Of KI perturbed, The theoretical study of the physical properties of JT centres (impurities, defects, etc. )embedded in a crystal is usually done in the framework of the cluster model [I]. In such a model the localized electron of the centre, which is in a degenerate state by definition of the JT effect, is assumed to interact only with a single localized vibrational mode, any lattice dispersion of frequency being neglected. This model is known to take into account several important features of the JT induced properties [I] but not the details of the optical spectra (Raman, infrared, vibronic sidebands of the absorption/emission spectra), which are very sensitive to the dynamics of the host crystal.
In a recent paper [2] the problem of the lattice dynamics perturbed by a JT centre was studied and resolved in the weak interaction limit by using the Green function method. It was found that a JT centre generates a very peculiar perturbation on the dynamical matrix.
On the other hand, a similar result can be obtained when the absorption lineshape due to a singletmultiplet transition (the dynamic JT effect) is studied. In this case the JT perturbed lattice dynamics is that relative to the excited electronic degenerate state. In both cases we conclude that the JT interaction induces a perturbation on the force constant matrix, which can be eventually seen in the structures of the phonon densities. Such a result, which will be presented in details in a forthcoming paper, can be deduced in the limit of the weak interaction from the absorption lineshape theory proposed some years ago (ref. [3] hereafter called MT).
In the present paper, after some consideration on the previous point, we present the computed one-phonon spectra for a (e x E) JT centre. As usual, (e x E) means that the electron is in a e-symmetry degenerate state and interacts with the E-symmetry displacements of the neighbour ions. Our aim is to see to what extent the JT dynamical perturbation can qualitatively modify the host lattice dynamics. So we do not apply it to any real impurity.
In the following we assume : 1) The JT centre is embedded in an ionic crystal. 2) The JT interaction is weak. 3) Only the n.n. of the JT centre are involved in the JT interaction.
In the present paper, after some consideration on the previous point, we present the computed one-phonon spectra for a (e x E) JT centre. As usual, (e x E) means that the electron is in a e-symmetry degenerate state and interacts with the E-symmetry displacements of the neighbour ions. Our aim is to see to what extent the JT dynamical perturbation can qualitatively modify the host lattice dynamics. So we do not apply it to any real impurity.
In the following we assume : 1) The JT centre is embedded in an ionic crystal. 2) The JT interaction is weak. 3) Only the n.n. of the JT centre are involved in the JT interaction.
The shape of the optical spectra is related to the symmetrized (E-symmetry in this case) projected phonon density p,(w) relative to the lattice dynamics, either of the ground electronic state (Raman and infrared spectra), or of the excited electronic state (vibronic sidebands). We remember that p,(o) is given by the imaginary part of the E-symmetry phonon propagator DE(a). So we first show the relevant JT induced processes which dress the unperturbed onephonon
propagator, and then we evaluated p,(o) from D,(o). By unperturbed propagator we mean the onephonon propagator when all the dynamical perturbations induced by the centre (change of mass and of force constants) are considered, but the JT interaction.
propagator, and then we evaluated p,(o) from D,(o). By unperturbed propagator we mean the onephonon propagator when all the dynamical perturbations induced by the centre (change of mass and of force constants) are considered, but the JT interaction.
In the cluster model, where one assumes that only the n.n. of the centre are involved in HL, as they are in He,, only two degenerate E-symmetry modes with the same frequency are representative of the whole lattice dynamics. So the commutator (6) loses its meaning. From the properties of the commutators (4) and (5) one can see that, while the electron and the
phonons are truly independent when He, = 0, they are so entangled for HeL # 0 to determined new coupled states, the so-called vibronic states. When the vibronic states are studied in the cluster model, the role played by HeL on the electronic states is emphasized but the possible modifications of HL are neglected [I].
phonons are truly independent when He, = 0, they are so entangled for HeL # 0 to determined new coupled states, the so-called vibronic states. When the vibronic states are studied in the cluster model, the role played by HeL on the electronic states is emphasized but the possible modifications of HL are neglected [I].
The opposite approach is here follow ed. In fact we compute the modifications of the one-phonon projected density of states pE(02), when the electron is supposed to be degenerate but structureless. Without the JT interaction, the unperturbed phonon propagator D;(t) (a dotted line in the diagram of figure 1) is given at T = 0 K by
where p;(02) is the unperturbed projected one-phonon density. (By &(w2) we mean its Hilbert transform.) In the harmonic approximation a phonon does not change frequency when a linear electron-phonon interaction is switched on and the only effect is a generation of many unperturbed phonons. When the electron is in a degenerate state, since the usual separation of the phonons from the electron (adiabatic approximation) is not valid anymore, the linear JT interaction modifies the frequency of the phonons. By following MT [3], one can prove that in the diagrams accounting for the many-phonon process there is always a class of higher order graphs (the crossing graphs) which give a contribution to the lower order graphs, when the electron line is removed by assuming that the electron is structureless.
