Pressure-induced two-step spin transition with structural symmetry breaking: X-ray diffraction, magnetic, and Raman studies

Shepherd, Helena J.; Bonnet, Sylvestre; Guionneau, Philippe; Bedoui, Salma; Garbarino, Gastón; Nicolazzi, William; Bousseksou, Azzedine; Molnár, Gábor

doi:10.1103/physrevb.84.144107

Cited by 53 publications

(58 citation statements)

References 60 publications

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“…48 As far as 1 is concerned, the two-step thermal transition with a HS to LS population ratio of 1/3 on the plateau has been uncovered using an Ising-like model with three interacting sublattices. 41,57 The width of the thermal transition plateau is quite large for 1 (50 K), and the two steps are associated with hysteresis behaviors, which confirm that indeed the intersublattice coupling might be strong in this material. Following the arguments of the dynamic Ising-like models discussed above, it is therefore not unexpected that a two-step relaxation process occurs in the dynamic regime at cryogenic temperature.…”

Section: Discussionsupporting

confidence: 55%

“…The P − T phase diagram of 1, as defined from a combination of singlecrystal x-ray diffraction, Raman spectroscopy, and magnetometry under hydrostatic pressures up to 16 kbar, reveals the presence of a two-step transition under pressure as well, with the same sequence of spin and crystallographic transitions. 57 Especially the crystal structure at a hydrostatic pressure of 4.6 kbar is completely isostructural with the intermediate state II determined at ambient pressure at 190 K. In addition, 1 exhibits a LIESST effect [T (LIESST) = 56 K], as evidenced by Raman spectroscopy performed at 77 K using a He-Ne excitation laser (632.8 nm), and confirmed by photomagnetic measurements. 41 Herein, we report the trapping of an ordered metastable [HS-LS-LS] superstructure for compound 1, which is furthermore detected as an intermediate state in the two-step relaxation following LIESST, from temperature-and timedependent x-ray crystallographic and photocrystallographic analysis.…”

Section: Imentioning

confidence: 58%

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Multimetastability, phototrapping, and thermal trapping of a metastable commensurate superstructure in a FeIIspin-crossover compound

et al. 2012

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The photoinduced switching and subsequent relaxation regime at cryogenic temperatures of the two-step spin-crossover compound [Fe(bapbpy)(NCS) 2 ] has been investigated by time-dependent photocrystallography. Upon photoexcitation from the low-spin (LS) state, a direct population of the metastable high-spin (HS) state occurs, without involving any intermediate structural state. The relaxation from the metastable HS state in isothermal conditions at 40 K proceeds in two successive steps associated with two symmetry breaking processes. The first step corresponds to the cooperative transformation to an intermediate superstructure, characterized by a long-range-ordered [HS-LS-LS] motif coupled to a commensurate displacive modulation, and concomitant with a tripling of the c axis of the unit cell (C2/c space group). The stabilization of the intermediate state is driven by strong molecule-lattice coupling. In the second stage, the intermediate state undergoes a transformation twinning triggered by lattice strain towards the LS state. The two-step relaxation is reminiscent of the two-step thermal transition of [Fe(bapbpy)(NCS) 2 ] and evidences multimetastability in the light-induced or relaxation regime. The long-range-ordered [HS-LS-LS] superstructure has also been trapped by rapid quench cooling to very low temperature, and has been structurally characterized.

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Section: Discussionsupporting

confidence: 55%

Section: Imentioning

confidence: 58%

Multimetastability, phototrapping, and thermal trapping of a metastable commensurate superstructure in a FeIIspin-crossover compound

et al. 2012

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“…This form of Hamiltonian (6) is due to a structure difference of adjacent two-spin clusters Cu1-R1 and Cu2-R2. Then, solving a set of linear equations arising from mapping onto spin Hamiltonian (6) results in the following J values:…”

Section: A the Cu(hfac) 2 L Me Casementioning

confidence: 99%

“…irradiation. [3][4][5][6] For instance, the classical case of SCO is a transition from the low spin state (S = 0, t 2g 6 ) to a high spin state (S = 2, t 2g 4 e g 2 ) of Fe(II) complexes with d 6 electron configuration with increasing temperature. 3,7 The SCO compounds represent prominent example of a bistability in the molecular crystals 8 and are attractive candidates for multifunctional materials 9,10 with potential applications to memory devices, temperature, optical, and pressure sensors, etc.…”

Section: Introductionmentioning

confidence: 99%

Exchange coupling transformations in Cu (II) heterospin complexes of “breathing crystals” under structural phase transitions

