Unhydrated Cr(V) peroxychromates M3CrO8 (M = Na, K, Rb):: Low-dimensional antiferromagnets exhibiting large specific heats at mK to 5 K temperatures

The alkali-metal Cr(V) peroxychromates with the general formula M3CrO8 (M = Na, K, Rb, and combinations thereof) and the 3d(1) ground state have been characterized by X-ray diffraction, variable frequency and variable temperature EPR, magnetic susceptibility, and thermodynamic techniques. K3CrO8 and Rb3CrO8 are isomorphous, with space groups as I (4) over bar 2m, whereas Na3CrO8 and K2NaCrO8 exhibit the orthorhombic space groups Cmc2 and Pbcm, respectively. Lattice parameters are presented for all of these compounds and are shown to depend sensitively on the cation. EPR measurements reveal that while the electronic ground state of Na3CrO8, K3CrO8, and Rb3CrO8 is 3d(x2-y2), that of the mixed salts, such as K2NaCrO8 and Rb2NaCrO8, is 3d(z)2 the origin of this switch is not yet clear. Specific heat measurements showed that K3CrO8 undergoes an antiferromagnetic ordering transition at T-N = 0.38 K and displays a broad peak around 0.8 K, which can be interpreted in terms of a Bonner-Fisher model. Rb3CrO8 behaves similarly, with a broad peak at 0.52 K. Na3CrO8 exhibits a a-like antiferromagnetic transition at 2.35 K, with K2NaCrO8 behaving similarly, with TN lower than 1.8 K. The exchange constants and the transition temperatures depend sensitively on the cation. These compounds all are found to exhibit large specific heats throughout the mK to 5 K temperature range, and the possibility exists of tuning the thermodynamic properties via dilution with compatible diamagnetic lattices such as M3NbO8. Their specific heat data indicate that they could find applications as tunable heat sinks and magnetic refrigerants in the mK to 5 K range.