X-ray single-crystal study of the low-temperature structure of

Abstract

A single-crystal x-ray study on the low-temperature structure of proves the existence of two different crystal structure distortions, as originally found by neutron scattering. One phase is hexagonal and does not differ much from the room temperature structure. It appears through a second-order phase transition. The other phase is orthorhombic and appears through a first-order phase transition. A model is proposed for the orthorhombic phase, which fits our x-ray measurements on as well as on . The model gives rise to a magnetic configuration that maps on to the row model of Zhang et al.

Crystal Structure and Magnetic Ordered States in the Distorted-Triangular-Lattice Antiferromagnet RbMnBr₃

“…The dielectric behaviors in the successive structural phase transitions and the diffraction patterns in each phase in KNiCl 3 are very similar to that in RbMnBr3, although another structure has been proposed for the V phase of KNiCh by Petrenko. [9] We believe that the latter phase of KNiCl:3 should be the same with our model. closely related to the present orthorhombic Pbea structure for phase V of RbMnBr3, which is shown in Fig.…”

Review/Synthèse: Triangular antiferromagnets

“…In KNiCl 3 dielectric anomalies indicating structural phase transitions are found at 274 K, 285 K, 561 K and 762 K [160]. A single crystal x-ray study on the low-temperature structure of KNiCl 3 [161] shows clearly the existence of two crystal structure distortions, as originally found by neutron scattering [130]. One phase (denoted as phase A) is hexagonal and does not differ much from the room temperature structure, the other phase (phase B) is orthorhombic.…”

“…In a phase A the unit cell is rotated through 90 • about the c axis from the room temperature unit cell and enlarged to √ 3a, √ 3a and c; in a phase B the low temperature unit cell has sizes 2a/ √ 3, a and c. The main feature of the phase B is a sinusoidal modulation of the ion chains in the basal plane: instead of the room temperature sequence 0-0-UP-0-0-UP-0-0, where "0" means ion in the basal plane and "UP" means ion slightly shifted above the basal plane along the c-axis, at low temperature the sequence is 0-UP-0-DN-0-UP-0-DN-0, where "DN" means the ion is shifted below basal plane. Possible space groups are P ca2 1 and P bcm [161].…”

Heisenberg magnet with modulated exchange

“…In some cases, as in KNiCl 3 , both phases are found to coexist at low temperature. 12 Perhaps, the most intriguing is the case of RbMnBr 3 , in which most experiments find the orthorhombic low-T phase, 13,14,15 and an incommensurate spiral spin structure with propagation vector Q = (1/3 + q, 1/3 + q, 1), 13,14,15,16 in place of the commensurate "triangular" antiferromagnetic order with Q = (1/3, 1/3, 1), which is characteristic of the non-distorted hexagonal materials CsMnBr 3 , CsNiCl 3 , etc. 17,18,19 In a magnetic field of about 3 T applied in the easy plane the spin structure becomes commensurate, with Q = (1/8, 1/8, 1).…”