High-pressure synthesis of quantum magnet M-YbTaO4 with a stretched diamond lattice

We report bulk magnetic properties of ytterbium tantalate in its monoclinic fergusonite modification, M-YbTaO4. The spin-1/2 Yb3+ ions in this phase are arranged on a geometrically frustrated “stretched diamond” lattice. M-YbTaO4 cannot be prepared at ambient pressure and was instead prepared in a belt-type apparatus at 6 GPa and 1800 oC. Susceptibility and specific heat data show no long-range ordering down to 1. 8 K and are consistent with a Jₑff = 1/2 Kramers doublet which splits in an applied field. Furthermore, under high-pressure synthesis the entire solid solution YbNbxTa (1–x) O4 (0 ≤ x ≤ 1) can be stabilised in the M phase, in contrast to ambient-pressure synthesis which favours the competing M′ phase for Ta-rich compositions. Subsequent annealing of the Nb-Ta mixed samples resulted in colour changes, suggesting oxygen deficiency in some of the as-prepared high pressure samples. There was little variation in the bulk magnetic properties upon varying either the Nb/Ta ratio or the annealing conditions.