X-Ray and Neutron Diffraction Study of Crystal Structure and Spin Arrangement of the Distorted-Triangular-Lattice Antiferromagnet $\mathrm{RbMnBr_3}$

Single-crystal X-ray and neutron diffraction study of the distorted-triangular-lattice antiferromagnet RbMnBr$_3$ was carried out in order to clarify the crystal structure below T$_4$ ∼222 K (phase V) and to find the origin of spin arrangement below T N =8.5 K. The phase-V diffraction pattern showed some systematic and accidental extinction of possible reflections with triple-domain formation. An orthorhombic structure of a space group symmetry P b c a with a unit cell 2a$_0$×${\sqrt {3a_0}}$×c$_0$ ( Z =8) was proposed. The triangular lattice of Mn$^{2+}$ ions is distorted in a novel form, coined as a zigzag-row model. Spin arrangement of this model was deduced using the mean-field theory. Spin structures of RbMnBr 3 in the incommensurately modulated LT (low-field triangular) and the commensurately modulated HT (high-field triangular) phases are described