3 research outputs found
Unraveling Hidden Mg–Mn–H Phase Relations at High Pressures and Temperatures by <i>in Situ</i> Synchrotron Diffraction
The Mg–Mn–H
system was investigated by <i>in situ</i> high pressure
studies of reaction mixtures MgH<sub>2</sub>–Mn–H<sub>2</sub>. The formation conditions of two complex hydrides with composition
Mg<sub>3</sub>MnH<sub>7</sub> were established. Previously known hexagonal
Mg<sub>3</sub>MnH<sub>7</sub> (h-Mg<sub>3</sub>MnH<sub>7</sub>) formed
at pressures 1.5–2 GPa and temperatures between 480 and 500
°C, whereas an orthorhombic form (o-Mg<sub>3</sub>MnH<sub>7</sub>) was obtained at pressures above 5 GPa and temperatures above 600
°C. The crystal structures of the polymorphs feature octahedral
[MnÂ(I)ÂH<sub>6</sub>]<sup>5–</sup> complexes and interstitial
H<sup>–</sup>. Interstitial H<sup>–</sup> is located
in trigonal bipyramidal and square pyramidal interstices formed by
Mg<sup>2+</sup> ions in h- and o-Mg<sub>3</sub>MnH<sub>7</sub>, respectively.
The hexagonal form can be retained at ambient pressure, whereas the
orthorhombic form upon decompression undergoes a distortion to monoclinic
Mg<sub>3</sub>MnH<sub>7</sub> (m-Mg<sub>3</sub>MnH<sub>7</sub>). The
structure elucidation of o- and m-Mg<sub>3</sub>MnH<sub>7</sub> was
aided by first-principles density functional theory (DFT) calculations.
Calculated enthalpy versus pressure relations predict m- and o-Mg<sub>3</sub>MnH<sub>7</sub> to be more stable than h-Mg<sub>3</sub>MnH<sub>7</sub> above 4.3 GPa. Phonon calculations revealed o-Mg<sub>3</sub>MnH<sub>7</sub> to be dynamically unstable at pressures below 5 GPa,
which explains its phase transition to m-Mg<sub>3</sub>MnH<sub>7</sub> on decompression. The electronic structure of the quenchable polymorphs
h- and m-Mg<sub>3</sub>MnH<sub>7</sub> is very similar. The stable
18-electron complex [MnH<sub>6</sub>]<sup>5–</sup> is mirrored
in the occupied states, and calculated band gaps are around 1.5 eV.
The study underlines the significance of <i>in situ</i> investigations
for mapping reaction conditions and understanding phase relations
for hydrogen-rich complex transition metal hydrides
Unraveling Hidden Mg–Mn–H Phase Relations at High Pressures and Temperatures by <i>in Situ</i> Synchrotron Diffraction
The Mg–Mn–H
system was investigated by <i>in situ</i> high pressure
studies of reaction mixtures MgH<sub>2</sub>–Mn–H<sub>2</sub>. The formation conditions of two complex hydrides with composition
Mg<sub>3</sub>MnH<sub>7</sub> were established. Previously known hexagonal
Mg<sub>3</sub>MnH<sub>7</sub> (h-Mg<sub>3</sub>MnH<sub>7</sub>) formed
at pressures 1.5–2 GPa and temperatures between 480 and 500
°C, whereas an orthorhombic form (o-Mg<sub>3</sub>MnH<sub>7</sub>) was obtained at pressures above 5 GPa and temperatures above 600
°C. The crystal structures of the polymorphs feature octahedral
[MnÂ(I)ÂH<sub>6</sub>]<sup>5–</sup> complexes and interstitial
H<sup>–</sup>. Interstitial H<sup>–</sup> is located
in trigonal bipyramidal and square pyramidal interstices formed by
Mg<sup>2+</sup> ions in h- and o-Mg<sub>3</sub>MnH<sub>7</sub>, respectively.
The hexagonal form can be retained at ambient pressure, whereas the
orthorhombic form upon decompression undergoes a distortion to monoclinic
Mg<sub>3</sub>MnH<sub>7</sub> (m-Mg<sub>3</sub>MnH<sub>7</sub>). The
structure elucidation of o- and m-Mg<sub>3</sub>MnH<sub>7</sub> was
aided by first-principles density functional theory (DFT) calculations.
Calculated enthalpy versus pressure relations predict m- and o-Mg<sub>3</sub>MnH<sub>7</sub> to be more stable than h-Mg<sub>3</sub>MnH<sub>7</sub> above 4.3 GPa. Phonon calculations revealed o-Mg<sub>3</sub>MnH<sub>7</sub> to be dynamically unstable at pressures below 5 GPa,
which explains its phase transition to m-Mg<sub>3</sub>MnH<sub>7</sub> on decompression. The electronic structure of the quenchable polymorphs
h- and m-Mg<sub>3</sub>MnH<sub>7</sub> is very similar. The stable
18-electron complex [MnH<sub>6</sub>]<sup>5–</sup> is mirrored
in the occupied states, and calculated band gaps are around 1.5 eV.
The study underlines the significance of <i>in situ</i> investigations
for mapping reaction conditions and understanding phase relations
for hydrogen-rich complex transition metal hydrides
Understanding Antiferromagnetic Coupling in Lead-Free Halide Double Perovskite Semiconductors
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 Cs2(Ag:Na)FeCl6 bulk semiconductor alloys grown by the hydrothermal method.
We show the importance of nonmagnetic monovalence ions at the BI 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 BI 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