Distorted tetrahedrons and libration mode in KH2AsO4(KDA)

On the basis of the order-disorder picture due to the locally distorted tetrahedrons, low-lying x(yz)y Raman spectra of KH2AsO4 (KDA) have been analyzed by the factorized form of complex dielectric function epsilon(nu) that includes one relaxation mode and two damped oscillator modes. The lowest relaxation mode and the next A(1) libration mode are due to the locally distorted AsO4 tetrahedrons, which are both forbidden if the AsO4 is non-distorted. The highest mode is an ordinary B-2(z) translational polar mode. From the spectral fitting analysis, the contribution of relaxation mode to the dielectric function in KDA is suppressed compared with KDP. The influence of the A(1) libration mode on the dielectric function in KDA is less than that of KDP. These are consistent with the lower T-c in KDA.