Strain coupling, microstructure dynamics, and acoustic mode softening in germanium telluride

GeTe is a material of intense topical interest due to its potential in the context of phase-change and nanowire memory devices, as a base for thermoelectric materials, and as a ferroelectric. The combination of a soft optic mode and a Peierls distortion contributes large strains at the cubic-rhombohedral phase transition near 625 K and the role of these has been investigated through their influence on elastic and anelastic properties by resonant ultrasound spectroscopy. The underlying physics is revealed by softening of the elastic constants by ∼30%-45%, due to strong coupling of shear and volume strains with the driving order parameter and consistent with an improper ferroelastic transition which is weakly first order. The magnitude of the softening is permissive of the transition mechanism involving a significant order/disorder component. A Debye loss peak in the vicinity of 180 K is attributed to freezing of the motion of ferroelastic twin walls and the activation energy of ∼0.07 eV is attributed to control by switching of the configuration of long and short Ge-Te bonds in the first coordination sphere around Ge. Precursor softening as the transition is approached from above can be described with a Vogel-Fulcher expression with a similar activation energy, which is attributed to coupling of acoustic modes with an unseen central mode that arises from dynamical clusters with local ordering of the Peierls distortion. The strain relaxation and ferroelastic behavior of GeTe depend on both displacive and order/disorder effects but the dynamics of switching will be determined by changes in the configuration of distorted GeTe6 octahedra, with a rather small activation energy barrier.EPSRC, NER

Elastic and anelastic relaxations accompanying relaxor ferroelectric behaviour of Ba<inf>6</inf>GaNb<inf>9</inf>O<inf>30</inf> tetragonal tungsten bronze from resonant ultrasound spectroscopy

Abstract Tetragonal tungsten bronze (TTB) structures offer some promise as lead-free ferroelectrics and have an advantage of great flexibility in terms of accessible composition ranges due to the number of crystallographic sites available for chemical substitution. The ferroic properties of interest are coupled with strain, which will be important in the context of stability, switching dynamics and thin film properties. Coupling of strain with the ferroelectric order parameter gives rise to changes in elastic properties, and these have been investigated for a ceramic sample of Ba6GaNb9O30 (BGNO) by resonant ultrasound spectroscopy. Room temperature values of the shear and bulk moduli for BGNO are rather higher than for TTBs with related composition which are orthorhombic at room temperature, consistent with suppression of the ferroelectric transition. Instead, a broad, rounded minimum in the shear modulus measured at ~1 MHz is accompanied by a broad rounded maximum in acoustic loss near 115 K and signifies relaxor freezing behaviour. Elastic softening with falling temperature from room temperature, ahead of the freezing interval, is attributed to the development of dynamical polar nanoregions (PNRs), whilst the nonlinear stiffening below ~115 K is consistent with a spectrum of relaxation times for freezing of the PNR microstructure.This work was supported by the strategic grant POSDRU/159/1.5/S/133255, Project ID 133255 (2014), co-financed by the European Social Fund within the Sectorial Operational Program Human Resources Development 2007 –2013. RUS facilities in Cambridge were established with funding from the Natural Environment Research Council (grants NE/B505738/1, NE/F017081/1) and from the Engineering and Physical Sciences Research Council (EP/I036079/1).This is the final version of the article. It first appeared from [Springer via https://doi.org/10.1007/s10973-015-5202-

