Influence of the cation size on the formation of alluaudite-type manganese arsenates: synthesis and characterization of Tl2Mn3(As2O7)2·2H2O, CsMn3(AsO4) (HAsO4)2·3H2O, and XMn3(AsO4)(HAsO4)2 (X=Na, K)

The crystal structure of HoB2C has been refined using powder X-ray diffraction data. The structure has B2C layers stacked directly above each other in the c-direction with Ho atoms sandwiched between layers. There is no evidence for a doubling of the c-axis, as was previously reported, from the X-ray or powder neutron data. The refined structure is close to having orthorhombic Pbam symmetry but with a slight monoclinic distortion (space group P112/m, Z=4, a=6.77454(8)?, b=6.78618(9)?, c=3.69604(3)? and (gamma)=90.091(1)°). Large anisotropic strain broadenings of the diffraction peaks are consistent with a model for [110] twinning.