Adam-Gibbs model in the density scaling regime and its implications for the configurational entropy scaling

To solve a long-standing problem of condensed matter physics with determining a proper description of the thermodynamic evolution of the time scale of molecular dynamics near the glass transition, we extend the well-known Adam-Gibbs model to describe the temperature-volume dependence of structural relaxation times, ${\tau}_{\alpha} (T,V)$. We employ the thermodynamic scaling idea reflected in the density scaling power law, ${\tau}_{\alpha}=f(T^{-1} V^{-\gamma } )$, recently acknowledged as a valid unifying concept in the glass transition physics, to discriminate between physically relevant and irrelevant attempts at formulating the temperature-volume representations of the Adam-Gibbs model. As a consequence, we determine a straightforward relation between the structural relaxation time ${\tau}_{\alpha}$ and the configurational entropy $S_c$, giving evidence that also $S_c (T,V)=g(T^{-1} V^{-\gamma} )$ with the exponent {\gamma} that enables to scale ${\tau}_{\alpha} (T,V)$. This important finding has meaningful implications for the linkage between thermodynamics and molecular dynamics near the glass transition, because it implies that ${\tau}_{\alpha}$ can be scaled with $S_c$

Adam-Gibbs model in the density scaling regime and its implications for the configurational entropy scaling

To solve a long-standing problem of condensed matter physics with determining a proper description of the thermodynamic evolution of the time scale of molecular dynamics near the glass transition, we have extended the well-known Adam-Gibbs model to describe the temperature-volume dependence of structural relaxation times, τα(T, V). We also employ the thermodynamic scaling idea reflected in the density scaling power law, τα = f(T−1V−γ), recently acknowledged as a valid unifying concept in the glass transition physics, to differentiate between physically relevant and irrelevant attempts at formulating the temperature-volume representations of the Adam-Gibbs model. As a consequence, we determine a straightforward relation between the structural relaxation time τα and the configurational entropy SC, giving evidence that also SC(T, V) = g(T−1V−γ) with the exponent γ that enables to scale τα(T, V). This important findings have meaningful implications for the connection between thermodynamics and molecular dynamics near the glass transition, because it implies that τα can be scaled with SC

Synthesis and temperature-dependent studies of a perovskite-like manganese formate framework templated with protonated acetamidine

We report the synthesis, crystal structure, thermal, dielectric, phonon and magnetic properties of the [CH3C(NH2)2][Mn(HCOO)3] (AceMn) compound. Our results show that this compound crystallizes in the perovskite-like orthorhombic structure, space group Imma. It undergoes a structural phase transition at 304 K into a monoclinic structure, space group P21/n. X-ray diffraction, dielectric, IR and Raman studies show that the ordering of the acetamidinium cations triggers the phase transition. Low-temperature magnetic studies show that this compound exhibits weak ferromagnetic properties below 9.0 K

Semiconducting properties of Cu5SbO6

Thermoelectric power, electrical resistivity, I V characteristics, relative electrical permittivity, dc magnetization and ac magnetic susceptibility measurements carried out on Cu5SbO6 showed p-type semiconducting behaviour with the activation energy of 0.24 eV as well as ferrimagnetic order with the Néel temperature of 5.2 K. The e ective magnetic moment of 5.857 B/f.u. revealed the orbital contribution to the magnetic moment. Large value of the relative electrical permittivity indicated that the Cu2+ ions with the unscreened and un lled electron shells are responsible for the polarizability and forming of electric dipoles

Effect of tantalum substitution on dielectric constant of ZnSb2-xTaxO6 solid solution (x=0.0,0.1,0.25,0.75,1.6)

The electrical measurements of ZnSb2xTaxO6 phases with x = 0:0; 0:1; 0:25; 0:75; 1:6 have revealed insulating behavior with strongly decreasing electrical conductivity when tantalum x content is increased. As a consequence, high values of relative permittivity "r and loss tangent tan( ) were observed, that decreased with increase of temperature and frequencies, for samples with low Ta content, below x = 0:75. In turn, for samples richer in tantalum, "r reaches 14, and tan( ) becomes as low as 0.02 for x = 1:6. These effects have been interpreted either by the framework of the relaxation processes, or by the spatial charge polarization leading to the low energy loss of materials

Supramolecular Structure of Phenyl Derivatives of Butanol Isomers

Wide-angle X-ray scattering patterns were recorded for a series of aliphatic butanol isomers (n-, iso-, sec-, tert-butanol) and their phenyl derivatives (4-phenyl-1-butanol, 2-methyl-3- phenyl-1-propanol, 4-phenyl-2-butanol, and 2-methyl-1-phenyl-2- propanol, respectively) to determine their atomic-scale structure with particular emphasis on the formation of supramolecular clusters. In addition, molecular dynamics simulations were carried out and yielded good agreement with experimental data. The combination of experimental and theoretical results allowed clarification of the origin of the pre-peak appearing at low scattering angles for the aliphatic butanols and its absence for their phenyl counterparts. It was demonstrated that the location of the hydroxyl group in the molecule of alkyl butanol, its geometry, and rigidity determine the morphology of the supramolecular clusters, while the addition of the aromatic moiety causes more disordered organization of molecules. The phenyl group significantly decreases the number of hydrogen bonds and size of the supramolecular clusters formed via the O−H···O scheme. The lower association ability of phenyl alcohols via H-bonds is additionally attenuated by the appearance of competing π−π configurations evidenced by the structural models

Dielectric relaxation and anhydrous proton conduction in [C2H5NH3][Na0.5Fe0.5(HCOO)3] metal-organic frameworks

Metal–organic frameworks (MOFs), in which metal clusters are coupled by organic moieties, exhibit inherent porosity and crystallinity. Although these systems have been examined for vast potential applications, the elementary proton conduction in anhydrous MOFs still remains elusive. One of the approaches to deal with this problem is the utilization of protic organic molecules, to be accommodated in the porous framework. In this work we report the temperature-dependent crystal structure and proton conduction in [C2H5NH3][Na0.5Fe0.5(HCOO)3] metal–organic frameworks using X-ray diffraction and broadband dielectric spectroscopic techniques. The detailed analysis of the crystal structure reveals disorder of the terminal ethylene groups in the polar phase (space group Pn). The structural phase transition from Pn to P21/n at T ≈ 363 K involves the distortion of the metal formate framework and ordering of EtA+ cations due to the reduction of the cell volume. The dielectric data have been presented in the dynamic window of permittivity formalism to understand the ferroelectric phase transition. The relaxation times have been estimated from the Kramers–Kronig transformation of the dielectric permittivity. A Grotthuss type mechanism of the proton conduction is possible at low temperatures with the activation energy of 0.23 eV. This type of experimental observation is expected to provide new prospective on the fundamental aspect of elementary proton transfer in anhydrous MOFs