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Solution-processable semiconductors with antiferromagnetic (AFM) order are attractive for future spintronics and information storage technology. Halide perovskites containing magnetic ions have emerged as multifunctional materials, demonstrating a cross-link between structural, optical, electrical, and magnetic properties. However, stable optoelectronic halide perovskites that are antiferromagnetic remain sparse, and the critical design rules to optimize magnetic coupling still must be developed. Here, we combine the complementary magnetometry and electron-spin-resonance experiments, together with first-principles calculations to study the antiferromagnetic coupling in stable Cs-2(Ag:Na)FeCl6 bulk semiconductor alloys grown by the hydrothermal method. We show the importance of nonmagnetic monovalence ions at the B-I site (Na/Ag) in facilitating the superexchange interaction via orbital hybridization, offering the tunability of the Curie-Weiss parameters between -27 and -210 K, with a potential to promote magnetic frustration via alloying the nonmagnetic B-I site (Ag:Na ratio). Combining our experimental evidence with first-principles calculations, we draw a cohesive picture of the material design for B-site-ordered antiferromagnetic halide double perovskites., Funding Agencies|Energimyndigheten [2022-06725, 2018-05973]; Swedish Research Council [KAW 2019.0082]; Knut and Alice Wallenberg Foundation [Dnr 48758-1, Dnr 48594-1]; Swedish Energy Agency [2009-00971]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [CTS 20:350]; Carl-Trygger Foundation [2023-05247]; Swedish Research Council (VR) [854843]; ERC [KAW-2018.0194]; Wallenberg Academy Scholar

Phonon-phonon and electron/exciton-phonon coupling play a vitally important role in thermal, electronic, as well as optical properties of metal halide perovskites. In this work, we evaluate phonon anharmonicity and coupling between electronic and vibrational excitations in novel double perovskite Cs2NaFeCl6 single crystals. By employing comprehensive Raman measurements combined with first-principles theoretical calculations, we identify four Raman-active vibrational modes. Polarization properties of these modes imply Fm (3) over barm symmetry of the lattice, indicative for on average an ordered distribution of Fe and Na atoms in the lattice. We further show that temperature dependence of the Raman modes, such as changes in the phonon line width and their energies, suggests high phonon anharmonicity, typical for double perovskite materials. Resonant multiphonon Raman scattering reveals the presence of high-lying band states that mediate strong electron-phonon coupling and give rise to intense nA(1g) overtones up to the fifth order. Strong electron-phonon coupling in Cs2NaFeCl6 is also concluded based on the Urbach tail analysis of the absorption coefficient and the calculated Frohlich coupling constant. Our results, therefore, suggest significant impacts of phonon-phonon and electron-phonon interactions on electronic properties of Cs2NaFeCl6, important for potential applications of this novel material., Funding Agencies|Knut and Alice Wallenberg Foundation [KAW 2019.0082]; Swedish Energy Agency [48758-1]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoeping University (Faculty Grant SFO-Mat-LiU ) [2009-00971]; Swedish Foundation for Strategic Research [UKR22-0029]; Swedish Research Council (VR) [2019-05551]; ERC (synergy grant FAST-CORR project) [854843]; Swedish Research Council [2018-05973]

A family of magnetic halide double perovskites (HDPs) have recently attracted attention due to their potential to broaden application areas of halide double perovskites into, e.g., spintronics. Up to date the theoretical modeling of these systems have relied on primitive approximations to the density functional theory (DFT). In this paper, we study structural, electronic and magnetic properties of the Fe3+-containing HDPs Cs2AgFeCl6 and Cs2NaFeCl6 using a combination of more advanced DFT-based methods, including DFT + U, hybrid-DFT, and treatments of various magnetic states. We examine the effect of varying the effective Hubbard parameter, U-eff, in DFT + U and the mixing-parameter, alpha, in hybrid DFT on the electronic structure and structural properties. Our results reveal a set of localized Fe(d) states that are highly sensitive to these parameters. Cs2AgFeCl6 and Cs2NaFeCl6 are both antiferromagnets with Neel temperatures well below room temperature and are thus in their paramagnetic (PM) state at the external conditions relevant to most applications. Therefore, we have examined the effect of disordered magnetism on the electronic structure of these systems and find that while Cs2NaFeCl6 is largely unaffected, Cs2AgFeCl6 shows significant renormalization of its electronic band structure., Funding Agencies|Knut and Alice Wallenberg Foundation [KAW 2019.0082]; Wallenberg Academy Scholar [KAW-2018.0194]; Swedish Government [2009-00971]; Swedish Government Strategic Research Areas in Materials Science on Functional Materials at Linkoping University [2009-00971]; Swedish Research Council [2019-05551, 2018-05973, 854843]; ERC [2022-06725]

