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1:2 B-site cation ordered Ba(Mg1/3Nb2/3)O3(BMN)ceramic was synthesized using conventional solid-state reaction at 1600℃ for 12 h.The structure parameters were obtained through Rietveld refinement of X-ray diffraction data.The Raman peak frequencies were obtained by Lorenz fitting on Raman spectrum.Four-parameter semiquantum model was used to fit the infrared reflectivity spectrum,and the fitted parameters were used to calculate the dielectric permittivity and dielectric loss tan.A total of 9 active Raman and 16 active IR modes were obtained using first-principle calculations based on density functional theory with local density approximation.All of the vibrational modes were assigned and represented by linear combinations of the symmetry coordinates deduced using group theory analysis.The Raman mode with the highest frequency A1g(4)(789 cm-1)can be described as the breathing vibration of NbO6.The IR modes Eu(1)(149 cm-1)and A2u(2)(212 cm-1),which can be described as the twisting vibrations of Ba–MgO6/Ba–NbO6 on the a–b plane and the stretching vibrations of Ba–MgO6/Ba–NbO6 along the c direction,respectively,are the dominant contributing modes to and tan.The dielectric property parameters obtained using IR spectrum fittings,first-principal calculations,and microwave measurements were compared.