87Rb NMR Research Articles

Stabilizirovannye polem magnitnye fazy v treugol'nom antiferromagnetike RbFe(MoO4)2

The magnetic H–T phase diagram of a quasi-two-dimensional easy plane antiferromagnet RbFe(MoO4)2 (S = 5/2) with an equilateral triangular lattice structure is studied with 87Rb NMR technique for field directed along hard axis C3. The studies confirm the two step transition from the low field umbrella-like incommensurate magnetic phase to the paramagnetic state observed recently (H. Mitamura et al., J. Magn. Magn. Mater. 400, 70 (2016)). The transitions were identified as a lambda anomaly in the spin-lattice relaxation rate and a step increase of magnetic susceptibility at intermediate transition. The 87Rb NMR study precludes the possibility of either V or fan spin textures in the new high field phase. The additional transition is presumably associated with loss of inter plane magnetic order before the transition to paramagnetic state of individual triangular planes.

Reducing the effects of weak homonuclear dipolar coupling with CPMG pulse sequences for static and spinning solids

The Carr-Purcell/Meiboom-Gill (CPMG) pulse sequence, initially introduced for measuring transverse relaxation time constants (T2), can provide significant signal enhancements for solid-state NMR (SSNMR) spectra. The proper implementation of CPMG for acquiring spectra influenced by chemical shift anisotropies (CSAs), first and/or second order quadrupolar interactions, or paramagnetic broadening has been well documented to date, as have the effects of heteronuclear dipolar coupling on CPMG echo trains and T2 lifetimes. Homonuclear dipolar coupling can also impact T2 lifetimes and CPMG echo trains; these effects have been thoroughly investigated for spectra of homonuclear dipolar coupled spin-1/2 nuclei typically acquired under static conditions that are predominantly influenced by dipolar broadening (e.g., 1H, 19F, etc.). In particular, it has been shown that short refocusing pulses with small flip angles can extend the effective T2 (T2eff, the observed T2 constant as impacted by experimental conditions) measured by CPMG sequences for strong homonuclear dipolar coupled spin-1/2 pairs under static conditions. To date, these effects have not been explored for (i) spin-1/2 nuclei that have significant CSAs and simultaneously feature weak homonuclear dipolar couplings, (ii) for quadrupolar nuclei that are also weakly homonuclear dipolar coupled, and (iii) for either of these cases under magic-angle spinning (MAS) conditions. Herein, we demonstrate that short refocusing pulses that cause small flip angles can reduce the attenuation of signal in CPMG echo trains resulting from dipolar dephasing caused by the weak homonuclear dipolar couplings. For both spin-1/2 and quadrupolar nuclei, this can lead to significant extensions in T2eff and signal enhancements of up to three times compared to conventional CPMG in favourable cases. These phenomena can occur under both static and magic-angle spinning (MAS) conditions, in the latter of which homonuclear couplings are reintroduced by rotational resonance (R2) recoupling. Experimental examples of 13C (I = 1/2), 2H (I = 1), 87Rb (I = 3/2), 23Na (I = 3/2), and 35Cl (I = 3/2) NMR under static and MAS conditions, as well as simulations of these phenomena, are shown and discussed.

High-field magnetic structure of the triangular antiferromagnet RbFe(MoO4)2

The magnetic H - T phase diagram of a quasi-two-dimensional antiferromagnet RbFe(MoO4)2 with an equilateral triangular lattice structure is studied with 87Rb NMR and neutron diffraction techniques. This combination of experimental techniques allows us to determine the ordered components of the magnetic moments on the Fe3+ ions within various high-field phases - the Y, UUD, V, and fan structures, stabilized in the compound by the in-plane magnetic field. It is also established that the transition from the V to the fan phase is of first-order, whereas the transition from the fan phase to the polarized paramagnetic phase is continuous. An analysis of the NMR spectra shows that the high-field fan phase of RbFe(MoO4)2 can be successfully described by a periodic commensurate oscillation of the magnetic moments around the field direction in each Fe layer combined with an incommensurate modulation of the magnetic structure perpendicular to the layers.

NMR spectroscopy probes microstructure, dynamics and doping of metal halide perovskites.

