Abstract
AbstractPhase transitions in ND4BeF3 crystals at T1 = 330 and T2 = 245 K were studied by specific heat (DSC) and X‐ray lattice parameters measurements in function of temperature. These ferroelastic transitions are accompanied by anomalous heat absorption, sudden changes in the unit cell parameters and crystal symmetry. Formation of domain structure in particular phases examined under a polarizing microscope is also described. Domain walls of the type (100), (010), and (001) observable at room temperature indicate on the triclinic symmetry of the Phase II. Above T1 the crystal becomes orthorhombic. First‐order phase transition at T2 causes the symmetry of the crystal in the Phase III to be monoclinic with the c‐axis unique. The phase transition diagram is proposed and compared with the phase situation in nondeuterated NH4BeF3 crystals.
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