Tuning Magnetic Properties with Pressure in Hybrid Organic

Article pubs.acs.org/JPCC

Tuning Magnetic Properties with Pressure in Hybrid Organic− Inorganic Materials: The Case of Copper Hydroxide Acetate Fan Yang,† Carlo Massobrio,‡ and Mauro Boero*,‡ †

School of Electronic and Electrical Engineering, Wuhan Textile University, Wuhan 430073, China Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), University of Strasbourg and CNRS, UMR 7504, 23 rue du Loess, F-67034 Strasbourg, France

‡

S Supporting Information *

ABSTRACT: The magnetic response and related spin topology of the hybrid organic−inorganic material copper hydroxide acetate Cu2(OH)3(CH3COO)·H2O are studied as a function of an applied external pressure within first-principles approaches. We show that structural changes induced by high pressure affect the Cu−O−Cu angles, particularly when the bridging O atom belongs to the CH3COO chains, with a sharp transition occurring above 2.75 GPa. These geometrical modifications are responsible for a transition from an antiferromagnetic (at the ambient pressure ground state) to a ferromagnetic state at high pressure (∼7 GPa). Our results are in agreement with the experimental outcome. They provide a guideline for applications of these composite systems in nanoelectronics and disclose new frontiers in the design of memory devices based on this family of hybrid lamellar materials.

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INTRODUCTION In organic−inorganic lamellar hybrid materials, a wide variety of organic chains can be inserted within the host architecture, i.e., between the transition metal ion-based oxide layers. This allows for a fine-tuning of the physical and chemical properties of the resulting system, mainly because of the flexibility and the chemical nature of the organic chains. The use of an external macroscopic quantity, such as pressure, is a practical tool to tune the properties of these materials and, ultimately, control their magnetic character.1 This is a promising path toward applications in next-generation nanoelectronics, especially memory switches and data storage devices.2 A representative prototype of these lamellar organic−inorganic materials is copper hydroxide acetate, Cu2(OH)3(CH3COO)·H2O. Such a compound consists of triangular arrays of CuII hydroxide building blocks forming a two-dimensional host structure able to accommodate an organic moiety, specifically CH3COO− spacers.3,4 The distance between two adjacent Cu hydroxide layers depends on the chemical nature and the conformation of the inserted organic chains.2 If an external pressure is applied along the crystallographic direction orthogonal to the Cu hydroxide planes, structural changes occur in the organic chains as a response to the reduction of the interlayer distance. Eventually, increasing values of the pressure are able to modify the magnetic properties of the material. In former studies,2 a comparison of the magnetic behavior of Cu2(NO3)(OH)3 and Cu2(OH)3(CH3COO)·H2O has shown for both an antiferromagnetic (AF) character as a ground state. However, the replacement of NO3− with the longer CH3COO− acetate units has the net effect of promoting the arising of a weak © 2014 American Chemical Society

ferromagnetic (F) intralayer character, as opposed to the persisting AF character of Cu2(NO3)(OH)3. In our previous investigations,3,4 first-principles approaches5,6 based on the density functional theory7 (DFT) have been useful to complement the missing structural information concerning the position of atoms, escaping experimental probes, and to provide insight into the bonding properties and the spin topology of Cu2(OH)3(CH3COO)·H2O. An early X-ray powder diffraction study of Cu2(OH)3(CH3COO)·H2O had highlighted the role of water molecules in between the layers, which can be easily and reversibly removed upon moderate heating.8 However, due to the lack of synthesized single crystals of suitable quality, the set of atomic coordinates remained long time incomplete. Recent developments in theoretical approaches and the increasing power of computational facilities allowed us to reach a comprehensive atomiclevel insight into both structural and magnetic properties of this material3 and to complement experiments by providing the missing coordinates information. The predictive power of these pioneering studies was then confirmed one year later,9 when another set of measurements was reported for this same system, thus providing a clear description of the structure of this material. Stimulated by recent experiments,10 we focus here on the effects of an external pressure, orthogonal to the copper hydroxide planes, on the spin topology of the Cu-based layers. Tuning the local magnetic character of this class of hybrid Received: April 9, 2014 Revised: July 24, 2014 Published: July 28, 2014 18700

