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 2 , Cdl 2 (4H), Pbl 2 (2H, 4H)are studied within 10 to 300 K temperature range. The NQR frequency grows weakly with temperature in CdI 2 , and CdBr 2 , whereas it decreases in PbI 2 . For PbI 2 the frequency depends linearly on the temperature beginning at T≧20K. 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 2 and CdBr 2 is shown to be due to the fact that in these crystals the ionic and covalent contributions to the gradient are of opposite sign. In PbI 2 the temperature dependence of the NQR frequency is associated with the peculiarities of the phonon spectrum of the layer crystals.

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