project No. MSM 0021627501

High-pressure vibrational and optical study of Bi2Te3

We report an experimental and theoretical lattice dynamics study of bismuth telluride (Bi2Te3) up to 23 GPa together with an experimental and theoretical study of the optical absorption and reflection up to 10 GPa. The indirect bandgap of the low-pressure rhombohedral (R-3m) phase (α-Bi2Te3) was observed to decrease with pressure at a rate of −6 meV/GPa. In regard to lattice dynamics, Raman-active modes of α-Bi2Te3 were observed up to 7.4 GPa. The pressure dependence of their frequency and width provides evidence of the presence of an electronic-topological transition around 4.0 GPa. Above 7.4 GPa a phase transition is detected to the C2/m structure. On further increasing pressure two additional phase transitions, attributed to the C2/c and disordered bcc (Im-3m) phases, have been observed near 15.5 and 21.6 GPa in good agreement with the structures recently observed by means of x-ray diffraction at high pressures in Bi2Te3. After release of pressure the sample reverts back to the original rhombohedral phase after considerable hysteresis. Raman- and IR-mode symmetries, frequencies, and pressure coefficients in the different phases are reported and discussed.This work has been done under financial support from Spanish MICINN under projects MAT2008-06873-C02- 02, MAT2007-66129, Prometeo/2011-035, MAT2010-21270-C04-03/04, and CSD2007-00045 and supported by the Ministry of Education, Youth and Sports of the Czech Republic (MSM 0021627501)

Termoelektrické a magnetické vlastnosti monokrystalického Bi2Se3 dopovaného Cr - výzkum energetického filtrování

Thermoelectric effects are one of the promising ways to utilize waste heat. Novel approaches have appeared in recent decades aiming to enhance thermoelectric conversion. The theory of energy filtering of free carriers by inclusions is among the latest developed methods. Although the basic idea is clear, experimental evidence of this phenomenon is rare. Based on this concept, we searched suitable systems with stable structures showing energy filtering. Here, we report on the anomalous behavior of Cr-doped single-crystal Bi2Se3 that indicates energy filtering. The solubility of chromium in Bi2Se3 was studied, which is the key parameter in the formation process of inclusions. We present recent results on the effect of Cr-doping on the transport coefficients on a wide set of single crystalline samples. Magnetic measurements were used to corroborate the conclusions drawn from the transport and X-ray measurements.Termoelektrické jevy jsou jednou ze slibných cest k zužitkování odpadního tepla. V posledních dekádách se objevily nové přístupy s cílem zlepšení termoelektrické konverze. Teorie energetického filtrování volných nositelů inkluzemi náleží mezi poslední rozvinuté metody. Ačkoliv je základní idea jasná, experimentální pozorování tohoto fenoménu je řídké. S ohledem na tento koncept jsme tak hledali vhodné systémy se stabilními strukturami, které by vykazovaly energetické filtrování. V článku pojednáváme o anomálním chování monokrystalu Bi2Se3 dopovaného Cr, který naznačuje energetické filtrování. Byla studována rozpustnost chromu v Bi2Se3, která je klíčovým parametrem v procesu tvorby inkluzí. Prezentujeme nové výsledky vlivu dopování Cr na transportní koeficienty na široké sérii monokrystalických vzorků. Magnetických měření bylo užito k podpoře výsledků koncipovaných na základě transportních a RTG měření

Monokrystaly SnSe dopované arsenem: Ambivalentní dopování a interakce s intrinsickými defekty

We performed ambivalent doping study on single crystals of two sets, SnSe1-xAsx and Sn1-xAsxSe, with the aim to explore the interaction of doping species with intrinsic defects. We found that As atoms substitute preferentially for Se atoms in both sets forming the extrinsic substitutional point defect As-Se. In the first set, As lowers the concentration of Sn vacancies, V-Sn, by an order of magnitude compared to undoped stoichiometric SnSe crystal. The remaining Sn vacancies are preferentially coordinated with As atoms. Importantly, a very low concentration of As led to healing process of hosting structure in terms of intrinsic point defects and eventual SnSe2 inclusions. This is reflected in an increase of the Hall mobility and drop of the Hall concentration. In the second set, the concentration of Sn vacancies markedly increases upon doping in contrast to the first set. Additionally, the coordination of Sn vacancies by As atoms is less evident due to the high concentration of vacancies. The substitutional defect As-Se is a deep-level defect that produces no free carriers at room temperature. Moreover, the coupling of V-Sn to As-Se defects increases their activation energy. This results in an unprecedentedly low Hall concentration in SnSe which stays below 10(16) cm(-3) for x = 0.0075. The present study indicates that doping of SnSe is a rather complex process that generally includes a strong interaction of doping atoms with the hosting structure. On the other hand, such doping allows adjustment of the type and concentration of defects. The present study reveals a general tendency of point defects to clustering, which modifies the properties of point defects markedly.Byla provedena studie ambivalentního dopování na dvou sériích monokrystalů SnSe: SnSe1-xAsx a Sn1-xAsxSe, jejímž cílem bylo zkoumání interakce dopantu s intrinsickými defekty. Bylo zjištěno, že atomy As v obou sériích přednostně substituují atomy Se za vzniku extrinsických substitučních bodových defektů AsSe. V první sérii As řádově snižuje koncentraci cínových vakancí VSn v porovnání s nedopovaným stechiometrickým krystalem SnSe. Zbylé Sn vakance se přednostně obklopují atomy As. Důležitým poznatkem je, že velmi nízká koncentrace As vede k uzdravování struktury SnSe ve smyslu intrinsických bodových defektů a případných inkluzí SnSe2. To se odráží v nárůstu Hallovy pohyblivosti a poklesu Hallovy koncentrace. Na rozdíl od první série ve druhé sérii koncentrace Sn vakancí spolu se vzrůstajícím obsahem As znatelně narůstá. Navíc obklopení Sn vakancí As atomy je méně zřetelné díky vysoké koncentraci vakancí. Substituční defekt AsSe je hlubokým defektem, který za teploty místnosti neprodukuje volné nositele náboje. Navíc párování VSn s defekty AsSe zvyšuje aktivační energii vakancí. To má za následek bezprecedentně nízké hodnoty Hallovy koncentrace v SnSe, které se pohybují pod 1016 cm-3 pro x = 0,0075. Předkládaná studie ukazuje, že dopování SnSe je velmi komplexní proces, který obecně zahrnuje silnou interakci dopujících atomů s hostitelskou strukturou. Na druhou stranu takovýto doping dovoluje upravení typu a koncentrace defektů. Předkládaná studie odhaluje obecnou tendenci bodových defektů vytvářet klastry, které významně upravují vlastnosti bodových defekt