We have performed a comprehensive, first-principles high-throughput study of the magnetic properties of halide double perovskites, Cs2BBCl-6, with magnetic ions occupying one or both B and B sites. Our findings indicate a general tendency for these materials to exhibit antiferromagnetic ordering with low Neel temperatures. At the same time, we reveal a few potential candidates that predicted to be ferromagnetic with relatively high Curie temperatures. Achieving ferromagnetic coupling might be feasible via simultaneously alloying at B and B sites with magnetic 3d and nonmagnetic 5d ions. With this approach, we discover that Cs2HgCrCl6, Cs2AgNiCl6, and Cs2AuNiCl6 have high Curie temperatures relative to their peers, with the latter two exhibiting half metallic behavior. Further, this study illuminates the underpinning mechanism of magnetic exchange interactions in halide double perovskites, enabling a deeper understanding of their magnetic behavior. Our findings, especially the discovery of the compounds with robust half metallic properties and high Curie temperatures, hold promise for potential applications in the field of spintronics., Funding Agencies|Knut and Alice Wallenberg Foundation [KAW 2019.0082]; Swedish Government Strategic Research Areas in Materials Science on Functional Materials at Linkoeping University [2009-00971]; Swedish Research Council [2019-05551, 854843, KAW-2018.0194]; ERC [2022-06725]; Wallenberg [2018-05973]

Hydrogenation reactions at gigapascal pressures can yield hydrogen-rich materials with properties relating to superconductivity, ion conductivity, and hydrogen storage. Here, we investigated the ternary Na-Si-H system by computational structure prediction and in situ synchrotron diffraction studies of reaction mixtures NaH-Si-H-2 at 5-10 GPa. Structure prediction indicated the existence of various hypervalent hydridosilicate phases with compositions NamSiH(4+m) (m = 1-3) at comparatively low pressures, 0-20 GPa. These ternary Na-Si-H phases share, as a common structural feature, octahedral SiH62- complexes which are condensed into chains for m = 1 and occur as isolated species for m = 2, 3. In situ studies demonstrated the formation of the double salt Na-3[SiH6]H (Na3SiH7, m = 3) containing both octahedral SiH62- moieties and hydridic H-. Upon formation at elevated temperatures (>500 degrees C), Na3SiH7 attains a tetragonal structure (P4/mbm, Z = 2) which, during cooling, transforms to an orthorhombic polymorph (Pbam, Z = 4). Upon decompression, Pbam-Na3SiH7 was retained to approx. 4.5 GPa, below which a further transition into a yet unknown polymorph occurred. Na3SiH7 is a new representative of yet elusive hydridosilicate compounds. Its double salt nature and polymorphism are strongly reminiscent of fluorosilicates and germanates., Funding Agencies|This research was supported by the Swedish Research Council (VR) through project 2019-06063 and the Bundesministerium fuer Bildung und Forschung (BMBF)-German Federal Ministry of Education and Research (Grant No. 05K20OLA awarded to HK) and the Deutsche Fo [2019-06063]; Swedish Research Council (VR) [05K20OLA]; Bundesministerium fuer Bildung und Forschung (BMBF)-German Federal Ministry of Education and Research [277832266]; Deutsche Forschungsgemeinschaft [2019-05551]; VR [2009-00971]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkping University; Knut and Alice Wallenberg Foundation [854843]; ERC

