Temperature Dependence of Nuclear Quadrupole Resonance in Layer‐Type Crystals

Abstract

The temperature dependences of the NQR frequencies of halides in layer-type crystals of CdBr,, Cd12(4H), Pb12(2H, 4H) are studied within 10 to 300 K temperature range. The NQR frequency grows weakly with temperature in CdI, and CdBr,, whereas it decreases in PbI,.For PbI, the frequency depends linearly on the temperature beginning at T 2 20 K. The difference of the NQR frequencies of nonequivalent nuclei in (4H) crystals corresponds to the calculated differences of the ion contributions to the gradient, while the sum of the calculated ion and covalent contributions does not account for the observed values of the NQR frequencies. This discrepancy is suggested to arise from the strong polarization of the covalent bonds by the internal electric field in the layer crystals. The growth of the NQR frequency with increasing temperature in CdI, and CdBr, is shown to be due to the fact that in these crystals the ionic and covalent contributions t o the gradient are of opposite sign. I n PbI, the temperature dependence of the NQR frequency is associated with the peculiarities of the phonon spectrum of the layer crystals. Formulation of the Problem and Experimental ResultsNuclear quadrupole resonance (NQR) spectra have been reported in a number of layer-t,ype crystals and, in particular, in CdBr, and Cd12(4H) [l, 21 (hereafter, the abbreviation in brackets stands for the crystalline modification of compounds showing polytypism [3]). Although there is an essential ionic bonding in such crystals, the point charge model was shown to be inadequate for calculating the observed NQR frequency values and the covalent contribution to the intracrystalline field gradient was estimated [ 11. The temperature dependence of NQR frequency was not investigated previously in detail for layer-type compounds.I n paper [4] it was shown that layer-type crystals feature a specific spectrum of acoustic phonons, in particular, "bending" vibrations causing some peculiar-1) Prospekt Nauki 144, 252650 Kiev-28, USSR.

Solid-State 127I NMR and GIPAW DFT Study of Metal Iodides and Their Hydrates: Structure, Symmetry, and Higher-Order Quadrupole-Induced Effects

“…There are two unique iodide sites per unit cell (Figure , inset). Both 127 I NQR − and 113 Cd NMR − data exist for CdI 2 (4H).…”

I127 NQR spectra of Pb1–xCdxI2 and (BiI3)(1–x)(PbI2)x of mixed layered semiconductors

“…Note that q zz is defined by contributions from the covalent bonds of the ion and from the point charges of the lattice ions q i zz [12]. It was shown [2,13] that the temperature dependence of the NQR frequency in the case of zero EFGT asymmetry parameter and if q i zz contribution from the point charges can be neglected and this may be used to interpret long wave low-frequency phonon spectra of crystals.…”

Nuclear quadrupole resonance spectroscopy with a femtotesla diamond magnetometer

“…The temperature dependence of NQR frequencies provides insight into crystal structure (35)(36)(37)(38)(39), and it can be used to validate the interpretation of spectra. We studied the temperature dependence of the 3.6-MHz NaNO 2 NQR transition near room temperature by controlling the apparatus temperature with a Peltier element and measuring the ambient temperature with a thermistor located near the sample (Materials and Methods).…”

“…NQR spectroscopy is a solid-state analysis technique that provides a unique chemical fingerprint based on the coupling of nuclear quadrupole moments to their local electric field gradients (27,28). NQR spectroscopy is used to identify powder substances in ambient conditions for security (29)(30)(31) and pharmaceutical (32)(33)(34) applications and to study the temperature-dependent properties of single-crystal materials (35)(36)(37)(38)(39). These applications typically require the ability to detect kilohertz to megahertz frequencies, at low-bias magnetic fields ≲1 mT, with femtotesla sensitivity (40).…”