Optical,X-ray,and calorimetric studies of phase transitions in ND4BeF3

Phase 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.