Solid-state magic-angle spinning NMR spectroscopy is a powerful technique to probe atomic-level microstructure and structural dynamics in metal halide perovskites. It can be used to measure dopant incorporation, phase segregation, halide mixing, decomposition pathways, passivation mechanisms, short-range and long-range dynamics, and other local properties. This Review describes practical aspects of recording solid-state NMR data on halide perovskites and how these afford unique insights into new compositions, dopants and passivation agents. We discuss the applicability, feasibility and limitations of 1H, 13C, 15N, 14N, 133Cs, 87Rb, 39K, 207Pb, 119Sn, 113Cd, 209Bi, 115In, 19F and 2H NMR in typical experimental scenarios. We highlight the pivotal complementary role of solid-state mechanosynthesis, which enables highly sensitive NMR studies by providing large quantities of high-purity materials of arbitrary complexity and of chemical shifts calculated using density functional theory. We examine the broader impact of solid-state NMR on materials research and how its evolution over seven decades has benefitted structural studies of contemporary materials such as halide perovskites. Finally, we summarize some of the open questions in perovskite optoelectronics that could be addressed using solid-state NMR. We, thereby, hope to stimulate wider use of this technique in materials and optoelectronics research.

Perspectives of fast magic-angle spinning 87 Rb NMR of organic solids at high magnetic fields.

We report solid-state 87 Rb NMR spectra from two Rb-ionophore complexes obtained with fast magic-angle spinning (MAS) (up to 60 kHz) at 21.1 T. These Rb-ionophore complexes containing macrocycles such as benzo-15-crown-5 and cryptand [2.2.2] are typical of organic Rb salts that exhibit very large 87 Rb quadrupole coupling constants (close to 20 MHz). We have also obtained static 87 Rb NMR spectra for these two compounds and determined both 87 Rb quadrupole coupling and chemical shift tensors. The experimental 87 Rb NMR tensor parameters are compared with those obtained by quantum chemical computations. Our results demonstrate that the combination of fast MAS (60 kHz or higher) and a high magnetic field (21.1 T or higher) is sufficient to produce high-quality solid-state 87 Rb NMR spectra for organic Rb solids at the natural abundance level. We anticipate that, with additional 87 Rb isotope enrichment (up to 99%), the sensitivity of solid-state 87 Rb NMR will be 400 times higher than 39 K NMR, which makes the former an attractive surrogate probe for studying K+ ion binding in biological systems.

Metallic State in Rubidium-Loaded Low-Silica X Zeolite

The ground state of alkali metals when the particle size decreases from bulk to nanometric atomic clusters is inevitably accompanied by quantum effects that can suppress their pristine metallic state. We demonstrate that the metallic nature of rubidium clusters confined and arrayed in the framework of insulating low-silica X zeolite is preserved. The $^{87}$Rb NMR spin-lattice relaxation assigned to rubidium clusters in supercages shows a Korringa behavior from 190~K down to 10~K, which is compatible with a macroscopic observation of low electrical resistivity. The density of states at the Fermi level is found to be enhanced compared to the analogous sodium case, consistent with a Holstein-Hubbard model of alkali-loaded zeolites.

Fast 2D Diffusion in Nanocrystalline RbSn2F5 as Seen By Impedance and 19f NMR Spectroscopy

The functionality of sensors or batteries relies in many cases on mobile ions such as Li+, Na+, O2- or F-, which are subjected to rapid self-diffusion processes. The dynamic parameters as well as the shape of the underlying motion correlation function depend on the dimensionality of the diffusion pathways. Model compounds help to unambiguously proof the concepts and models developed to describe low dimensional diffusion.[1-3] The pseudo-binary fluoride RbSn2F5 with trigonal symmetry constitutes such a model substance. Here, we present a new mechanochemical route to synthesise nanocrystalline RbSn2F5 via a one-pot reaction in a planetary mill. Soft annealing transforms the as prepared product into microcrystalline RbSn2F5. We used X-ray diffraction, thermal analyses and high-resolution 87Rb, 19F , and 119Sn NMR spectroscopy to characterize structure and ion dynamics in detail. In particular, we took advantage of AC impedance spectroscopy and time-domain NMR measurements to record diffusion-induced spin-lattice-relaxation rates in both the laboratory and rotating frame of reference to monitor long-range as well as short-ranged F anion transport processes in RbSn2F5. It turned out that only an NMR spectral density function developed for 2D diffusion yields an activation energy being in accordance with that derived from conductivity measurements. Our results confirm Richards[4,5] semi-empirical NMR relaxation model for 2D uncorrelated jump diffusion, not only for microcrystalline RbSn2F5 but also for the nanocrystalline, mechanosynthesized sample. Financial support by the Austrian Federal Ministry for Science, Research and Economy as well as the Christian-Doppler Forschungsgesellschaft is highly appreciated.

Phase Segregation in Cs-, Rb- and K-Doped Mixed-Cation (MA)x(FA)1-xPbI3 Hybrid Perovskites from Solid-State NMR.

