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The birefringent and piezooptic properties of (MgSiF,) . 6 H,O crystals are investigated. The secondorder phase transition at = 353 K is observed for the first time. It is shown that the investigated crystals possess properties characteristic of an incommensurate phase in the temperature range 298 to 353 K. The transition temperatures are found to be strongly dependent on the history of the sample. MCCJlenOBaHLI nBynpeJIOMnR€OI4Me II ~L~~O O~T M Y~C K H~ CBOiiCTBa KpHCTannOB (MgSiF,) ' 6 H,O. Bnepmre Ha6nWoAaJIcR @a3OBbla UepeXOn npII = 353 K. nOKa3aH0, YTO MCCJIcAyeMLle KpIICTaJIJIbI 0 6 n a n a m~ CBOfiCTBaMII, XapaKTepHLIMIl AJlR HeCOpa3MepHOfi ($a361 B TeMIIepaTypHOfi 06nac-r~ 298 DO 353 K. 06HapyXeHa CHJILHaR 3aBPiCHMOCTh TeMnepaTyp ($a30BLIX IIepeXOnOB OT IIpeaLlCTOpHH o6pasua.

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