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Optical, X‐ray, and calorimetric studies of phase transitions in ND 4 BeF 3
Author(s) -
Waśkowska A.,
Pietraszko A.,
Czapla Z.
Publication year - 1987
Publication title -
crystal research and technology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.377
H-Index - 64
eISSN - 1521-4079
pISSN - 0232-1300
DOI - 10.1002/crat.2170220714
Subject(s) - triclinic crystal system , orthorhombic crystal system , phase transition , monoclinic crystal system , crystallography , crystal (programming language) , phase diagram , crystal structure , condensed matter physics , phase (matter) , ferroics , chemistry , materials science , quantum phase transition , quantum critical point , physics , organic chemistry , computer science , programming language
Phase transitions in ND 4 BeF 3 crystals at T 1 = 330 and T 2 = 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 T 1 the crystal becomes orthorhombic. First‐order phase transition at T 2 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 NH 4 BeF 3 crystals.