X-ray investigation of lateral hetero-structures of inversion domains in LiNbO3, KTiOPO4 and KTiOAsO4

In this paper periodically domain-inverted (PDI) ferroelectric crystals are studied using high-resolution X-ray diffraction. Rocking curves and reciprocal-space maps of the principal symmetric Bragg reflections in LiNbO3 (LN) (Λ = 5μm), KTiOPO4 (KTP) (Λ = 9μm) and KTiOAsO4 (KTA) (Λ = 39μm) are presented. For all the samples strong satellite reflections were observed as a consequence of the PDI structure. Analysis of the satellites showed that they were caused by a combination of coherent and incoherent scattering between the adjacent domains. Whilst the satellites contained phase information regarding the structure of the domain wall, this information could not be rigorously extracted without a priori knowledge of the twinning mechanism. Analysis of the profiles reveals strain distributions of Δd/d = 1.6 × 10-4 and 2.0 × 10-4 perpendicular to domain walls in KTP and LN samples, respectively, and lateral correlation lengths of 63μm (KTP), 194μm (KTA) and 10μm (LN). The decay of crystal truncation rods in LN and KTP was found to support the occurrence of surface corrugations

Drivers of Holocene palsa distribution in North America

Palsas and peat plateaus are climatically sensitive landforms in permafrost peatlands. Climate envelope models have previously related palsa/peat plateau distributions in Europe to modern climate, but similar bioclimatic modelling has not been attempted for North America. Recent climate change has rendered many palsas/peat plateaus in this region, and their valuable carbon stores, vulnerable. We fitted a binary logistic regression model to predict palsa/peat plateau presence for North America by relating the distribution of 352 extant landforms to gridded modern climate data. Our model accurately classified 85.3% of grid cells that contain observed palsas/peat plateaus and 77.1% of grid cells without observed palsas/peat plateaus. The model indicates that modern North American palsas/peat plateaus are supported by cold, dry climates with large seasonal temperature ranges and mild growing seasons. We used palaeoclimate simulations from a general circulation model to simulate Holocene distributions of palsas/peat plateaus at 500-year intervals. We constrained these outputs with timings of peat initiation, deglaciation, and postglacial drainage across the continent. Our palaeoclimate simulations indicate that this climate envelope remained stationary in western North America throughout the Holocene, but further east it migrated northwards during 11.5–6.0 ka BP. However, palsa extents in eastern North America were restricted from following this moving climate envelope by late deglaciation, drainage and peat initiation. We validated our Holocene simulations against available palaeoecological records and whilst they agree that permafrost peatlands aggraded earliest in western North America, our simulations contest previous suggestions that late permafrost aggradation in central Canada was climatically-driven

Thin layer diffraction

Many of the challenges in X-ray diffraction arise from the requirement to produce detailed information on very thin layers. This paper illustrates the present limits in the analysis of X-ray diffraction profiles with examples of epitaxial and polycrystalline layers. One of the primary uses of X-ray diffraction of epitaxial layers is in the determination of composition and thickness, but this can be fraught with problems for the unwary yet a very powerful technique when the correct procedures are used. Often the assumptions concerning epitaxial quality are rather ambitious and this paper will consider the influence of imperfect epitaxy on the subsequent interpretation. The latter part of this paper will discuss the limits in polycrystalline diffraction analysis for the study of nano-structures and the initial stages of the recrystallisation of amorphous Si.

X-ray scattering from semiconductors

287 p. : ill. ; 23 cm

X-ray scattering from semiconductors

287 p. : ill. ; 23 cm

Controls on saturated hydraulic conductivity in a degrading permafrost peatland complex

Abstract Permafrost peatlands are vulnerable to rapid structural changes under climatic warming, including vertical collapse. Peatland water budgets, and therefore peat hydraulic properties, are important determinants of vegetation and carbon fluxes. Measurements of hydraulic properties exist for only a limited number of permafrost peatland locations, primarily concentrated in North America. The impacts of thaw-induced collapse upon properties such as horizontal saturated hydraulic conductivity (Kh), and thus lateral drainage, remain poorly understood. We made laboratory determinations of Kh from 82 peat samples from a degrading Swedish palsa mire. We fitted a linear mixed-effects model (LMM) to establish the controls on Kh, which declined strongly with increasing depth, humification and dry bulk density. Depth exerted the strongest control on Kh in our LMM, which demonstrated strong predictive performance (r2 = 0.605). Humification and dry bulk density were influential predictors, but the high collinearity of these two variables meant only one could be included reliably in our LMM. Surprisingly, peat Kh did not differ significantly between desiccating and collapsed palsas. We compared our site-specific LMM to an existing, multi-site model, fitted primarily to boreal and temperate peatlands. The multi-site model made less skillful predictions (r2 = 0.528) than our site-specific model, possibly due to latitudinal differences in peat compaction, floristic composition and climate. Nonetheless, low bias means the multi-site model may still be useful for estimating peat Kh at high latitudes. Permafrost peatlands remain underrepresented in multi-site models of peat hydraulic properties, and measurements such as ours could be used to improve future iterations. Key Points Depth and humification are important controls for horizontal saturated hydraulic conductivity in a degrading Swedish palsa complex Peat hydraulic properties did not significantly differ between desiccating and collapsed areas of the palsa complex An existing model, trained on lower-latitude peatlands, predicted horizontal saturated hydraulic conductivity adequately, with low bia