Abstract
La
0.4
Pr
0.3
Ca
0.1
Sr
0.2
MnO
3
has been investigated as a potential candidate for room temperature magnetic refrigeration. Results from X-ray powder diffraction reveal an orthorhombic ...structure with Pnma space group. The electronic and chemical properties have been confirmed by X-ray photoelectron spectroscopy and ion-beam analysis. A second-order paramagnetic to ferromagnetic transition was observed near room temperature (289 K), with a mean-field like critical behaviour at low field and a tricritical mean-field like behaviour at high field. The field induced crossover in critical behaviour is a consequence of the system being close to a first-order magnetic transition in combination with a magnetic field induced suppression of local lattice distortions. The lattice distortions consist of interconnected and weakly distorted pairs of Mn-ions, where each pair shares an electron and a hole, dispersed by large Jahn–Teller distortions at Mn
3+
lattice sites. A comparatively high value of the isothermal entropy-change (3.08 J/kg-K at 2 T) is observed and the direct measurements of the adiabatic temperature change reveal a temperature change of 1.5 K for a magnetic field change of 1.9 T.
The reaction of (Cp*Mo)2(μ‐Cl)2B2H6 (1) with CO at room temperature led to the formation of the highly fluxional species {Cp*Mo(CO)2}2{μ‐η2:η2‐B2H4} (2). Compound 2, to the best of our knowledge, is ...the first example of a bimetallic diborane(4) conforming to a singly bridged Cs structure. Theoretical studies show that 2 mimics the Cotton dimolybdenum–alkyne complex {CpMo(CO)2}2C2H2. In an attempt to replace two hydrogen atoms of diborane(4) in 2 with a 2e W(CO)4 fragment, {Cp*Mo(CO)2}2 B2H2W(CO)4 (3) was isolated upon treatment with W(CO)5⋅thf. Compound 3 shows the intriguing presence of B2H2 with a short B−B length of 1.624(4) Å. We isolated the tungsten analogues of 3, {Cp*W(CO)2}2B2H2W(CO)4 (4) and {Cp*W(CO)2}2B2H2Mo(CO)4 (5), which provided direct proof of the existence of the tungsten analogue of 2.
Different ways 2 B special: Dinuclear Group 6 metal complexes of diborane(4) and diborene(2) in unusual bonding situations have been synthesized (see core structures). The highly fluxional species {Cp*Mo(CO)2}2{μ‐η2:η2‐B2H4} was found to mimic the Cotton dimolybdenum–alkyne complex {CpMo(CO)2}2C2H2. Treatment with W(CO)5⋅thf gave {Cp*Mo(CO)2}2B2H2W(CO)4 containing B2H2 with a short B−B length of 1.624(4) Å.
The inherent tendency of BR fragments to undergo coupling is utilized to predict M 2 B 10 H 10 and M 2 @B 10 H 8 complexes (where M = Mn and Fe). Electronic structure analysis of Mn 2 B 10 H 10 (7) ...shows that the metal d-orbitals stabilize the interlocked boron wheel structure, forming an unprecedented geometrical pattern with Möbius aromaticity. The two additional electrons in Fe 2 @B 10 H 10 (8) stabilize a twisted 10boraannulene structure. The removal of 2H from 7 and 8 leads to the planar structures Mn 2 @B 10 H 8 (11) and Fe 2 @B 10 H 8 (10), respectively. The stability of the planar arrangements is due to multicentered (σ + π) bonding, where π-donation occurs from the M 2 (M = Fe and Mn) unit to the borocyclic unit. The presence of 10π electrons in M 2 @B 10 H 8 relates it to naphthalene, having Hückel π-aromaticity. The condensation of naphthalene to graphene in two dimensions suggests the ability to build the different metal boride monolayers FeB 5 and Fe 2 B 5 , considering Fe 2 @B 10 as the building block, bringing this molecular boron chemistry into the solid state. One of the predicted monolayers, β-Fe 2 B 5 , is found to be the global minimum in the planar arrangement based on a USPEX crystal structure search algorithm. Electronic structure analysis further shows that the stabilization mechanism in the molecular building block remains unaltered in the solid state.
