Limits of the quasiharmonic approximation in MgO: Volume dependence of optical modes investigated by infrared reflectivity and 
<i>ab initio</i>
 calculations

Calandrini, Eugenio; Paulatto, Lorenzo; Antonangeli, Daniele; He, Fei; Lobo, R. P. S. M.; Capitani, Francesco; Brubach, Jean‐Blaise; Roy, Pascale Le; Vincent, Laëtitia; Giura, P.

doi:10.1103/physrevb.103.054302

Cited by 9 publications

(6 citation statements)

References 49 publications

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“…However, more recent studies have shown a complex picture of the limit of quasiharmonic approximation (e.g. Giura et al, 2019;Calandrini et al, 2021), and a detailed discussion on the limit is out of the scope of this study.…”

Section: P -T -G Relationshipmentioning

confidence: 99%

Shear properties of MgO inferred using neural networks

et al. 2023

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Abstract. Shear properties of mantle minerals are vital for interpreting seismic shear wave speeds and therefore inferring the composition and dynamics of a planetary interior. Shear wave speed and elastic tensor components, from which the shear modulus can be computed, are usually measured in the laboratory mimicking the Earth's (or a planet's) internal pressure and temperature conditions. A functional form that relates the shear modulus to pressure (and temperature) is fitted to the measurements and used to interpolate within and extrapolate beyond the range covered by the data. Assuming a functional form provides prior information, and the constraints on the predicted shear modulus and its uncertainties might depend largely on the assumed prior rather than the data. In the present study, we propose a data-driven approach in which we train a neural network to learn the relationship between the pressure, temperature and shear modulus from the experimental data without prescribing a functional form a priori. We present an application to MgO, but the same approach works for any other mineral if there are sufficient data to train a neural network. At low pressures, the shear modulus of MgO is well-constrained by the data. However, our results show that different experimental results are inconsistent even at room temperature, seen as multiple peaks and diverging trends in probability density functions predicted by the network. Furthermore, although an explicit finite-strain equation mostly agrees with the likelihood predicted by the neural network, there are regions where it diverges from the range given by the networks. In those regions, it is the prior assumption of the form of the equation that provides constraints on the shear modulus regardless of how the Earth behaves (or data behave). In situations where realistic uncertainties are not reported, one can become overconfident when interpreting seismic models based on those defined equations of state. In contrast, the trained neural network provides a reasonable approximation to experimental data and quantifies the uncertainty from experimental errors, interpolation uncertainty, data sparsity and inconsistencies from different experiments.

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Section: P -T -G Relationshipmentioning

confidence: 99%

Shear properties of MgO inferred using neural networks

et al. 2023

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“…The only physical quantity directly comparable between experimental and modeling results is the splitting of the longitudinal and transverse optical modes, i.e., the LO/TO splitting. This mode separation originates from the degeneracy elimination between the LO and TO phonons at the Brillouin-zone center [207]. In this regard, there is an overall reasonable agreement of reported measurements with predicted data (Table 3.1).…”

Section: Mgssupporting

confidence: 80%

“…The goal of this study is to model the reflectivity, which is characterized by a frequency band of high reflectivity, i.e., the reststrahlen band [207]. The top and bottom of this band are defined by the transverse (ω T O ) and longitudinal (ω LO ) optical modes, respectively.…”

Section: Computational Detailsmentioning

confidence: 99%

“…Here, it is clearly observed that a self-consistency among the required elements entering in the calculation of ω T O and ω LO is highly desirable, in particular, the LO/TO splitting is rather sensitive to the Born effective charge, as it enters the NAC as Z 2 . An accurate determination of the TO mode is rather significant for the modeling of the reflectivity as it controls the edge of the reststrahlen band [207].…”

Section: Systemmentioning

confidence: 99%

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Ab initio unraveling of structural, magnetic, and optical properties of monosulfides and vacancy clustering in perovskite heterostructures

