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> 3 σ (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.

Source: Acta Crystallographica Section B - September 9, 2019 Category: Chemistry Authors: Bolotina, N. Yakubovich, O. Shvanskaya, L. Dimitrova, O. Volkov, A. Vasiliev, A. Tags: hydrothermal synthesis modulated crystal structure phosphates β -tridymite derivatives polymorphism transition metals morphotropic series antiferromagnetism pseudo-merohedral twins research papers Source Type: research