CHAPTER 1
Nuclear Quadrupole Resonance Spectroscopy
BY K.B. DILLON
1 Introduction
This chapter reports on the pure nuclear quadrupole resonance (NQR) results for quadrupolar (I > 1/2) nuclei in inorganic or organometallic solids. There has been a small decrease in the number of articles published compared with the previous year, with no major conference in this area taking place. One notable feature has been an increase in publications for some nuclei, such as 115In, and a decrease for the halogens Cl, Br and I. The structure and strength of hydrogen bonds in inorganic solids have been reviewed, including results obtained by NQR spectroscopy. Nuclear magnetic resonance techniques, including NQR, as a means for the non-destructive characterisation of materials, have been surveyed. More specialised reviews have appeared on NQR (and NMR) studies of Cu nuclei in YbInCu4 under high pressure, and on recent NQR and high-field NMR results for 25As nuclei in crystalline and glassy samples of arsenic chalcogenides.
Patent applications have been filed for a transmit-receive coil system for NQR signal detection in substances and components thereof, for a receiving system for high q antennas in NQR and a method of detecting substances, for a scanner for NQR measurements and a method of detection for substances containing quadrupolar nuclei, for improvements to apparatus for NQR measurements, and for polarisation-amplified 14N NQR detection of TNT and other explosives in mines by using the quadrupole-quadrupole solid effect.
The normal format is followed in the more detailed sections, with results for main group elements followed by those for transition metals and lanthanides.
2 Main Group Elements
2.1 Group 13 (Gallium-69 and Indium-115). – NQR spectra of 69Ga and 115In nuclei have been recorded for some GaSe and InSe layer compounds. The spectra were analysed, starting from polytypes of these compounds and the probability of formation of ordered structural fragments in the basic crystal modifications. As part of a study of Ni-Y intermetallics (Y = Al, Ga, In, Ti), 69Ga and 115In NQR frequencies (as appropriate) have been measured for Ni2Ga3, ε-Niln and Ni2ln3. The electric field gradient (efg) parameters were computed in advance, enabling rapid location of the experimental signals. The Ni2Y3 structure requires two distinct Y sites, and these were observed for Y = Ga and In. In the Ga compound, the linewidth for the Ga(2) site was more than three times that for the Ga(1) site. The lineshapes appeared distinctly Lorentzian for both compounds, possibly indicating the onset of intersite hopping at room temperature (RT). Line assignments were confirmed from the computed values. Two distinguishable In sites were also found for ε-NiIn, and the results were in excellent agreement with computations using WIEN 97.
Unconventional superconductivity has been deduced in CeIn3 from 115In NQR, including spin-lattice relaxation rates (SLR), and AC-susceptibility measurements, as a function of temperature (T) between 0.05 and 100 K, and a pressure P of 2.65 GPa. A superconducting transition was found at Tc = 95 mK, very much lower than the onset temperature Tonset = 0.15 K at zero resistance. No coherence peak was observed just below Tc in the T-dependence plot, consistent with unconventional superconductivity. 115In NQR and AC-susceptibility data have been similarly reported for CeRhIn5, and 63Cu NQR results for YbInCu4, under pressure. CeRhIn5 showed a P-induced phase transition from antiferromagnetic (AF) ordering to a superconducting state. The studies revealed a homogeneous coexistence of these two types of orderings near the phase boundaries. The results for YbInCu4, which shows a first order valence transition at 42 K and ambient P, indicated a ferromagnetically ordered ground state after the valence transition was suppressed by P. The P-dependence of the 115In NQR parameters has been monitored at various T for CeRhIn5. Some AC-susceptibility measurements were also described. The compound underwent a superconducting transition at Tc ~ 2.1 K for P = 1.6 GPa. The Néel temperature TN was reduced above P = 1.23 GPa, accompanied by emergent pseudogap behaviour. The results at 1.6 GPa revealed that antiferromagnetism and superconductivity coexist microscopically. This coexistent state was suggested to persist down to P ~ 1.5 GPa. The superconductivity did not show any trace of gap opening in the low-lying excitations below the onset temperature 2 K, and the results indicated its unconventional characteristics.
