The significance of the Jahn-Teller effect for the phase transitions of K₂NbCl₆ and Rb₂NbCl₆

Abstract

X-ray powder measurements were used to investigate the phase transitions and crystal structures of K2NbCl6 and Rb2NbCl6. In both cases, the metrical changes of the unit cell are accompanied by a compressive distortion of the NbCl6 2− groups from O h to D 4h symmetry. This static manifestation of the Jahn-Teller effect is ascribed to electro static interactions with the cations. Thus, Rb2NbCl6 trans forms from Fm-3m (Z = 4) to I4/mmm (Z = 2) due to a cation-supported compression in the c direction, whereas K2NbCl6 expands along c when converting from Fm-3m to P4/mnc (Z = 2). On further cooling a second transition leads to Cccm (Z = 4) where b as the shortest axis is approximately parallel to the direction of the octahedral contraction. The oxidation state of Nb4+ (d 1) is checked via bond-length bond-strength correlations and magnetic measurements.

Ordering of hidden multipoles in spin-orbit entangled 5d1 Ta chlorides

“…As seen from Fig. 2a, a large effective moment of 0.96 B, comparable to those of double perovskite oxides, is observed in the 4d analogue of Rb2TaCl6, Rb2NbCl6 [16], which can be attributed to the increased hybridization and the reduced spin-orbit coupling. Since the Cl-3p level is closer to the Nb-4d level than Ta The Curie-Weiss temperature of |Θ| ~ 30 K implies the possible ordering of multipoles and the pseudo-dipolar moments around 30 K. Indeed, we find anomalies in the small Curie-Weiss-like contribution in χ(T), which reflects the uncanceled magnetic dipolar moments and can be used as a marker for the pseudo-dipolar and the multipolar moments.…”

Strong tuning of magnetism and electronic structure by spin orientation

“…As shown in Fig. 1(b-c), the crystal structure of K 2 NbCl 6 is tetragonal when temperature is below 282 K [34]. The slight orthorhombic distortion at low temperature [34] will not affect our conclusion, as demonstrated in Supplemental Materials (SM) [35] (see, also, references [34,[36][37][38][39][40][41] therein).…”

Manipulation of Jeff=3/2 states by tuning the tetragonal distortion

“…3(a-c). Unlike perovskite oxides where oxygen octahedra are connected via the corner-, edge-, or face-sharing manner, these M Cl 6 octahedra are nearly isolated [26]. In this sense, the electron hoppings between M ions are significantly suppressed, leading to narrow bands near the Fermi level.…”

“…In this sense, the electron hoppings between M ions are significantly suppressed, leading to narrow bands near the Fermi level. According to their experimental structures [20,26], all M Cl 6 octahedra in these hexachlorides are tetragonally distorted, i.e., with the static Jahn-Teller Q 3 mode distortion. In addition, the d-p hybridization in chloride family is much weaker than that in oxides, which makes the d orbital levels more pure.…”

“…Furthermore, quantum phase transitions between the S = 1/2 and J eff = 3/2 states, as well as between different J eff = 3/2 states, are tuned by epitaxial strain. Although the influences of Jahn-Teller distortion on physical properties have been touched more or less in these SOC systems [20][21][22][23][24][25][26][27], its essential role to the J eff states, especially how to manipulate the J eff states via the strain, has not been reported yet.…”