Structural phase transition to disorder low-temperature phase in [Fe(ptz)₆](BF₄)₂ spin-crossover compounds

Abstract

In the spin-crossover compound [Fe(ptz)(6)](BF(4))(2) (where ptz=1-n-propyltetrazole) six different phases are observed. When a single crystal is slowly cooled from high temperatures to those below 125 K, the reflections broaden into diffuse maxima and split into two maxima along the c* direction [Kusz, Gütlich & Spiering (2004). Top. Curr. Chem. 234, 129-153]. As both maxima are broad along the c* direction, the short-range order exists only along the c direction and in the ab plane the structure remains long-range ordered. In this disordered phase additional satellite reflections appear. Upon heating above 135 K, the diffuse maxima return to their previous shape and this process is completely reversible. Rapidly cooled samples, on the other hand, do not show such splitting and the symmetry remains R\bar 3, despite a jump in lattice parameters. We use a special technique to analyse the disorder model of the slowly cooled samples, which consists of layered domains shifted in the hexagonal ab plane. The low-spin disordered phase was solved in a novel approach to accommodate the very unusual twinning and refined in the non-standard space group C\bar 1. In contrast to the ordered low-spin phase, the Fe ion is in a non-centrosymmetric coordination polyhedron and two of the six propyl groups change their conformation.

Piezo‐ and Photo‐Crystallography Applied to Spin‐Crossover Materials

“…Apart from revealing the pressure-temperature phase diagram of this material, already identified as very intricate in isobaric conditions, 71 this study confirmed the interest in getting high pressure structural data at low temperatures, which is one of the remaining challenges in the field.…”

“…94 The recent construction of He cryostats with coupled laser excitation has allowed the development of photo-crystallography of SCO compounds. 64,65,71,84,[94][95][96][97][98][99][100] A nice illustration of the information one can obtain from detailed analysis of the molecular structures of photoinduced states is provided by the comparison of the structural changes in the two polymorphs of [Fe(PM-BiA) 2 (NCS) 2 ], denoted BIA I and BIA II, 94 whose T(LIESST) values are quite different (78 K for BIA I and 35 K for BIA II). The evolution of structural parameters at the intramolecular level during thermal conversion, and after photoexcitation at low temperature, are shown in Figure 20.4.…”

Switchable slow relaxation of magnetization in the native low temperature phase of a cooperative spin-crossover compound

“…In both structures, the [Fe(mtz) 6 ] 2+ cations form similar supramolecular hexagonal layers, reminiscent of analogue tetrazole complexes, 18,19 through C tz –H···A hydrogen bonds with the triflate anions (Fig. 2, Table S3†).…”

General Landau theory of non-symmetry-breaking and symmetry-breaking spin transition materials

“…A discontinuous symmetry breaking phase transition coupled to a spin-crossover corresponds also to the case of the [Fe(ptz)6](BF4)2 system. [53][54][55][56][57][58] At atmospheric pressure this system exhibits a spin conversion coupled to a first-order ferroelastic phase transition and both phenomena occur simultaneously, 54 while both phenomena occurs separately under pressure. 59,60 V Non-symmetry-breaking stepwise phase transitions Non-symmetry breaking stepwise phase transitions occur in systems for which the crystalline lattice is made of sub-lattices with active molecular sites independent by symmetry.…”