dx.doi.org/10.1021/jp503489g | J. Phys. Chem. C 2014, 118, 18700−18705

The Journal of Physical Chemistry C

Article

text and in the tables, the spin density to which we refer is intended to be the standard definition

material by an external macroscopic thermodynamical variable is prone to give access to a wealth of applications in volatile/ nonvolatile memory nanodevices, an important component in the challenging postscale technology, where cutting-edge miniaturization is mandatory.11 To shed some light into the issue of spin transition as a response to pressure, we performed atomic-scale simulations, paying particular attention to the DFT level adopted, with specific emphasis on the functional describing the exchangecorrelation interaction. This is a crucial issue for the reliability of the simulations, particularly when subtle spin distribution details are involved. Our calculations are performed in a wide range of pressure, ranging from 0 to 7.5 GPa, providing a detailed insight into both the structure and the related electronic properties of copper hydroxide acetate. In particular, we are able to highlight the peculiarity of the piezomagnetic character of Cu2(OH)3(CH3COO)·H2O. More precisely, this system displays a clear structural transition in the Cu−O−Cu angles for pressures above 2.75 GPa. Once this threshold is overcome, further increase of pressure confers a distinct F character to the Cu-based layers forming the scaffold of this composite material.

ρs (x) = ρα (x) − ρβ (x) =

∑ |ψiα(x)|2 −∑ |ψi β(x)|2 i

i

(1)

where α and β indicate the up- and down-spin components, respectively, and ψiα,β(x) are the Kohn−Sham orbitals of the system. The pressure, evaluated as one-third of the trace of the associated stress tensor,19 was computed for each structure obtained upon NVE dynamics for which the external (constant) pressure was imposed by shrinking the system along the c crystallographic direction, thus preserving the angles and the symmetry of the cell. The lattice parameters used for each run at different pressure were extrapolated from experimental data (see Tables S1 and S2 in the Supporting Information). Further check was done by fitting the total energy E of the system as a function of the volume of the simulation cell and by computing the pressure in the standard way as P = dE/dV. This is summarized in the Supporting Information (Figure S1).

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RESULTS AND DISCUSSION For each of the two exchange-correlation functional adopted in this work, we analyzed the spin topology resulting after dynamical relaxation of the pristine structure. In particular, we performed 32 distinct relaxations for the BLYP functional and 21 for the HCTH/120 functional for each value of the pressure indicated in the ongoing discussion and for different values of the total spin S. The scope of such a procedure is to allow the electronic degrees of freedom, hence the spin density distribution, to explore the multiple minima landscape typical of these systems. In fact, at fixed (relaxed) atomic configuration, this class of lamellar hybrid materials can take different spin topologies on the single copper hydroxide planes, whose local spin Σ can be either in an AF state (Σ = 0) or in a F one (Σ ≠ 0) with nearly the same total energy for a global singlet spin state (S = 0), as explained in ref 3. We recall that in a delocalized basis set such as PWs usually one does not attribute a specific spin character to an atomic site or a set of atomic sites. Moreover, even in a localized basis set approach, any a priori choice of the local spin topology could result in an unjustified biasing imposed to the system. To avoid this, different initial random values are attributed to the fictitious electronic degrees of freedom at the beginning of the simulation, and the system is allowed to evolve freely upon damped dynamics. The actual spin topology is then detected at the end of such a procedure, which, in turn, requires a number of different separate simulations, as indicated above, to accumulate statistics. The general picture arising from this approach amounts to different values of Σ corresponding to an identical total spin S, as described in detail elsewhere.3 All these equivalent spin states yield total energies separated by 0.01 eV or less, confirming the near-degenerate character of the various local spin topologies.3 A set of increasing values of the pressure, ranging from ambient pressure to about 7.5 GPa, were applied to the system. To this aim, the simulation cell underwent a progressive shrinking along the c lattice parameter, i.e., the crystallographic direction orthogonal to the copper hydroxide layers (see Figure 1 and Supporting Information, Figure S1 and Tables S1 and S2). In the absence of any external pressure, the spin states S = 2 and S = 4 of the system are characterized by total energies