T-dependence of the 115In NQR parameters, including the SLR, from 0-40 K has been monitored for the heavy-fermion superconductor CeCoIn5 (Tc 2.3 K). Some 59Co NMR data were also obtained. SLR measurements revealed that the magnetic nature was characterised by strong AF spin fluctuations in the vicinity of the quantum critical point (QCP). The anomalous T-dependence of the SLR for 115In nuclei could be well explained by the anisotropic spin-fluctuation model. AF spin fluctuations were found to be suppressed by a small magnetic field, reinforcing the view that CeCoIn5 is located close to the QCP. The effect on the Néel temperature TN caused by substitution of Ir for Rh in CeRh1-xIrxIn5 (x = 0, 0.25 or 0.45) has been studied by 115In NQR, including the T-dependence of the SLR. TN was found to increase slightly on substitution of Ir for Rh up to x = 0.45. This resembled the behaviour in hydrostatically pressurised CeRhIn5 below 1.0 GPa. These measurements were extended to CeRh0.5Ir0.5In5 and CeRhIn5 under pressures up to 1.6 GPa; LaRhIn5 and LaIrIn5 were also used as reference compounds. TN decreased with application of P = 1.6 GPa or replacing half of Rh with Ir, probably due to an increase in bandwidth. In CeRh0.5Ir0.5In5, unconventional superconductivity set in well below TN, with Tc ~ 1 K. The 115In SLR as a function of T has been followed for Ce (Ir, Rh) In5, and for analogous La (Ir, Rh) In5. The SLR was higher by an order of magnitude for the Ce compounds, indicating strong magnetic fluctuations. CeIrIn5 was located near the QCP, with quasi-2D spin fluctuations. For this compound, the SLR varied with T3 below Tc = 0.40 K, indicating unconventional superconductivity with a line-node gap. CeIrIn5 was much more itinerant than CeRhIn5.
2.2 Group 14 (Germanium-73). – 73Ge NQR (and NMR) data have been obtained for a 73Ge-enriched sample of UGe2 at ambient P and 1.3 GPa. At the latter P, the T-dependence of the SLR from 0.01-100 K demonstrated the onset of a superconducting transition at Tc = 0.55 K. The lack of a coherence peak just below Tc, followed by T3-like behaviour in the T-dependence plot, provided evidence for an unconventional superconducting state that coexists with the ferromagnetic state on a microscopic scale.
2.23 Group 15 (Nitrogen-14, Arsenic-75, Antimony-121 and -123, and Bismuth-209). – The technology of NQR detection with particular reference to 14N nuclei has been described, and some experimental results presented. It has been demonstrated theoretically and experimentally that irradiation of a powder sample containing spin-1 nuclei by two of the three characteristic NQR frequencies can result in several echo signals at the third NQR frequency. Experiments were carried out for NaNO2 and the organic compound RDX at 27°C. The principal echo had the same shape and time of occurrence as the echo produced after a pair of single-frequency excitations, but the other two echoes were not equivalent to any single-frequency echo. The time of occurrence and shape of these secondary echoes were determined by the correlation of the distribution in one transition frequency with the distribution in the second transition frequency. This correlation in turn was determined by the correlation between the distributions of the efg components, which was itself determined by the type and concentration of crystal defects present. Optimal conditions for observing these secondary echoes were derived. Very good agreement was found between theory and experiment. This could represent a promising method for the study of internal strains and defects in materials containing spin-1 nuclei.
Transient processes observed previously in a single crystal of NaNO2 subjected to the pulse sequences MW-2 and MW-4, and their modifications with 180° flip angle of the pulses, have been explained theoretically in the frame of a two-particle model. The nature of the echo signals in the effective field of multi-pulse sequences received by phase inversion of the pulses, or by introducing an additional 180° pulse, was connected with re-focusing of the accumulated digressions of the flip angle from the ideal 180° pulse. Experimental results from powdered samples of NaNO2 and an organic compound at RT showed single and multiple echoes in the effective field of various pulse sequences. Multiple echoes in the envelope of the NQR signal have been obtained in a field of multipulse sequences for powdered N-containing samples, including NaNO2, at 293 K. Echo signals were observed over a wide range of pulse rotation angles. An analogue of the magic NMR echo in solid-state NMR could be obtained under certain conditions. The pulsed spin-locking effect has been observed in 14N NQR from polycrystalline NaNO2 at 297 K, using a phase coupled pulse sequence (PCPS). The dependence of the effective relaxation time on the PCPS parameters was also investigated. This dependence and the spin-echo signal behaviour were very similar to those in a conventional pulsed spin-locking experiment. Various composite pulse sequences as used in powder NMR have been applied to the NQR of 14N nuclei in a powdered sample of NaNO2 at 22°C. Best outcomes were achieved using the composite pulse (45)0 (90)180 (164)0 in the majority of cases. An experimental and theoretical study has been described of quasistationary and stationary states established in a quadrupolar spin system subjected to a chain of identical pulses that can be preceded by a preparatory pulse. A theoretical expression was obtained for the magnetisation of the spin system that took into account off-resonance conditions. Experimental measurements on powdered samples of NaNO2 and RDX at 22°C were described, and the results compared with theoretical calculations. Possible applications for the detection of N-containing explosives were discussed.
