ABSTRACT
While phosphotellurite glasses have superior properties over SiO2-based glasses for many applications in optoelectronics and photonic devices, their high hydroxyl content limits their use in the mid-infrared range. This drawback can be overcome by fluoride addition to the formulation. In this work, we report the preparation, optical, and structural characterization of new glasses in the ternary system TeO2-xNaF-NaPO3 having the compositions 0.8TeO2-0.2[xNaF-(1 - x)NaPO3] and 0.6TeO2-0.4[xNaF-(1 - x)NaPO3] (0 ≤ x ≤ 1) obtained by the traditional melt-quenching method and labeled as T8NNx and T6NNx, respectively. Differential scanning calorimetry (DSC) reveals high thermal stability against crystallization, with Tx-Tg varying from 80 to 130 °C, depending on fluoride/phosphate ratios. Raman spectroscopy suggests that the network connectivity increases with increasing phosphate concentration. 125Te, 23Na, 31P, and 19F NMR spectroscopy provides detailed structural information about Te-O-P, Te-F, Te-O-Te, P-O-P, and P-F linkages and the charge compensation mechanism for the sodium ions. The present study is the first comprehensive structural characterization of a fluorophosphotellurite glass system.
ABSTRACT
Knowledge of the role of landscapes in shaping genetic connectivity and divergence is essential for understanding patterns of biogeography and diversity. This is particularly relevant for the Andes region, a major biodiversity hotspot of relatively recent origin. We examined the phylogeography and landscape genetics of the Andean wax palm Ceroxylon echinulatum (Arecaceae) that occurs in two narrow bands of montane forests on each side of the Andes in Ecuador and northeastern Peru. First, we tested the hypothesis of C. echinulatum being a geographic cline species crossing the Andes in the Amotape-Huancabamba zone (AHZ) of southern Ecuador/northern Peru, as indicated by observations on fruit morphology. Second, we assessed the timeframe of cross-Andean divergence, and third, we investigated the impact of contemporary and historical landscape features on observed spatio-genetic patterns. Individual-based Bayesian clustering (BC) identified a northeastern, southeastern, southwestern, and northwestern cluster, with areas of genetic discontinuity coinciding with the Andes and the Giron-Paute deflection. F-statistics derived from BC suggested an east-to-west dispersal history. Population-based analyses revealed strong genetic structuring at both small and large geographic scales. Interpopulation relationships and Mantel tests strongly supported the cline model with cross-Andean dispersal in the AHZ. Along the cline, gene flow measured as F(ST) was mainly limited by distance, with less but significant impact of climatic friction. Coalescent analysis revealed that cross-Andean divergence took place during the Quaternary. Significant historical isolation (R(ST) > F(ST)) was found in the southwestern population. The current study illustrates a joint effect of founder dynamics, divergence by distance and historical isolation on patterns of Andean diversity and distribution.