COMPUTATIONAL METHOD We use a first-principles molecular dynamics (FPMD) approach5,6 in which the exchange and correlation functionals are described either in terms of a Becke12 and Lee, Yang, and Parr13 (BLYP) prescription or a Hamprecht, Cohen, Tozer, and Handy14 (HCTH/120) one. Both these functionals have been shown to be particularly suited to characterize the hybrid material on which we focus here4 and are capable of providing with sufficient accuracy energetics and hydrogen bonding properties of heterogeneous and organic materials.15 Valence electrons, including semicore 3d states for the Cu transition metal, are treated explicitly and represented in a plane-wave (PW) basis set with an energy cutoff of 90 Ry and the Brillouin zone sampled at the Γ point. The core−valence interaction is described in terms of norm-conserving numerical pseudopotentials of the Trouiller−Martins type.16 The system used in all simulations is composed of 72 atoms (8 Cu, 24 O, 8 C, and 32 H) in a periodically repeated simulation box. The lattice parameters are identical to the ones reported in ref 8 and complemented by our former study,3 namely a = 5.6025 Å, b = 6.1120 Å, c = 18.747 Å, and β = 91°012′. Equilibrium structures are obtained by damped FPMD. Such a dynamical approach allows the system to search for stable local minima on the Born−Oppenheimer surface in a more flexible way with respect to standard static geometry optimization. This is a necessary step whenever no educated guesses are possible for the actual structure, as in the case in this class of hybrid system. The convergence was checked by monitoring the residual forces acting on the atoms. Whenever the maximum force component was smaller than 10−3 hartree/ au, the system did no longer undergo significant structural modifications and the relaxation was retained as achieved. For all the structures, the total spin multiplicity 2S + 1 was set to three different values, 1 (S = 0), 5 (S = 2), and 9 (S = 4), resulting in local spin distribution compatible with the assigned total S value. For instance, in the first case (S = 0), the local spin distribution are all consistent with the global AF character in the absence of applied external pressure.17,18 All across the 18701



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Figure 2. Relative energies of the system at S = 2 and S = 4, computed with respect to the ground state S = 0, as a function of the applied pressure for the two DFT functionals considered. The solid line with filled circles refers to the BLYP functional, while the dashed line with filled triangles refers to the HCTH/120 functional.

Table 1. Relative Energies of the System at Increasing Pressure from 0 to 7.5 GPa Calculated by Using the Two Different DFT Functionals Considered ES=2 − ES=0 (eV)

Figure 1. Geometry and example of spin topology for the system at a pressure of 7.5 GPa for the spin state S = 0. The O1, O2, O3, and O4 sites in the Cu−Oi−Cu moieties are labeled according to what has been adopted hereafter in all figures and tables.

above the S = 0 ground state by 0.51 eV (BLYP), 0.17 eV (HCTH (S = 2) and 1.40 eV (BLYP), 0.39 eV (HCTH) (S = 4). A closer look reveals that the AF configuration (S = 0) is the ground state from ambient pressure to about 5 GPa. Namely, at pressure equal to 5 GPa, the difference E(S = 4) − E(S = 0) is 0.432 eV for the BLYP functional and 0.283 eV in the case in which the HCTH/120 functional is adopted. Nonetheless, for pressure larger than 2.75 GPa we remark that the energy difference between the spin states S = 0, S = 2, and S = 4 reduces significantly becoming smaller and smaller. In the case of the HCTH functional, a slight inversion of stability of high spin states with respect to the S = 0 state arises for the largest pressure applied (7.5 GPa), in agreement with the experimental outcome.10 These results are summarized in Figure 2 and Table 1. In this respect, within the numerical accuracy allowed by exchange-correlation density functionals,20 the BLYP and the HCTH schemes give different results. The general picture provided by these simulations is that, although the overall energetic trend is generally preserved, the intimate details of the magnetic character of the system depend crucially on the accuracy in the description of the nature of chemical bonding of the material under external stress. Specifically, the better the accuracy in the description of the exchange part (to which spin properties are sensitive), the closer the agreement with experiments. Now, in BLYP approaches, the exchange is the standard expression proposed by Becke12 in his original work. Instead, in the HCTH/120 functional, the exact Hartree exchange is still included at an empirical level according to eq 11 of ref 14, but upon careful reparameterization with a selfconsistent procedure on the training set of atoms and molecules specified there. A conclusion that can be drawn at

