Li oxide garnets are among the most promising candidates for solid-state electrolytes in novel Li ion and Li metal based battery concepts. Cubic Li7La3Zr2O12 stabilized by a partial substitution of ...Zr4+ by Bi5+ has not been the focus of research yet, despite the fact that Bi5+ would be a cost-effective alternative to other stabilizing cations such as Nb5+ and Ta5+. In this study, Li7-xLa3Zr2-xBixO12 (x = 0.10, 0.20, ..., 1.00) was prepared by a low-temperature solid-state synthesis route. The samples have been characterized by a rich portfolio of techniques, including scanning electron microscopy, X-ray powder diffraction, neutron powder diffraction, Raman spectroscopy, and 7Li NMR spectroscopy. Pure-phase cubic garnet samples were obtained for x ≥ 0.20. The introduction of Bi5+ leads to an increase in the unit-cell parameters. Samples are sensitive to air, which causes the formation of LiOH and Li2CO3 and the protonation of the garnet phase, leading to a further increase in the unit-cell parameters. The incorporation of Bi5+ on the octahedral 16a site was confirmed by Raman spectroscopy. 7Li NMR spectroscopy shows that fast Li ion dynamics are only observed for samples with high Bi5+ contents.
In the present work, the oxidation and spin state of Fe and the local structure around Fe in the supercapacitor birnessite with different contents of the Fe dopant were investigated using Mössbauer ...spectroscopy. It was found that Fe ions were exclusively present as high spin Fe
3+
in octahedral coordination with about 70% iron occupying the Mn
3+
positions and about 30% iron occupying the Mn
4+
positions in the MnO
6
octahedra for all Fe-doped birnessite samples. Based on these new findings, the trend of typical cell parameters, selected bond lengths of the Fe-doped birnessites and their corresponding quadrupole splittings in the Mössbauer spectra were well explained by considering both the weakened Jahn–Teller effect during the replacement of Mn
3+
by Fe
3+
and the expansion of octahedra during the replacement of Mn
4+
by Fe
3+
. The present work offers some new insights into the understanding of the mechanism of the heterogeneous atomic doping on the crystal structure of birnessite, with importance for both mineralogy and material science.
Here, we present a comprehensive study that encompasses changes within the crystal and magnetic structure in the brownmillerite-type phase Ca
2
Fe
2
O
5
induced by the substitution of Fe
3+
with Ga
...3+
. 61 synthetic single-crystal samples of Ca
2
Fe
2−
x
Ga
x
O
5
0.00 ≤
x
≤ 1.328 have been investigated by single-crystal X-ray diffraction at 25 °C. We find that pure Ca
2
Fe
2
O
5
and samples up to
x
~ 1.0 have space group
Pnma
,
Z
= 4, whereas samples with
x
> 1.0 show
I
2
mb
symmetry,
Z
= 4. The Raman spectroscopic measurements exhibit that the change from
Pnma
to
I
2
mb
space group symmetry is reflected by a significant shift of two Raman modes below 150 cm
−1
. These Raman modes are obviously linked to changes in the Ca–O bond lengths at the phase transition.
57
Fe Mössbauer spectroscopy was used to characterize the cation distribution and magnetic structure as a function of composition and temperature. Thereby, the strong preference of Ga
3+
for the tetrahedral site is verified, as an independent method besides XRD. At room-temperature, Ca
2
Fe
2−
x
Ga
x
O
5
solid solution compounds with 0 ≤
x
≤ 1.0 are antiferromagnetic ordered, as revealed by the appearance of magnetically split sextets in the Mössbauer spectra; samples with higher Ga
3+
contents are paramagnetic. Over and above, the substitution of Fe
3+
by Ga
3+
results in the appearance of sharp, additional magnetic hyperfine split sextets, which can be attributed to cluster configurations within the individual tetrahedral chains. The temperature-dependent (20–720 K) Mössbauer study reveals a transition from the magnetically ordered to the paramagnetic state at a temperature of about 710 K for the Ca
2
Fe
2
O
5
end-member.