Hybrid (organic–inorganic) multication lead halide perovskites hold promise for a new generation of easily processable solar cells. Best performing compositions to date are multiple-cation solid alloys of formamidinium (FA), methylammonium (MA), cesium, and rubidium lead halides which provide power conversion efficiencies up to around 22%. Here, we elucidate the atomic-level nature of Cs and Rb incorporation into the perovskite lattice of FA-based materials. We use 133Cs, 87Rb, 39K, 13C, and 14N solid-state MAS NMR to probe microscopic composition of Cs-, Rb-, K-, MA-, and FA-containing phases in double-, triple-, and quadruple-cation lead halides in bulk and in a thin film. Contrary to previous reports, we have found no proof of Rb or K incorporation into the 3D perovskite lattice in these systems. We also show that the structure of bulk mechanochemical perovskites bears close resemblance to that of thin films, making them a good benchmark for structural studies. These findings provide fundamental understanding of previously reported excellent photovoltaic parameters in these systems and their superior stability.

Crystal and Electronic Structures of Complex Bismuth Iodides A3Bi2I9 (A = K, Rb, Cs) Related to Perovskite: Aiding the Rational Design of Photovoltaics

Ternary bismuth halides form an interesting functional materials class in the context of the closely related Pb halide perovskite photovoltaics, especially given the significantly reduced toxicity of Bi when compared with Pb. The compounds A3Bi2I9 (A = K, Rb, Cs) examined here crystallize in two different structure types: the layered defect-perovskite K3Bi2I9 type, and the Cs3Cr2Cl9 type. The latter structure type features isolated Bi2I93– anions. Here, the crystal structures of the ternary iodides are redetermined and a corrected structural model for Rb3Bi2I9, as established by single crystal X-ray diffraction and solid state 87Rb NMR spectroscopy and supported by density functional theory (DFT) calculations is presented. A variety of facile preparation techniques for single crystals, bulk materials, as well as solution-processed thin films are described. The optical properties and electronic structures are investigated experimentally by optical absorption and ultraviolet photoemission spectroscopy and c...

Identification of Various Coexisting Phases in Superconducting and Non-superconducting Samples of RbxFe2−ySe2

87Rb NMR, muon spin relaxation (μSR), and neutron diffraction studies have been carried out to investigate how the superconducting (SC) fraction is determined in RbxFe2−ySe2 samples. We observe several phases with different compositions, crystal structures, and magnetic properties of each sample. Samples containing a phase with superconductivity (SC phase) are separated into two phases, an antiferromagnetic (AFM) phase with the block-type antiferromagnetism (AFM1 phase; space group I4/m, Neel temperature TN1 ∼ 550 K) and the SC phase (I4/mmm, SC transition temperature Tc ∼ 30 K). This SC phase has a very small amount of Fe vacancies, if any. In samples having no SC phase, besides the AFM1 phase, there are two other phases, an AFM phase (AFM2 phase; Pmna, TN2 ∼ 150 K) and a phase with an appreciable amount of Fe vacancy (I4/mmm; non-SC phase). Thus, the presence of the coexisting phases is an origin of the complicated physical behavior dependent on the \((x,y)\) values observed in the RbxFe2−ySe2 samples.

Multinuclear NMR studies on the effect of electrostatic and hydrophobic interactions on bindings to counterions to weakly acidic and basic polyelectrolytes

23Na, 35Cl, 81Br, and 87Rb NMR have been applied to clarify the influence of electrostatic and hydrophobic interactions on the counterion binding equilibria of weakly acidic and basic polyelectrolytes with negatively and positively charged polyions, respectively. Na+ and Rb+ ions capable of binding to weakly acidic polymers (i.e., polyacrylate acid (PAA)), and Cl− and Br− ions capable of binding to weakly basic polymers (i.e., poly(N-vinylimidazole) (PVIm) and polyallylamine (PAAm)) were used as representative examples to measure these interactions. The results revealed that the longitudinal and transverse NMR relaxation rates of the Cl−, Br− and the Rb+ counterions in the PVIm and PAA systems were faster than those of free Cl−, Br− and Rb+ ions when the polyions were completely neutralized. The binding interactions between the Cl−, Br− and the Rb+ ions and PVIm and PAA were found to be hydrophobic in nature because the magnitudes of these effects were found to be dependent on the hydrophobicities of the polyelectrolytes and the counterions. Although a slight hydrophobic interaction was detected between the Rb+ ions and PAA, where the degree of neutralization of the polyacid (α) was 1, Na+ ions also started to bind to PAA at α = 0.65, where the linear charge density corresponded to the Bjerrum length.

