A new polar symmetry of huebnerite (MnWO4) with ferrodistortive domains

Theoretical spin-wave dispersions in the antiferromagnetic phase AF1 of MnWO4 based on the polar atomistic model in P2

“…At zero field, three successive antiferromagnetic phase transitions are observed: the commensurate magnetic structure AF1 with propagation vector k = (± 1 4 , 1 2 , 1 2 ) is present below 8 K, the incommensurate elliptical spiral spin structure AF2 existing in 8 ∼ 12.3 K induces ferroelectric order, and the incommensurate collinear sinusoidal spin structure AF3 is only observed in a narrow temperature range below 13.5 K [17,19,20]. The fundamental crystal structure of MnWO 4 is monoclinic, and the corresponding space group has been believed to be P2/c until our structural studies confirmed the true symmetry P2 [21][22][23][24] and the two different spin-canting configurations at two Mn 2+ sublattices, as a symmetrical consequence of the di-rect polar subgroup relation between P2 and P2/c. This noncollinear spin-canting texture in AF1 of MnWO 4 is in contrast to the previous collinear magnetic model (↑↑↓↓) where the magnetic moments at two Mn sites are aligned collinearly along the easy axis with a common angle of 35 • -37 • against a-axis on the (a-c) plane [17,19,20].…”

“…The fundamental crystal structure of MnWO 4 is monoclinic, and the corresponding space group has been believed to be P2/c until our structural studies confirmed the true symmetry P2 [21][22][23][24]. For the observation of h0l reflections with l = arbitrary integer (strong when l = even; extremely weak when l = odd), the structure exhibits no gliding mirror plane c perpendicular to the crystallographic b axis, thus cannot belong to P2/c.…”

“…For the observation of h0l reflections with l = arbitrary integer (strong when l = even; extremely weak when l = odd), the structure exhibits no gliding mirror plane c perpendicular to the crystallographic b axis, thus cannot belong to P2/c. As shown extensive neutron and x-ray diffraction including Renninger scans [21][22][23][24], the true space group symmetry for huebnerite (MnWO 4 )-type structures is the polar space group P2. The most relevant finding in our structure model is the presence of faint intrinsic polarity in b, which may be further remedied due to ferrodistortive domains formed about c * , as claimed in [21].…”

“…As shown extensive neutron and x-ray diffraction including Renninger scans [21][22][23][24], the true space group symmetry for huebnerite (MnWO 4 )-type structures is the polar space group P2. The most relevant finding in our structure model is the presence of faint intrinsic polarity in b, which may be further remedied due to ferrodistortive domains formed about c * , as claimed in [21]. Indeed, the site character between two independent polar atomic sites for Mn 2+ in P2, denoted as Mn a and Mn b , fairly differs from each other in terms of bonding valence sums [25].…”

Two spin-canting textures in the antiferromagnetic phase AF1 of MnWO4based on the new polar atomistic model inP2

“…The improper fer roelectricity in the crystallographic b axis is induced in an incommensurately modulated helical spin system of Mn 2+ aligned in the ( ac) plane. The crystallographic space group symmetry was believed to be P2/c until our extensive struc tural investigations confirmed the true symmetry P2 using single crystal neutron and synchrotron xray diffraction data including Renninger scans [9][10][11].…”