the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Feb 2025
Automated wideline nuclear quadrupole resonance of mixed-cation lead halide perovskites
Abstract. Nuclear quadrupole resonance (NQR) is a sister technique to NMR that is extremely sensitive to local crystal composition and structure. Unfortunately, in disordered materials this sensitivity also leads to very large linewidths, presenting a technical challenge and requiring a serious time investment to get a full spectrum. Here we describe our newly developed, automated NQR setup to acquire high-quality wideline spectra. Using this setup, we carried out 127I NQR on three mixed cation lead halide perovskites (LHPs) of the form MAxFA1−xPbI3 (MA = methylammonium, FA = formamidinium, x = 0.25,0.50,0.75) at various temperatures. We achieve a signal-to-noise of up to ~ 400 for lineshapes with a full-width-half-maximum of ~ 2.5 MHz acquired with a spectral width of 20 MHz in the course of two to three days. The spectra, which at least partially exhibit features encoding structural information, are interpreted using a statistical model. This model finds a degree of MA–MA, FA–FA clustering (0.2 ≤ S ≤ 0.35). This proof-of-principle for both the wideline NQR setup and the statistical model widens the applicability of an underutilised avenue of non-invasive structural research.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Magnetic Resonance.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.- Preprint
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RC1: 'Comment on mr-2025-2', Anonymous Referee #1, 27 Feb 2025
The manuscript by Wolffs et al., “Automated wideline nuclear quadrupole resonance of mixed-cation lead halide perovskites”, describes an automated nuclear quadrupole resonance (NQR) spectrometer capable of wideline acquisitions. The authors apply this system to analyze mixed cation lead halide perovskites, which is a topic of interest to the community. The authors make use of an eATM autotuning robot in combination with a home-built probe, and a Varian console to acquire broad NQR lines with little human input. Interestingly, the authors linearized the power output across a broadband range and collected beautiful NQR lines of >2 MHz in linewidth. The authors also provided a nice model for extracting structural information from the resulting NQR data. Overall, the authors have written a clear and interesting manuscript presenting their NQR approach to investigating lead halide perovskites, and the manuscript should be considered for publication subject to a few minor revisions / suggestions:
- Formula (1) of page 2 is correct and provides the NQR frequencies for a spin I = 5/2 nucleus when h = 0, as stated by the authors. There is nothing inherently incorrect in the statements made by the authors. However, I worry that this may confuse readers new to the field of NQR, as eta (h) will influence the transition frequencies when h ≠ 0. I suggest adding a statement to clarify that the NQR frequencies are influenced when h ≠ 0.
- On page 2, line 35: The authors state that NQR has re-emerged partly due to lead halide perovskites. I suggest to the authors to add a short line explaining what NQR spectroscopy offers from other techniques, such as solid-state NMR and X-ray diffraction.
- On page 3, line 55: The authors state that the NQR spectrum becomes “too wide to easily acquire with regular, commercially available equipment”. To my knowledge, there are no regular commercial NQR instruments that are available, and commercial solid-state NMR console (Bruker, Varian, Tecmag) can be adapted to NQR. I suggest rephrasing this to simply saying that the broader NQR spectra are even more difficult to observe. The “commercial” aspect may not be appropriate here, as current solid-state NMR consoles are sufficiently flexible to be adapted to a specific challenge, such as the case of this work where a Varian NMR console was used.
- On page 6, line 163: The authors cite Pyykko’s paper from 2008, but there has since been a 2017 update. The differences in the quadrupole moment of 127I is not large so it is not worth repeating the calculations for this small correction, but perhaps worth noting for future work. Unless there was a reason why the 2008 value was chosen.
- On page 6, line 166: Could the authors please define the number of turns, pitch (distance between turns), and wire gauge used to construct the NQR coil? I also suggest reporting the probe’s Q or the bandwidth of the resonator (this is distinct from the bandwidth of the probe), but I will leave the latter up to the author’s discretion.
- The authors measured the v2 NQR frequency of 127I. There is a clear benefit to this approach in terms of s/n. I suggest clarifying in the manuscript why v2 was chosen rather than v1.
- Page 18, line 312: The authors mentioned distortions twice in the sentence: “Distortions in the spectrum below the tried and tested bandwidth of 159–179MHz exhibit spectral distortions that are tentatively attributed …” which could be clarified.
- As broad NQR lines in mixed halide perovskites have been acquired using similarly automated NQR instrumentation (Aebli et al., 2021), to my understanding, the most novel part of this work appears to be in the model used to interpret the NQR data. Could the authors clarify the novel aspect for the hardware in the manuscript?
Citation: https://doi.org/10.5194/mr-2025-2-RC1 -
AC1: 'Reply on RC1', Arno Kentgens, 23 Mar 2025
We thank Anonymous Referee 1 for the positive evaluation of our work and the detailed comments for improvement. Our point-by-point answers to the comments are:
- Formula (1) of page 2 is correct and provides the NQR frequencies for a spin I = 5/2 nucleus when h = 0, as stated by the authors. There is nothing inherently incorrect in the statements made by the authors. However, I worry that this may confuse readers new to the field of NQR, as eta (h) will influence the transition frequencies when η ≠ 0. I suggest adding a statement to clarify that the NQR frequencies are influenced when h ≠ 0.
We agree with the reviewer and added a statement to clarify that the NQR frequencies are influenced when η ≠ 0 on page 2 of the revised manuscript.