The crystal structure of KScP2O7 Redhammer, Günther J; Tippelt, Gerold
Acta crystallographica. Section E, Crystallographic communications,
09/2020, Letnik:
76, Številka:
Pt 9
Journal Article
Recenzirano
Odprti dostop
Single crystals of KScP2O7, potassium scandium diphosphate, were grown in a borate flux. The title compound crystallizes isotypically with KAlP2O7 in space-group type P21/c, Z = 4. The main building ...block is an {ScP2O11}9- unit, forming layers parallel to (001). These layers are stacked along 001 via common corners of octa-hedral and tetra-hedral units to span up large hepta-gonal cavities that host the potassium cations with a coordination number of 10. The P-O-P bridging angle increases with increasing size of the octa-hedrally coordinated M III cation, as do the K-O distances within a series of KM IIIP2O7 compounds (M III = Al to Y with ionic radii r = 0.538 to 0.90 Å).Single crystals of KScP2O7, potassium scandium diphosphate, were grown in a borate flux. The title compound crystallizes isotypically with KAlP2O7 in space-group type P21/c, Z = 4. The main building block is an {ScP2O11}9- unit, forming layers parallel to (001). These layers are stacked along 001 via common corners of octa-hedral and tetra-hedral units to span up large hepta-gonal cavities that host the potassium cations with a coordination number of 10. The P-O-P bridging angle increases with increasing size of the octa-hedrally coordinated M III cation, as do the K-O distances within a series of KM IIIP2O7 compounds (M III = Al to Y with ionic radii r = 0.538 to 0.90 Å).
Li7La3Zr2O12 garnet (LLZO) belongs to the most promising solid electrolytes for the development of solid-state Li batteries. The stability of LLZO upon exposure to air is still a matter of ...discussion. Therefore, we performed a comprehensive study on the aging behavior of Al-stabilized LLZO (space group (SG) Ia3¯d) and Ga-stabilized LLZO (SG I4¯3d) involving 98 powder and 51 single-crystal X-ray diffraction measurements. A Li+/H+ exchange starts immediately on exposure to air, whereby the exchange is more pronounced in samples with smaller particle/single-crystal diameter. A slight displacement of Li from interstitial Li2 (96h) toward the regular tetrahedral Li1 (24d) sites occurs in Al-stabilized LLZO. In addition, site occupancy at the 96h site decreases as Li+ is exchanged by H+. More extensive hydration during a mild hydrothermal treatment of samples at 90 °C induces a structural phase transition in Al-LLZO to SG I4¯3d with a splitting of the 24d site into two independent tetrahedral sites (i.e., 12a and 12b), whereby Al3+ solely occupies the 12a site. Li+ is preferably removed from the interstitial 48e site (equivalent to 96h). Analogous effects are observed in Ga-stabilized LLZO, which has SG I4¯3d in the pristine state.
NaSICON (Na Super‐Ionic CONducting) structured materials are among the most promising solid electrolytes for Li‐ion batteries and `beyond Li‐ion' batteries (e.g. Na and K) due to their superior ionic ...conductivities. Although this material has been well known for decades, its exact phase behaviour is still poorly understood. Herein, a starting material of Na3Sc2(PO4)3 single crystals is used, grown by flux methodology, where Na is subsequently chemically replaced by Ag, in order to take advantage of the higher scattering contrast of Ag. It is found that the NaSICON‐type compound shows two phase transitions from a low‐temperature monoclinic α‐phase to a monoclinic β‐phase at about 180 K and to a rhombohedral γ‐phase at about 290 K. The framework of Sc2(PO4)33− is rigid and does not change significantly with temperature and change of symmetry. The main driving force for the phase transitions is related to order–disorder phenomena of the conducting cations. The sensitivity of the phase behaviour on the ordering of these ions suggests that small compositional changes can have a great impact on the phase behaviour and, hence, on the ionic conductivity of NaSICON‐structured materials.
