A commensurately modulated crystal structure and the physical properties of a novel polymorph of the caesium manganese phosphate CsMnPO4.

A novel modification of the CsMnPO4 β-phase was achieved by hydrothermal synthesis at 553 K. The compound crystallizes in the monoclinic system with the basic unit-cell parameters a = 11.0699 (4), b = 11.0819 (6), c = 9.1106 (3) Å, γ = 119.480 (5)o; the modulation vectors are q1 = 0.4a* and q2 = 0.4b*. The structure was determined based on single-crystal X-ray diffraction data obtained from a pseudo-merohedral twin using a superspace approach in the (3 + 2)D symmetry group P11a(a1,b1,0)0(a2,b2,0)0 and refined to R = 0.083 for 10 266 reflections with I > 3σ(I). It is considered as a low-temperature polymorph of CsMnPO4 with the same UUUDDD-type layer topology built by MnO4 and PO4 tetrahedra, and stacked in a framework in the same manner as β-tridymite. Large open channels parallel to the [110] and [001] directions incorporate Cs atoms. All Cs atoms are distributed along the asuper = 55.35 (1) and bsuper = 55.41 (1) axes of the large unit cell with pseudo periods of asuper/5 and bsuper/5 which are broken mainly by the positions of oxygen atoms (orientation of Mn- and P-centered tetrahedra). The β-phase is discussed as a member of the morphotropic series of manganese phosphates with large cations of AMnPO4, where A = Cs, Rb, K and Ag. The title compound is an antiferromagnet with the Neel temperature TN = 4.5 K.