ES=4 − ES=0 (eV)

pressure (GPa)

BLYP

HCTH/120

BLYP

HCTH/120

0 (c = 18.747 Å) 2.025 (c = 18.077 Å) 2.75 (c = 17.963 Å) 3.0 (c = 17.746 Å) 5.0 (c = 17.108 Å) 7.5 (c = 16.312 Å)

0.51 0.50 0.48 0.49 0.35 0.09

0.17 0.14 0.27 0.09 0.18 −0.05

1.40 0.77 0.62 0.71 0.43 0.17

0.39 0.32 0.31 0.15 0.28 −0.03

this point is that the use of hybrid functionals, where the exact exchange is explicitly computed and not parametrized, would most likely lead to an even better accuracy. In view of the results obtained here, the structural transformation corresponding to the different pressures will be described for the HCTH/ 120 functional only. Results obtained within the BLYP functional are provided in the Supporting Information (Tables S3 and S4 and Figures S2 and S3) Upon compression, the simulation cell has shown a slight monotonic decrease in terms of Cu−O distances, especially in the case in which O belongs to the CH3COO chain, until a value of about 2.75 GPa was reached (Figure 3). During this stage, we remarked that the general conformation of the chains remains unchanged, since no isomerization involving sharp angle transitions occurs. However, the volume shrinking is accompanied by a monotonic decrease of the Cu− O−Cu angles, where O can belong either to a CH3COO chain or to an OH hydroxyl group (Figures 4 and 5). For the range of pressures up to 2.75 GPa, the shortening of the Cu−O bonds mentioned above can vary from 0.09 to 0.21 Å (see Tables 2, 3, and 4) with respect to the equilibrium values found for this same system at ambient pressure.3 Such a shrinking is nearly insensitive to the spin state of the system and is compatible with the typical Cu−O bonds oscillations.21 The material is able to cope with the additional stress by adopting these reduced bond lengths and by relying on the angular flexibility. From the standpoint of the bond lengths, HCTH/120 and BLYP results turn out to be numerically indistinguishable. Instead, the angles present slight differences between the two functionals, as a comparison between Figures 18702



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Table 2. Main Structural Data and Spin Density Projected on the O Atoms for Increasing Pressures in Spin State S = 0 for the HCTH/120 Functionala exptl press. (GPa) computed press. (GPa) Cu−O1−Cu (deg) O spin Cu−O1 (Å) Cu−O2−Cu (deg) O spin Cu−O2 (Å) Cu−O3−Cu (deg) O spin Cu−O3 (Å) Cu−O4−Cu (deg) O spin Cu−O4 (Å)

Figure 3. Cu−O bond distances at different pressure and for the two spin states S = 0 and S = 4 in the case of HCTH/120 calculations. For clarity, each CuOi (i = 1−4) curve is rigidly shifted along the y-axis of the amount indicated in parentheses, whereas the actual numerical values are reported in Table 2.

0 0.18

2.025 3.32

2.75 3.42

3.0 3.60

5.0 4.35

7.5 5.80

95.9

89.6

89.4

89.9

86.3

83.4

0.105 2.19 95.7

−0.025 2.04 89.4

−0.022 2.10 89.6

−0.021 2.11 90.1

−0.006 2.13 83.7

−0.008 2.15 84.2

0.110 2.38 99.0

−0.044 2.12 93.3

−0.040 2.17 92.8

−0.036 2.18 93.2

−0.010 2.17 90.7

0.002 2.15 90.6

−0.024 2.20 99.2

0.022 2.02 94.1

0.018 2.08 93.9

−0.016 2.09 94.2

−0.031 2.09 89.0

−0.039 2.10 85.4

0.021 2.21

−0.005 2.00

0.001 2.03

0.001 2.03

−0.010 2.00

−0.001 2.01

a

The experimental pressure is used to estimate the lattice parameters, while the computed pressure is the one resulting upon calculation of the stress tensor. The O atoms considered here are those belonging to the CH3COO moiety.

