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Abstract
New compounds AMOXO4 (A = Na, K, Rb, Tl; M = Sb, Ta; X = Ge, Si) were prepared by solid state synthesis and as single crystals by the flux method. Their crystal structure and ferroelectric properties are similar to those of KTiOPO4, but optical nonlinearity is much lower. For TlSbOGeO4 an antiferroelectricity was supposed to exist in the interval between two phase transitions at Tc = 272 K and TN = 286 K. According to values of Curie-Weiss constant (C = 5·103) the phase transitions in TlSbOGeO4 have features of order-disorder transformation while in KTaOGeO4 the value C = 1.2·105 corresponds to displacive one. Special role of the monovalent cations in KTiOPO4-type compounds is discussed taking into consideration data on ferroelectric, nonlinear optical properties and structure.