High-pressure synthesis of quantum magnet <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>M</mml:mi> <mml:mtext>−</mml:mtext> <mml:msub> <mml:mi>YbTaO</mml:mi> <mml:mn>4</mml:mn> </mml:msub> </mml:mrow> </mml:math> with a

We report bulk magnetic properties of ytterbium tantalate in its monoclinic fergusonite modification, <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mi>M</a:mi> <a:mtext>−</a:mtext> <a:msub> <a:mi>YbTaO</a:mi> <a:mn>4</a:mn> </a:msub> </a:mrow> </a:math> . The spin- <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mfrac> <b:mn>1</b:mn> <b:mn>2</b:mn> </b:mfrac> <b:mo> </b:mo> <b:msup> <b:mrow> <b:mi>Yb</b:mi> </b:mrow> <b:mrow> <b:mn>3</b:mn> <b:mo>+</b:mo> </b:mrow> </b:msup> </b:math> ions in this phase are arranged on a geometrically frustrated “stretched diamond” lattice. <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mrow> <c:mi>M</c:mi> <c:mtext>−</c:mtext> <c:msub> <c:mi>YbTaO</c:mi> <c:mn>4</c:mn> </c:msub> </c:mrow> </c:math> cannot be prepared at ambient pressure and was instead prepared in a belt-type apparatus at 6 GPa and <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:mrow> <d:msup> <d:mn>1800</d:mn> <d:mo>∘</d:mo> </d:msup> <d:mi mathvariant="normal">C</d:mi> </d:mrow> </d:math> . Susceptibility and specific heat data show no long-range ordering down to 1.8 K and are consistent with a <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"> <f:mrow> <f:msub> <f:mi>J</f:mi> <f:mi>eff</f:mi> </f:msub> <f:mo>=</f:mo> <f:mfrac> <f:mn>1</f:mn> <f:mn>2</f:mn> </f:mfrac> </f:mrow> </f:math> Kramers doublet which splits in an applied field. Furthermore, under high-pressure synthesis the entire solid solution <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"> <g:mrow> <g:msub> <g:mi>YbNb</g:mi> <g:mi mathvariant="normal">x</g:mi> </g:msub> <g:msub> <g:mi>Ta</g:mi> <g:mrow> <g:mn>1</g:mn> <g:mo>−</g:mo> <g:mi mathvariant="normal">x</g:mi> </g:mrow> </g:msub> <g:msub> <g:mi mathvariant="normal">O</g:mi> <g:mn>4</g:mn> </g:msub> </g:mrow> </g:math> ( <k:math xmlns:k="http://www.w3.org/1998/Math/MathML"> <k:mrow> <k:mn>0</k:mn> <k:mo>≤</k:mo> <k:mi>x</k:mi> <k:mo>≤</k:mo> <k:mn>1</k:mn> </k:mrow> </k:math> ) can be stabilized in the <l:math xmlns:l="http://www.w3.org/1998/Math/MathML"> <l:mi>M</l:mi> </l:math> phase, in contrast to ambient-pressure synthesis which favors the competing <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:msup> <m:mi>M</m:mi> <m:mo>′</m:mo> </m:msup> </m:math> 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.