Quaternary
nonlinear optical single crystals Ag
x
Ga
x
Ge1–x
Se2 (x = 0.250, 0.167) were grown
by the Bridgman method in a four-zone furnace. The thermal expansion
behavior of Ag
x
Ga
x
Ge1–x
Se2 (x = 0.25, 0.167) was studied by the method of single-crystal
X-ray diffraction from 150 to 295 K and powder X-ray diffraction in
the range of 298–773 K. Both results show the crystals have
positive linear thermal expansion coefficients in different directions
and a positive volume thermal expansion coefficient, and it is observed
that they satisfy the relationship of α
a
> α
c
> α
b
and α
V
≈
α
a
+ α
b
+ α
c
for the orthorhombic
structure. It is found that the Ag
x
Ga
x
Ge1–x
Se2 (x = 0.25, 0.167) unit cells varying with
temperature were mainly dominated by variations in framework geometry
(AgSe4 tetrahedron), and the thermal motion of Ag atoms
in the AgSe4 tetrahedron. As it was revealed, according
to the powder X-ray diffraction, it is found that the isotropic thermal
atomic displacement parameter of the Ag atoms is much larger than
those of the Se and Ga(Ge) atoms in the AgSe4 tetrahedron.
Furthermore, anisotropic atomic displacement parameters (ADPs) of
Ag atoms are extracted from the single-crystal diffraction; the ADPs
along the a axis, b axis, and c axis have a significant difference, which means the thermal
vibration of Ag atoms is anisotropic. It is of great significance
for improving crystal growth technology and understanding the thermal
properties of this kind of crystals.