K39andSe77NMR study of the paraelectric-to-incommensurate phase transition ofK2SeO4

The ${}_{}{}^{39}{}_{}{}^{}\mathrm{K}$ quadrupole-coupling and chemical-shift tensors have been determined from the angular dependences of the ${}_{}{}^{39}{}_{}{}^{}\mathrm{K}$ line shifts of the ${}_{}{}^{39}{}_{}{}^{}$K±(1/2⇆∓ 1) / 2 central NMR transitions in the paraelectric (P) and incommensurate (I) phases of ${\mathrm{K}}_2$ ${\mathrm{SeO}}_4$. The main effect of the P-I phase transition on these tensors is the appearance of nonzero off-diagonal elements $V_{\mathrm{ab}}$ and $V_{\mathrm{bc}}$ which reflects the destruction of mirror planes by frozen-in soft-mode displacements along the b axis. From the angular dependences of the ${}_{}{}^{77}{}_{}{}^{}\mathrm{Se}$ line shifts the ${}_{}{}^{77}{}_{}{}^{}\mathrm{Se}$ chemical-shift tensor has been determined in the paraelectric phase of ${\mathrm{K}}_2$ ${\mathrm{SeO}}_4$. In contrast to the ${}_{}{}^{39}{}_{}{}^{}\mathrm{K}$ quadrupole-coupling and chemical-shift tensors it remains unaffected on going through $T_I$ and changes only slightly at $T_C$.