Optical studies of phase transitions in oxyfluoride (NH4)2NbOF5

• Published in: Physics of the solid state Vol. 52; no. 10; pp. 2168 - 2172
• Main Authors: Mel’nikova, S. V., Laptash, N. M., Aleksandrov, K. S.
• Format: Journal Article
• Language: English
• Published: Dordrecht SP MAIK Nauka/Interperiodica 2010
• Subjects: Lattice Dynamics and Phase Transitions; Physics; Physics and Astronomy; Solid State Physics; Oxyfluoride; Twin Boundary; Optical Indicatrix; Twin Structure; Single Domain State
• ISSN: 1063-7834; 1090-6460
• DOI: 10.1134/S1063783410100240

Polarization-optical studies and measurements of the birefringence Δ
n
and the angle of rotation of the optical indicatrix for the (NH
4
)
2
NbOF
5
crystal have been carried out in the temperature range 100–350 K. Two anomalies of the birefringence have been revealed at the temperatures
T
01
= 258 K and
T
02
≈ 219 K. According to the twinning pattern, the crystal undergoes successive changes in symmetry: orthorhombic ↔ monoclinic 1 ↔ monoclinic 2. The twofold axis of the monoclinic phases (or the normal to the plane) is directed along [001]
or
. The effect of the uniaxial compression along [011]
or
and the electric field
E
≈ 25 kV/cm along [100]
or
on the twin structure has been studied. The ferroelastic phase transition at
T
01
is due to the appearance of the shear deformation
x
4
(
T
) and is accompanied by significant anomalies of the birefringence. Strong pretransition phenomena mask the jumps in the birefringence Δ
n
(
T
) and in the angle of rotation of the indicatrix φ(
T
) at
T
01
.