Insulators containing CuCl4X22−(X=H

Abstract

The origin of the difference in structure between compounds containing CuCl 4 X 2 2− (X = H

Jahn–Teller and Non-Jahn–Teller Systems Involving CuF64– Units: Role of the Internal Electric Field in Ba2ZnF6:Cu2+ and Other Insulating Systems

“…Moreover, the appearance of the three minima typically associated to a JT problem now involves the vibronic coupling between non-degenerate levels and thus if orthorhombic minima were present, which is not assured, it could only be explained through the use of a full pseudo Jahn-Teller model. 6,[63][64][65] At the time of publication we found a recent paper 66 claiming that the properties of K2ZnF4:Cu 2+ can be explained on the basis of the JT effect in the same way as KZnF3:Cu 2+ . This assertion is supported in Ref.…”

Changing the Usual Interpretation of the Structure and Ground State of Cu2+-Layered Perovskites

“…where  2 +  2 = 1. As an electron in a pure 3z 2 -r 2 orbital and D4h symmetry gives no contribution to gZ -g0 in second-order perturbations 69 the positive experimental gZ -g0 = 0.05 value found in (CH3NH3)2CdCl4:Cu 2+ can mainly be ascribed to the admixture embodied in Eq. (1) induced by the D4h  D2h symmetry reduction.…”

“…As in that case it has been measured gZ(x 2 -y 2 ) = 2.37 then we can estimate  2  13% for (CH3NH3)2CdCl4:Cu 2+ . Moreover, it has been pointed out 69 that  can also be estimated from experimental gY -gX and <g> values through the relation gY -gX  (4/3) (<g> -g0)…”

“…Indeed in NH4Cl crystals grown in acidic solutions the CuCl4(H2O)2 2complex is formed whose local symmetry below T = 20 K is not tetragonal but orthorhombic [69][70][71][72] . Again, if Z corresponds to the Cu 2+ -O 2direction the wavefunction of the unpaired electron has been proved to display a strong 3z 2 -r 2 character 69,71 . Indeed, it has been derived  2  10% although the estimated orthorhombic distortion RL -RS  0.50 Å is a bit higher than that calculated for (CH3NH3)2CdCl4:Cu 2+ 69 .…”

“…Moreover, there is a negative contribution to the ground state force constant 66 that plays a key role for reaching an unstable situation, a matter commented on in section 3.5. The microscopic origin of the orthorhombic instability of CuCl4(H2O) 2complex in NH4Cl has previously been discussed in some detail 69 .…”