The development of a fully ab initio theory for the chemical vapor deposition (CVD) synthesis of two-dimensional (2D) materials is a prominent challenge in computational chemistry and materials ...science. Here, quantum-mechanical density functional theory calculations are used to discover the mechanisms underlying the nucleation and growth of monolayer 2H molybdenum disulfide (MoS2) during organometallic CVD. Starting with molybdenum hexacarbonyl (Mo(CO)6) and hydrogen sulfide (H2S) as molecular precursors, we elucidate processes such as the decomposition of Mo(CO)6 to Mo(CO)3, sulfidation of Mo(CO)3, formation of metallic trigonal-phase (1T) Mo–S clusters, transition to semiconducting hexagonal-phase 2H MoS2, and the competition between the growth of Mo- and S-zigzag edges that lead to triangular and hexagonal flakes. We demonstrate thermodynamic and kinetic control over the formation of Mo- and S-zigzag edges. Additionally, we find the removal of hydrogen (H2) to be the rate-determining step in the growth process. We further compute the free energy of formation of the investigated Mo–S clusters on amorphous SiO2, demonstrating the important role played by the SiO2 substrate in the initial stages of nucleation and growth. We also show the feasibility of forming Mo–S clusters with more than two Mo atoms on the SiO2 surface. Our work lays the foundation for developing fully ab initio models of 2D material synthesis.
The electronic structure of (η5-Cp)2Zr(NH2-BB-NH2) (3b) suggests that it could be a candidate for having a boron-boron triple bond in the cyclic system; however, computational studies shows that 3b ...is a very high energy isomer on its potential energy surface. Replacement of amines with tricoordinate nucleophilic boron groups (η5-Cp)2ZrB(PH3)2-BB-B(PH3)2 (3c) reduces the relative energy dramatically. The BB triple bond arises through the donation of two electrons from the metal fragment, ZrCp2, to the in-plane π-bonding orbital of the central B-B unit. Strong σ-donating and chelating bis-phosphine ligands (Me2P(CH2)nPMe2), which stabilize donor-acceptor bonding interaction in gem-diborene L2B-BBr2 (10), would be a good choice along the synthetic path towards 3d, (η5-Cp)2ZrB4(Me2P(CH2)3PMe2)2. A comparison of the energetics of the reaction leading to a cyclic boryne system (3d), with the linear boryne isomer (B2NHCPh)2 shows that the angle strain from cyclization is compensated by stabilization from the metal.
The inherent tendency of BR fragments to undergo coupling is utilized to predict M
2
B
10
H
10
and M
2
@B
10
H
8
complexes (where M = Mn and Fe). Electronic structure analysis of Mn
2
B
10
H
10
(7) ...shows that the metal d-orbitals stabilize the interlocked boron wheel structure, forming an unprecedented geometrical pattern with Möbius aromaticity. The two additional electrons in Fe
2
@B
10
H
10
(8) stabilize a twisted 10boraannulene structure. The removal of 2H from 7 and 8 leads to the planar structures Mn
2
@B
10
H
8
(11) and Fe
2
@B
10
H
8
(10), respectively. The stability of the planar arrangements is due to multicentered (σ + π) bonding, where π-donation occurs from the M
2
(M = Fe and Mn) unit to the borocyclic unit. The presence of 10π electrons in M
2
@B
10
H
8
relates it to naphthalene, having Hückel π-aromaticity. The condensation of naphthalene to graphene in two dimensions suggests the ability to build the different metal boride monolayers FeB
5
and Fe
2
B
5
, considering Fe
2
@B
10
as the building block, bringing this molecular boron chemistry into the solid state. One of the predicted monolayers, β-Fe
2
B
5
, is found to be the global minimum in the planar arrangement based on a USPEX crystal structure search algorithm. Electronic structure analysis further shows that the stabilization mechanism in the molecular building block remains unaltered in the solid state.
The design of (1) Möbius aromatic interlocked boron wheel Mn
2
B
10
H
10
, (2) Hückel aromatic boron analogs of naphthalene (M
2
@B
10
H
8
; M = Mn and Fe), and (3) metal boride monolayers (FeB
5
and Fe
2
B
5
), creating a molecules to materials continuum.
In order to find a highly efficient, environment-friendly magnetic refrigerant, direct measurements of the adiabatic temperature change Δ T adb is required. Here, in this work a simple setup for the ...Δ T adb measurement is presented. Using a permanent magnet Halbach array with a maximum magnetic field of 1.8 T and a rate of magnetic field change of 5 T/s, accurate determination of Δ T adb is possible in this system. The operating temperature range of the system is from 100 K to 400 K, designed for the characterization of materials with potential for room temperature magnetic refrigeration applications. Using the setup, the Δ T adb of a first-order and two second-order compounds have been studied. Results from the direct measurement for the first-order compound have been compared with Δ T adb calculated from the temperature and magnetic field dependent specific heat data. By comparing results from direct and indirect measurements, it is concluded that for a reliable characterization of the magnetocaloric effect, direct measurement of Δ T adb should be adopted.
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