Table 3. Main Structural Data and Spin Density Projected on the O Atoms for Increasing Pressures in Spin State S = 2 for the HCTH/120 Functionala exptl (GPa) computed press. (GPa) Cu−O1−Cu (deg) O spin Cu−O1 (Å) Cu−O2−Cu (deg) O spin Cu−O2 (Å) Cu−O3−Cu (deg) O spin Cu−O3 (Å) Cu−O4−Cu (deg) O spin Cu−O4 (Å)

Figure 4. CuOCu angles, for each O belonging to a CH3COO group, as a function of pressure for the HCTH/120 functional calculations. For clarity, each CuOiCu (i = 1−4) curve is rigidly shifted along the yaxis. The corresponding numerical values are listed in Table 1.

0 0.18

2.025 3.32

2.75 3.42

3.0 3.60

5.0 4.35

7.5 5.80

95.8

88.5

89.6

90.0

85.8

83.3

0.103 2.18 95.7

0.077 2.10 89.5

0.080 2.10 89.9

0.073 2.08 90.4

0.041 2.14 82.8

0.033 2.13 83.7

0.113 2.38 99.1

0.081 2.21 93.9

0.077 2.17 92.1

0.078 2.15 93.5

0.034 2.19 90.8

0.035 2.14 90.5

0.022 2.20 99.1

−0.020 2.08 94.3

0.119 2.10 92.9

−0.011 2.06 94.4

0.035 2.09 89.7

−0.031 2.09 85.1

−0.026 2.22

0.006 2.03

0.106 2.04

0.003 2.00

0.009 2.00

−0.006 2.01

a


realized either by AF (Σ = 0) or by F layers (Σ ≠ 0). These two topologies are nearly degenerate and were fully characterized in a former work3 to which we refer for any further details. The situation changes abruptly when the pressure overcomes the value of 2.75 GPa. From the structural point of view, we observe that all the Cu−O distances suddenly revert back almost to their original values. Such an abrupt change is indicated by the discontinuity shown in Figure 3 and hold for any spin state of the system. At the same time, all the Cu−O− Cu angles undergo an abrupt variation, characterized by an increase of slope in the interval 2.75−3 GPa (see Figures 4 and

Figure 5. CuOCu angle, for each O belonging to an OH group, as a function of pressure for the HCT/120 functional calculations. As in the former figure, for clarity, each CuOiCu (i = 1−4) curve.

4 and 5 and the corresponding S2 and S3 in the Supporting Information shows. By looking at the electronic structure, we noticed that no significant changes in the spin distribution and in the relative stability of the various spin topologies arise. More specifically, the ground state is still the one with zero total spin (S = 0) 18703



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available experimental outcome23 and supports the notion of a pressure-controlled spin-switch mechanism. The only warning we have to put forward is the larger value of the theoretical pressure at which such a transition occurs with respect to the experimental one (about 1.5 GPa). This is not unexpected in DFT-based approaches, particularly when molecular solids are involved. Such an overestimation is generally ascribed, on one hand, to the fact that only approximated exchange-correlation functionals are available and, on a lesser extent, to the approximations in the core−valence interactions introduced by the use of pseudopotentials.24,25 The observed spin-switch mechanism is induced by geometrical modifications, and these, in turn, are promoted by an applied external pressure, hence a macroscopic variable behaving as an easily tunable parameter. On a general perspective, a change in the total spin and its associated space distribution in a molecular system is not completely uncommon. Indeed, it has been observed in different cases, from simple methylene26 to hydro- and alkoxo-bridged Cu compounds,27 and even more complex synthetic organic systems,28,29 where the flexibility of the organic moiety plays indeed a major role in localizing spin densities on specific parts of the system upon local distortions. However, in those cases, it was unclear how such a spin transition can be controlled to make applications feasible. This piece of work demonstrates that one can move from fundamental ideas to a practical tool for triggering such a transition, providing a guideline in the realization of a new generation of memory devices for future nanotechnological applications.