- On page 2, line 35: The authors state that NQR has re-emerged partly due to lead halide perovskites. I suggest to the authors to add a short line explaining what NQR spectroscopy offers from other techniques, such as solid-state NMR and X-ray diffraction.
At the end of page 2 we have added a few sentences about the specific advantage of halide NQR to study LHPs compared to NMR and XRD.
- On page 3, line 55: The authors state that the NQR spectrum becomes “too wide to easily acquire with regular, commercially available equipment”. To my knowledge, there are no regular commercial NQR instruments that are available, and commercial solid-state NMR console (Bruker, Varian, Tecmag) can be adapted to NQR. I suggest rephrasing this to simply saying that the broader NQR spectra are even more difficult to observe. The “commercial” aspect may not be appropriate here, as current solid-state NMR consoles are sufficiently flexible to be adapted to a specific challenge, such as the case of this work where a Varian NMR console was used.
We agree with the reviewer and removed the reference to commercial equipment, now only pointing out the technical challenge to record extremely wideline NQR spectra of LHPs with occupational disorder.
- On page 6, line 163: The authors cite Pyykko’s paper from 2008, but there has since been a 2017 update. The differences in the quadrupole moment of 127I is not large so it is not worth repeating the calculations for this small correction, but perhaps worth noting for future work. Unless there was a reason why the 2008 value was chosen.
We agree with the reviewer that future calculations should use the newer quadrupole moment, but the reported difference in the quadrupole moment of 127I is only 1.2%. We have therefore not repeated the calculations and left the article unmodified on this point.
- On page 6, line 166: Could the authors please define the number of turns, pitch (distance between turns), and wire gauge used to construct the NQR coil? I also suggest reporting the probe’s Q or the bandwidth of the resonator (this is distinct from the bandwidth of the probe), but I will leave the latter up to the author’s discretion.
We have added a detailed description of the coil to the description of the wideline NQR setup on page 6, line 171 we now give the probe Q, and the number of turns, length and wire gauge.
- The authors measured the v2 NQR frequency of 127I. There is a clear benefit to this approach in terms of s/n. I suggest clarifying in the manuscript why v2 was chosen rather than v1.
Indeed we clearly chose v2 for the better S/N at the higher frequency and clarified our choice for the second NQR resonance in section1.3, line 65.
- Page 18, line 312: The authors mentioned distortions twice in the sentence: “Distortions in the spectrum below the tried and tested bandwidth of 159–179MHz exhibit spectral distortions that are tentatively attributed …” which could be clarified.
We thank the referee for spotting the error and have rephrased the sentence (page19 line 336).
- As broad NQR lines in mixed halide perovskites have been acquired using similarly automated NQR instrumentation (Aebli et al., 2021), to my understanding, the most novel part of this work appears to be in the model used to interpret the NQR data. Could the authors clarify the novel aspect for the hardware in the manuscript?
With apologies to both the referee and the authors of Aebli et al. (2021) if we are wrong, but it is our impression that the VOCS experiments in the Aebli paper were done without automation. There is no reference to the use of an ATM system or any means of automation in the paper. However, we acknowledge that the experiments by Mozur et al (2020) are using an ATM which we explicititly reference in line 61. Unfortunately Mozur et al. did not provide details about the approach to record undistored spectra in an automated way. We have added a detailed description of our automated approach on page 10 of the revised manuscript; we added VT operation, lineairized the rf-field strength, and adapted the order in which record the spectra at different frequencies to get a reliable performance from the ATM thus recording overall lineshapes that are not distorted by technical imperfections. Finally, we have separated the operation of the ATM from the pulse program so that remote operation and corrections thereof are facilitated. Therefore we believe to have improved upon the NQR approach in a way that contributes to the field.
Citation: https://doi.org/10.5194/mr-2025-2-AC1
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RC2: 'Comment on mr-2025-2', Anonymous Referee #2, 28 Feb 2025
The manuscript titled “Automated Wideline Nuclear Quadrupole Resonance of Mixed-Cation Lead Halide Perovskites” by Wolf et al. presents an outstanding study. The authors have developed an innovative method, including the use of an eATM robot to automate the recording of wide-line NQR spectra. This technique is successfully applied to conduct a ¹²⁷I NQR study on three mixed-cation lead halide perovskites (LHPs) at varying temperatures. Furthermore, they provide an effective approach for analyzing the NQR spectra, yielding valuable insights into the structural properties of these materials. This work makes a significant contribution to the magnetic resonance literature and should be considered a valuable addition to the field. The authors may wish to consider the following point:
In the experimental section on NQR (page 8), it would be helpful to include the dimensions of the sample holder and specify whether the sample was positioned at the center of the RF coil to minimize RF inhomogeneity.
Citation: https://doi.org/10.5194/mr-2025-2-RC2 -
AC2: 'Reply on RC2', Arno Kentgens, 23 Mar 2025
We thank the reviewer for the positive evaluation of our work and the suggestion for improvement:
- In the experimental section on NQR (page 8), it would be helpful to include the dimensions of the sample holder and specify whether the sample was positioned at the center of the RF coil to minimize RF inhomogeneity.
We have added details about the coil and the requested details on sample holder dimensions and sample placement on page 8 (~line 200) of the revised manuscript.
Citation: https://doi.org/10.5194/mr-2025-2-AC2
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AC2: 'Reply on RC2', Arno Kentgens, 23 Mar 2025
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