Magnetic structures and phase transitions of Nickel-Bromine boracite Ni₃B₇O₁₃Br. a neutron powder diffraction study

Abstract

The nuclear crystal structure and the magnetic ordering of the two ferroelectric/weakly ferromagnetic phases of Nickel-Bromine boracite with the Shubnikov groups of Pc′a2′₁ (30-21K) and P1 (below 21K)— postulated previously on the basis of magnetoelectric and optical data—have been investigated by neutron powder diffraction. The neutron spectra confirm the onset of magnetic ordering at 30 K but do not resolve the transition to the triclinic symmetry. The observed neutron intensities of the ortho-rhombic and triclinic phase, collected at 24 K and 8 K, respectively, have been interpreted in terms of the magnetic space group Pc′a2′₁ by an essentially antiferromagnetic ordering of the nickel ion moments. The magnetic structure refinements allowed to distinguish two different types of the magnetically ordered nickel ions among the three inequivalent nickel sites. Two possible magnetic structure models and a potential frustration phenomenon during spin ordering are discussed.

Key Words:

• boracite
• neutron diffraction
• magnetic structure
• ferroelectric
• ferromagnetic
• ferroelastic