Table 4. Main Structural Data and Spin Density Projected on the O Atoms for Increasing Pressures in Spin State S = 4 for the HCTH/120 Functionala exptl press. (GPa) computed press. (GPa) Cu−O1−Cu (deg) O spin Cu−O1 (Å) Cu−O2−Cu (deg) O spin Cu−O2 (Å) Cu−O3−Cu (deg) O spin Cu−O3 (Å) Cu−O4−Cu (deg) O spin Cu−O4 (Å)

0 0.183

2.025 3.321

2.75 3.418

3 3.602

5 4.353

7.5 5.795

95.8

88.6

88.7

89.6

86.0

83.9

0.105 2.18 95.7

0.079 2.10 89.3

0.073 2.10 88.8

0.064 2.11 90.0

0.043 2.14 82.7

0.038 2.13 83.8

0.114 2.38 98.3

0.08 2.21 92.2

0.072 2.19 91.3

0.069 2.18 92.8

0.038 2.19 89.4

0.044 2.14 90.0

0.132 2.21 98.8

0.116 2.12 93.0

0.114 2.11 92.5

0.107 2.10 93.7

0.114 2.14 86.1

0.118 2.14 82.8

0.141 2.22

0.12 2.04

0.11 2.04

0.113 2.03

0.097 2.03

0.091 2.03

a


5). It looks like the system reached its maximum possible compression of bond lengths, and in these conditions, to minimize further the internal stress, the compound has to resort to drastic angular changes, taking advantage of the flexibility of the chains. On a major extent, this is exactly what happens for the angles in which the O atoms belong to the long CH3COO moiety, as summarized in Tables 2 and 3 for S = 0 and S = 4, respectively, and graphically in Figures 3, 4, and 5. On the other hand, angles in which the O atom belongs to an OH group are less affected by further compression, as the slope in the angle/ pressure plot (Figure 5) shows a less pronounced discontinuity and even a slight decrease in intensity. In one case (Cu−O4− Cu) we observed even an increase in the angle, made possible by a partial folding of the longer chains occurring when the system reduces its internal stress. Beyond, the transition point, and up to 7.5 GPa, the behaviors of angles and distances do no longer show any sign of dramatic change, and all proceed monotonically. Interesting features can be remarked also in the spin density values ρs(Oi) on the O atoms bridging two Cu sites. The decrease of the Cu−Oi−Cu (i = 1−4) angles with pressure is accompanied by a general reduction of ρs(Oi) in all spin states. In particular, in the case S = 0 (Table 2) we observe in one case a drastic reduction of about 2 orders of magnitude in ρs(O1), from +0.105 to −0.008, accompanied by a spin inversion at high pressure. Coming to the S = 2 spin state, smaller variations up to 1 order of magnitude characterize the trend of ρs(Oi) for increasing pressure, as reported in Table 3, when comparing the P= 0 and the P = 7.5 GPa situations. Instead, a reduction of ρs(Oi) up to 50% with no spin inversion characterizes the S = 4 state (Table 4). Given this scenario, we can infer that high pressures perturb the AF state more than the F one, thus making the latter energetically more accessible. Indeed, the values of the total energy for the F state approach those of the AF state, until a transition in the magnetic state arises22 at values of the order of 7 GPa, as shown above when describing the content of Table 1 and Figure 2. This is consistent with the

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CONCLUSIONS

We show that in Cu2(OH)3(CH3COO)·H2O the net effect of an external pressure is to shrink initially all the structural parameters of the system, without affecting significantly neither the magnetic character of the material nor the gross geometrical features. Basically, the flexible chains adapt to adsorb the stress added to the structure. Then, when a critical value of the pressure is overcome, amounting to about 2.75 GPa in the present case, these chains undergo a major conformational change which triggers further minimization of the internal stress. The Cu−O−Cu angles (O ∈ CH3COO) experience an abrupt and permanent modification making a transition to low values which, in the end, by increasing the pressure, give rise to a change in the spin topology, favoring high spin states. To recover the experimental behavior, an accurate description of the exchange interaction turns out to be crucial. This is particularly evidenct in the case of the HCTH functional, where the exact exchange is still empirically included as in any DFT approach making use of Becke’s formula, but the self-consistent reparameterization of the exchange prefactor allows for a better description of the exchange interaction. In the case of the hybrid lamellar system targeted in the present study, the novelty stems from the use of an external macroscopic quantity (the pressure) to induce a spin transition, thus making possible the realization of a spin switch. Therefore, the interplay between the Cu-based layers and the chemical composition of the hosted chains provide a versatile probe to be engineered for applications in next-generation molecular electronics devices. 18704



