你是 做 这个材料的 ？？
锂离子电池？？？？

Pnma和Pnmb是2个不同的空间群，前者是62号空间群，后者是53号空间群。它们都属于Orthorhombic（正交）晶系。

个人总结，它们的相同点和不同点如下：

（1）2个空间群都是centro-symmetric（中心对称）的空间群。
（2）两者的基本对称元素如下：1）都是 Primitive lattice；2）a 轴的垂直方向都有 n-glide plane（n型的滑移面）；   
        3）b 轴的垂直方向都有 m-mirror plane（对称面）；4）c 轴的垂直方向上，前者是有 a-glide plane（a 型的滑移
        面），后者是有 b-glide plane（b 型的滑移面）。
（3）Pnmb 不是53号空间群的标准形式，其标准形式是Pmna。但这个并不影响 Pnma 和 Pnmb 的比较。

关于滑移面的解释如下：

－－Glide planes a, b and c, which after reflecting in the plane translate an object by 112 of the length of a, b and c basis vectors, respectively。
      Because of this, for example, glide plane, a, can be perpendicular toeither b or c, but it cannot be perpendicular to a. Similarly, glide plane, b, cannot be perpendicular to b, and glide plane, c, cannot be perpendicular to c. Since the translation is always by 112 of the corresponding basis vector, these planes produce two symmetrically equivalent objects within one full length of the corresponding basis vector (and within one unit
cell), i.e. their order is 2.

－－Glide plane n, which after reflecting in the plane translates an object by 112 of the length of the diagonal between the two basis vectors located in the plane parallel to n. For example, glide plane, n, perpendicular to c will translate an object by 1/2(a + b). Glide plane n, results in two symmetrically equivalent objects within the full length of the diagonal vector (and within one unit cell) and its order is 2，

多谢2楼高手