<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">Cu</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>impurities in fluorite-type crystals: Mechanisms favoring an off-center motion

Fernández, Pablo García; Aramburu, J. A.; Barriuso, M. T.; Moreno, M.

doi:10.1103/physrevb.69.174110

Cited by 28 publications

(26 citation statements)

References 49 publications

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“…This potential increases the smaller the lattice constant is and the larger the charge of the impurity ion is. 5,7 Thus, the trends proposed by this model are in agreement with the results shown in Table I. ͑c͒ The PJT effect can produce a distortion along the t 1u mode.…”

supporting

confidence: 79%

“…2 Therefore if the JT effect was the cause of the off-center distortion in 6 In the case of d 9 impurities in sites with octahedral symmetry the JT distortion along the e g mode usually involves displacements of around 0.1 Å. As seen in Table I, density functional theory ͑DFT͒ calculations 5,7 indicate that the off-center distortions can be much larger and of the order of 1 Å.…”

mentioning

confidence: 96%

“…8 In the case of the Cu 2+ impurity in BaF 2 the electronic T 2g ground state couples to A 2u excited states through a t 1u vibrational mode to move off-center. Combining DFT calculations with a PJT model, we have shown 5,7 that there are two main types of excited states contributing to the distortion. One involves the excitation of one electron from the charge-transfer levels to the metal-d t 2g ones to produce an A 2u state and the other excites one electron from the t 2g d-shell of the metal to the close p-shell one.…”

mentioning

confidence: 99%

“…One involves the excitation of one electron from the charge-transfer levels to the metal-d t 2g ones to produce an A 2u state and the other excites one electron from the t 2g d-shell of the metal to the close p-shell one. DFT calculations indicate 5,7 that the latter channel is very important in Fe + or Ni + impurity centers in the sense that if that coupling is suppressed the distortion is destroyed. This PJT contribution is less important for divalent ions because the 3d-4p separation in the free ion ͑6.5 eV for Ni + and 15 eV for Cu 2+ ͒ increases substantially with the ion charge.…”

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confidence: 99%

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Comment on “Molecular structure and dynamics of off-center Cu2+ ions and strongly coupled Cu2+–Cu2+ pairs in BaF2 crystals: Electron paramagnetic resonance and electron spin relaxation studies” [J. Chem. Phys. 127, 124705 (2007)]

García‐Fernández¹,

Aramburu²,

Moreno³

et al. 2008

The Journal of Chemical Physics

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In the recent study of Cu2+-doped BaF2 Hoffmann et al. demonstrated using electron paramagnetic resonance spectroscopy that the Cu2+ impurity experiences an off-center shift from the substitutional position and ascribe this distortion to the Jahn–Teller effect. In this comment, we propose that the off-center distortion observed for BaF2:Cu2+ is driven by pseudo-Jahn–Teller interactions which are qualitatively different and in competition with the usual Jahn–Teller effect. It is also stressed that off-center phenomena in a given host lattice are not necessarily enhanced by decreasing the ionic radius of the impurity.

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supporting

confidence: 79%

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confidence: 96%

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confidence: 99%

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confidence: 99%

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Comment on “Molecular structure and dynamics of off-center Cu2+ ions and strongly coupled Cu2+–Cu2+ pairs in BaF2 crystals: Electron paramagnetic resonance and electron spin relaxation studies” [J. Chem. Phys. 127, 124705 (2007)]

García‐Fernández¹,

Aramburu²,

Moreno³

et al. 2008

The Journal of Chemical Physics

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“…22,23 This method, whose application has been largely ignored in this field, allows one to rationalize the different behaviors observed for N 2 H 7 + and H 5 O 2 + . Many works on molecular and solid-state systems [22][23][24][25] show that the analysis of the excited states coupled to the ground state via appropriate vibrational modes provide valuable knowledge on the causes giving rise to asymmetric ͑off-center͒ atomic displacements. Moreover, conclusions on the electrostatic/covalent ͑or localized/delocalized͒ nature of strong HBs can be extracted from this analysis.…”

