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Marked changes in the Nb4d band in the X-ray photo-electron spectrum accompany the semiconductor-metal transition in NbO2. The Nb4d band progressively shifts towards the O2p band with increase in temperature.
The use of the photoacoustic effect in the investigation of first- and second-order phase transitions has been examined. Changes in the amplitude of the photoacoustic signal across the phase transition are compared with changes in thermal properties such as specific heat and thermal diffusivity. The systems studied include NaNO2, TlNO3, CsNO3, NH4NO3, BaTiO3, CoO, Cu2HgI4, VO2 and V3O5. The current photoacoustic studies are discussed in the light of the theoretical models available.
ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTChemistry for the concerned citizen. The case of IndiaC. N. R. Rao Cite this: J. Chem. Educ. 1972, 49, 5, 321Publication Date (Print):May 1, 1972Publication History Received3 August 2009Published online1 May 1972Published inissue 1 May 1972https://doi.org/10.1021/ed049p321Request reuse permissionsArticle Views40Altmetric-Citations2LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (2 MB) Get e-Alertsclose Get e-Alerts
An unusual set of anomalous functional properties of rocksalt crystals of Group IV chalcogenides were recently linked to a kind of bonding termed as metavalent bonding (MVB) which involves violation of the 8‐N rule. Precise mechanisms of MVB and the relevance of lone pair of Group IV cations are still debated. With restrictions of low dimensionality on the possible atomic coordination, 2D materials provide a rich platform for exploration of MVB. Here, we present first‐principles theoretical analysis of the nature of bonding in five distinct 2D lattices of Group IV chalcogenides MX (M: Sn, Pb, Ge and X: S, Se, Te), in which the natural out‐of‐plane expression of the lone pair versus in‐plane bonding can be systematically explored. While their honeycomb lattices respecting the 8‐N rule are shown to exhibit covalent bonding, their square and orthorhombic structures exhibit MVB only in‐plane, with cationic lone pair activating the out‐of‐plane structural puckering that controls their relative stability. Anomalies in Born‐effective charges, dielectric constants, Grüneisen parameters occur only in their in‐plane behaviour, confirming MVB is confined strictly to 2D and originates from p‐p orbital interactions. Our work opens up directions for chemical design of MVB based 2D materials and their heterostructures.
No abstract is provided for this article.
No abstract is provided for this article.
Manganates of the compositions Nd0.5-xLaxCa0.5MnO3 and Pr0.5-xLaxCa0.5 MnO3, with the average A-site cation radius, ⟨rA⟩, in the range 1.17-1.20 Å, are charge ordered at ordinary temperatures (TCO~240 K), and undergo a re-entrant transition to a ferromagnetic state on cooling (TC<TCO). The ferromagnetic Curie temperature, TC, of the re-entrant transition increases markedly with x or ⟨rA⟩ in the two series of compounds. The plots of TC and TCO against ⟨rA⟩ show that the two curves intersect around a ⟨rA⟩ value of 1.195 Å, below which TCO>TC. Site disorder due to the size mismatch of the A-site cations, σ2, has a marked effect on the re-entrant transition temperature, TC. Thus, in a series of manganates of the type Ln0.5A0.5MnO3 with a fixed ⟨rA⟩ value of 1.185 Å, the TC decreases markedly with increase in site disorder, suggesting that the re-entrant transition can be entirely suppressed at a sufficiently high value of σ2. Between TCO and TC, the CO and FM states are likely to coexist, the coexistence temperature regime decreasing with increasing ⟨rA⟩, and increasing with σ2 at a fixed ⟨rA⟩.
Prompted by a literature report that thin films of Pr0.5Ca0.5Ba2Cu3O7-δ are superconducting, the structure and bulk properties of orthorhombic Pr1-xCaxBa2Cu3O7-δ and tetragonal PrBa2-xLaxCu3O7-δ have been investigated. Both these systems do not show superconductivity. Tetragonal Pr1-xCaxBaLaCu3O7-δ, on the other hand, shows superconductivity when x=1.0 and 0.8 with onset Tc's of 66 K and 46 K respectively; compositions with x < 0.6 are semiconducting. Since the hole concentration in PrBa2Cu3O6.9, PrBaLaCu3O7.42 and CaBaLaCu3O6.9 are comparable, the absence of superconductivity in the first two cuprates is likely to be due to the location of the 4f level of Pr3+ in the vicinity of the Cu-O band.
No abstract is provided for this article.
N2 chemisorbs on (100), (110) and (111) single-crystal faces at 80 K without any dissociation. A molecular precursor species at the threshold of dissociation is formed at 80 K on a Ni(110) surface covered with atomic carbon or nitrogen; the precursor species dissociates at higher temperatures. At low C concentrations, however, dissociative adsorption occurs on Ni(110) at 80 K without forming any precursor species, accompanied by molecular chemisorption on the open Ni sites.
Internal vibration modes of bis-(alkylammonium) tetrachlorometallates(II) and the corresponding alkylammonium chlorides have been studied through their phase transitions using infrared spectroscopy. The studies show that the vibrational states of alkylammonium ions change markedly through the phase transitions. Spectra of the analogous tetrabromometallates and alkylammonium bromides also confirm this behaviour. There is appreciable motion of the alkylammonium ions in the high-temperature phases; thus, CH3NH+3 ions are essentially undistorted in these phases. The low-temperature, ordered phases show evidence of stronger hydrogen bonding of the cations and for the presence of C—N torsional modes.
