The unique physical property of negative thermal expansion (NTE) is not only interesting for scientific research but also important for practical applications. Chemical modification generally tends to weaken NTE. It remains a challenge to obtain enhanced NTE from currently available materials. Herein, we successfully achieve enhanced NTE in Pb(TiV)O by improving its ferroelectricity. With the chemical substitution of vanadium, lattice tetragonality (c/a) is highly promoted, which is attributed to strong spontaneous polarization, evidenced by the enhanced covalent interaction in the V/Ti-O and Pb-O2 bonds from first-principles calculations. As a consequence, Pb(TiV)O exhibits a nonlinear and much stronger NTE over a wide temperature range with a volumetric coefficient of thermal expansion α = -3.76 × 10/°C (25-550 °C). Interestingly, an intrinsic giant volume contraction (∼3.7%) was obtained at the composition of Pb(TiV)O during the ferroelectric-to-paraelectric phase transition, which represents the highest value ever reported. Such volume contraction is well correlated to the effect of spontaneous volume ferroelectrostriction. The present study extends the scope of the NTE family and provides an effective approach to explore new materials with large NTE, such as through adjusting the NTE-related ferroelectric property in the family of ferroelectrics.
An aqueous extract of Phyllanthus niruri (Euphorbiaceae) inhibited human immunodeficiency virus type-1 reverse transcriptase (HIV-1-RT). The inhibitor against HIV-1-RT in this plant was purified by combination of three column chromatographies, Sephadex LH-20, cellulose, and reverse-phase high-performance liquid chromatography. The inhibitor was then identified by nuclear magnetic resonance (NMR) spectra as repandusinic acid A monosodium salt (RA) which was originally isolated from Mallotus repandus. The 50% inhibitory doses (ID50) of RA on HIV-1-RT and DNA polymerase alpha (from HeLa cells) were 0.05 microM and 0.6 microM, respectively, representing approximately a 10-fold more sensitivity of HIV-1-RT compared with DNA polymerase alpha. RA was shown to be a competitive inhibitor with respect to the template-primer while it was a noncompetitive inhibitor with respect to the substrate. RA as low as 10.1 microM inhibited HIV-1-induced cytopathogenicity in MT-4 cells. In addition, 4.5 microM of RA inhibited HIV-1-induced giant cell formation of SUP-T1 approximately 50%. RA (2.5 microM) inhibited up to 90% of HIV-1 specific p24 antigen production in a Clone H9 cell system.
The discovery of unusual negative thermal expansion (NTE) provides the opportunity to control the common but much desired property of thermal expansion, which is valuable not only in scientific interests but also in practical applications. However, most of the available NTE materials are limited to a narrow temperature range, and the NTE effect is generally weakened by means of various modifications. Here, we report an enhanced NTE effect that occurs over a wide temperature range ( V = -5.24 × 10 -5 °C -1 , 25-575 C), and this NTE effect is accompanied by an abnormal enhanced tetragonality, a large spontaneous polarization, and a G-type antiferromagnetic ordering in the present perovskite-type ferroelectric of (1-x)PbTiO3-xBiCoO3. Specifically, for the composition of 0.5PbTiO3-0.5BiCoO3, an extensive volumetric contraction of ~4.8 % has been observed near the Curie temperature of 700 °C, which represents the highest level in PbTiO3-based ferroelectrics.According to our experimental and theoretical results, the giant NTE originates from a synergistic effect of the ferroelectrostriction and spin-crossover of cobalt on the crystal lattice. The actual NTE mechanism is contrasted with previous functional NTE materials, in which the NTE is simply coupled with one ordering such as electronic, magnetic, or ferroelectric ordering. The present study sheds light on the understanding of NTE mechanisms and it attests that NTE could be simultaneously coupled with different orderings, which will pave a new way toward the design of large NTE materials.
PbVO 3 features giant tetragonal distortion (c/a = 1.23) and spontaneous polarization (P s = 101 μC/cm 2 ) originating from the ordering of d xy orbital of V 4+ with single d electrons. A dedicated study of Cr-substituted PbVO 3 prepared by high-pressure synthesis afforded a well-defined variation in c/a ratio. Ensuing changes in atomic structure were correlated to ferroelastic domain switching and a temperature-induced structural transition to a cubic phase accompanied with an insulator-to-metal transition and a negative thermal expansion.
Perovskite-oxide Bi 1−x Pb x NiO 3 for 0.60 ≤ x ≤ 0.80 was found to show a polar orthorhombic-to-nonpolar orthorhombic phase transition accompanied by negative thermal expansion. Bi 1−x Pb x NiO 3 showed successive crystal structure changes depending on the amount of Pb. As the amount of Pb increased, the crystal structure changed from a triclinic one with Bi 3+ /Bi 5+ long-range ordering to an orthorhombic one with Bi 3+ /Bi 5+ short-range ordering; then, it changed into a polar orthorhombic structure without Bi 3+ / Bi 5+ ordering and finally to a polar LiNbO 3 -type one. The key to the inversion symmetry breaking in PbNiO 3 , where both 6s 2 lone-pair and Jahn−Teller active cations are absent, is the high-valency state of Pb 4+ . Our results suggest that the polar orthorhombic phase can be realized by using high-valence A-site cations in addition to controlling the tolerance factor.
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