High-pressure synthesis of quantum magnet M − YbTaO 4 with a stretched diamond lattice

We report bulk magnetic properties of ytterbium tantalate in its monoclinic fergusonite modification, M − YbTaO 4 . The spin- 1 2 Yb 3 + ions in this phase are arranged on a geometrically frustrated “stretched diamond” lattice. M − YbTaO 4 cannot be prepared at ambient pressure and was instead prepared in a belt-type apparatus at 6 GPa and 1800 ∘ C . Susceptibility and specific heat data show no long-range ordering down to 1.8 K and are consistent with a J eff = 1 2 Kramers doublet which splits in an applied field. Furthermore, under high-pressure synthesis the entire solid solution YbNb x Ta 1 − x O 4 ( 0 ≤ x ≤ 1 ) can be stabilized in the M phase, in contrast to ambient-pressure synthesis which favors the competing M ′ phase for Ta-rich compositions. Subsequent annealing of the Nb-Ta mixed samples resulted in color 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.