1,720,965 research outputs found
Role of alkali ions in the near-zero thermal expansion of NaSICON-type AZr2(PO4)3 (A=Na,K,Rb,Cs) and Zr2(PO4)3 compounds
No full textZero thermal expansion (ZTE) is a rare phenomenon of great importance in the field of materials design. AZr2(PO4)3 (A = Na, K, Rb, Cs) exhibit near-zero thermal expansion. In this work, we perform first principles calculations in AZr2(PO4)3 (A = Na, K, Rb, Cs) and Zr2(PO4)3 compounds to elucidate the effects of A cations on the thermal expansion behavior. Structural and vibrational analysis shows that the near-zero thermal expansion is strongly related to the dynamics of the "lantern" units of [Zr2(PO4)3], where the ZrO6 and PO4 polyhedra are rigid. The calculated Gruneisen parameters reveal that different A-site alkali metal atoms heavily affect the phonon modes of A atoms and the rotation of ZrO6 and PO4 polyhedra, as well as the acoustic phonon modes, thus resulting in a different thermal expansion behavior of AZr2(PO4)3 (A = Na, K, Rb, Cs). The absence of A atoms in Zr2(PO4)3 facilitates the structural flexibility and therefore the occurrence of a stronger negative thermal expansion. This work provides insights into the modulation of thermal expansion of these NaZr2(PO4)3-type compounds
Insight into the Relationship between Negative Thermal Expansion and Structure Flexibility: The Case of Zn(CN)2-Type Compounds
No full textHigh structure flexibility can lead to large negative thermal expansion (NTE), but the reason is not clear. In this work, first-principles calculations have been carried out to investigate the relationship between NTE and structure flexibility in Zn(CN)2-type compounds. Smaller bulk modulus corresponds to larger compressibility, thus making the crystal structure more flexible and more suitable for NTE. It indicated that the ionic nature of the bond and the bond length jointly affect the structural flexibility and then act on the transverse vibration of C and N atoms. The results of lattice dynamic suggested that higher structural flexibility promotes a greater number of low-frequency optical modes with negative Grüneisen parameters, resulting in a larger NTE. This work also gives us new insight into the design of NTE materials
Control of Thermal Expansion in TaVO5 by Double Chemical Substitution
No full textThe control of thermal expansion is an important andchallengingissue. Focusing attention on the class of AMO(5) negativethermal expansion (NTE) materials, an approach to control their thermalexpansion is still missing. In this work, the thermal expansion ofTaVO(5) has been controlled from strong negative to zeroto positive by double chemical substitution, i.e., Ti and Mo replaceTa and V elements, respectively. A joint study of temperature-dependentX-ray diffraction, X-ray photoelectron spectroscopy, and first-principlescalculations has been performed to investigate the thermal expansionmechanism. With the increasing substitution of Ti and Mo atoms, thevalence state always remains balanced, and the volume decreases togetherwith a lattice distortion, which leads to the suppression of the NTE.Lattice dynamics calculations confirm that the negative Gru''neisenparameters of the low-frequency modes weaken and the thermal vibrationsof the polyhedral units diminish after the substitution of Ti andMo atoms. The present work successfully achieves a tailored thermalexpansion in TaVO5 and draws a possible way to controlthe thermal expansion of other NTE materials.We adopt the double-element chemicalsubstitution to realizethe control of thermal expansion from strong negative to zero to positivein a TaVO5 system, which causes more lattice distortionby simultaneously part replacing the A site (octahedron) and M site(tetrahedron)
Octahedral tilt distortion in negative thermal expansion in the fluorides CaZrF6 and ScF3
No full textUnderstanding the mechanism of negative thermal expansion (NTE) and controlling the thermal expansion properties of materials at the atomistic level have long captured the attention of scholars. In this regard, the NTE mechanism of fluorides such as monometallic ScF3 and bimetallic fluorides like CaZrF6 is particularly interesting; despite sharing similar crystal structure, CaZrF6 exhibits a thermal expansion coefficient twice that of ScF3. Why is this so? Here, we investigate the structural vibrations of these two substances in real and reciprocal space at atmospheric and zero pressure, respectively, using machine-learning based deep potentials in combination with density-functional theory. Our study reveals that the structural flexibility of CaZrF6 results in a greater number of vibration modes exhibiting negative Gruneisen parameters. Moreover, the combined impact of octahedral distortion and tilt vibration leads to a substantially greater NTE for CaZrF6 compared to ScF3. This work offers insights into understanding and tailoring NTE by considering polyhedral distortion in other open-framework materials
Understanding Large Negative Thermal Expansion of NdFe(CN)6 through the Electronic Structure and Lattice Dynamics
No full textA large negative thermal expansion (NTE) (αv = -4.1 × 10-5 K-1, 100-525 K) has been discovered in NdFe(CN)6. Here, the synchrotron X-ray diffraction and lattice dynamics calculations using the density functional theory were conducted to understand the NTE in NdFe(CN)6. The information obtained on the bond nature of the Nd-N≡C-Fe linkage and on the atomic thermal vibrations suggests that the transverse vibrations of the -N≡C- group, in particular from N atoms, produced the NTE in NdFe(CN)6. This is corroborated by the calculated Grüneisen parameters, which confirm the relationship between NTE and CN atomic vibrations. The results provide a helpful contribution toward the realization of new materials with negative or controllable thermal expansion
Critical Role of Nonrigid Unit and Spiral Acoustical Modes in Designing Colossal Negative Thermal Expansion
No full textExploring the relationship between thermal expansion and structural complexity is a challenging topic in the study of modern materials where volume stability is required. This work reports a new family of negative thermal expansion (NTE) materials, AM(CN)(4) with A = Li and Na and M = B, Al, Ga, and In. Here, the compounds of LiB(CN)(4) and NaB(CN)(4) were only synthesized; others were purely computationally studied. A critical role of nonrigid vibrational modes and spiral acoustical modes has been identified in NaB(CN)(4). This understanding has been exploited to design the colossal NTE materials of NaM(CN)(4) (M = Al, Ga, In). A joint study involving synchrotron X-ray diffraction, Raman spectroscopy, and first-principles calculations has been conducted to investigate the thermal expansion mechanism. It has been found that the A atoms can either increase the symmetry of the crystal structure, inducing stronger NTE, or lower the crystal symmetry, thus resulting in positive thermal expansion. Conversely, the M-site atoms do not affect the crystal structure. However, as the radius of the M atoms increases, the ionic nature of the C-M bonds strengthens and the CN vibrations become more flexible, thereby enhancing the NTE behavior. This study provides new insights to aid in the discovery and design of novel NTE materials and the control of thermal expansion
Giant Negative Thermal Expansion in Ultralight NaB(CN)4
No full textNegative thermal expansion (NTE) is crucial for controlling the thermomechanical properties of functional materials, albeit being
relatively rare. This study reports a giant NTE (aV=-9.2·10-5 K-1, 100-200 K; aV=-3.7·10-5 K-1, 200-650 K) observed in NaB(CN)4, showcasing interesting ultralight properties. A comprehensive investigation involving synchrotron X-ray diffraction, Raman spectroscopy, and first-principles calculations has been conducted to explore the thermal expansion mechanism. The findings indicate that the low-frequency phonon modes play a primary role in NTE, and non rigid vibration modes with most negative Grüneisen parameters are the key contributing factor to the giant NTE observed in NaB(CN)4. This work presents a new material with giant NTE and ultralight mass density, providing insights for the understanding and design of novel NTE materials
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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