Brillouin Scattering in Ferroelectric NH₄HSO₄

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Brillouin shifts and the line widths in NH₄HSO₄ which concern the longitudinal phonons propagating along the a^*, b and c directions were measured in the temperature range −115∼30°C. The Brillouin shift caused by the pure longitudinal acoustic phonons propagating along the b axis shows an abrupt decrease and the line width increases below the upper paraelectric-ferroelectric transition temperature. The quasi-longitudinal component along the a^* axis shows a small anomaly in the shift near the transition temperature. No unambiguous anomalies were found in the shift and the line width of the quasi-longitudinal component along the c axis.