Elastic and anelastic relaxations associated with phase transitions in EuTiO3

Elastic and anelastic properties of single crystal samples of EuTiO3 have been measured between 10 and 300 K by Resonant Ultrasound Spectroscopy at frequencies in the vicinity of 1 MHz. Softening of the shear elastic constants C44 and by ~20-30% occurs with falling temperature in a narrow interval through the transition point, Tc = 284 K, for the cubic - tetragonal transition. This is accounted for by classical coupling of macroscopic spontaneous strains with the tilt order parameter, in the same manner as occurs in SrTiO3. A peak in the acoustic loss occurs a few degrees below Tc and is interpreted in terms of initially mobile ferroelastic twin walls which rapidly become pinned with further lowering of temperature. This contrasts with the properties of twin walls in SrTiO3 which remain mobile down to at least 15 K. No further anomalies were observed that might be indicative of strain coupling to any additional phase transitions above 10 K. A slight anomaly in the shear elastic constants, independent of frequency and without any associated acoustic loss, was found at ~140 K. It marks a change from elastic stiffening to softening with falling temperature and perhaps provides evidence for coupling between strain and local fluctuations of dipoles related to the incipient ferroelectric transition. An increase in acoustic loss below ~80 K is attributed to the development of dynamical magnetic clustering ahead of the known antiferromagnetic ordering transition at ~5.5 K. Detection of these elastic anomalies serves to emphasise that coupling of strain with tilting, ferroelectric and magnetic order parameters is likely to be a permeating influence in determining the structure, stability, properties and behaviour of EuTiO3.RUS facilities were established in Cambridge through a grant from the Natural Environment Research Council of Great Britain to MAC, which is gratefully acknowledged (NE/B505738/1). LJS acknowledges the support of the National Science Centre (NCN) through Grant MAESTRO No. DEC-2012/04/A/ST3/00342. CP acknowledges Financial support in Greece through grants EURYI and MEXT-CT-2006-039047 grants, and in Singapore through Award No. NRF-CRP-4-2008-04 of the Competitive Research Programme.This is the accepted version. The final version is published of the final version by APS here: http://journals.aps.org/prb/abstract/10.1103/PhysRevB.90.054119

Elastic and anelastic relaxation behaviour of perovskite multiferroics II: PbZr0.53_{0.53}Ti0.47_{0.47}O3_3 (PZT)–PbFe0.5_{0.5}Ta0.5_{0.5}O3_3 (PFT)

Elastic and anelastic properties of ceramic samples of multiferroic perovskites with nominal compositions across the binary join PbZr$_{0.53}$Ti$_{0.47}$O$_3$–PbFe$_{0.5}$Ta$_{0.5}$O$_3$ (PZT–PFT) have been assembled to create a binary phase diagram and to address the role of strain relaxation associated with their phase transitions. Structural relationships are similar to those observed previously for PbZr$_{0.53}$Ti$_{0.47}$O$_3$–PbFe$_{0.5}$Nb$_{0.5}$O$_3$ (PZT–PFN), but the magnitude of the tetragonal shear strain associated with the ferroelectric order parameter appears to be much smaller. This leads to relaxor character for the development of ferroelectric properties in the end member PbFe$_{0.5}$Ta$_{0.5}$O$_3$. As for PZT–PFN, there appear to be two discrete instabilities rather than simply a reorientation of the electric dipole in the transition sequence cubic–tetragonal–monoclinic, and the second transition has characteristics typical of an improper ferroelastic. At intermediate compositions, the ferroelastic microstructure has strain heterogeneities on a mesoscopic length scale and, probably, also on a microscopic scale. This results in a wide anelastic freezing interval for strain-related defects rather than the freezing of discrete twin walls that would occur in a conventional ferroelastic material. In PFT, however, the acoustic loss behaviour more nearly resembles that due to freezing of conventional ferroelastic twin walls. Precursor softening of the shear modulus in both PFT and PFN does not fit with a Vogel–Fulcher description, but in PFT there is a temperature interval where the softening conforms to a power law suggestive of the role of fluctuations of the order parameter with dispersion along one branch of the Brillouin zone. Magnetic ordering appears to be coupled only weakly with a volume strain and not with shear strain but, as with multiferroic PZT–PFN perovskites, takes place within crystals which have significant strain heterogeneities on different length scales.RUS facilities in Cambridge were established with funding from the Natural Environment Research Council (Grants NE/B505738/1, NE/F017081/1). The present work was supported by Grant No. EP/ I036079/1 from the Engineering and Physical Sciences Research Council. We thank Dr. Sam Crossley for his assistance with dielectric analysis and the use of his software to run those measurements. JAS gratefully acknowledges the hospitality of the Max Planck Institute for Chemical Physics of Solids. The Nanopaleomagnetism lab has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007– 2013)/ERC Grant Agreement 320750. SED and HS acknowledge support from the Winton Programme for the physics of sustainability. HS also acknowledges support from the Funai Foundation for Information Technology and the British Council Japan Association. Part of the work was carried out at the University of Puerto Rico, supported by the DOEEBSCoR project DEG02-ER46526

Ecology and Biogeography of Free-Living Nematodes Associated with Chemosynthetic Environments in the Deep Sea: A Review