2015

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Family of "breathing crystals" is the polymer-chain complexes of Cu(hfac) 2 with nitroxides. The polymer chains consist of one-, two-or three-spin clusters. The "breathing crystals" experience simultaneous magnetic and Jahn-Teller type structural phase transitions with change of total cluster spin and drastic change of bond lengths (ca. 10-12%). For the first time the intra-cluster magnetic couplings in "breathing crystals" have been calculated both by band structure methods GGA+U and hybrid DFT (B3LYP and PBE0) for the isolated exchange clusters. The temperature dependence of the magnetic coupling constant was calculated for two polymer-chain compounds of the "breathing crystal" family -C 21 H 19 CuF 12 N 4 O 6 with the chains containing two-spin clusters and C 22 H 21 CuF 12 N 4 O 6 with the chains of alternating three-spin clusters and one-spin sites. It was found that adding a Hubbard-like parameter not only to the copper 3d electrons but also to the oxygen 2p electrons (GGA+U d +U p approach) results in an improved description of exchange coupling in the "breathing crystal" compounds. At the same time treatment of the isolated clusters by a large basis hybrid DFT with high computational cost provides a similar quality fit of the experimental magneto-chemical data as that for the GGA+U d +U p band structure calculation scheme. Our calculations also showed that in spite of the abrupt transformation of the magnetic coupling constant under the phase transition, the band gap in the "breathing crystals" remains about the same value with temperature decrease. C 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

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“…On the other hand, several studies aimed also to achieve PIPT in spin-crossover solids at the macroscopic scale using short laser pulses [7][8][9][10][11][12][13][14][15][16]. It was shown that a single laser pulse (ns or fs) can drive the system from one spin state to the other, but the microscopic details of this phenomenon remain largely unexplored-in particular for the most interesting cooperative systems.Compound 1 is a mononuclear iron(II) complex displaying a two-step thermal and pressure-induced spin transition [17][18][19][20][21]. Both steps involved in the transition are first order and show wide hysteresis loops of 6 and 25 K at atmospheric pressure.…”

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confidence: 99%

Triggering a Phase Transition by a Spatially Localized Laser Pulse: Role of Strain

et al. 2012

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We report here the optical microscopic imaging of a first-order phase transition induced by a nanosecond laser pulse (532 nm) in a single crystal of the molecular spin-crossover complex [FeðbapbpyÞðNCSÞ 2 ]. The transition starts with the formation of a high spin domain in the region irradiated by the focused laser beam, followed by the subsequent growth or contraction of the initial domain. Remarkably, in otherwise identical experimental conditions one can observe either the irreversible transition of the whole crystal or merely the formation of a transient domain-depending on which region of the crystal is excited. This observation as well as the rather slow dynamics suggest that the main control parameter is the inhomogeneous accommodation strain, which destabilizes the photoinduced domain. DOI: 10.1103/PhysRevLett.109.135702 PACS numbers: 64.70.KÀ, 07.60.Pb, 64.60.My, 75.30.Wx Photoinduced phase transition (PIPT) phenomena have received much attention in various types of solids such as organic charge transfer compounds, metal oxides, inorganic complexes, and even in metals [1][2][3][4]. In these materials, the strong electron-lattice or spin-lattice interactions have been shown to play a key role in driving the photoinduced transformation. Experimental evidence indicating that relatively weak photoexcitation can trigger a real macroscopic phase transition has been inferred in these systems from the huge photoconversion efficiencies as well as from the occurrence of characteristic nonlinear phenomena such as a threshold behavior. Here, we report on a conceptually different investigation where the phase transition was triggered by a spatially localized laser pulse within a comparatively large single crystal. Compared to previous studies on PIPT phenomena, our work brings in two new experimental features:(1) only a small part of the crystal is photoexcited by the focused laser beam, and (2) the phase change is not averaged for the whole crystal by a point or spectroscopic array detector but followed instead by an imaging charge-coupled device.For our experiments, we have chosen the molecular spin-crossover compound [Fe II ðbapbpyÞðNCSÞ 2 ] (1) (where bapbpy ¼ N-{6-[6-(pyridin-2-ylamino)pyridin-2-yl]pyridin-2-yl}pyridin-2-amine). Spin-crossover (SCO) materials of transition metal complexes are benchmark examples for PIPT. They exhibit bistability between the so-called low spin (LS) and high spin (HS) electronic configurations, which display strikingly different physical properties, including mass density, magnetic susceptibility, optical density, etc. [5]. Switching between the two molecular spin states can be induced by changing the sample temperature, applying external pressure or an intense magnetic field, and also by light irradiation. Light-induced excited spin state trapping in the solid state was first demonstrated by Decurtins et al. [6]. This so-called ''LIESST effect'' is basically a molecular phenomenon and can be observed only at cryogenic temperatures. On the other hand, several studies aimed a...

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Pressure-induced two-step spin transition with structural symmetry breaking: X-ray diffraction, magnetic, and Raman studies

Cited by 53 publications

References 60 publications

Multimetastability, phototrapping, and thermal trapping of a metastable commensurate superstructure in a FeIIspin-crossover compound

Multimetastability, phototrapping, and thermal trapping of a metastable commensurate superstructure in a FeIIspin-crossover compound

Exchange coupling transformations in Cu (II) heterospin complexes of “breathing crystals” under structural phase transitions

Triggering a Phase Transition by a Spatially Localized Laser Pulse: Role of Strain

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