The linear and nonlinear optical properties of
B
a
G
a
4
S
7
,
B
a
G
a
4
S
e
7
,
B
a
G
a
2
G
e
S
6
, and
B
a
G
a
2
G
e
S
e
6
are reviewed, highlighting their potential for frequency downconversion of powerful laser sources from the near- to the mid-IR. Examples of successful applications are summarized and briefly discussed.