K2SiH6, crystallizing in the cubicK(2)PtCl(6) structure type (Fm3 ̅m), features unusual hypervalent SiH6 (2-) complexes. Here, the formation of K2SiH6 athigh pressures is revisited by in situ synchrotron diffraction experiments,considering KSiH3 as a precursor. At the investigated pressures,8 and 13 GPa, K2SiH6 adopts the trigonal (NH4)(2)SiF6 structure type (P3 ̅m1) upon formation. The trigonal polymorphis stable up to 725 & DEG;C at 13 GPa. At room temperature, the transitioninto an ambient pressure recoverable cubic form occurs below 6.7 GPa.Theory suggests the existence of an additional, hexagonal, variantin the pressure interval 3-5 GPa. According to density functionaltheory band structure calculations, K2SiH6 isa semiconductor with a band gap around 2 eV. Nonbonding H-dominatedstates are situated below and Si-H anti-bonding states arelocated above the Fermi level. Enthalpically feasible and dynamicallystable metallic variants of K2SiH6 may be obtainedwhen substituting Si partially by Al or P, thus inducing p- and n-typemetallicity, respectively. Yet, electron-phonon coupling appearsweak, and calculated superconducting transition temperatures are <1K. The formation of K2SiH6 at high pressuresstarting from KSiH3 or mixtures of KH and KSiH3 is investigated by in situ synchrotron diffraction experiments.Between 6.5 and 13 GPa, K2SiH6 adopts the trigonal(NH4)(2)SiF6 structure type (P3 ̅m1), which is stable up to 725 & DEG;C at 13 GPa. At room temperature, the transition into an ambientpressure-recoverable cubic form occurs below 6.5 GPa., Funding Agencies|Swedish Research Council (VR) [2019-06063]; Bundesministerium fuer Bildung und Forschung (BMBF)-German Federal Ministry of Education and Research [05K20OLA]; Deutsche Forschungsgemeinschaft [277832266]; BMBF [05K16WC2, 05K13WC2]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [854843]; Knut and Alice Wallenberg Foundation; ERC (synergy grant FASTCORR project) [2019-05551]; [2009-00971]; [2022-06725]; [2018-05973]; Swedish Research Council [2019-06063] Funding Source: Swedish Research Council

The solid Earths inner core (IC) is a sphere with a radius of about 1300 km in the center of the Earth. The information about the IC comes mainly from seismic studies. The composition of the IC is obtained by matching the seismic data and properties of candidate phases subjected to high pressure (P) and temperature (T). The close match between the density of the IC and iron suggests that the main constituent of the IC is iron. However, the stable phase of iron is still a subject of debate. One such iron phase, the body-centered cubic phase (bcc), is dynamically unstable at pressures of the IC (330-364 GPa) and low T but gets stabilized at high T characteristic of the IC (5000-7000 K). So far, ab initio molecular dynamics (AIMD) studies attempted to compute the bcc elastic properties for a small (order of 102) number of atoms. The mechanism of the bcc stabilization cannot be enabled in such cells and that has led to erroneous results. Here we apply AIMD to compute elastic moduli and sound velocities of the Fe bcc phase for a 2000 Fe atom computational cell, which is a cell of unprecedented size for ab initio calculations of iron. Unlike in previous ab initio calculations, both the longitudinal and the shear sound velocities of the Fe bcc phase closely match the properties of the IC material at P = 360 GPa and T = 6600 K, likely the PT conditions in the IC. The calculated density of the bcc iron at these PT conditions is just 3% higher than the density of the IC material according to the Preliminary Earth Model. This suggests that the widely assumed amount of light elements in the IC may need a reconsideration. The anisotropy of the bcc phase is an exact match to the most recent seismic studies., Funding Agencies|Swedish Research Council (VR) [2019-05551, 2017-03744, 2019-06063]; Olle Engkvist Byggmastare Foundation; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]; Knut and Alice Wallenberg Foundation; ERC [854843]; Swedish Research Council [2016-07213]

We show that elastic moduli of higher (third and fourth) order of prestressed single crystals at high pressure, obtained from the Gibbs free energy, enter the relation between the Cauchy stress and Lagrange finite-strain tensor the same way they do for the second-order elastic moduli and, therefore, directly describe the nonlinear elasticity of prestressed crystals., Funding Agencies|Russian Foundation for Basic Research [19- 02-00871]; Russian Science Foundation [18-12-00492]; Swedish Government Strategic Research Areas in Materials Science on Functional Materials at Linkoping University [2009 00971]; Swedish Research Council (VR) [2019-05551]; Knut and Alice Wallenberg Foundation; ERC (Synergy Grant FAST- CORR) [854843]