Lattice Dynamics and Thermodynamics of Molybdenum from First-Principles Calculations
of the ultrahigh pressure scale. The equation of state (EOS) of Mo at high pressure is being used as a calibration standard to the ruby fluorescence in diamond anvil compression (DAC) experiments.1 Because of its important position in the field of material science and condensed matter science, Mo has attracted tremendous experimental and theoretical interest in its wide range of properties recently. At ambient condition, Mo is in body-centered-cubic (bcc) structure and melts at 2890 K.2 But what is the most stable phase of Mo under ultrahigh pressure? Experimentally, the shock wave (SW) acoustic velocity measurements showed that there was a sharp break on the Hugoniot curve at about 210 GPa (at a calculated temperature of 4100 K), which indicated that a solid-solid phase transition occurred prior to melting at 390 GPa (at a calculated temperature of 10 000 K).3 To compare with the SW data, Vohra and Ruoff investigated the static compression of Mo by energy-dispersive X-ray diffraction and found that the bcc Mo was stable up to 272 GPa at 300 K.4 The phase transition in shock compression at 210 GPa was not observed. By further X-ray diffraction investigation, Ruoff et al. showed that Mo was stable in a bcc structure up to at least 560 GPa at room temperature.5 Theoretically, Moriarty suggested that Mo was stable in the bcc structure up to 420 GPa, where it transformed to a hexagonal close-packed (hcp) structure and then at 620 GPa to a face-centered closepacked (fcc) structure.6 Later, Boettger7 and Christensen et al.8 showed that the hcp phase of Mo was not stable, and the bcc phase transformed directly to the fcc phase at 700 GPa.
Belonoshko et al. confirmed the results of Boettger and showed the transition pressure was 720 GPa at zero pressure.9 By calculating the Gibbs free energies of the bcc and fcc Mo in the pressure range from 350 to 850 GPa at room temperatures up to 7500 K, Belonoshko et al. found that Mo had lower free energy in the fcc phase than in the bcc phase at elevated temperatures.10 Shortly after these new results were reported, Cazorla et al. found that the hcp Mo was noticeably more stable above 350 GPa at high temperature by calculating the Helmholtz and Gibbs free energies of the bcc, fcc, and hcp phases.11 The other intriguing problem is the melting properties. For the transition metals, such as Mo, Ta, and W, there are enormous discrepancies in melting curves between laser-heated DAC12-17 and SW3,18 methods. As for Mo (as well as Ta and W), several thousand degrees of discrepancies exist in extrapolating from DAC pressures of around 100 GPa12-15,17 to SW pressure of 390 GPa.18 As is known that the overestimation of the melting temperature exists in SW experiments, Errandonea13 corrected the SW data by considering 30% superheating. The revised melting temperatures are located at 7700 ( 1500 K (390 GPa), also much larger than the melting temperature (just above 4000 K) at this pressure extrapolated from DAC experiments. Results from the empirical and phenomenological melting models are dependent on the selection of the parameters.13,19-22 It is shown that the choice of different sets of parameters leads to huge differences in the melting temperatures at high pressure. In addition, theoretical results are consistent with the SW data at high pressure but diverge from DAC data below 100 GPa. All these results are inadequate to explain the extreme discrepancies in extrapolating the DAC data to the SW data.9-11,23 The highpressure melting curve of Mo still remains inconclusive up to now.
The low-temperature phonon spectrum of Mo as measured in inelastic neutron scattering experiments exhibited a variety of anomalies: the large softening near the H point, the T2 branch at the N point, and the longitudinal branch did
not display the rounded dip near q ) 2/3 [111], which was typical for "regular" monatomic bcc metals.24-26 With temperature increasing, the H point phonon displayed anomalous stiffening, which had been proposed to arise from either intrinsic anharmonicity of the interatomic potential or electron-phonon coupling.26 Theoretically, using the molecular dynamics (MD) simulations with environment-dependent tight-binding parametrization, Haas et al. reproduced the weakening of the phonon anomalies as the temperature increased.27 Later, Farber et al. determined the lattice dynamics of Mo at high pressure to 37 GPa using high-resolution inelastic X-ray scattering (IXS).28 Meanwhile, they calculated the quasiharmonic phonon spectrum up to the highest experimental pressure by linear response theory. Both the experimental and theoretical results showed an obvious decrease in the relative magnitude of the H point phonon anomaly under compression. Recently, Cazorla et al. obtained the phonon dispersion curves of the bcc Mo at zero pressure using the small displacement method,29 but under larger compression, there are no experimental and theoretical studies. The first-principles density functional theory is very successful in predicting the high-pressure behavior of phonon dispersion relations and their concomitant anharmonic effects.30 It is more effective to connect the phonon properties directly to the lattice dynamics under pressure, temperature, or their combination. One of the main purposes of this work is to investigate the lattice dynamics and thermodynamics of Mo under high pressure and temperature.
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