Section: Introductionmentioning

confidence: 99%

Pseudo-Jahn-Teller origin of the low barrier hydrogen bond in N2H7+

García‐Fernández

García-Canales

Garcı́a-Lastra

et al. 2008

The Journal of Chemical Physics

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The microscopic origin and quantum effects of the low barrier hydrogen bond ͑LBHB͒ in the proton-bound ammonia dimer cation N 2 H 7 + were studied by means of ab initio and density-functional theory ͑DFT͒ methods. These results were analyzed in the framework of vibronic theory and compared to those obtained for the Zundel cation H 5 O 2 + . All geometry optimizations carried out using wavefunction-based methods ͓Hartree-Fock, second and fourth order Möller-Plesset theory ͑MP2 and MP4͒, and quadratic configuration interaction with singles and doubles excitations ͑QCISD͔͒ lead to an asymmetrical H 3 N-H +¯N H 3 conformation ͑C 3v symmetry͒ with a small energy barrier ͑1.26 kcal/ mol in MP4 and QCISD calculations͒ between both equivalent minima. The value of this barrier is underestimated in DFT calculations particularly at the local density approximation level where geometry optimization leads to a symmetric H 3 N¯H +¯N H 3 structure ͑D 3d point group͒. The instability of the symmetric D 3d structure is shown to originate from the pseudo-Jahn-Teller mixing of the electronic 1 A 1g ground state with five low lying excited states of A 2u symmetry through the asymmetric ␣ 2u vibrational mode. A molecular orbital study of the pseudo-Jahn-Teller coupling has allowed us to discuss the origin of the proton displacement and the LBHB formation in terms of the polarization of the NH 3 molecules and the transfer of electronic charge between the proton and the NH 3 units ͑rebonding͒. The parallel study of the H 5 O 2 + cation, which presents a symmetric single-well structure, allows us to analyze why these similar molecules behave differently with respect to proton transfer. From the vibronic analysis, a unified view of the Rudle-Pimentel three-center four-electron and charge transfer models of LBHBs is given. Finally, the large difference in the N-N distance in the D 3d and C 3v configurations of N 2 H 7 + indicates a large anharmonic coupling between ␣ 2u -␣ 1g modes along the proton-transfer dynamics. This issue was explored by solving numerically the vibrational Schrödinger equation corresponding to the bidimensional E͓Q͑␣ 2u ͒ , Q͑␣ 1g ͔͒ energy surface calculated at the MP4 / 6-311+ + G ** level of theory.

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Red Shift in Optical Excitations on Layered Copper Perovskites under Pressure: Role of the Orthorhombic Instability

Carrasco‐Busturia

Sánchez-Movellán

Tygesen

et al. 2022

Chemistry A European J

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The red shift under pressure in optical transitions of layered compounds with CuCl64− units is explored through first‐principles calculations and the analysis of available experimental data. The results on Cu2+‐doped (C2H5NH3)2CdCl4, that is taken as a guide, show the existence of a highly anisotropic response to pressure related to a structural instability, driven by a negative force constant, that leads to an orthorhombic geometry of CuCl64− units but with a hole displaying a dominant 3z2‐r2 character (z being the direction perpendicular to the layer plane). As a result of such an instability, a pressure of only 3 GPa reduces by 0.21 Å the longest Cu2+‐Cl− distance, lying in the layer plane, while leaving unmodified the two other metal‐ligand distances. Owing to this fact, it is shown that the lowest d‐d transition would experience a red shift of 0.34 eV while the first allowed charge transfer transition is also found to be red shifted but only by 0.11 eV that reasonably concurs with the experimental value. The parallel study on Jahn‐Teller systems CdCl2:Cu2+ and NaCl:Cu2+ involving tetragonal elongated CuCl64− units shows that the reduction of the long axis by a pressure of 3 GPa is three times smaller than that for the layered (C2H5NH3)2CdCl4:Cu2+ compound. Accordingly, the optical transitions of such systems, which involve a positive force constant, are much less sensitive to pressure than in layered compounds. The origin of the red shift under pressure undergone by the lowest d‐d and charge transfer transitions of (C2H5NH3)2CdCl4:Cu2+ is discussed in detail.

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Cu2+impurities in fluorite-type crystals: Mechanisms favoring an off-center motion

Cited by 28 publications

References 49 publications

Comment on “Molecular structure and dynamics of off-center Cu2+ ions and strongly coupled Cu2+–Cu2+ pairs in BaF2 crystals: Electron paramagnetic resonance and electron spin relaxation studies” [J. Chem. Phys. 127, 124705 (2007)]

Comment on “Molecular structure and dynamics of off-center Cu2+ ions and strongly coupled Cu2+–Cu2+ pairs in BaF2 crystals: Electron paramagnetic resonance and electron spin relaxation studies” [J. Chem. Phys. 127, 124705 (2007)]

Pseudo-Jahn-Teller origin of the low barrier hydrogen bond in N2H7+

Red Shift in Optical Excitations on Layered Copper Perovskites under Pressure: Role of the Orthorhombic Instability

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