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91.
An Unusual Phase Transition Driven by Vibrational Entropy Changes in a Hybrid Organic–Inorganic Perovskite
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Wenjuan Wei Prof. Wei Li Dr. Keith T. Butler Dr. Guoqiang Feng Prof. Christopher J. Howard Prof. Michael A. Carpenter Prof. Peixiang Lu Prof. Aron Walsh Prof. Anthony K. Cheetham 《Angewandte Chemie (International ed. in English)》2018,57(29):8932-8936
The driving forces for the phase transitions of ABX3 hybrid organic–inorganic perovskites have been limited to the octahedral tilting, order–disorder, and displacement. Now, a complex structural phase transition has been explored in a HOIP, [CH3NH3][Mn(N3)3], based on structural characterizations and ab initio lattice dynamics calculations. This unusual first‐order phase transition between two ordered phases at about 265 K is primarily driven by changes in the collective atomic vibrations of the whole lattice, along with concurrent molecular displacements and an unusual octahedral tilting. A significant entropy difference (4.35 J K?1 mol?1) is observed between the low‐ and high‐temperature structures induced by such atomic vibrations, which plays a main role in driving the transition. This finding offers an alternative pathway for designing new ferroic phase transitions and related physical properties in HOIPs and other hybrid crystals. 相似文献
92.
Manganese Tetraboride,MnB4: High‐Temperature Crystal Structure,p–n Transition, 55Mn NMR Spectroscopy,Solid Solutions,and Mechanical Properties
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Dr. Arno Knappschneider Dr. Christian Litterscheid Dr. Jakoah Brgoch Dr. Nathan C. George Dr. Sebastian Henke Prof. Anthony K. Cheetham Dr. Jerry G. Hu Prof. Ram Seshadri Prof. Dr. Barbara Albert 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(22):8177-8181
The structural and electronic properties of MnB4 were studied by high‐temperature powder X‐ray diffraction and measurements of the conductivity and Seebeck coefficient on spark‐plasma‐sintered samples. A transition from the room‐temperature monoclinic structure (space group P21/c) to a high‐temperature orthorhombic structure (space group Pnnm) was observed at about 650 K. The material remained semiconducting after the transition, but its behavior changed from p‐type to n‐type. 55Mn NMR measurements revealed an isotropic chemical shift of ?1315 ppm, confirming an oxidation state of Mn close to I. Solid solutions of Cr1?xMnxB4 (two phases in space groups Pnnm and P21/c) were synthesized for the first time. In addition, nanoindentation studies yielded values of (496±26) and (25.3±1.7) GPa for the Young’s modulus and hardness, respectively, compared to values of 530 and 37 GPa obtained by DFT calculations. 相似文献
93.
We examine using Monte Carlo simulations, photon transport in optically ‘thin’ slabs whose thickness L is only a few times the transport mean free path l*, with particles of different scattering anisotropies. The confined geometry causes an auto-selection of only photons with
looping paths to remain within the slab. The results of the Monte Carlo simulations are borne out by our analytical treatment
that incorporates directional persistence by the use of the Ornstein-Uhlenbeck process, which interpolates between the short
time ballistic and long time diffusive regimes. 相似文献
94.
Obliquely propagating altra-low-frequency dust-electromagnetic waves in a self-gravitating, warm, magnetized, two fluid dusty
plasma system have been investigated. Two special cases, namely, dust-Alfvén mode propagating parallel to the external magnetic
field and dustmagnetosonic mode propagating perpendicular to the external magnetic field have also been considered. It has
been shown that effects of self-gravitational field, dust fluid temperature, and obliqueness significantly modify the dispersion
properties of these ultra-low-frequency dust-electromagnetic modes. It is also found that in parallel propagating dust-Alfvén
mode these effects play no role, but in obliquely propagating dust-Alfvén mode or perpendicular propagating dust-magnetosonic
mode the effect of self-gravitational field plays destabilizing role whereas the effect of dust/ion fluid temperature plays
stabilizing role. 相似文献
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98.
Thomas D. Bennett Dr. Jin‐Chong Tan Dr. Stephen A. Moggach Raimondas Galvelis Dr. Caroline Mellot‐Draznieks Prof. Barbara A. Reisner Dr. A. Thirumurugan Dr. David. R. Allan Prof. Anthony K. Cheetham 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(35):10684-10690
The dense, anhydrous zeolitic imidazolate frameworks (ZIFs), Zn(Im)2 ( 1 ) and LiB(Im)4 ( 2 ), adopt the same zni topology and differ only in terms of the inorganic species present in their structures. Their mechanical properties (specifically the Young’s and bulk moduli, along with the hardness) have been elucidated by using high pressure, synchrotron X‐ray diffraction, density functional calculations and nanoindentation studies. Under hydrostatic pressure, framework 2 undergoes a phase transition at 1.69 GPa, which is somewhat higher than the transition previously reported in 1 . The Young’s modulus (E) and hardness (H) of 1 (E≈8.5, H≈1 GPa) is substantially higher than that of 2 (E≈3, H≈0.1 GPa), whilst its bulk modulus is relatively lower (≈14 GPa cf. ≈16.6 GPa). The heavier, zinc‐containing material was also found to be significantly harder than its light analogue. The differential behaviour of the two materials is discussed in terms of the smaller pore volume of 2 and the greater flexibility of the LiN4 tetrathedron compared with the ZnN4 and BN4 units. 相似文献
99.
Tan JC Civalleri B Lin CC Valenzano L Galvelis R Chen PF Bennett TD Mellot-Draznieks C Zicovich-Wilson CM Cheetham AK 《Physical review letters》2012,108(9):095502
Using Brillouin scattering, we measured the single-crystal elastic constants (C(ij)'s) of a prototypical metal-organic framework (MOF): zeolitic imidazolate framework (ZIF)-8 [Zn(2-methylimidazolate)(2)], which adopts a zeolitic sodalite topology and exhibits large porosity. Its C(ij)'s under ambient conditions are (in GPa) C(11)=9.522(7), C(12)=6.865(14), and C(44)=0.967(4). Tensorial analysis of the C(ij)'s reveals the complete picture of the anisotropic elasticity in cubic ZIF-8. We show that ZIF-8 has a remarkably low shear modulus G(min) < or approximately 1 GPa, which is the lowest yet reported for a single-crystalline extended solid. Using ab initio calculations, we demonstrate that ZIF-8's C(ij)'s can be reliably predicted, and its elastic deformation mechanism is linked to the pliant ZnN(4) tetrahedra. Our results shed new light on the role of elastic constants in establishing the structural stability of MOF materials and thus their suitability for practical applications. 相似文献
100.
Lee C Mellot-Draznieks C Slater B Wu G Harrison WT Rao CN Cheetham AK 《Chemical communications (Cambridge, England)》2006,(25):2687-2689
Experimental and computational studies indicate that the formation of a series of zinc 4-cyclohexene-1,2-dicarboxylates takes place under thermodynamic rather than kinetic control. 相似文献