Background: Here, insight is provided into the present knowledge on free-living nematodes associated with chemosynthetic environments in the deep sea. It was investigated if the same trends of high standing stock, low diversity, and the dominance of a specialized fauna, as observed for macro-invertebrates, are also present in the nematodes in both vents and seeps. Methodology: This review is based on existing literature, in combination with integrated analysis of datasets, obtained through the Census of Marine Life program on Biogeography of Deep-Water Chemosynthetic Ecosystems (ChEss). Findings: Nematodes are often thriving in the sulphidic sediments of deep cold seeps, with standing stock values ocassionaly exceeding largely the numbers at background sites. Vents seem not characterized by elevated densities. Both chemosynthetic driven ecosystems are showing low nematode diversity, and high dominance of single species. Genera richness seems inversely correlated to vent and seep fluid emissions, associated with distinct habitat types. Deep-sea cold seeps and hydrothermal vents are, however, highly dissimilar in terms of community composition and dominant taxa. There is no unique affinity of particular nematode taxa with seeps or vents. Conclusions: It seems that shallow water relatives, rather than typical deep-sea taxa, have successfully colonized the reduced sediments of seeps at large water depth. For vents, the taxonomic similarity with adjacent regular sediments is much higher, supporting rather the importance of local adaptation, than that of long distance distribution. Likely the ephemeral nature of vents, its long distance offshore and the absence of pelagic transport mechanisms, have prevented so far the establishment of a successful and typical vent nematode fauna. Some future perspectives in meiofauna research are provided in order to get a more integrated picture of vent and seep biological processes, including all components of the marine ecosystem

Elastic and anelastic relaxation behaviour of perovskite multiferroics I: PbZr<inf>0.53</inf>Ti<inf>0.47</inf>O<inf>3</inf> (PZT)–PbFe<inf>0.5</inf>Nb<inf>0.5</inf>O<inf>3</inf> (PFN)

Perovskites in the ternary system PbTiO₃ (PT)–PbZrO₃ (PZ)–Pb(Fe₀.₅Nb₀.₅)O₃ (PFN) have attracted close interest because they can display simultaneous ferroelectric, magnetic and ferroelastic properties. Those with the most sensitive response to external fields are likely to have compositions near the morphotropic phase boundary (MPB) which lies close to the binary join Pb(Zr₀.₅₃Ti₀.₄₇)O₃ (PZT)–PFN. In the present study, the strength and dynamics of strain coupling behaviour which accompanies the development of ferroelectricity and (anti)ferromagnetism in ceramic PZT–PFN samples have been investigated by resonant ultrasound spectroscopy. Elastic softening ahead of the cubic–tetragonal transition does not fit with models based on dispersion of the soft mode or relaxor characteristics but is attributed, instead, to coupling between acoustic modes and a central peak mode from correlated relaxations and/or microstructure dynamics. Softening of the shear modulus through the transition by up to ~50 % fits with the expected pattern for linear/quadratic strain/order parameter coupling at an improper ferroelastic transition and close to tricritical evolution for the order parameter. Superattenuation of acoustic resonances in a temperature interval of ~100 K below the transition point is indicative of mobile ferroelastic twin walls. By way of contrast, the first-order tetragonal–monoclinic transition involves only a small change in the shear modulus and is not accompanied by significant changes in acoustic dissipation. The dominant feature of the elastic and anelastic properties at low temperatures is a concave-up variation of the shear modulus and relatively high loss down to the lowest temperature, which appears to be the signature of materials with substantial local strain heterogeneity and a spectrum of strain relaxation times. No evidence of magnetoelastic coupling has been found, in spite of the samples displaying ferromagnetism below ~550 K and possible spin glass ordering below ~50 K. For the important multiferroic perovskite ceramics with compositions close to the MPB of ternary PT-PZ-PFN, there must be some focus in future on the role of strain heterogeneity.RUS facilities in Cambridge were established with funding from the Natural Environment Research Council (Grants NE/B505738/1, NE/F017081/1). The present work was supported by Grant No. EP/I036079/1 from the Engineering and Physical Sciences Research Council. We thank Dr. Sam Crossley for his assistance with dielectric analysis and the use of his software to run those measurements. JAS gratefully acknowledges the hospitality of the Max Planck Institute for Chemical Physics of Solids. The Nanaopaleomagnetism lab has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007–2013)/ERC Grant Agreement 320750. SED and HS acknowledge support from the Winton Programme for the physics of sustainability. HS also acknowledges support from the Funai Foundation for Information Technology and the British Council Japan Association