Microscopic magnetic modeling for the SS=1/2 alternating chain compounds Na3_3Cu2_2SbO6_6 and Na2_2Cu2_2TeO6_6

The spin-1/2 alternating Heisenberg chain system Na$_3$Cu$_2$SbO$_6$ features two relevant exchange couplings: $J_{1a}$ within the structural Cu$_2$O$_6$ dimers and $J_{1b}$ between the dimers. Motivated by the controversially discussed nature of $J_{1a}$, we perform extensive density-functional-theory (DFT) calculations, including DFT+$U$ and hybrid functionals. Fits to the experimental magnetic susceptibility using high-temperature series expansions and quantum Monte Carlo simulations yield the optimal parameters $J_{1a}$ = $-$217 K and $J_{1b}$ = 174 K with the alternation ratio $\alpha = J_{1a}/J_{1b} \simeq$ $-$1.25. For the closely related system Na$_2$Cu$_2$TeO$_6$, DFT yields substantially enhanced $J_{1b}$, but weaker $J_{1a}$. The comparative analysis renders the buckling of the chains as the key parameter altering the magnetic coupling regime. Numerical simulation of the dispersion relations of the alternating chain model clarify why both antiferromagnetic and ferrromagnetic $J_{1a}$ can reproduce the experimental magnetic susceptibility data.Comment: published version: 11 pages, 8 figures, 5 tables + Supplemental materia

Europium Phosphate, Europium Arsenate, and Europium Antimonate - Correlation of Crystal Structure and Physical Properties

Crystals of EuPO4, EuAsO4, and EuSbO4 were grown by chemical vapor transport. These compounds exhibit complex anions and they crystallize in different structures. The modelling of the chemical transport experiment was carried out with the program TRAGMIN. By means of different spectroscopic methods structural and electronic influences of the complex anion on the electronic state of the europium ion were investigated. These results were in good agreement with calculations of the electronic structure. The magnetic susceptibility shows the typical van Vleck paramagnetism of trivalent europium. The specific heat capacity was determined in the range from 1.9 to 320 K. The conformity of these results was checked on the basis of specific heat determination through differential scanning calorimetry (DSC) in the temperature range 235 to 473 K. The optical bandgaps, received from UV/Vis spectroscopy are in good agreement with calculated ones