X-ray diffraction studies indicate that 24 A3LXZ7 compounds have the same hexagonal crystal structure, with space group P63. For A3CuSiS7, the crystallographic parameters are as follows (A and a and c given): La, 10.28, 5.755 A.; Ce, 10.19, 5.735 A.; Pr, 10.15, 5.71 A.; Nd, 10.08, 5.70 A.; Sm, 10.00, 5.68 A.; Gd, 9.93, 5.67 A.; Tb, 9.90, 5.67 A.; Dy, 9.86, 5.665 A.; Ho, 9.81, 5.66 A.; Er, 9.78, 5.66 A.; Y, 9.84, 5.68 A. For A3CuSiSe7, they are: La, 10.61, 6.08 A.; Ce, 10.54, 6.06 A.; Pr, 10.51, 6.03 A.; Nd, 10.46, 6.02 A.; Sm, 10.40, 6.00 A.; Gd, 10.34, 5.99 A.; Tb, 10.31, 5.98 A.; Dy, 10.28, 5.97 A. For La3CuGeS7 they are: 10.30, 5.83 A.; La3AgSiS7, 10.39, 5.76 A.; La3AgGeS7, 10.36, 5.85 A.; La3CuGeSe7, 10.62, 6.14 A.; La3AgSiSe7, 10.70, 6.13 A. The ratio of c/a decreases with increasing at. number of substituent.