Emergence of superconductivity and magnetic ordering tuned by Fe-vacancy in alkali-metal Fe chalcogenides RbxFe2-ySe2

Samples of RbxFe2-ySe2 exhibiting superconductivity [superconducting (SC) samples] undergo a phase-separation into two phases, a Fe-vacancy ordered phase with antiferromagnetic (AFM) transition at TN1~500 K (AFM1 phase) and a phase with little Fe- vacancy and SC transition at Tc~30 K (SC phase). The samples of RbxFe2-ySe2 exhibiting no SC behaviour (non-SC samples) are phase-separated into three phases, the AFM1 phase, another AFM phase with TN2 ~150 K (AFM2 phase), and a paramagnetic phase with no SC transitions (paramagnetic non-SC phase). In this paper, we present the experimental results of magnetic susceptibility, electrical resistivity, and NMR measurements on single crystals of RbxFe2-ySe2 to reveal physical properties of these co-existing phases in the SC and non-SC samples. The 87Rb and 77Se NMR spectra show that the Fe vacancy concentration is very small in the Fe planes of the SC phase, whereas the AFM2 and paramagnetic non-SC phases in non-SC samples have larger amount of Fe vacancies. The randomness induced by the Fe vacancy in the non-SC samples makes the AFM2 and paramagnetic non-SC phases insulating/semiconducting and magnetically active, resulting in the absence of the superconductivity in RbxFe2-ySe2.

Paramagnetic shimming for wide-range variable-field NMR

We propose a new passive shimming strategy for variable-field NMR experiments, in which the magnetic field produced by paramagnetic shim pieces placed inside the magnet bore compensates the inhomogeneity of a variable-field magnet for a wide range of magnet currents. Paramagnetic shimming is demonstrated in 7Li, 87Rb, and 45Sc NMR of a liquid solution sample in magnetic fields of 3.4, 4.0, and 5.4T at a fixed carrier frequency of 56.0MHz. Since both the main-field inhomogeneity and the paramagnetic magnetization are proportional to the main-magnet current, the resonance lines are equally narrowed by the improved field homogeneity with an identical configuration of the paramagnetic shim pieces. Paramagnetic shimming presented in this work opens the possibility of high-resolution variable-field NMR experiments.

Solid State Complex Chemistry: Formation, Structure, and Properties of Homoleptic Tetracyanamidogermanates RbRE[Ge(CN2)4] (RE = La, Pr, Nd, Gd)

Tetracyanamidometallates with the general formula RbRE[T(CN2)4] (RE = La, Pr, Nd, Gd; T = Si, Ge) were prepared by solid state metathesis reactions starting from stoichiometric mixtures of RECl3, A2[TF6], and Li2(CN2). Reactions were studied by differential thermal analysis that showed ignition temperatures between 360 and 390 °C for the formation of RbGd[T(CN2)4] with T = Si and Ge. The powder diffraction patterns of RbRE[Ge(CN2)4] were indexed isotypically to the already known RbRE[Si(CN2)4] compound. IR spectra of RbLa[Ge(CN2)4] were measured and compared with those of RbLa[Si(CN2)4]. (73)Ge, (87)Rb, and (139)La solid state NMR measurements and density functional theory calculations were used to verify the novel homoleptic [Ge(CN2)4](4-) ion. Luminescence properties of Eu(3+), Ce(3+), and Tb(3+) doped samples are reported.

Study of ferroelectric–paraelectric phase transition of RbH2AsO4 single crystals by 87Rb NMR relaxation time measurements

The temperature dependences of the resonance frequency, line width, and spin–lattice relaxation time T1 of the ferroelectric and paraelectric phases of RbH2AsO4 crystals are investigated. The 87Rb NMR line at the phase transition temperature TC (=110K) splits into two lines, which indicates the occurrence of phase transition from tetragonal to orthorhombic phase. In addition, T1 for the 87Rb nucleus undergoes remarkable changes near TC, which indicates drastic alterations of the spin dynamics at TC. At TC, changes occur in the resonance frequency and T1, which are related to variations in the symmetry of the environments of the 87Rb ions.

Phase separation of antiferromagnetism and superconductivity in Rb x Fe2−y Se2 observed by using Rb NMR

We have carried out 87Rb NMR measurements on single crystals with and without the superconducting (SC) transition to clarify whether the SC and the antiferromagnetic (AFM) states are microscopically coexistent or just phase-separated in the Rb-intercalated iron-selenide Rb x Fe2−y Se2. The 87Rb NMR spectra observed at temperature below 300 K for these crystals are clearly separable into two parts with different structures and widths. The broader spectrum can be explained by the presence of a region with an AFM spin structure proposed by neutron scattering studies. The narrower spectrum in the crystal with the SC transition can be understood from the temperature dependences of the spectral frequency-position and shape to stem from the SC region with T c = 30.8 K. Thus, we can clearly conclude that the superconductivity and the antiferromagnetism in the present system are phase-separated. Also, the crystal without the SC transition has two phase-separated regions with respective magnetic transitions at ∼500 and ∼50 K.