The superionic conducting Na Super‐Ionic CONducting‐type (NaSICON‐type) material Ag3Sc2(PO4)3 was studied by single‐crystal X‐ray diffraction between 100 and 520 K. Two phase transitions were found, at 177 and 291 K. Full structural characterization of the three polymorphs is given for the first time.
Using a low temperature synthesis protocol, it was possible to obtain phase-pure synthetic aegirine (NaFeSi2O6) at temperatures as low as 130 °C, albeit only with rather long synthesis times of ~200 ...h; at 155 °C, a nano-crystallite shaped phase-pure material is formed after 24 h. These are, to the best of our knowledge, the lowest temperatures reported so far for phase-pure aegirine synthesis. Powder X-ray diffraction (PXRD) was used to characterize phase purity, structural state and microstructural properties (size and strain) of the as-synthesized (130–230 °C) and heat treated (300–900 °C) samples, via Rietveld analysis of powder patterns. Melting was observed at 999 °C. With increasing synthesis temperature, crystallite size linearly increased from 10 nm to 30 nm at 230 °C, while unit cell parameters decreased. The microstrain was very small. Additional heat treatment of as synthesized samples showed that the crystallite size remained rather unaffected up to 700 °C. The lattice parameters, however, already changed at low temperatures and successively became smaller, indicating increasing ordering towards more regular arrangements of building units. This was confirmed by 57Fe Mössbauer spectroscopy, where a distinct decrease of the quadrupole splitting with increasing synthesis temperature was found. Finally, Raman spectroscopy showed that some weakly-developed pre-ordering effects were present in the samples, which appeared to be amorphous in PXRD, while well-resolved spectra appeared as soon as the long-range ordered crystalline state could be found with X-ray diffraction.
Spinel-type Li0.64Fe2.15Ge0.21O4, lithium diiron(III) germanium tetra-oxide, has been formed as a by-product during flux growth of an Li-Fe-Ge pyroxene-type material. In the title compound, lithium ...is ordered on the octa-hedral B sites, while Ge(4+) orders onto the tetra-hedral A sites, and iron distributes over both the octa-hedral and tetra-hedral sites, and is in the trivalent state as determined from Mössbauer spectroscopy. The oxygen parameter u is 0.2543; thus, the spinel is close to having an ideal cubic closed packing of the O atoms. The title spinel is compared with other Li- and Ge-containing spinels.
A total of 30 synthetic samples of the Ca2Fe2-xAlxO5, 0.00 ≤ x ≤ 1.34 solid solution series have been investigated by single crystal X-ray diffraction at 25°C. Pure Ca2Fe2O5 and samples up to x = ...0.56 have space group Pnma, Z = 4, whereas samples with x > 0.56 show I2mb symmetry, Z = 4. The substitution of Fe3+ by the smaller Al3+ cation decreases unit-cell parameters and average octahedral and tetrahedral bond lengths and induces distinct changes in the O-atom coordination of the interstitial Ca atom. Discontinuities in the structural parameters vs. the Al3+tot content and changes in slope of these quantities are associated with the phase transition. The essential difference between the two modifications is the cation-O atom-cation angle within the planes of corner sharing octahedra, which is close to 180° in I2mb, but ≈184° in the Pnma phase, and the existence of two different orientations of the tetrahedral chains in Pnma as opposed to one in I2mb. At low overall Al3+ concentrations Al3+ preferentially enters the tetrahedral site until ≈2/3 of it is filled. Additional Al3+ cations, substituted for Fe3+, are equally distributed over octahedral and tetrahedral sites. At high temperature pure Ca2Fe2O5 transforms to a body-centered structure at 724(4) °C. Substituting Al3+ for Fe3+ linearly decreases the transition temperature by 15°C per 0.1 Al3+ down to 623(5)°C for x = 0.65.
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