T-dependence studies from 77-298 K have been reported for 75As NQR from Co(AsF6)2.2AsF3. Three resonances were detected at 298 K. The two lower frequency lines showed a nearly linear T-dependence with a relatively small positive T coefficient to 180 K, where the signals disappeared in the noise, and were attributed to the two AsF6 octahedra. The high frequency line had a negative T coefficient and was visible to 77 K; it was assigned to AsF3 pyramids. A very short relaxation time was found for the low frequency lines, probably due to the paramagnetic Co2+ ions as efficient relaxation centres. In the related compounds Co(AsF6)2, Co(AsF6)2.2SbF3, Co(AsF6)2.2SO2 and Mn(AsF6)2 the signals were broadened beyond detectability by paramagnetic local fields. The T-dependence of 75As NQR parameters, including the SLR, from the Kondo semiconductor CeRhAs has been monitored from 0-300 K, to study the successive phase transitions found recently, gap formation and their interplay. Some 75As NMR data were also presented. For the three well-resolved signals from the low T phase IV below 165 K, (T1T)-1 showed an activation type T-dependence, suggesting a gap opening over the entire Fermi surface, in contrast with the V-shaped gap in isostructural CeNiSn and CeRhSb. The evaluated gap and bandwidth were an order of magnitude larger in CeRhAs. In phase III, between 165 and 235 K, the NQR frequency changed discontinuously. Only one signal was detected in phase II at RT.
Pulsed NQR measurements for 75As nuclei have been reported for crystalline and glassy As2S3 and As2Se3 at 77 and 300 K. As a function of pulse separation, the decays of the NQR Hahn echoes following a 90°-180° pulse sequence showed damped oscillations superimposed on an exponential decay. The damped oscillations were explained in terms of indirect (J) coupling between two As nuclei via polarised electrons on S or Se. Values of J couplings were obtained from the periods of the oscillations and calculations of the most probable transitions using second-order perturbation theory. The value estimated for 2J (75As-S-75As) in crystalline As2S3 by this means compared well with an empirical estimate from a value for 2J (31P-As-31P). There were insufficient experimental data on 2J (X-Se-X) to make a similar empirical estimate for the Se compound. 75As NQR (and high field NMR) lineshapes in glassy As2S3 and As2O3 have been compared before and after photodarkening (As2S3) or X-ray irradiation (As2O3). No significant changes were found. The results were inconsistent with a microscopic model of photodarkening that involved the switching of a large number of 75As-chalcogen bonds. NQR experiments on 75As nuclei, including measurement of T1 values, have been used to study local structural order in amorphous As-Se systems. Regions were identified with As atoms bound to zero, two or three Se atoms. Bonding at lower As concentrations was governed by preferential bonding (chemical ordering) between As and Se, but at higher As concentrations was governed by both chemical ordering and statistical effects. Above 45% As, the spin-echo spectra were narrower and peaks corresponding to different numbers of As-As bonds appeared, possibly revealing As-rich small clusters formed to release local strains. The T1 values suggested that the rigidity decreased dramatically for As concentrations >40%, in contradiction with the predictions of mean field theory, which thus cannot be applied to such systems.
NaFe4Sb12 and KFe4Sb12 have been synthesised and their properties investigated by various physical techniques, including Sb NQR for the Na compound. Five lines were observed, assigned to two transitions for 121Sb and three for 123Sb, showing only one crystallographic site. Below Tc (85 K), all the lines decreased sharply to zero, demonstrating the onset of ferromagnetic ordering through an increase of the internal field at the Sb site. A single crystal of CdSb has been grown, and its Zeeman Sb NQR spectra obtained at 77 and 300 K. The orientation of the efg about the unit-cell crystal axes was established for the Sb atoms, which are all crystallographically equivalent. Features seen previously in the 209Bi NQR spectrum of a Bi4Ge3O12 single crystal were taken as an indication of the presence of ordered local magnetic fields in the compound. Experimental data necessary for the computer simulation of the NQR spectra in external magnetic fields were obtained. Four spatially different orientations of Sb-Sb bonds and the corresponding efg qzz axes were found.
121Sb and 123Sb NQR spectra and the SLR have been recorded as a function of T from 0–150 K for YbSb2 in both the normal and superconducting (Tc 1.4 K) states. Two different Sb sites were found. In the superconducting state, the SLR had an exponential T-dependence, indicating the occurrence of s-wave superconductivity. 1/T1T was T-independent between 2 and 150 K in the normal state, consistent with T-independent susceptibility and almost no contribution from Yb ions. The results showed that the SLR was determined by magnetic interactions. 121Sb, 123Sb NQR frequencies and the SLR between 0 and 150 K have been monitored for the filled-skutterudite compound PrRu4Sb12 in both the normal and superconducting (Tc 1.3 K) states. The T-dependence of the resonance frequencies revealed an energy scheme in the cubic crystal electric field of the Pr3+ ions, consistent with an energy separation of ~70 K between the ground and first excited states. In the normal state, the Korringa relationship was valid. These results could be understood in terms of a conventional Fermi liquid. In the superconducting state, the SLR showed a distinct coherence peak just below Tc, followed by an exponential decrease, showing that the compound is a typical weak-coupling s-wave conductor. This is in strong contrast with the heavy-fermion superconductor PrOS4Sb12. The results highlighted the fact that the Pr-4f2 derived nonmagnetic doublet plays a key role in the unconventional electronic and superconducting properties of the Os compound.