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Ribozymes: First-Principles Molecular Dynamics Simulations of a Fully Hydrated Model System. J. Chem. Theory Comput. 2005, 1, 925− 934. (16) Troullier, N.; Martins, J. L. Efficient Pseudopotentials for PlaneWave Calculations. Phys. Rev. B 1991, 43, 1993−2006. (17) Rabu, P.; Rouba, S.; Laget, V.; Hornick, C.; Drillon, M. Ferro/ Antiferromegnetism Mediated by Interlayer Organic Spacers in layered Copper(II) Compounds. Chem. Commun. 1996, 1107−1108. (18) Laget, V.; Hornick, C.; Rabu, P.; Drillon, M.; Turek, P.; Ziessel, R. Multilayered Ferromagnets Based on Hybrid Organic-Inorganic Derivatives. Adv. Mater. 1998, 10, 1024−1028. (19) Parrinello, M.; Rahman, A. Crystal Structure and Pair Potentials: A Molecular Dynamics Study. Phys. Rev. Lett. 1980, 45, 1196−1199. (20) Johnson, B. G.; Gill, P. M. W.; Pople, J. A. The Performance of a Family of Density Functional Methods. J. Chem. Phys. 1993, 98, 5612− 5626. (21) Wu, H.; Desai, S. R.; Wang, L.-S. Chemical Bonding between Cu and Oxygen-Copper Oxides vs O2 Complexes: A Study of CuOx (x=0−6) Species by Anion Photoelectron Spectroscopy. J. Phys. Chem. A 1997, 101, 2103−2111. (22) An additional calculation done on the system of ref 9 has shown that the spin state S = 4 is more stable than the S = 0 one by 0.14 eV, corroborating the notion of the arising of a ferromagnetic state replacing the antiferromagnetic one when the system is under pressure. (23) Rousseau, R.; Boero, M.; Bernasconi, M.; Parrinello, M.; Terakura, K. Ab Initio Simulations of Phase Transitions and Dissociation of H2S at High Pressure. Phys. Rev. Lett. 2000, 85, 1254−1257. (24) Boero, M.; Andreoni, W. Trimethylene Isomers and Propene: Structural and Vibrational Properties from Density Functional Theory. Chem. Phys. Lett. 1997, 265, 24−34. (25) Massobrio, C.; Ruiz, E. Localized orbitals vs. PseudopotentialPlane Wave Basis Sets: Performances and Accuracy for Molecular Magnetic Systems. Monatsh. Chem. 2003, 134, 317−326. (26) Ruiz, E.; Alemany, P.; Alvarez, S.; Cano, J. Toward the Prediction of magnetic Coupling in Molecular Systems: Hydro- and Alkoxo-Bridged Cu(II) Binuclear Complexes. J. Am. Chem. Soc. 1997, 119, 1297−1303. (27) Pulizzi, F. Spintronics. Nat. Mater. 2012, 11, 367 and references therein. (28) Gervasio, F. L.; Laio, A.; Parrinello, M.; Boero, M. Charge Localization in DNA Fibers. Phys. Rev. Lett. 2005, 94, 158103. (29) Boero, M.; Gervasio, F. L.; Parrinello, M. Charge Localisation and Hopping in DNA. Mol. Simul. 2007, 33, 57−60.

ASSOCIATED CONTENT

S Supporting Information *

Further details about the calculation of pressure, structural parameters, and HCTH functional results. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone +33-3-88107142; Fax +33-3-88107247; e-mail mauro. [email protected] (M.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge fruitful discussions with P. Rabu and G. Rogez (IPCMS) and computational facilities provided by GENCIDARI, under allocations x2013095071 and x2013096092, and HPC EQUIP@MESO at the University of Strasbourg. This work was financially supported by the National Natural Science Foundation of China (no. 11304234).

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REFERENCES

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Tuning Magnetic Properties with Pressure in Hybrid Organic

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