Dynamics of Ag+ Ions in RbAg4I5 Probed Indirectly via 87Rb Solid-State NMR

Solid-state 87Rb NMR has been used to determine the ionic hopping rate of Ag+ ions in a powdered sample of α-RbAg4I5 as a function of temperature from 20 to 250 °C. In this phase, Rb is a stationary framework atom, which does not take part in ionic conduction in this material. At the same time 87Rb has a large quadrupole moment, making 87Rb NMR capable of detecting mobile species in close proximity. Simulation of the static 87Rb NMR powder pattern under the influence of Ag+ motion was performed using the EXPRESS software. Lineshape simulations were used to extract an ionic hopping rate for each temperature, and changes in the line shape were correlated to changes in ion mobility, which increases with temperature. Ag+ hopping rates were found to be in the range of 7.0 ± 0.5 kHz to 30 ± 2 kHz within the temperature window of 20 to 100 °C, resulting in an activation energy of 17 ± 3 kJ/mol (0.18 ± 0.03 eV), for silver ion hopping. This result is in agreement with previous studies, suggesting that this indire...

Phenomenon of high-temperature phase transition of RbH2PO4 crystals studied by 1H, 31P, and 87Rb NMR

The high-temperature phase transition phenomenon of RbH2PO4 single crystals were investigated by using a FT NMR spectrometer. The molecular dynamics of each ion in the high-temperature phase was studied by examining the relaxation times and the shifts of the resonance lines of the 1H, 31P, and 87Rb ions. The behavior that gives rise to the strong temperature dependence for spin-lattice relaxation time for 31P near 350K (=TP) in RbH2PO4 is probably related to the re-orientational motion of the PO4 groups. As far as the surface and the spin-lattice relaxation time is concerned, the high temperature phenomenon exhibited near Td (=400K) is not related to a physical change, but is instead related to a chemical change.

Synthesis and Structure Resolution of RbLaF4

The synthesis and structure resolution of RbLaF(4) are described. RbLaF(4) is synthesized by solid-state reaction between RbF and LaF(3) at 425 °C under a nonoxidizing atmosphere. Its crystal structure has been resolved by combining neutron and synchrotron powder diffraction data refinements (Pnma,a = 6.46281(2) Å, b = 3.86498(1) Å, c = 16.17629(4) Å, Z = 4). One-dimensional (87)Rb, (139)La, and (19)F MAS NMR spectra have been recorded and are in agreement with the proposed structural model. Assignment of the (19)F resonances is performed on the basis of both (19)F-(139)La J-coupling multiplet patterns observed in a heteronuclear DQ-filtered J-resolved spectrum and (19)F-(87)Rb HMQC MAS experiments. DFT calculations of both the (19)F isotropic chemical shieldings and the (87)Rb, (139)La electric field gradient tensors using the GIPAW and PAW methods implemented in the CASTEP code are in good agreement with the experimental values and support the proposed structural model. Finally, the conductivity of RbLaF(4) and luminescence properties of Eu-doped LaRbF(4) are investigated.

Superconducting and normal-state properties of single-crystalline Tl0.47Rb0.34Fe1.63Se2as seen viaSe77andRb87NMR

We report both 77Se and 87Rb NMR studies on Tl0.47Rb0.34Fe1.63Se2 single-crystalline superconductors (T_c about 32 K). Singlet superconductivity is decisively determined by a sharp drop of the Knight shift K(T) below T_c, after subtracting the superconducting diamagnetic effect. However, the Hebel-Slichter coherence peak below T_c is not observed in the spin-lattice relaxation rate 1/T_1, even with a low in-plane NMR field of 2.6 Tesla. Just above T_c, no evidence of low-energy spin fluctuation is found in the spin-lattice relaxation rate on both the 77Se and the 87Rb sites. Upon warming, however, the Knight shifts and the spin-lattice relaxation rates of both nuclei increase substantially with temperature. In particular, the Knight shift is nearly isotropic and follow a function fit of K=a+bT^2 from T_c up to 300 K. These normal state properties may be an indication of thermally activated spin fluctuations. Our observations should put a strong constraint to the theory of magnetism and superconductivity in the newly discovered iron-based superconductors.