4 research outputs found
Cationic Effect on Pressure Driven Spin-State Transition and Cooperativity in Hybrid Perovskites
Hybrid
or metal–organic framework (MOF) perovskites of general
composition, ABX<sub>3</sub>, are known to show interesting properties
that can lead to a variety of technological applications. Our first-principles
study shows they are also potential candidates for exhibiting cooperative
spin-state transitions upon application of external stimuli. We demonstrate
this by considering two specific Fe-based MOF perovskites, namely
dimethylammonium iron formate, [CH<sub>3</sub>NH<sub>2</sub>CH<sub>3</sub>]Â[FeÂ(HCOO)<sub>3</sub>], and hydroxylammonium iron formate,
[NH<sub>3</sub>OH]Â[FeÂ(HCOO)<sub>3</sub>]. Both the compounds are found
to undergo high-spin (<i>S</i> = 2) to low-spin (<i>S</i> = 0) transition at FeÂ(II) site upon application of moderate
strength of hydrostatic pressure, along with large hysteresis. This
spin-state transition is signaled by the changes in electronic, magnetic,
and optical properties. We find both the transition pressure and the
width of the hysteresis to be strongly dependent on the choice of
A-site cation, dimethylammonium or hydroxylammonium, implying that
tuning of spin-switching properties is achievable by chemical variation
of the amine cation in the structure. Our findings open up novel functionalities
in this family of materials of recent interest, which can have important
usage in sensors and memory devices
Cationic Effect on Pressure Driven Spin-State Transition and Cooperativity in Hybrid Perovskites
Hybrid
or metal–organic framework (MOF) perovskites of general
composition, ABX<sub>3</sub>, are known to show interesting properties
that can lead to a variety of technological applications. Our first-principles
study shows they are also potential candidates for exhibiting cooperative
spin-state transitions upon application of external stimuli. We demonstrate
this by considering two specific Fe-based MOF perovskites, namely
dimethylammonium iron formate, [CH<sub>3</sub>NH<sub>2</sub>CH<sub>3</sub>]Â[FeÂ(HCOO)<sub>3</sub>], and hydroxylammonium iron formate,
[NH<sub>3</sub>OH]Â[FeÂ(HCOO)<sub>3</sub>]. Both the compounds are found
to undergo high-spin (<i>S</i> = 2) to low-spin (<i>S</i> = 0) transition at FeÂ(II) site upon application of moderate
strength of hydrostatic pressure, along with large hysteresis. This
spin-state transition is signaled by the changes in electronic, magnetic,
and optical properties. We find both the transition pressure and the
width of the hysteresis to be strongly dependent on the choice of
A-site cation, dimethylammonium or hydroxylammonium, implying that
tuning of spin-switching properties is achievable by chemical variation
of the amine cation in the structure. Our findings open up novel functionalities
in this family of materials of recent interest, which can have important
usage in sensors and memory devices
Structure–Property Relationships in α‑, β′‑, and γ‑Modifications of Mn<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>
The
manganese orthophosphate, Mn<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>, is characterized by the rich variety of polymorphous modifications,
α-, β′-, and γ-phases, crystallized in monoclinic <i>P</i>2<sub>1</sub>/<i>c</i> (<i>P</i>2<sub>1</sub>/<i>n</i>) space group type with unit cell volume
ratios of 2:6:1. The crystal structures of these phases are constituted
by three-dimensional framework of corner- and edge-sharing [MnO<sub>5</sub>] and [MnO<sub>6</sub>] polyhedra strengthened by [PO<sub>4</sub>] tetrahedra. All compounds experience long-range antiferromagnetic
order at Neel temperature <i>T</i><sub>N</sub> = 21.9 K
(α-phase), 12.3 K (β′-phase), and 13.3 K (γ-phase).
Additionally, second magnetic phase transition takes place at <i>T</i>* = 10.3 K in β′-phase. The magnetization
curves of α- and β′-modifications evidence spin-floplike
features at <i>B</i> = 1.9 and 3.7 T, while the γ-Mn<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub> stands out for an extended one-third
magnetization plateau stabilized in the range of magnetic field <i>B</i> = 7.5–23.5 T. The first-principles calculations
define the main paths of superexchange interaction between Mn spins
in these polymorphs. The spin model for α-phase is found to
be characterized by collection of uniform and alternating chains,
which are coupled in all three directions. The strongest magnetic
exchange interaction in γ-phase emphasizes the trimer units,
which make chains that are in turn weakly coupled to each other. The
spin model of β′-phase turns out to be more complex compared
to α- or γ-phase. It shows complex chain structures involving
exchange interactions between Mn2 (Mn2′, Mn2″) and Mn3
(Mn3′, Mn3″). These chains interact through exchanges
involving Mn1 (Mn1′, Mn1″) spins
Crystal Structure, Physical Properties, and Electronic and Magnetic Structure of the Spin <i>S</i> = <sup>5</sup>/<sub>2</sub> Zigzag Chain Compound Bi<sub>2</sub>Fe(SeO<sub>3</sub>)<sub>2</sub>OCl<sub>3</sub>
We report the synthesis and characterization
of the new bismuth iron selenite oxochloride Bi<sub>2</sub>FeÂ(SeO<sub>3</sub>)<sub>2</sub>OCl<sub>3</sub>. The main feature of its crystal
structure is the presence of a reasonably isolated set of spin <i>S</i> = <sup>5</sup>/<sub>2</sub> zigzag chains of corner-sharing
FeO<sub>6</sub> octahedra decorated with BiO<sub>4</sub>Cl<sub>3</sub>, BiO<sub>3</sub>Cl<sub>3</sub>, and SeO<sub>3</sub> groups. When
the temperature is lowered, the magnetization passes through a broad
maximum at <i>T</i><sub>max</sub> ≈ 130 K, which
indicates the formation of a magnetic short-range correlation regime.
The same behavior is demonstrated by the integral electron spin resonance
intensity. The absorption is characterized by the isotropic effective
factor <i>g</i> ≈ 2 typical for high-spin Fe<sup>3+</sup> ions. The broadening of ESR absorption lines at low temperatures
with the critical exponent β = <sup>7</sup>/<sub>4</sub> is
consistent with the divergence of the temperature-dependent correlation
length expected for the quasi-one-dimensional antiferromagnetic spin
chain upon approaching the long-range ordering transition from above.
At <i>T</i><sub>N</sub> = 13 K, Bi<sub>2</sub>FeÂ(SeO<sub>3</sub>)<sub>2</sub>OCl<sub>3</sub> exhibits a transition into an
antiferromagnetically ordered state, evidenced in the magnetization,
specific heat, and Mössbauer spectra. At <i>T</i> < <i>T</i><sub>N</sub>, the <sup>57</sup>Fe Mössbauer
spectra reveal a low saturated value of the hyperfine field <i>H</i><sub>hf</sub> ≈ 44 T, which indicates a quantum
spin reduction of spin-only magnetic moment Δ<i>S</i>/<i>S</i> ≈ 20%. The determination of exchange interaction
parameters using first-principles calculations validates the quasi-one-dimensional
nature of magnetism in this compound