An Unusual Phase Transition Driven by Vibrational Entropy Changes in a Hybrid Organic–Inorganic Perovskite

Abstract

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Configurational Entropy Driven High‐Pressure Behaviour of a Flexible Metal–Organic Framework (MOF)

“…Intriguingly, the chemical variability of MOFs and coordination networks more gen-erally allows for studying the underlying free energy landscape as a function of small chemical changes, paving the ground for a fundamentally motivated approach for materials design with (smart) dynamic responses to external stimuli. [4,5] A subclass of MOFs, so-called flexible MOFs, shows large structural flexibility with volume changes exceeding DV = 20 % as response to temperature and pressure variation, and guest adsorption. [5,6] Intense research efforts have shown that macroscopic parameters such as topology, [7,8] dispersion interactions and vibrational entropy [9][10][11][12] as determined by microscopic chemical interactions all contribute to structural flexibility; however, the targeted synthesis of flexible MOFs which concerns the manipulation of macroscopic thermodynamics via chemical changes on a microscopic level is still beyond our knowledge.…”

First-principles study of the structural, electronic, magnetic, and ferroelectric properties of a charge-ordered iron(ii)-iron(iii) formate framework

“…Metal-organic frameworks (MOFs), consisting of organic cations, a network of metal ions coordinated by organic bridging ligands, have attracted a lot of attention because of their multifunctional properties, such as ferroelectricity, magnetism, and conductivity. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] In particular, the combination of ferroelectric and magnetic orders (i.e., multiferroicity) in a single-phase material is of great technological and fundamental importance. 1-3, 13, 18-21 Furthermore, by considering their diversity in chemical compositions and structural topologies, MOFs are considered as highly promising candidates for multifunctional materials for their potential application in gas absorption and separation, ionic exchange and identification, conductive and catalytic, magnetic and even ferroelectric properties.…”

Free energy predictions for crystal stability and synthesisability

“…where ω νq is the phonon frequency for band ν and wavevector q. While the absolute value of S vib can easily exceed 100 k B , 18 in balanced reactions such as Eq. 4, the resulting change…”

“…Indeed, there are many examples of temperature-driven (first-order) phase transitions that are well described within the QHA, including the monoclinic-to-tetragonal phase transition in ZrO 2 27 and the α-to-β transition in elemental Sn, 28 through to more complex hybrid organic-inorganic crystals. 18…”

“…Within the harmonic approximation (HA), the vibrational entropy is given bywhere ω ν q is the phonon frequency for band ν and wavevector q . While the absolute value of S vib can easily exceed 100 k B , 18 in balanced reactions such as eqn (4), the resulting changeis smaller and typically in the range 0–5 k B .…”