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 M 3 CrO 8 (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. K 3 CrO 8 and Rb 3 CrO 8 are isomorphous, with space groups as I 4̄2 m, whereas Na 3 CrO 8 and K 2 NaCrO 8 exhibit the orthorhombic space groups Cmc 2 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 Na 3 CrO 8, K 3 CrO 8, and Rb 3 CrO 8 is 3d x 2 - y 2, that of the mixed salts, such as K 2 NaCrO 8 and Rb 2 NaCrO 8, is 3d z 2; the origin of this switch is not yet clear. Specific heat measurements showed that K 3 CrO 8 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. Rb 3 CrO 8 behaves similarly, with a broad peak at 0.52 K. Na 3 CrO 8 exhibits a λ-like antiferromagnetic transition at 2.35 K, with K 2 NaCrO 8 behaving similarly, with T N 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 M 3 NbO 8 . Their specific heat data indicate that they could find applications as tunable heat sinks and magnetic refrigerants in the mK to 5 K range.