Chmeruk

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First-principles modeling techniques offer the ability to simulate a wide range of systems under different physical conditions, such as temperature, pressure, and composition, without relying on empirical knowledge. Density functional theory (DFT), a quantum mechanical method, has become an exceptionally successful framework for materials science modeling. Employing DFT makes it possible to gain valuable insights into the fundamental state of a system, enabling the reliable determination of equilibrium crystal structures. Over time, DFT has become an essential tool that can be incorporated into various schemes for predicting the properties of a material related to its structure, insulating/metallic behavior, magnetism, and optics. DFT is regularly applied in numerous fields, spanning from fundamental subjects in condensed matter physics to the study of large-scale phenomena in geosciences. In the latter, the effectiveness of DFT stems from its ability to simulate the properties found on the Earth, other planets, and meteorites, which may pose challenges for their direct study or laboratory investigation. In this thesis, a comprehensive examination of a family of monosulfides and a perovskite heterostructure was conducted. These materials are relevant for their potential applications in technology, energy harvesting, and in the case of monosulfides, their speculated abundance on the planet Mercury. Firstly, a DFT approach was used to analyze two non-magnetic monosulfides, CaS and MgS. We determined their structural properties and then focused on the modeling of their reflectivity in the infrared region. The calculation of the reflectivity considered both harmonic and anharmonic contributions. In the harmonic limit, the non-analytic correction was employed to accurately determine the LO/TO splitting, which is necessary to delimit the retstrahlend band, that is, the maximum of the reflectivity. The anharmonic effects given by up to three-phonon and isotopic scatterings, which were included using perturbation theory, primarily smeared the reflectivity spectra edges in the high-wave region. Secondly, four polymorphs of MnS were studied using a combination of first-principles methods to simulate their antiferromagnetic (AFM) and paramagnetic (PM) states. The integration of DFT+$U$ with special quasirandom structures (SQS) supercells, and occupation matrix control techniques was crucial for achieving convergence, structural optimization accuracy, and obtaining finite energy band gaps and local magnetic moments in the PM phases. The addition of the Hubbard $U$ correction was necessary to treat the highly-correlated Mn $d$-electrons. The success of our approach was clear based on our electronic structure predictions for the PM rock-salt B1-MnS polymorph. Experimentally this phase has been observed to be an insulator, but multiple \emph{ab initio} works resulted previously in metallic behavior. Our computations, on the other hand, predicted insulating and magnetic properties that compare well with available measurements. Additionally, the pressure-field stability of the four MnS polymorphs was studied. In the case of the PM phases, B1-MnS was identified to be the most stable up to about 21 GPa, then transforming into the B31-MnS polymorph. This finding was in close agreement with high-pressure experiments reporting a similar phase transformation. The optical properties of B1-, B4-, and B31-MnS were also simulated. The SQS technique was used to obtain soft-mode-free phonon band structures within the harmonic approximation. Then, the anharmonic effects were included, and the reflectivity was calculated for B1-MnS and B4-MnS. In both cases, a good agreement for the LO/TO splitting was achieved in comparison to experimental results. Lastly, the oxygen-deficient heterostructure of LaAlO$_{3-\delta}$ /SrTiO$_{3-\delta}$ was investigated also employing DFT+$U$, with a particular emphasis on the potential impact of vacancy clustering at the interface. Six distinct configurations of pairs of vacancies were studied and their energies were compared to find the most stable one. The orbital reconstruction of Ti orbitals was also examined based on their location with respect to the vacancies and the local magnetic moments were calculated. The final results showed that linearly arranged vacancies located opposite to Ti ions give the most energetically stable configuration.

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“…The QHA is easy to implement and only requires 0 K calculations [14]. However, its temperature range of validity is difficult to assess and can strongly depend on the materials under consideration, since it is now well established that anharmonic effects, beyond the QHA, can be important far below the melting temperature [19][20][21].…”

Section: Introductionmentioning

confidence: 99%

Sound velocities and thermodynamical properties of hcp iron at high pressure and temperature

Bouchet¹,

Bottin²,

Antonangeli³

et al. 2022

J. Phys.: Condens. Matter

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Sound velocities and thermodynamical properties of hcp iron have been computed using ab initio calculations over an extended density and temperature range, encompassing the conditions directly relevant for the Earth’s inner core. At room temperature, and up to 350 GPa, an excellent agreement is obtained between present results and experimental data for many thermodynamical quantities: phonon density of states, vibrational entropy, heat capacity, Grüneisen parameter and thermal expansion. With increasing temperature, along an isochore, we observe a strong decrease of the phonon frequencies, demonstrating that intrinsic anharmonic effects cannot be neglected. We also carefully compare previous theoretical data for the sound velocities and try to explain the discrepancies observed with experiments. Finally, we propose a temperature dependant Birch’s law that we compare with previous experimental work.

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Limits of the quasiharmonic approximation in MgO: Volume dependence of optical modes investigated by infrared reflectivity and ab initio calculations

Cited by 9 publications

References 49 publications

Shear properties of MgO inferred using neural networks

Shear properties of MgO inferred using neural networks

Ab initio unraveling of structural, magnetic, and optical properties of monosulfides and vacancy clustering in perovskite heterostructures

Sound velocities and thermodynamical properties of hcp iron at high pressure and temperature

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